Coastal Erosion Study; Omaui, New River Estuary, Southland

Transcription

1 Coastal Erosion Study; Omaui, New River Estuary, Southland REPORT FOR Environment Southland DATE June 2007 CLIENT REFERENCE 157SRC.1135 AUTHOR Derek Todd COPIES SUPPLIED TO Environment Southland

2 EXECUTIVE SUMMARY The aim of this study, which has been commissioned by Environment Southland, is to gain an understanding of the processes responsible for the past patterns of channel and shoreline movements around Omaui village on the south shore of the entrance to the New River Estuary; to determine whether these processes have changed; and if so, what are the implications for future shoreline movements. The study highlights that instability of the entrance to the New River Estuary is a natural process, which has been occurring for over 150 years of recorded history, including before a 25% reduction in estuary area as a result of major reclamation works at the head of the estuary at the beginning of the last century. This historical instability of the entrance can be considered as regular cycles of morphological change, with a natural progressive movement of the ebb channel to the south over a period of time and associated with a narrowing or closing of this channel and the opening of a secondary northern channel, followed by a return to a more central position of the main channel. These cycles of morphological change appear to be driven by three processes: 1) The low ratio of the tidal prism to the longshore transport rate which allows the ebb tide delta to encroach on the main ebb channel on the southern side of the delta; 2) Different flow routes of the flood (SE over the northern end of the delta) and ebb tide that limits the ability to maintain a wide ebb channel on the southern side of the inlet; and 3) Major river flood events opening up the southern ebb channel through the delta and relocating it to a more central location. The contemporary aerial photographs show that prior to 1962 the ebb channel and delta configuration underwent cycles of migration similar to those described in the historical accounts. Shoreline erosion along the Omaui shore was associated with this inlet instability, with the most eroded vegetation position at the village and along the Holocene dune foreland to the east being in the 1952 and 1956 photographs. During the 1962 to 1978 period, the delta and channel formation moved to a classic third stage of the mouth morphology cycle with a large ebb delta, a 400 m southern shift in the channel location along the western edge of the Holocene dune foreland to the east of Omaui village, and a well developed northern channel along the Oreti shore. Advance of the shoreline as measured by the position of the vegetation line occurred along the Omaui shore, and shoreline erosion was occurring along the Oreti shore. Since 1978 the past cycles appear to have been broken with the main ebb channel remaining relatively static in this southern position with only a narrow beach being present at Omaui, Environment Southland i

3 which has been insufficient to perform a natural buffering function against erosion during events that combine strong westerly conditions with high water levels. The presence and location of secondary channels through the central and southern regions of delta, the orientation of the shoreline, and a higher than average coastal storm frequency within a number of time periods also contributed to maintaining high rates of erosion on the Omaui shoreline through the 1980 s and 1990 s. As a result, since this time the vegetation line in the dune area to the east of the village has eroded over 60 m, such that by 2004 it had retreated to a similar position as it was in At Omaui village and to the west, where there has also been large beach losses, shoreline retreat has been less due to the more resistant nature of the hinterland. It is considered that this current trend has occurred due to the channel becoming entrenched in this position, possibility as a result of a cluster of flood events in the late 1970 s to early 1980 s, or due to mouth morphology having gained a state of stable equilibrium, and any changes, such as floods, results in a feedback loop returning the channel rapidly back to this position. The key question is whether the ebb channel will remain in its current static position, therefore maintaining narrow beach widths at Omaui and allowing high energy westerly events to attack the shoreline. Clearly while the channel remains in its current position, erosion will continue, particularly on the Holocene dune environment to the east of the village. This area has the least resistance to erosion and the highest exposure, so erosion will continue to be higher in this area under this scenario. At the village, and to the west, the now nearly total removal of the sand beach and the exposure of more resistant rock shoreline will continue to slow the rate of further erosion of the hinterland. The re-development of a beach at Omaui is dependant on the movement of the ebb channel away from its current position. Given that the channel has become entrenched in this location, and that very significant flood events in the recent past failed to achieve this outcome, this does not appear likely under the current process environment. Even if the channel was to re-locate away from the Omaui shore, or the beach was re-established, by either natural or artificial means, it needs to be emphasised that morphological instability of the mouth of the inlet would still occur due to the nature of relationship between the process variables at the site. Environment Southland ii

4 CONTENTS Page No. 1.0 Introduction Project Outline Background on Estuary Inlets Investigations Methodology Physical Components of the New River Estuary Phsical Setting Geology Marine Processes Tidal Parameters Extreme Sea Levels Tsunami Wind and Wave Climate Currents Sediment Supply Longshore Sediment Transport Fluvial Processes Flood Flows Sediment Supply Estuary Sedimentation and Reclamation Estuary Sedimentation Estuary Reclamation Historical Changes Results of Literature Review Information Sources Stability of the Entrance Channel Prior to Estuary Reclamation Stability of the Entrance Channel Following Estuary Reclamation Preceptions of Long-Term Residents Shoreline Changes Inlet Channel Changes Near Omaui Village Estuary Changes Changes Outside the Estuary Flood Effects 45 Environment Southland iii

5 3.2.6 Tidal Currents and Sea Events Summary Aerial Photograph Analysis Changes in Position of Estuary Mouth Features Shoreline Vegetation Beach Width Channel Width and Ebb Tide Delta Location Identification of Patterns, Cycles and Trends Influence of Extreme Fluvial and Coastal Events Since Flood Events Marine Events Summary of Major Findings from Aerial Photograph Analysis Conclusions Patterns of Shoreline Movement Processes Responsible for Observed Patterns Implications for further Shoreline Movements References 75 APPENDICES A: Aerial Photographs used in the Analysis B: Shoreline Mapping Baselines and Measurement Profiles C: Flood Records for Rivers Discharging into the New river Estuary D: Old Charts of the Entrance of the New River Estuary E: Summary of Historical Changes in New river Mouth Estuary 1830's to 1940's Environment Southland iv

6 Introduction 1.1 Project Outline DTec Consulting Ltd have been commissioned by Environment Southland to investigate and report on the processes responsible for the current shoreline erosion at Omaui village, which is located on the south shore of the entrance to the New River Estuary ( see Figure 1, p9). Aerial photographs over the past 60 years show that while the entrance channel to the estuary has predominantly been on the south side of the inlet, its position has fluctuated over time resulting in periods of accretion and erosion of the beach and shoreline at Omaui. In the past these fluctuations in the channel and shoreline position have been often referred to as being in cycles of varying lengths of time from 7 years to 30 years. However, the feeling of long-term local Omaui residents is that in recent years the cycle has been broken, with the sand volumes on the beach in front of the village being at an all time minimum from which it will not recover. This is of concern to the residents and the Invercargill City Council, as continued shoreline retreat will effect beach access, recreational amenity values, road stability, and the oxidation ponds for the village. The aim of this study is to gain an understanding of the processes responsible for the past patterns of channel and shoreline movements around Omaui village; to determine whether these processes have changed; and if so, what are the implications for future shoreline movements. 1.2 Background on Estuary Inlets An inlet is considered to be a region connecting two or more large water bodies by a relatively short and narrow channel. The water bodies may be, as in the case at Omaui, the ocean and an estuary. The scientific literature contains many definitions of estuaries. Hume & Herdendorf (1988) suggest that perhaps the most widely used is that of Pritchard (1967), which states that an estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which seawater is measurably diluted with fresh water derived from land drainage. Therefore, within the physical setting of an estuary, which is determined by the geology and geomorphology of the surrounding land and coast, the behaviour of the estuary is a function of the complex interactions between the marine and fluvial processes. The key fluvial process variables include the river flow and sediment discharge, while key marine processes variables include the tidal range, waves approach, and sediment transport. Due to Environment Southland 1

7 climatic variability, the majority of these processes variables can experience large scale variations within short periods of time, which may or may not occur con-currently. This creates difficulties for the interpretation of process and response interactions within estuaries. Estuary responses to the interaction of coastal and fluvial processes can include: Changes in location of estuary channels Sedimentation or erosion of the estuary basin Changes in the size and location of the tidal deltas Variations on how sediment is by-passed between the coast and estuary (tidal bypassing 1 or bar bypassing 2 Instability of the inlet location Changes to the size of the inlet (morphological instability). For the current study it is the last three of these responses which are of particular interest. Clearly human changes to the coastal or fluvial processes, or to the estuary configuration, can also be the catalyst for changes in the estuary behaviour. However, within the complex interactions of estuary behaviour, the scientific literature on tidal inlets recognises three relationships between some of the estuary parameters which are of relevance for the stability of the inlet mouth channel and the ocean part of the estuary environment, that being the ebb tide delta 3 and associated channels. Therefore these relationships are of significance to this study. These relationships are: 1. A relationship between the inlet gorge channel cross-sectional area and the tidal prism 4 of the estuary, which is often written: A = CΩ n, where A is the inlet channel cross-sectional area at mid tide, Ω is the tidal prism in spring tides, and C & n are constants. This relationship has been observed in many inlets through out the world, and was established for some New Zealand inlets by Furkert (1947) and Heath (1975), hence is often referred to as the Furkert-Heath relationship. Hume & Herdendorf (1993) extended this work to 82 inlets, and redefined the relationship on estuary type. Unfortunately the New 1 When sand is transported into the estuary on the flood tide, where some may be deposited, and then returned to sea and the littoral system by the ebb tide discharge. 2 Sand bypasses the estuary entrance on an offshore bar under the combined influence of waves and currents. 3 Bar or shoal formed in the ocean on the outside the inlet mouth Environment Southland 2

8 River Estuary was not included in the analysis. However, this analysis found that the relationship was strongest (r 2 =0.98) for barrier enclosed estuary, such as the New River Estuary, with the relationship being: A = 2.46 x 10-4 Ω The A-Ω relationship has often been use to characterise the morphological stability of the inlets, with the inference being that those estuaries which do not fit the relationship were unstable, and would therefore be undergoing either erosion or deposition in the inlet gorge or the estuary body to order to achieve this stability. As pointed out by Hume & Herdendorf (1988b) these adjustments may be by either a flood scouring the inlet throat, depositing sediment in the estuary or eroding the estuary banks; or by storm waves action depositing littoral material in the inlet. However, Hume & Herdendorf (1988b), also point out that because a range of physical factors may influence inlet throat dimensions, the A-Ω relationship should not be used as the sole descriptor of instability. 2. The above A-Ω relationship also indicates that one of the main factors determining the size of the entrance is the ability for flow through the entrance to transport sediment. Hence, for inlet stability the tidal flows through the entrance have to be sufficient to transport the sediment deposited there by wave generated and ocean currents. Based on this concept Bruun & Gerritsen (1960) introduced the Ω/M tot ratio to evaluate the relative degree of overall stability of the tidal inlet, where M tot is the total annual littoral drift carried to the inlet. Bruun (1978) classified the likely entrance conditions at estuaries according to this ratio as being: Ω/M tot Ratio Entrance Condition Ω/M tot > 300 Little or no bar outside the inlet throat. Good entrance conditions and flushing 150 < Ω/M tot > 300 Little ocean bar 100 < Ω/M tot > 150 Low ocean bar, navigation problems usually minor 50 < Ω/M tot > 100 Wider and higher ocean bar, increasing navigation problems 20 < Ω/M tot > 50 Wide and shallow ocean bar, navigation difficult Ω/M tot < 20 Very shallow ocean bar, navigation very difficult, entrance unstable Hence, large ratio values are associated with tidal by-passing, and low ratio values are associated with bar by-passing. 4 The total amount of water which flows into or out of the estuary with the movement of the tide, excluding any freshwater flows. Also called the tidal compartment. Environment Southland 3

9 Bruun (1978) also reported that for stable inlets on sandy coasts the mean maximum velocity through the inlet throat was generally in a narrow range of 1 m/s ± 15% (approx 2 knots). This is the termed the critical velocity for sediment transport in the inlet throat, being capable of moving all sediment sizes up to a diameter of 10 mm that may be deposited into the inlet by littoral drift. Hume & Herdendorf (1992) measured a slightly greater range for estuaries on the north-east coast of New Zealand, with those associated with higher velocities occurring at coasts with high rates of littoral drift, and deeper gorge channels. In the medium to shorter term, Escoffier (1940) found that there were two states of inlet area being in equilibrium with the flow velocities through the mouth; one being a state of stable equilibrium, and the other an unstable equilibrium. In stable equilibrium, any change in inlet area results in a feedback loop returning the channel back to an equilibrium position. In this state if inlet area decreases, then velocity increases, resulting in scouring back to equilibrium area; or if inlet area increases, then velocity drops and deposition occurs to return the area back to equilibrium. In an unstable state, two situations can occur; if the inlet area decreases, velocities also decrease until the inlet closes; or the velocity increases (e.g. during a river flood) resulting in an increase in the inlet area with scour, followed by a decrease in the velocity such that the state of stable equilibrium is reached. 3. A linear relationship between the volume of sand on the ebb tide delta and the volume of the tidal prism. This relationship has been observed for several tidal deltas on different coasts of the USA (e.g. Pacific, Atlantic, Gulf), and Hicks & Hume (1996) established the following values for the relationship for 17 North Island New Zealand tidal deltas: V = 1.37x10-3 Ω 1.32 (Sin θ) 1.33 where V is the volume of the ebb tide delta (m 3 ), Ω is the volume of the spring tidal prism (m 3 ), and θ is the angle of the ebb tide jet with respect to the shoreline orientation. Hicks & Hume (1996) suggest that the strong influence on the size of the ebb tide delta by the tidal prism appears to be due to; (1) the momentum and hence off-shore extent of the ebb jet is directly related to the ebb discharge, and (2) the delta deposit scales with the length of the ebb jet. Due to the high correlation between the inlet cross-section area and the tidal prism, there is also a linear relationship between the size of the ebb delta and the inlet area. Environment Southland 4

10 Hicks & Hume (1996) found that the angle of the ebb jet with respect to the shoreline was a secondary control on the on the size of the ebb delta, greater angle resulting in higher volumes in the delta. Hicks & Hume considered this to be due to the more shore-normal the ebb outflow, the greater the water depth and the more space available for a deposit to grow on top of the background nearshore topography, and as the ebb jet angle becomes more acute, there is less direct conflict of tidal and wave energy. The results of the research also suggested that for estuaries with similar tidal prisms, the ebb tide delta volume also appeared to increase with decreasing wave energy. However, a relationship between these two parameters could not be determined from the data available. In relation to the current study, it can be seen from the above relationships that changes in the size of the tidal prism, such as a reduction due to large scale reclamation at the head of the New River Estuary, will have a large influence in the size of the inlet channel at Omaui and the size of the ebb tide delta at the entrance to the inlet. These long-term adjustments are likely to be marked by periods of instability in the mouth channel and ebb tide delta form. In the shorter-term, inlet channel changes and instability could also be driven by fluctuations in the coastal sediment load being delivered to the mouth of the estuary, and in the river discharge into the estuary. 1.3 Investigation Methodology The methodology involved the following three step approach: 1. Review of information on changes in the position of the inlet channels and shorelines at the mouth of the New River Estuary. This was further divided into two components: A literature review of books and documents which give accounts of changes in the bar, channel and shorelines at the estuary mouth. Principle sources of this type of information were NRETAC (1973), Smith (1924), and Rennie (1980). Interviews of long-term local residents of the Omaui area. This was largely accompanied by a collective meeting with the following residents: Dorothy Gilbert, Murray Sharp, Bill Smellie, Lindsay Russell, Bruce Allen, Geoff Piercy, Alan and Sue Campion; and telephone conversations with Bob Ferguson and Graham Metzger. The oldest memories of Omaui dated back 65 years (to 1941), and at least two others had memories over a 50 year period. 2. Mapping of shoreline and channel changes at the entrance to the New River Estuary from aerial photographs. The area of interest included the south shore from the Pilot Station to Environment Southland 5

11 Mokomoko Inlet, the entrance channel and ebb tidal bar at the mouth of the channel, and the western and southern shores of Sandy Point on the north side of the inlet channel. A total of 16 photograph dates were used spanning 60 years from 1947 to Unfortunately not all of the photo dates covered all of the study area, particularly 2000, which missed out the ebb tide delta, and 2003, 2005, and 2006 which only covered the southern shore or parts of it. All photographs and a full list of the photograph dates, sources and area of coverage is presented in Appendix A. Not all of the photographs used were ortho-rectified to correct errors in measurement due to distortion of the photographs. However, to minimise the errors each of the photograph runs were geo-referenced using the LINZ aerial ortho-photos flown in 1998 as the georeferencing benchmark since these are ortho-rectified images. This geo-referencing involved locating common reference points, including Bombay Rock, on each set of photographs to provide relative positioning, and then using computer software to warp the second photo in each case to the 1998 locations. On each photo date the locations of the following features were digitised: The vegetation line on both the north and south shore of the estuary entrance The edge of the inlet channel as marked by exposed sand along each shore of the estuary entrance. The location of the ebb tide delta and any channels through the bar. The resulting shorelines, channel edges and ebb tide delta positions were overlaid on the 1998 base photo, with the changes representing a time series of response of these features to changes or variability in the tidal prism or marine and fluvial parameters that drive these responses. Changes in shoreline vegetation and channel edge position were electronically measured Tide levels for at the time of each of photograph were obtained to assist with interpretation of changes in the channel and bar locations. When the locations of these features was uncertain, dashed lines were used in the mapping. To quantify the changes in shoreline positions, at each date the distance from fixed baselines established on either side of the mouth to the vegetation and wetted line (e.g. channel edge) was measured at 50 m intervals for a 3 km length of shore on either side of mouth as shown in Appendix B. The results from the individual profiles were then averaged for sections of shore with similar aspects, with the Omaui shore being divided into five areas, and the Sandy Point shore into three areas. Environment Southland 6

12 3. Examine the record of changes in the tidal prism and variability in the marine and fluvial drivers to see if there is any temporal correlation with the changes in the inlet channel. Information on changes of the tidal prism came from information on reclamations and estuary sedimentation contained in NRETAC (1973), Blakely (1973) and Thoms (1981). Information on longshore transport was interpolated from the 20 year ( ) hindcast of deep water waves around New Zealand, which has been generated by the WAM wave generation model (Gorman & Laing 2000, Gorman et al 2003). This hindcast model provides 3 hourly wave statistics for 53 cells around New Zealand at o x o spacing, hence over the 20 year period of the model there are 58,440 modelled waves. For the Foveaux Strait area the wave model results were validated against the 153 day wave buoy record, with a good correlation be obtained. For inshore sites, an interpolation process as described by Gorman et al (2003) is used to transform these deep water waves to the required inshore location. A parallel contour approximation is then used to modify the resulting waves for refraction effects to the water depth at the inshore site. Wave statistics of significant wave height, mean wave period, longshore energy flux magnitude and direction are then calculated for the inshore site. For the study area, these transformations were applied to get the wave climate at an inshore site with a 10 m water depth located 2.5 km due west from the entrance to the New River Estuary. However, only six of the aerial photograph dates corresponded with the 20 year wave hindcast record from WAM. In order to extend the time series of wave data, the results of a second wave hindcast model from NOAA 5 was used for the period March 1997 to November 2006 (9.8 years). This hindcast model supplies three hourly deepwater significant wave heights, peak periods, and directions at 1 o latitude intervals around New Zealand, with the most appropriate site for the New River Estuary being NWW3 site located at longitude o W latitude 47 o S, in 90 m of water approximately 13.5 km west of Stewart Island. The refraction of these waves as they cross Foveaux Strait was assumed to be similar to earlier 20 year data set, so the correlation between the two data sets over the two years of overlapping record ( ) was used to transform the recent deep water data to a comparable inshore wave climate. A further five aerial photograph 5 NOAA: National Oceanic & Atmospheric Administration, US Department of commerce. The wave hindcast programme is Wave Watch 3 (NWW3) Environment Southland 7

13 dates are covered by this wave data set, therefore, in total 11 of the 16 aerial photographs are covered by some form of wave time series data. Historical information on flood events up to 1953 was obtained from Cowie (1957), although this was largely restricted to the Oreti catchment. Environment Southland provided flows for flood events since 1977 in the Oreti and Waihopai Rivers, and since 1982 for the Makarewa River. Therefore total flow flows into the New River Estuary are only available for events since this later date, however 10 of the 16 aerial photographs was taken during this period. Environment Southland 8

14 2.0 Physical Components of the New River Estuary 2.1 Physical Setting The New River Estuary is located on the south coast of Southland, discharging into Foveaux Strait in the lee of Stewart Island. The estuary has a current area of km 2 of which km 2 is exposed at low tide (Thoms 1981) 6. However, the estuary used to be larger, with 12.2 km 2 having been reclaimed over the last 115 years around the city of Invercargill, which is located at the head of the estuary (see Figure 1). The estuary has a catchment area of 3950 km 2, primarily of Southland plains, and receives discharge from two principal rivers, the Oreti River, whose catchment area covers 89% of the estuary catchment, and the Waihopai River, which flows through the city of Invercargill to discharge into the head of the estuary. Five smaller creeks discharge into the eastern side of the estuary. From the geomorphic classification of New Zealand estuaries by Hume & Herdendorf (1988), the New River estuary is a barrier enclosed estuary of the single spit enclosed type. This type of estuary is commonly situated adjacent to a stable rock headland, where wave refraction results in a low energy stable site for the inlet. However, strong tidal currents and waves at the inlet constantly change its form within this stable location. At the New River Estuary, the stable down-drift headland (Steep Head) extents around 3 km further west into Foveaux Strait that the northern shore at Sandy Point (See Figure 1), which is the southern limit of a large log-spiral shaped or zeta form sand bay which extends 10 km northwest to the rock headland at Howells Point, Riverton. The sheltering effect of Stewart Island on large southerly waves, and the generally shallow water depths in Foveaux Strait adds to the low-energy environment at the mouth of the estuary. Due to off-set configuration of the shoreline around the estuary mouth, and the location of the ebb tide jet along the shore of the south headland, the ebb-tide delta is restricted to the northern side of the inlet entrance (see Figure 1). This triangular or L shaped delta or bar is common for these inlet configurations, and are classified as high angle half-deltas by Hicks & Hume (1996). The size of delta in Figure 1 is around 2 km 2, and it southern extent varies in response to changes in inlet channel and sea conditions. Historically these changes in the extent of the bar have caused problems for ships entering the estuary. 6 Ballinger (1972) gives the estuary area as being 41.4 km 2. however check measurements show that this is incorrect, and the area given by Thoms (1981) is correct. Environment Southland 9

15 Waihopai River Dunns Rd Bushy Point Oreti River X Daffodil Bay Gibb 1978 shoreline movement site X Sandy Point Owi Pt Ebb tide Delta Entrance Point Bombay Rock Pilot Station Steep Head Mokomoko Inlet Figure 1: The New River Estuary. Omaui is on the south coast just outside the entrance channel to the estuary. Photo Date January Omaui village is located at the eastern end of the southern headland, across the entrance channel from the ebb tide delta (see Figure 1). Therefore, technically the village is located on the ebb tide channel outside of the estuary, with the inlet channel proper starting around 1 km further east, just past Bombay Rock. This section of shoreline is the western edge of a Holocene dune field, which extends east to Mokomoko inlet. At the throat of the inlet (narrowest point), the entrance channel is in the order of 1 km wide. Environment Southland 10

16 2.2 Geology The geology of the New River Estuary is described by NRETAC (1973), Blakely (1973) and Thoms (1981). The estuary is geological young, being formed during the most recent higher sea levels during the late Pleistocene. The only areas of older more resistant rocks being found in the Omaui-Mokomoko area, including the outcrops located in the entrance channel to the estuary, which are known as Bombay, Guiding Star, and Mokomoko Rocks (NRETAC 1973). These are ancient volcanic rocks that date from around 280 million years ago. Elsewhere, the land around the estuary consists of a complex series of sedimentary deposits which range in age from Pliocene (upper Tertiary) to Recent (NRETAC 1973). presented in Figure 2 (next page). A geological stretch map from this report is As can be seem from this figure, the remnants of historical shorelines when sea level were in the order of 25ft (7.6 m) and 6ft (1.8 m) above the current level, together with a series of beach storm ridges laid down at a time when sea level dropped from this latest peak, and a prevailing westerly transport of coastal sediment, have influenced the size and shape of the modern estuary. The estuary at the time of the +7.6m shoreline, would have been much smaller than present, occupying only the lower drowned river valley of the Waihopai River (Thoms 1981), with the mouth of the Oreti River at the time being around Wallacetown to the north of the confluence with the Makarewa River (NRETAC 1973). By the time of the +1.8 m shoreline, which has been carbon dated at 4660±85 years, the mouth of the Oreti River had moved south to be around the confluence with Makarewa Rivers, hence still not discharging into the estuary. This mouth is then considered to have closed due to coastal drift and/or a drop in river flow resulting from a postglacial loss of a large portion of water supply due to recession of Von branch of Wakatipu glacier and subsequent beheading of upper reaches of the river. This forced the lower Oreti River into a new course to the south between the newly formed storm ridges and the cliffed west face of the Otatara Ridge marine sequence, which was laid down in an earlier period of higher sea level. The age of this course of the lower river is put in the range of 5,000 to 1,000 years BP (Blakely 1973), which is taken to be the age of the modern estuary. Environment Southland 11

17 Figure 2: Geological Sketch Map from NRETAC (1973) Environment Southland 12

18 2.3 Marine Processes In the context of this study, the key marine processes are tidal flows into and out of the mouth of the estuary, the wave climate and refraction patterns together with currents in Foveaux Strait driving longshore sediment transport, and the rate for sediment supply to the mouth area Tidal Parameters The NZ tide tables (LINZ) give the tidal range at the entrance to the New River entrance as being 2.4 m in spring tides and 1.8 m in neap tides. These ranges are different from those given by Thoms (1981), who appears to have used the tide heights in chart datum as tidal range. The tidal prism, being the volume of water exchanged between the ocean and the estuary in each tide, is calculated from the product of the tidal range and the area of estuary covered and uncovered with water in each tidal cycle. Thoms (1981) calculated the tidal prism of the New River Estuary in spring tides to be 72.6 million m 3. However this appears to be an over approximation due to not making allowance for the tidal flats not being covered to the full depth of the tidal range. Applying the method of tidal prism calculation used by Heath (1975), which makes allowance for bed slope of the mudflats by approximating that they will be covered on average to a depth of half the tidal range, produces the following tidal prism volumes at spring and neap tides: Tidal prism = (subtidal area x tidal range) + (mudflats area x tidal range x 0.5) For Spring Tide: Tidal prism = million m 3 For Neap Tide: Tidal prism = million m 3 Given that New Zealand experiences semi-diurnal tides, this volume of water passes through the inlet entrance every 6.25 hours, therefore giving a mean discharge through the inlet of 2,300 m 3 /s during spring tides and 1,720 m 3 /s during neap tides. These tidal flows are in the order of times larger that the normal fluvial flows into the estuary, hence it clear that the tidal component completely dominates the flows in and out of the estuary. From the A-Ω relationship from Hume & Herdendorf (1993) 7 for New Zealand barrier enclosed estuaries, the resulting stable cross-sectional area of the inlet throat at mid tide should be the order of 3,480 m 2. This is a considerably larger cross-section area than the 780 m 2 obtained by Thoms (1981). There are two reasons why it is considered that the area given by Thoms is too low: 7 A=2.46x10-4 Ω Environment Southland 13

19 1. The resulting mean tidal velocity through the channel would have had to have been in the order of 2.9 m/s (approx 5.6 knots). This is nearly three times the critical maximum velocity for stability from Bruun (1978), and more than double the mean for any other of the 32 New Zealand barrier enclosed estuaries investigated by Hume & Herdendorf (1993). 2. The width of the channel in aerial photographs around the time of Thoms study (1979, 1980, 1981), which were all taken during low tide conditions, was in the range of m wide, therefore implying a mean depth across the channel at mid tide of 1 m or less. This is considered to be too shallow for the main tidal channel at this state of the tide. In comparison, the above discharge and calculated inlet cross-section area from the A-Ω relationship gives a mean spring tidal velocity through the inlet of 0.66 m/s (approx 1.3 knots), which is similar to the average mean tidal velocity found for the 32 New Zealand barrier enclosed estuaries investigated by from Hume & Herdendorf (1993). This is considered to be a much more realistic estimate of the tidal velocity that implied by throat cross-sectional area given by Thoms. This velocity is still able to initiate the transport of all sand sized sediment, hence is theoretically sufficient to prevent deposition of littoral drift material in the inlet throat. Also from the tidal prism we can calculate the theoretical volume of the ebb tide delta from the relationship given in section 1.2. The ebb jet along the headland from Omaui to Steep Head has an angle in respect to the orientation of Oreti Beach in the order of 60 degrees, therefore the resulting theoretical ebb tide delta volume should be in the order of x 10 6 m Extreme Sea Levels Extreme sea levels can result from large astronomical tides and storm surge. They result in shortterm increases in the volume of water passing through the estuary inlet, which increases flow velocities in the inlet and may result in channel scour and changes to the ebb tide delta, and hence influence the morphological stability of the inlet. Their influence will be greatest when they occur in combination, or in association with high river flows. Large Astronomical Tides The largest astronomical tides occur during the equinoxes, when all the gravitational pull of the earth and moon are at an annual maximum. These equinoctial high tides at the entrance to the New River Entrance are in the order of 1.5 m above MSL, which is 0.3 m above the MHWS. The influence of these extreme tide levels on estuary water levels can be established from the record of high water levels recorded at Stead Street presented in Bradley (2001) and reproduced in Table 1 Environment Southland 14

20 (page 18) Applying the NIWA tide forecaster 8 on the record shows that of the 15 events in which estuary water levels are recorded to have greater that 2 m above MSL, seven occurred when predicted tides were above 1.4 m, and 80% when predicted tides were above MHWS (1.2 m). On the day of the highest recorded water levels at Stead Street, 2.57 m above MSL, (1 June 1958), predicted tide levels were 1.37 m above MSL, while the highest predicted tide in association with an extreme water level was 1.57 m (29 March 1998) when water levels reached 2.51 m, which is the third highest on record. In these maximum water level events, the recorded levels are in the range of m above predicted tide, however this can not be taken as purely storm surge influence due to the contribution of river flows. For example, Bradley (2001) gives the actual sea level at Bluff in the event on 29 March 1998 as being 1.67 m above MSL, hence storm surge taken as the difference between actual and predicted tides, was only 0.10 m, whereas estuary water levels were 0.94 m above predicted tide level. High flows in the both the Makarewa and Waihopai Rivers coincided with this extreme tide event, therefore are likely to have contributed to 0.84 m higher water level in the estuary, along with storm surge estuary effects. What is also of relevance from the data in Table 1 is that in only one event in (16 May 1994) was tide not the significant contributor to water level. This highlights the importance of tides in extreme water level events. For the 29 March 1998 sea level, the volume of water involved in tidal exchanges through the mouth of the estuary (e.g. excluding river flow), would have been in the order of x 10 6 m 3, which is 24% greater than the normal spring tidal prism. Storm Surge Storm surge is the super-elevation of sea level along a coast due to the effects of low atmospheric pressure and high onshore winds. The adjustment of sea level in response to air pressure changes is termed the inverse barometric effect, which in general terms equates to a 1 cm rise in sea level for every millibar (mb) drop in air pressure below the mean value at the sea surface of 1014 mb. There is a corresponding fall in sea level when air pressure is above the mean value. The response of sea level to pressure changes occurs relatively slowly, taking in the range of 2-12 hours, and affects large areas in a fairly uniform manner. The adjustment of water levels on the continental shelf due the affect of on-shore wind is termed wind stress, with the response generally being much faster and increases for shallower water depths. For shallow estuaries, such as the New River Estuary, these wind stress effects can be increased by the formation of seiches, which are long period standing waves or oscillations of water level in closed and semi-closed water basins 8 NIWA tide forecaster: Environment Southland 15

21 forced by an extreme disturbance to the water surface, such as high winds. The magnitude of these oscillations is controlled by the depth and geometry of the basin (the shallower and longer the greater the amplitude of the oscillation), and for a open ended basin like an estuary, the greatest vertical movements in water level (the antinodes of the wave form) are found at the landward end of the estuary. The combined influence of wind stress and seiche formation are referred to as estuary effects. A third adjustment is due to effect of coastal trapped waves, which are long period waves whose behaviour is controlled by the Earth s rotation and that are trapped on the continental shelf by refraction. A fourth adjustment, the near shore increase in water level due to wave set-up, can also be considered a component of storm surge. However, in most assessments it is included in the estimates of wave run-up. The upper limit for open coast storm surge heights around New Zealand is commonly stated to be around m (de Lange 1996, Bell et al 2000), of which m is inverse barometric effect, m is wind stress effect, and m is a coastal trapped wave effect (de Lange 1996). However there are variations in the magnitude around New Zealand as the inverse barometric effect is greatest on the West Coast and least on the East Coast, and reduces from north to south. Conversely, wind stress tends to be greater in the south. Unfortunately there does not appear to have been any detailed assessment of storm surge magnitudes or potential on the south coast. However, Goring (2004) reported that the largest storm surge from seven years of record at the Dog Island sea level recorder (6km from the entrance to Bluff Harbour) was 0.51 m on 16 April From the difference between recorded tide levels given in Bradley (2001) and predicted tide levels, storm surge at Bluff Harbour on the same day was less, at 0.38 m. However, water level in the New River Estuary were 0.65 m above predicted tide, highlighting the potential for estuary effects to increase storm surge levels within shallow water estuaries. A more extreme example of the potential influence of these estuary effects is 29 March 1998, when calculated storm surge at Bluff was only 0.1 m, but water levels in the estuary were 0.94 m above predicted tide level. However, high flows in the Makarewa and Waihopai Rivers are likely to have also contributed to the extreme estuary water levels recorded. A coastal storm was also running at the time, with the mean significant wave height over the day being 3.95 m from a westerly direction, and a maximum significant wave height of 5.08 m. Unfortunately, apart from the above two events and one further event on 16 June 1999, the contribution of storm surge on extreme water levels in the New River Estuary is unknown. Maximum differences between recorded estuary levels at Stead Street (Bradley 2001, and Table 1) and predicted tides were 1.30 m on 16 May Although this event coincided with a high flow Environment Southland 16

22 event the Makarewa and Waihopai Rivers, it is considered that the majority of the rise in estuary levels were due to sea conditions and estuary effects, as significant wave heights at the 10 m contour on this day were close to maximum possible, averaged above 7 m from the south west and winds were averaging around 10 m/s (20 knots) from the a WSW direction. On the day of maximum recorded estuary water level (2.57 m above MSL on 1 June 1958), the contribution of storm surge, estuary effects or river flow to the 1.2 m difference between water level and predicted tide is unknown. Cowie (1957) reported flooding in Invercargill from very high estuary levels driven by strong gale force winds in January 1935, 1938 and 1939, which suggests a component of storm surge and/or estuary effects was involved, particularly in the 1938 event when predicted high tides were only 0.8 m above MSL. Maximum Sea Levels From a joint probability analysis of tides and surge at Dog Island, Goring (2004) calculated that a 50 year return period sea level from a combination of these two processes was 1.69 m above MLOS 9, and the 100 year return period sea level was 1.73 m above MLOS. At these sea levels the volume of water involved in tidal exchanges through the mouth of the New River Estuary would be in the order of: For 50 year Return Period: x 10 6 m 3 ; 32% greater than the spring tidal prism. For 100 year Return Period: x 10 6 m 3 ; 35% greater than the spring tidal prism. It is notable that the above extreme sea level are considerably lower that the corresponding return period estuary water levels at Stead Street which are given by Bradley (2001) as being 2.78 m for a 50 year return period and 2.90 m for a 100 year return period. The difference represents the contribution of estuary effects and river flows to the estuary water levels. The maximum recorded estuary water levels from Bradley (2001) and the predicted tide levels at the time are presented in Table 1. 9 MLOS: Mean Level of the Sea. This is slightly different to MSL elevation as used in surveying due to the effects of long-term sea level rise. Environment Southland 17

23 Table 1: Maximum Recorded Water Levels in the New River Estuary. (From Bradley 2001) Date Estuary Water Predicted Water Level Estuary Ratio to Level at Stead Tide Above Tide Discharge in Spring tidal Street (m) Height (m) Height (m) Ebb tide (m 3 ) Prism 1 June x Mar x Mar x May x Sep x May x Feb x Apr x May x Apr x Jul x Mar x Sep x May x Dec x Environment Southland has maintained a water level recorder on the Stead Street Bridge since Extreme levels from before this time are from manual observations. Also presented in Table 1 is the ebb tide discharge at the time of these extreme water levels, and the ratio of this discharge to the spring tide prism of 51.7 x 10 6 m 3. As can be seem from this ratio, the volume discharged on the 1 st June 1958 would have been in the order of 90% larger than the normal spring tidal prism. The mean discharge associated with this volume would be around 4,370 m 3 /s, with a mean velocity for the throat area given by the A-Ω relationship in the order of 1.25 m/s, which is sufficient to initiate scour of the channel banks an bed to increase the channel size. With estuary waters of 2 m, the corresponding mean tidal velocities through the inlet throat would be in the order of 1 m/s, which also be sufficient to initiate channel scour. So these extreme water level conditions are examples of the second of the Escofficer (1940) Unstable conditions would result in an increase in the inlet area with scour. While these conditions are followed by a decrease in the velocity such that the state of stable equilibrium is returned, it is the conditions and timing of these unstable states which are of interest in this study. Environment Southland 18

24 2.3.3 Tsunami As with extreme sea levels, Tsunamis can result very large short-term increases in the volume of water passing through the estuary inlet. Since they are travelling at high velocities, they have the potential for promoting large changes in the morphology of the inlet channel and the ebb tide delta. Their influence will be greatest when they occur in combination with high tides. At least six tsunamis have been recorded on the Southland Coast since the 1820s (de Lange & Healy 1986; Fraser 1998; Goring pers. com ). Of these, the most significant is considered to be the event in the 1820s, when a tsunami is reported to have drowned a group of Maoris walking along the Orepuki beach. This supposedly had a wave height of around 10 m (Fraser 1998) but based on more recent tsunami modelling (see below) this is debateable. The largest tsunamis with water level records on the Southland coast are events generated by large earthquakes on the south American coast in August 1868, May 1877 and May Maximum water level variations of 1.5 m were reported at Bluff in the 1868 and 1877 tsunamis, and the same at Invercargill in the 1868 event (Fraser 1998). The records for the 1960 event show water level variations of 0.5 m at Bluff, and 1 m at Colac Bay. Based on five modelled tsunami scenarios for Southland, Downes et al. (2006) found that Tsunami triggered by Puysegur Subduction Zone earthquakes caused the highest water elevations along the Southland coast generally 1 to 2 m with some locations experiencing 2 to 4 m.. Berryman et al (2005) gives 100 and 500 year return periods tsunami wave heights along the Southland coast from all sources (near and far field) as being between 0-2 m and 2-4 m respectively Wind and Wave Climate Rennie (1980) noted that the wind regime reflects the location and latitude of the area, being in the roaring forty s wind belt, experiencing predominantly westerly airflow as well as the frequent north-east migration of sub-polar low pressure systems. Rennie presented wind data from three local NZMS 11 sites; Centre Island, Invercargill Airport, and Tiwai Point; however the duration of the wind records was no given. Although the sites showed different wind direction frequencies, with Invercargill Airport experiencing more northerly than the other two sites 12, the data showed the following general patterns: 10 Derek Goring, Mulgor Consulting Ltd: Presentation to the New Zealand Coastal Society Conference, Kaikoura, November New Zealand Meteorological Service 12 The predominance of northerly wind at the airport is shown on the wind rose in Figure 2 on p12 of this report. Environment Southland 19

25 Wind from the westerly quarter occur 50% of the time, with due west being the most dominant at 27%. Around 50% of these westerly winds have speeds greater than 21 knots (39 km/hr). This is classed as a strong breeze or stronger on the Beaufort Scale, at which large waves can form at sea. These westerly directions account for 60% of all the winds above this speed. Rennie indicated that the Central Island site was the most relevant for winds influencing waves arriving on the Invercargill coast, and that this site experienced a greater frequency of both westerly and high velocity winds than the other two sites. In relation to the frequency of strong winds, Rennie reported that Watts (1947) found that the Southland coastal region received 28 gale force winds (< 62/km/hr) per year and Davies (1977) claimed the area is subject to 10-20% gales per year. Environment Southland have 3.5 years of continuous hourly wind data from mid 2003 to the end of 2006 for a site in Miller Street, Invercargill. This data also shows a dominance of winds from the westerly quarter, occurring 57% of the time. Wind speeds over this period were not as strong as reported by Rennie (1980), with no gale force winds being recorded, and only 0.3% of the recordings being greater that 39 km/hr (Strong breeze). However, it is considered that this site is not a good indicator of wind strength (Dallas Bradley, ES, pers com). All strong winds were however from the westerly quarter. This dominant westerly wind climate drives the wave climate, with long period swell being generated in the area between 47 o S and 60 o S, which arrives at the southland coast as WSW and southwest waves. From summarising sea reports and shore observations, Pickrill & Mitchell (1979) described the wave climate to the south of New Zealand as being extremely high energy, with the prevailing deep water wave being a westerly through to south-westerly, m high and second periods. They noted that very rarely did wave heights and periods drop beneath 2 m and 6 seconds respectively, however the range of conditions is extremely variable. A 153 day data set (May-September 1989) from a wave buoy located in 90 m of water depth to the west of Foveaux Strait give a mean significant wave height 13 of 2.6 m (Gorman et al 2003), while 9.8 years of hindcast waves (March 1997 November 2006) from NOAA in a similar location gives a mean and maximum deepwater significant wave height of 3.2 m and 9.5 m respectively. Over 90% of the waves in the NOAA hindcast were from the westerly quarter, of which 10% had significant wave heights greater than 5 m. 13 Mean height of the highest 1/3 of the waves. Environment Southland 20

26 A more quantitative assessment of the inshore wave climate relevant to the New River Estuary can be defined from the 20 year ( ) wave hindcast from the WAM wave generation model. From the transformations described in Section 1.3, graphs of the resulting wave climate at an inshore site with a 10 m water depth located 2.5 km due west from the entrance to the New River Estuary are presented in Figures 3, 4, and 5. As can be seem from Figure 3, wave approach direction reflects the wind direction, refraction effects across the relatively shallow water of Foveaux Strait and the blocking effects of Stewart Island on more southerly waves. The resulting distribution is strongly uni-modal, with 95% of all waves being from directions between WSW and West. However since this direction is nearly normal to the shoreline orientation, small variations in the direction within this approach window could have a large influence on whether the direction of wave induced sediment transport is to the north or to the south. W 42% WSW 45% SW 9% SSW 3% Wave Approach Direction Figure 3: Modelled (WAM) wave approach directions at location 46.5 o S, o E in 10 m water depth off the mouth of the New River Estuary. Environment Southland 21

27 As shown in Figure 4, the mean significant wave height over the 20 years of modelled record is 1.95 m, indicating that the inshore waves are much reduced from those observed in deep water due to the process of refraction across the relatively gently sloping seabed to the west of Foveaux Strait. However, the distribution is positively skewed due to the frequent westerly passage of low pressure systems, with 16% of the modelled waves having significant wave heights of over 3m, and 2% over 5 m. It is possible for very large storm swells to still be present at the mouth of the estuary, with significant wave heights being modelled to exceed 8 m during six storm events over the 20 year period. These wave heights are the maximum possible in this water depth (10 m). Mean wave period is more normally distributed (Figure 5), with a mean period of 8.7 seconds, and a maximum of 14.6 seconds. Figure 4: Modelled (WAM) significant wave heights in 10 m water depth off the mouth of the New River Estuary. Environment Southland 22

28 Figure 5: Modelled (WAM) mean wave period in 10 m water depth off the mouth of the New River Estuary. The 20 year time series of significant wave heights is presented in Figure 6. Although there is considerable variability to the monthly means, the record displays a weak seasonality with generally lower mean wave heights in period November to March and higher mean wave heights in April to October. The graph also shows that during the years 1984 to 1985 and 1989 wave heights were considerably lower that the long term average, while 1982, 1994 and 1997 experienced mean annual height of 2.3 m or more. Environment Southland 23

29 Wave Height (m) Significant Wave Height Distribution Monthly Means Annual Means Year Figure 6: Significant Wave Height Distribution from Wave Hindcast to 10 m Water Depth at the Mouth of the New River Estuary Environment Southland 24

30 2.3.5 Currents As well as waves, sediment transport in Foveaux Strait is also influenced by currents, of which there are three sources in the Strait:, wind driven currents, tidal currents, and the Southland Current, which as a branch of the Tasman current flows eastward around the bottom of the South Island carrying water from west of New Zealand to the subtropical convergence region near the Chatham Rise. Tidal stream velocities in Foveaux Strait on bathymetric charts for the area (LINZ chart NZ69), have maximum velocities at a site in the middle of the Strait opposite Colac Bay of 82 cm/s to the east during rising spring tide conditions, and 67 cm/s to the west in falling spring tide conditions. However, these are surface velocities, hence of little relevance for seabed sediment transport. Chiswell (1996) reported current meter results obtained during 1993 from 23 m water depth at one site in Foveaux Strait. Although the Southland Current is unidirectional to the east through the Strait, this study found that due to the shallow water depths (33 m ) the direction of the current near the sea bed is still strongly influenced by wind, and flowed to the west 30 % of the time. It was presumed that this reversal is due to the influence of westward ebb tide flows during times when wind direction is not from the west. Maximum current velocities at this depth were 25.1 cm/s to the east and 18.5 cm/s to the west, with a mean net current of 3.0 cm/s to the east. Although this net current is low, the maximum velocities in either direction are sufficient to entrain and transport sediment finer than very coarse sand Sediment Supply The size and extent of the sand spit and dunes at the southern end of Oreti Beach (e.g. Sandy Point) and on the south side of the entrance channel between Omaui and Mokomoko Inlet, clearly indicates that in recent geological time there has been large supplies of sand to the entrance of the New River Estuary. Assuming a mean dune height of 10 m within these areas, the accumulated sediment volume is estimated to be in the order of 13.0 x 10 7 m 3. Also assuming that this accumulation dates from around the time of relative sea level standstill 6500 years ago, gives an annual accumulation rate in the order of 20,000 m 3 /yr. The source of this sand has assumed to be the reworking of ancient sea floor sediments in Foveaux Strait (Kirk 1980), which are transported to the west and shoreward by the dominant western waves and currents. Sampling of seabed sediment in Foveaux Strait by Cullen (1967) found coarse sands to the middle of Oreti Beach, and medium to fine sand at the mouth of the New River Estuary, which is consistent with the sediment sizes found on the beach. Environment Southland 25

31 In his national inventory of shoreline movements, Gibb (1978) included two sites on the Oreti Beach spit (map reference: NZMS 260 E & ), both of which showed accretion in excess of 3.5 m/yr for the period 1907 to 1963 (e.g. advance of at least 200 m). These results suggest that rapid rates of sediment supply accumulation were still occurring in contemporary time periods. However, Rennie (1980), cast doubt on the use of Gibb s rates for long-term trends, as they covered the period of maximum influence of an successful dune restoration programme in this area in the 1920 s and 1930 s, which involved the extensive use of scrub groynes and marram planting to combat the large scale erosion due to vegetation removal by grazing and rabbits in the 1880 s and 1890 s. Comparing shoreline positions from 1865 and 1974 cadastral plans, Rennie shoreline erosion order of m over this longer period at the same sites as Gibb measured accretion. Rennie also undertook aerial photograph comparisons of more recent shorelines ( ), finding much slower rates of shoreline erosion (-1 m/yr) or stability for the area south of Dunns Road, and accretion for the beach to the north. No attempt has been made in this current study to resolve the conflict between the results of Gibb (1978) and Rennie (1980) regarding the shoreline movements. However, shoreline movements measured from aerial photographs since 1947 (see section 4.1.1) do not support the findings of Rennie. It is noted that the accuracy of old shorelines positions on historical cadastral plans as used by both authors, is questionable. Rennie (1980) also disagreed that sea bed was the source of modern sand progradation, as the strong tidal streams and currents in the Strait should have already removed all the surplus seabed sands, and that the Waiau River is now the more likely supplier of sand along with backshore erosion. This change in dominant sediment supply is consistent with a large decrease in beach accumulation rate and possible switch to erosion described by Rennie, but is not supported by other investigations Longshore Sediment Transport As established in section 1.2, the annual longshore transport rate influences the stability of the tidal inlet and the difficulty to navigate over the ebb tide delta by the Ω/M tot Ratio. Unfortunately the wave refraction analysis undertaken by Rennie (1980) for the south coast did not include the south end of Oreti Beach and the New River Estuary area due to difficulty of assessing the effects of Stewart Island. However, he did imply that it would have a more sheltered wave climate, thereby reducing rates of sediment transport, and hence the ebb tide delta is likely to be a net sediment deposition area. An assessment of longshore sediment transport rates can be obtained from the 20 year inshore wave hindcast from the WAM model. Figures 7 and 8 (pages 26 & 27) show time series of Environment Southland 26

32 the resulting net annual and monthly transport rates, with positive values being northward transport and negative values being southward transport. As can be seen from these plots, there is dominant net southward transport, with the mean annual net rate being 4.8 x 10 6 m 3 /yr and the mean monthly net rate being 4.02 x 10 5 m 3 /month. However, it can also be seen that there is considerable variability in the magnitude from year to year and month to month, with some months having a net transport to the north. However this only occurred in 20 months (17%) over the total 20 year record, with the most frequent being in June and March. On an annual basis the largest net southward transport occurred in 1994, involving an estimated 8 x 10 6 m 3 over the year, while net southward transport of less than 2 x 10 6 m 3 occurred in 1985, 1989 and This suggests that sediment transport to the ebb tide delta at the mouth of the estuary should have been a maximum in 1994, and a minimum in 1985, 1989, and These dates correspond with the distribution of annual maximum and minimum wave heights respectively. Only in the above years of minimum transport did the Ω/M tot Ratio exceed the criteria of Ω/M tot < 20, hence according to the classification of Bruun (1978) in all other years the entrance would be classified as being unstable, with a very shallow ocean bar making navigation very difficult. This would appear to be the normal condition of the entrance to the New River Estuary. On a monthly basis, the lowest mean southward transport volumes, hence greatest theoretical stability in the entrance channel occurs in February, March and June. Net southward transport in these months is under 300,000 m 3 /month, which is less than 70% of the monthly mean over the total record. The months with the highest mean monthly southward transport, hence greatest instability in the entrance channel, were August to October when mean monthly volumes were in excess of 500,000 m 3 /month, equivalent to 32 55% higher that the monthly mean over the total record. A comparison with the monthly mean wave heights, indicates that this early spring peak in southward transport appears to be driven by a greater frequency of more westerly waves during this period than greater wave heights. The low monthly southward transport in June appears to be also driven by wave approach direction (e.g. less westerly waves) as wave heights are above the monthly mean for the whole record while southward transport is only 55% of the monthly mean for the total record. By comparison, the low southward transports in February and March appear to be a function of reduced mean wave heights at this time of the year. Environment Southland 27

33 Net Southward Longshore transport (m3/yr) Annual Net Longshore Transport ( ) E E E E E E E E E+06 Figure 7: Mean Annual Longshore Transport Volumes in 10 m water Depth at the Mouth of the New River Estuary Environment Southland 28

34 Longshore transport (m3/month) Monthly Net Longshore Transport ( ) 1.000E E+05 North Sediment Transport 0.000E E E E E E+06 South Sediment Transport E+06 Figure 8: Mean Monthly Longshore Transport Volumes in 10 m water Depth at the Mouth of the New River Estuary Environment Southland 29

35 2.4 Fluvial Processes Flood Flows As stated in section 2.1, two principle rivers, the Oreti and the Waihopai Rives discharge into the New River Estuary. While the mean annual flow of these two rivers are not large, it is the temporal distribution of combined flood discharges which are important for inlet mouth behaviour, as they have the ability to significantly increase the size of the ebb tide discharge, which may result in increased bed scour in the inlet channel, changes in the orientation of the ebb tide jet, and hence changes to the size and dimensions of the ebb tide delta. Data on river flows in flood events in both the Oreti and Waihopai Rivers is available from Environment Southland flow recordings stations since For the Oreti, the recording site used (Wallacetown) is upstream of the confluence with the Makarewa River, so flows from this river have to be added to the Oreti flow records. A more downstream recording site at Dunns Road was not used due to tidal influences on the flood flows at this site. Prior to 1977, information on flood heights is available for events back to 1957 in the Oreti Catchment Flood Warning Information Booklet (Environment Southland 2003). These height have been converted to flows by high flow rating curves. Dates and notes of large flood events prior to 1953 is available in Cowie (1957). It is recognised that without flow records it is hard to compare the frequency and magnitude of early flood events to recent flows, in part due to the construction of flood stopbanks since the formation of Catchment Boards in the 1940 s reducing the frequency of land flooding, and increasing the volumes of water retained in the river channels in high flow events. However, an estimated ranking of flood events having a theoretical combined discharge into the New River Estuary of above 1000 m 3 /s is presented in Table 2. For the most extreme of these events it is likely that the peak discharge to the estuary is actually limited to around 1500 m 3 /s due to the narrow river channel leading to the estuary resulting in ponding in the downstream reaches (Bradley, SRC, pers com). Also included in Table 2 are indicative return period of the combined discharge, which is based on the sum of the discharges from the individual rivers for the same return period. It recognised that such an approach would over estimate the combined return period, so these can be considered to be conservative estimates. The combined flood record from all of the above sources is presented in Appendix C, which contains 45 events. Environment Southland 30

36 Table 2: Combined Peak Discharges into New River Estuary Greater that 1000 m 3 /s Date Estimate Combined Peak Discharge into Estuary 29 March m 3 /s (1) Predicted High Tide Level (above MSL) Notes 0.7 Estimated from flows given in Cowie (1957) for Oreti plus 50 year return period flow for Makarewa and Waihopai Estimated year return period for combined flow to estuary. 28 January m 3 /s 0.77 Estimated 40 year return period for flow to estuary 18 November m 3 /s 0.77 Estimated 30 year return period for flow to estuary Spring 1878???? Flood heights in Southland generally just a little below March 1913 levels. 15 October m 3 /s 1.29 Estimated 15 year return period for flow to estuary 26 August m 3 /s 1.37 Estimated 10 year return period for flow to estuary June 1935???? 1.23 Biggest flood post 1913 to Widespread flooding across Oreti catchment March 1861???? Widespread flooding across Southland 11 March m 3 /s 0.75 Estimated 7.5 year return period for flow to estuary 21 February m 3 /s 0.72 Estimated 5 year return period for flow to estuary 31 October m 3 /s 0.95 Estimated 4.5 year return period for flow to estuary 1 June m 3 /s 0.95 Estimated 4.5 year return period for flow to estuary 18 January m 3 /s 1.29 Estimated 4.2 year return period for flow to estuary 14 January m 3 /s 0.98 Estimated 4.2 year return period for flow to estuary 3 May m 3 /s 1.44 Estimated 4 year return period for flow to estuary 16 May m 3 /s 1.13 Estimated 3.5 year return period for flow to estuary April 1968 >997 m 3 /s Oreti & Makarewa flows only. Waihopai <30 m 3 /s Estimated 3.2 year return period for flow to estuary September 1972 >982 m 3 /s Oreti & Makarewa flows only. Waihopai <30 m 3 /s Estimated 3.2 year return period for flow to estuary Note: (1) Two flow estimates for the Oreti from Cowie (1957). The highest being 2380 m 3 /s at Winton, and the lower being 1500 m 3 /s further downstream at Oporo. It is assumed that the drop in flow is due to the large amount of water which escaped from the river channel between the two sites. Also included in Table 2 is the predicted high tide levels for the flood dates. From a flood perceptive, it is fortunate that the three highest combined river discharge events occurred when there were neap tides, and that none of the listed events occurred in association with equinoctial high tides. Although five of the listed discharge events occurred when predicted tides were greater that MHWS, none of the dates correspond with the dates for the highest 50 estuary level recorded at the Stead Street as given in Bradley (2001). This confirms that tide and estuary effects are a greater influence on extreme estuary water levels that river events. Environment Southland 31

37 The other point regarding flood flows that may be relevant for estuary mouth stability is the temporal distribution of these events, particularly clusters of events or long periods without floods. The temporal distribution over the whole record since 1861 is shown in Figure 9, and more detailed for the period of flow recordings since 1977 in Figure 10. Although care is needed when interpreting frequencies obtained from different methods of reporting, it appears that there was a cluster of floods events in the mid 1930 s to early 1940 s, and again in the late 1970 s to early 1980 s. Little can be read into the lack of events shown in Figure 9 for the periods 1880 to 1910, and from the mid 1940 s to mid 1950 s, as these are outside of the periods covered by the various reports, so may not be an accuracy reflect of conditions during those periods. However, from the recent record in Figure 10, we can confidently say that frequency of large discharges since 1985 has been much less than in the previous 8 years, with only 8 events in 17 years compared with 12 events the eight year period 1977 to Sediment Supply Sediment, will be discharged into the estuary by the rivers as either suspended or bed load, particularly in flood events. Due to the drop on velocities from the river channel to the estuary, a large amount of this material will be deposited within the estuary, which depending on tidal flows, may not be discharged to sea, hence contribute to estuary sedimentation. Blakely (1973) suggested that the Oreti in normal flow (28 m 3 /s) contributed an estimated 50 tonnes/day of sediment to the estuary with the rate increasing to 2200 tonnes/day for very large extreme flood flows of 2500 m 3 /s (e.g. > 100 year return period). By contrast, the Waihopai was estimated to supply 0.2 tonnes/day in normal flows of 0.6 m 3 /s. Environment Southland 32

38 Flow into Estuary (m3/s) Number of floods Flood Frequency Years Figure 9: Temporal Distribution of Reported Floods in Catchments discharging into the New River Estuary Flood Flows to New River Estuary Date Figure 10: Temporal Distribution of Combined Flows in the New River Estuary above 750 m 3 /s Environment Southland 33

39 2.5 Estuary Sedimentation and Reclamation From the discussion in section 1.2, it is clear that changes in the size of the tidal prism will have a large influence in the size of the inlet channel and the ebb tide delta. Two possible causes of change in the tidal prism are sedimentation of the estuary and reclamation Estuary Sedimentation There is a long history of sedimentation within the New River Estuary, which as documented in NRETAC (1973) was hindering the development of a port at Invercargill as long ago as the early 1880 s. As a result dredging of the channel at the Invercargill jetty occurred as early as 1887, although this and subsequent schemes to combat the reduction in channel depths in the upper estuary were unsuccessful. Due to a combination of sedimentation and difficulty in crossing the bar at the estuary mouth, the last commercial vessel to use the port was in 1939 (NRETAC 1973). Thoms (1981) suggested three source areas for sediment in the estuary: Medium fine marine sands deposited on the ebb and flood deltas, on the flanks of the main estuary channels, and in the Mokomoko Inlet. While the majority of this sand is likely to have entered the harbour by tidal currents (e.g. tidal bypassing), there was also large contributions from wind blown sand from Sandy Point in the 1880 s to early 1900 s. For example Smith (1924) records dune movements into the estuary of 250 m in the vicinity of Daffodil Bay between 1907 and 1924, and 190 m at Whalers Bay. Blakely (1973) considered that this wind blown source was still relevant, but Thoms (1981) does not specify mention this as a source. Oreti River. Thoms (1981) suggested that this source supplied a large range of sediment sizes, while Blakely (1973) suggested that it was primarily silts and clays. This river borne sediment predominantly settled out in the lower river channel due to flocculation processes where the river flow met the salt wedge, with some then being flushed into the estuary in larger river flows and transported on the flood tide to the Waihopai Arm, where it settled out on tidal flats and bound by vegetation (Spartina). Waihopai River, supplying predominantly mud sized sediments into the Waihopai Arm. In considering the relative importance of these sources, Thoms (1981) concluded that majority of the fine river borne sediment is ultimately flushed out of the estuary by ebb tide currents, Environment Southland 34

40 and that up to 70% of the sediment accretion in the estuary was derived from the ocean floor rather than from the land. To determine where in the estuary sedimentation was occurring, Blakely (1973) undertook a comparison of the 1856 soundings to 1969 aerial photographs. He found that except for the change from a large pool above the confluence of the Oreti & Waihopai Arms of the estuary (e.g. opposite Bushy Point) to mud flats and sand bars covered with Spartina, and a change in the offset of the curve of the entrance of the Oreti River into the estuary; the pattern of estuary channels and shorelines was essentially unchanged. Blakely (1973) also suggested that the change in the Oreti and Waihopai Arms from mainly sandy flats free of vegetation to partially vegetated flats with organic silts had occurred in relatively recent times over the last years. From rod experiments, Thoms (1981) estimated the rate of deposition on the inter-tidal flats to be in the order of 30,000 m 3 /yr, with most deposition occurring in the upper reaches of the Waihopai Arm, and a net loss from the western shore of the main estuary due to wave action. In the context of the volume of the estuary, this deposition rate is very small, only accounting for a 6% loss in the spring tidal prism over a 100 year period. However, this deposition does imply, particularly if it has predominantly occurred over the last years, that the A-Ω relationship may not be in complete equilibrium with relatively recent adjustments in the throat inlet Estuary Reclamation The history of reclamations in the New River Estuary is well documented in NRETAC (1973), which notes that that first reported reclamation occurred in 1865, when 2 acres (0.8 ha) were reclaimed on the eastern side for railway purposes. Agricultural reclamations are reported to have started in 1890, and by 1902 the New River Harbour Reclamation Act was passed that authorised the reclamation of the tidal flats. By 1910 a scheme to reclaim 890 hectares using prison labour was started, with the land used for the prison farm and later some as the Invercargill Airport. The southern boundary of the reclamation was the causeway for the Otatara tramway, which is now Stead Street. This reclamation also included western side of the upper Waihopai Arm (area known as the Rifle Range), with 1064 hectares having been reclaimed by From 1920 to the 1970 s a further 145 hectares of industrial land on the eastern side of the Waihopai River and south of Tweed Street has been reclaimed, bringing the total area of tidal flats reclaimed to 1210 hectares (12.1 km 2 ) (Ballinger 1972). This Environment Southland 35

41 reclamation represented a 26% loss in the estuary area, and a 14.5 million m 3 reduction in the spring tidal prism (28%). This is a significant reduction in the tidal prism, which from the A-Ω relationship should have resulted in a corresponding reduction in the inlet throat area of around 900 m 2. The majority of this adjustment should have occurred in response to the pre 1920 s reclamations, therefore has had over 85 years to occur. The most likely long-term adjustments in the throat to accommodate this reduction in area are considered to be a decrease in the channel width and depth. Thoms (1981) considered that the increase in numbers of vessels grounding in the estuary since 1915 was an indication of these types of channel responses to the reduction in the tidal prism due to reclamation. A significant change in the tidal prism as a result of reclamation would also effect the Ω/M tot ratio for bar stability and size. The smaller prism after reclamation would reduce this ratio, implying more instability of the entrance. This is examined further in sections and Environment Southland 36

42 3.0 Historical Changes 3.1 Results of Literature Review Information Sources The majority of the historical information on changes in condition of the mouth of the New River Estuary has been obtained from NRETAC (1973). This information is largely focused on how depths on the ebb tide bar and instability in the entrance channel effected shipping, and is gathered from a number of reports. Three early surveys of the mouth area are mentioned in the NRETAC (1973) publication, being : Capt Strokes of Acheron survey of entrance 1856: J.T Thomson (chief surveyor) fix limits of navigation channel. 1863: T. Heale (Chief surveyor, Southland), mapping of Shoals and channels. Although the first two of these charts are not included in NRETAC (1973) publication, they are used in a 1918 bathymetric chart 14, a copy of which is held by SRC. While this chart gives the position of the ebb tide delta and entrance channel as well as soundings in the channel, it does not differentiate from which of the two surveys the information is from. However from comparison with the frontispiece of Thoms (1981), which is a copy of a 1856 map 15 of the estuary, it is considered that the ebb tide delta shown in the 1918 chart is from the 1850 survey, and the shoals and soundings in the entrance channel are from the 1856 survey. The frontispiece to NRETAC (1973) is part of a map of Southland dated 1865 showing soundings in the estuary channel, and is credited to Wyld as the engraver. But a larger version of this map shows it was surveyed by Heale, hence it assumed that the soundings are from his 1863 survey. However, the shape of the ebb tide delta and position of the entrance channel in this map does not agree with the changes since the 1856 survey described in Heale s 1863 report (reproduced in NRETAC 1973), so again there is some confusion on the date of the information presented in the map. All of the above maps and charts (e.g. 1856, 1865 and 1918) are presented in Appendix D. 14 Approaches to Awarua or Bluff Harbour and New River Environment Southland 37

43 There is also confusion over the naming of some of the rocks in the entrance channel. The Heale (1863) report mentions both the Oberon Rocks, which are shown on the 1865 map, and the rocks on which the Guilding Star streamer struck. The Guilding Star rocks are shown on the 1918 chart, but this name must post date the 1850 and 1856 surveys, as the streamer only struck these rocks in However the confusing thing is than the streamer Oberon was renamed the Guilding Star after striking a rock in Bluff Harbour in 1861 (NRETAC 1973) Stability of the Entrance channel prior to Estuary Reclamation A summary of all of the historical information on the changes in the ebb delta and entrance channel to the estuary, and possible related events such as floods, tsunamis and reclamations, are presented Appendix E. From this table, it is clear that there were issues with entrance channel stability and suitability for shipping right from the earliest days of the establishment of Invercargill, well before the start of significant estuary reclamation in There are reports of the closure to shipping of the main ebb tide channel past Omaui for prolonged periods of time in the 1830 s, 1850 s, 1860 s and 1880 s. From the historical accounts these closures are preceded by growth of the ebb tide delta in a southerly direction constricting the channel depth and width as it is forced over to the hard shoreline at Omaui, eroding the sand dunes to the east and exposing the ancient volcanic rocks along the edge of the Holocene dune foreland (much the same as the current situation). At the same time, a normally blind channel on the northern side of the inlet must have become more pronounced and open to the sea, as vessels are recorded as using this channel for entry to the estuary. Heale (1863) described the mechanism for opening this blind channel as being the flood tide coming in from the north-west aided by the prevailing winds maintaining a shallow trough in this area of the ebb delta, and limiting the size of the flood tide flow through the southern channel. As the blind channel develops it results in a splitting of the ebb discharge, further limiting the ability for scour in the southern channel, allowing the ebb delta to progressively accumulate in this area and hence prolonging the time that the condition prevailed. Following some time, in the order of 18 months to three years, the lack of reports on channel instability suggests that southern channel was again clear for shipping, indicating there had been a natural scouring of this channel, and possibility a shift in position away from shore at Omaui. 15 The copy of the map in Thoms (1981) includes drawings of proposed training works at the estuary mouth and at the Invercargill wharf which post date It is assumed that this are from a later Thomson report (1881), which used the original 1856 map as a base. Environment Southland 38

44 From these accounts, the notion of cycles of entrance behaviour appears to be well founded. A sketch diagram of an indicative 3-stage cycle of change on the ebb delta and channels is shown in Figure KEY Sand Deposits Ebb Tide Flows Bombay Rock Other Rocks 2 3 Figure 11: Indicative cycle of change on the ebb delta and channels at the mouth of the New River Estuary. Note the change in the alignment of the southern ebb tide channel, with increasing angle to the orientation of the shoreline as the cycle develops. Environment Southland 39

45 What is missing from this cycle, is the process driver which re-initiates cycle back from stage 3 to stage 1, by eroding the southern extension of the ebb tide delta, closing the northern blind channel and allowing the southern channel to redevelop as the main shipping passage. The possibilities are that it was some event, either fluvial (e.g. flood) or marine (e.g. storm or tsunami), or that it was the result of trigger action due to exceeding some limit of stability/instability. Although records are patchy, there does appear to be a pattern of large flood events occurring between the recorded periods of limited southern channel access, which is stage 3 of the cycle. For example from Appendix E we can see that a large flood in 1861 occurred between the stage 3 conditions reported in 1856 and 1863, and again a large flood in 1877 between the stage 3 conditions reported in the 1860 s and the 1880 s. In relation to the cycle of mouth channel changes, the increased discharge on the ebb tide in these flood events could have acted in two ways; either initiating the start of the cycle by enlarging the blind channel, or completing the cycle by increased scour in the southern channel, which removes the southern end of the ebb tide delta and hence allows a straightened the channel in the vicinity of Omaui Village away from rock shore. This will be examined further in the contemporary period since 1947 covered by the aerial photographs. There were also two significant tsunami events (1868 and 1877) between the cycle stage 3 conditions in the 1860 s and the 1880 s. Records of the tsunami heights at Invercargill and Bluff, suggest that in both of these events 60% of the tidal prism may have entered and emptied from the estuary within 30 minutes to one hour. Given that there are no reports of the southern channel being constricted at these times, it is considered that the majority of these flows into and out of the estuary would have been via this channel, and due to the likely high velocities there would have most likely have been considerable scour of the channel and associated erosion of southern flanks of the ebb delta. As a result it is considered that tsunami events are most likely to have returned the estuary mouth to stage 1 of the cycle. There was also reports in the 1880 s that the large volumes of sand being blown into the estuary from Sandy Point at this time (due to effects of grazing and rabbits) was contributing to difficulty of shipping. While this undoubtedly contributed to sedimentation on the east side of Sandy Point (e.g. Owi & Daffodil Bay), and possibility in the estuary upstream of the flood tide delta, it is doubtful whether it contributed significantly to constriction of the entrance channel at Omaui due to the method (wind rather than littoral processes) and direction (east and SE rather than south) of transport. It is also noted that mouth instability and channel Environment Southland 40

46 constrictions were recorded in the 1850 s and 1860 s, before the denudation of vegetation on Sandy Spit. The degree of instability of the entrance conditions prior to the large scale reduce in the tidal prism due to estuary reclamations can be examined in terms of the relationships presented in section 1.2. Morphological stability of the inlet throat, which should be recalled is the channel east from Entrance Point past Mokomoko Inlet (see Figure 1, p10 ), from the A-Ω relationship. From the pre-reclamation tidal prism of x 10 6 m 3, the corresponding stable throat channel cross-sectional area should have been in the order of 4,375 m 2. From the 1918 bathymetric chart, the throat channel at Entrance Point in 1856 is shown to have mid-tide width of around 920 m, and a cross-sectional area in the order of 4,260 m 2. Therefore, this suggests that the inlet throat was relatively stable at this time. However, the entrance channel past the ebb tide delta at Omaui only had a cross-sectional area of 40% of the throat, indicating the degree to which the channel is constricted by the delta, and the large volume of tidal flow that is not contained in the channel, therefore not available to assist with maintaining the channel. Hence, while the inlet throat may have been morphologically stable, the main ebb channel appears to be a regularly been in a state of unstable equilibrium leading to closure as prescribed by Escoffier (1940) (see section 1.2). The stability of the entrance as determined by the Ω/M tot ratio. Assuming that the long-term average rate of longshore transport has not altered from that calculated from the 20 year wave hindcast in section 2.3.7, then the Ω/M tot ratio prior to the estuary reclamation would have been in the order of From the Bruun & Gerritsen (1960) classification (see section 1.2), this would result in an unstable entrance with a very shallow ocean bar, over which navigation would be very difficult. This appears to accurately describe the conditions which are reported to have prevalent for long periods at this time. Relationship of ebb tide delta volume to tidal prism. From this relationship the larger tidal prism prior to the reclamations in the estuary would have resulted in a greater volume of sand being stored in the ebb tide delta. This increase is in the order of 40% of the contemporary volumes calculated by the relationship. This increase in volume could be accommodated by either an increase in the area of the delta, or an increase in the elevation of delta. It is not possible to determine elevations, but a comparison of the historical maps with the recent aerial photographs indicates that historically the Environment Southland 41

47 delta did cover a larger area, being shown to extend to nearly opposite Steep Head. This suggests that the opportunity for the delta to constrict the channel was greater. So, it is concluded that instability of the entrance was a natural occurrence prior to the large scale estuary reclamation, and in fact may have been more unstable that at present due to the influence of a larger tidal prism Stability of the Entrance Channel Following Estuary Reclamation As pointed out in section 2.5.2, the reclamation of over 1000 hectares (25% of the estuary area) in the period reduced the tidal prism (Ω) by around the same percentage. From the A-Ω relationship this should have resulted in a reduction of around 900 m 2 in the inlet throat area and a corresponding reduction in the main ebb channel past Omaui, with the response likely to be reduction in both width and depth. It is considered that these channel responses would occur progressively over time, but unfortunately there are no charts or soundings on which to confirm the occurrence, magnitude, or rate of change. However, Thoms (1981) considered that the increase in numbers of vessels grounding in the estuary since 1915 was an indication of these types of channel responses were occurring. The record in Appendix E of the northern blind channel being used for vessel passage for extended periods in , and also suggests that reductions in the southern channel were occurring in the 30 years post reclamation. The reduction in the tidal prism would have also reduced the Ω/M tot ratio, implying that mouth instability was increased from the prereclamation period. However, the relationship between the tidal prism and the ebb tide delta volume indicates that the volume of the ebb tide delta should have decreased as result of the reclamations. To be constant with the other inlet stability relationships, this implies that the volume loss was accommodated by a reduction in delta width (e.g. west-east) rather than length (e.g. northsouth), with losses most likely to have been from the seaward side of the delta. From the relationship, the volume reduction should have been in the order of 6.5 X 10 6 m 3. The question is, where has this sand gone? Possible sinks include deposited in the estuary and hence contributing to the long-term shoaling of this environment, deposited along the flanks of the ebb and throat channel to facilitate width reductions, and transported south past steep head with increased bar-bypassing. The other possibility is that sediment supply to the ebb tide delta was reduced in this period due to the planting and dune control works being undertaken on Oreti Beach. For this to Environment Southland 42

48 occur there would need to be significant returns of sediment from dunes into the littoral zone by offshore winds. The almost total absence of these winds suggests that the dune control work had no influence on sediment supply to the ebb tide delta. From the records in Appendix E, it is considered most likely that the cycles of entrance channel behaviour described in the previous section were still occurring in this postreclamation phase, being super-imposed on the above medium term changes in entrance morphology. However, the duration of the cycles appear to have been changed with constrictions of the southern channel appearing to last a lot longer than in the pre-reclamation period. Unfortunately there are no accounts of changes in the estuary mouth environment following the potentially largest river input on record (March 1913), and any changes in the magnitude of the driving processing required to initiate changes in the stages of the cycles are unknown. However, the Pilot s log in 1926 notes the re-opening of the southern channel in association with a flood event in March, hence indicating these events may be responsible for initiating the return to stage one of the mouth development cycle. However, floods in 1918 and 1924, or the succession of floods from 1935 to 1941 reported in Cowie (1957) do not appear to have had the same effect, although this may be a function of their magnitude. 3.2 Perceptions of Long-Term Residents The discussion with long-term local residents of the Omaui area revealed some information on past changes in the environment around the estuary mouth over the last 50 years and on possible events responsible for those changes, that was not available in literature. The results of these discussions are summarised on various topics in the following notes. Some of these observations confirm the historical accounts, and some can be tested further by the aerial photograph analysis in section Shoreline Changes Whale bone and cooking pot found at a small creek to East of village, implies a little beach was present here at time of whaling (late 1830 s). This is in agreement with a accounts of difficulty in vessel passage associated with a southerly channel position in the 1830 s. Anchor chains found in cliff towards the Pilots station, suggest that sea was close into this area, probably during the whaling period (e.g. only limited sand beach present at this time). This again agrees with the accounts of the channel being hard to the south in the 1830 s. Environment Southland 43

49 Sea had been into where sewage pond is before the 1950 s, as found part of an old sailing ship in a pond in that area. This is in agreement with the cycles of channel movement noted in the literature in the 1850 s and 1860 s. Estimate 200m of dunes up to ft high (e.g. 3-4 m).were present in the mid 1960 s between the sewage pond and the mouth channel. Since this time the site has always been erosional, with very little accretion. Estimate 2 chain (40 m) eroded since 1976, with erosion quicker since the sewage pond constructed in the early 1980 s. This can be tested by the aerial photograph analysis in section 4. Dune accretion and advance into the channel along the western edge of the dune foreland between Omaui and Mokomoko Inlet also occurred in the 1960 s, with all the rocks being covered by dunes, which were fenced and & stocked. Maximum dune extend was thought to occur around the mid 1960 s. Since this time, three rows of dunes have been eroded away., with an estimate of 80 m erosion in the last years. This can also be tested by the aerial photograph analysis in section 4. Estimate loss of 15 ft (approx 5 m) of grass verge along road in front of the village in the last 20 years (e.g. since mid 1980 s) as the mouth channel has progressively moved south towards the village. Previously the height of sand on the beach at the village was only 2-3ft (approx 0.6 1m) below road level, with the sand totally covering the rocks. Were able to drive onto to beach from the road, and enough sand present to walk all the way to the Pilots station on sand. Now the drop to the beach from the road is in the order of m, which has created access issue for residents, and lot of rock is exposed, rather than sand Inlet Channel Changes Near Omaui Village Earliest recollection of area was 1941, when the channel was on the northern side of the inlet with a big wide beach present at the village to such an extent that you could walk around Bombay Rock. The lagoon in the sand bar was located further east at sand hills rather than at the village : The entrance channel migrate across the beach from the northwest corner of the dune foreland to form a lagoon at the village (as shown in 1947 photo, Appendix A). This lagoon was the swimming hole for the summer holiday makers at the village. Water only entered the lagoon during spring high tide cycles, and the lagoon was known to split into two and then reform as a single water body on occasions. The Environment Southland 44

50 sand bar between the lagoon and the estuary entrance channel had single on top of it, in which dotterels were observed to nest. Lagoon was always present in late 1940 s, then decreased in size in late 1950 s as the estuary entrance channel switched over to the north side of the inlet. At this time, it was possible to cycle around Bombay Rock on the extensive sand bar on the south side of the channel. Several references to cycles of channel shifts, beach and lagoon development at the village. These cycles referred to by various sources as being of 7 year, 21 year and 30 year durations. This is consistent with the historical accounts. Consider that the bar at the mouth of the estuary (ebb tide delta) was more extensive, and that it extended further into the estuary. It is now thought to be lower on the seaward side, which allows more wave attack into the estuary. This is consistent with the expected response of the ebb tide delta to a reduction in tidal prism with large scale reclamation, and suggests that these responses may well have still been occurring well after the major reclamation ceased in the 1920 s Estuary Changes Blame Catchment Board works in rivers for siltation of the estuary Consider that large amounts of sand has built up at the mouth of the Oreti River Changes Outside the Estuary Three Sisters beach (between Steep Head and Barracouta Point) has changed from a boulder beach to a sand beach over the last 50 years. Suggestion that this is where the sand that used in be in the extensive sand bar on the Omaui side of the entrance channel has migrated to. This is consistent with the reduction in the volume ebb tide delta with a reduction in the tidal prism Flood Effects Recollections of a high frequency of flood events in the 1950 s & 1960 s, and then again in the period. Suggest possible combination of these high flood frequency periods combined with greater frequency of westerly wind (effecting time required for flood draining due to backwater effects) effects the stability and position of the estuary inlet channel. This can be tested by the aerial photograph analysis in section 4. Environment Southland 45

51 At least one person considered that there was a change in the entrance channel and wave approach following the largest recorded flood in January Tidal currents and Sea Events Consider that generally the ebb tide had a greater velocity than the flood tide. One person recalled a large sea around (possibly 190 Chilean Tsunami in May 1960), which resulted in waves breaking onto the road at the village for around 3 hours at high tide. This suggests that there was little beach present in front of the village at this time. Reference made to the effect of spring tide combined with high westerly wind, forcing water ashore. 3.3 Summary Instability of the entrance to the New River Estuary is a natural process which was occurring well before 25% of the estuary area was reclaimed near the beginning of the last century. This instability appears to be driven by two inter-related processes; the different flow routes of the flood and ebb tide that limits the development of ebb tide channel at the southern side of the inlet, and the low ratio of the tidal prism to the longshore transport rate which allows the ebb tide delta to encroach on the main ebb channel. There are clear indications of a cycle of mouth development, with natural progressive movement of the entrance channel to the south and river floods appear to be important in returning the cycle back to the start. The mouth environment would have undoubtedly responded to the reduction in tidal prism from the reclamation of 1000 hectares of the estuary with a corresponding reduction in throat cross-section area, and probability a reduction in the volume of the ebb tide delta. It is most likely that these responses resulted in a short-medium term increase in entrance instability. However, this can not be verified by charts, maps, or personal accounts for the period following the reclamation. What is not known is whether these changes have altered the cycles of mouth development, and whether the responses in the mouth environment to the reclamation are now complete or still on-going. This could determine the range of contemporary variation in channel and shoreline positions. Environment Southland 46

52 4.0 Aerial Photography Analysis The analysis of the 16 aerial photographs covering 60 years since 1947 can be used in the following three ways to meet the aims of this study: 1. To quantify changes in position of the shoreline, ebb channel and ebb tide delta. 2. To identify temporal patterns or cycles of change, which can be examined alongside the databases of the marine and fluvial processes to establish the roles of these processes in determining spatial limits and timescales of the patterns or cycles of change. This can only be done for the photographs since 1978 due to limitations in the marine and fluvial databases. 3. To verify observations and perceptions. 4.1 Changes in Position of Estuary Mouth Features Shoreline Vegetation Position Changes in the position of shoreline vegetation is commonly used as an indicator of shoreline erosion and accretion, with the position altering in response to the frequency and intensity that salt water from high sea levels or wave run-up reaches that position. The vegetation limit on each of the aerial photographs is plotted on a 1998 photo base in Figure 12. From this figure it can be seen that the vegetation line on both sides of the entrance to the estuary has fluctuated in position over the last 60 years rather than having a constant trend of either erosion or advance. The greatest fluctuations have occurred in the area east of Omaui village to the first set of exposed ancient rocks on the dune foreland (referred to as the 1 st rocks). In this area the vegetation line position has fluctuated in the order of 80 m, with the most eroded position occurring in 1952, when the vegetation limit was at the current location of the oxidation pond, and the most advanced position occurring in The magnitude of change between each pair of photographs for the eight sections of shoreline identified around the entrance (see Appendix B) are presented in Table 3, and these are plotted from a base date of 2004 in Figure 13. It is recognised that it is more common for this sort of plot to be presented with the base date being the earliest date rather than the most recent date (2005 not used as not include the Sandy Point shoreline). However, it was felt that using the most recent as the base fits better to the perception of change held by most people (e.g. the shoreline was so many metres further in/out at some time in the past than it is now). Environment Southland 47

53 Figure 12: Vegetation Line Position from Aerial Photographs Environment Southland 48

54 Table 3: Mean Change in Position of Shoreline Vegetation Limit as Measured on Aerial Photographs. (All distances in metres) Oreti West Beach Oreti Spit Tip Oreti-Sandy Point West Omaui Village Omaui Village Village to 1 st Rock Between 1 st & 2 nd Rocks East of 2 nd Rock Oreti West Beach Oreti Spit Tip Oreti-Sandy Point West Omaui Village Omaui Village Village to 1 st Rock Between 1 st & 2 nd Rocks East of 2 nd Rock Accumulative Change from 2004 Base Date Environment Southland 49

55 Retreat Shoreline Position Relative to 2004 (m) Advance Cumulative Changes in Vegetation Position Comparied to 2004 base date Oreti West Beach Oreti spit tip Oreti-Sandy Point West of Omaui Village Omaui village Village to 1st Rock 1st-2nd Rock East of 2nd Rock Time Figure 13: Movements of the Vegetation Line on both sides of the mouth of the New River Estuary relative to the 2004 position Environment Southland 50

56 In terms of shoreline behaviour around the estuary mouth, the following observations can be made from data presented in Table 3 and Figure 13. On the Omaui side of the mouth, the position of the shoreline at the village and to the west has fluctuated in the order of ±20 m over the last 60 years, but with little net movement. This scale of movements is to be expected for a shoreline comprising of a thin sand wedge overlaying coarse gravel erosion products of resistant ancient bedrock material, which restricts the magnitude of shoreline retreat during periods of beach sand deficit. In contrast, the extensive Holocene sand dune area to the east of Omaui Village that faces to the north-west has experienced considerable fluctuations; with around 20 m of erosion in the late 1940 s to mid 1950 s, followed by a advance of up to 80 m until the late 1970 s, and then gradual retreat of up to 60 m since resulting in a return to around mid 1940 s positions in recent years. The greatest fluctuations occur at the southwestern end of this section of coast, immediately to the east of Omaui Village. As shown in Figure 14 (next page), this area appears to be at the eastern limit of the gravel material, and critically is marked by a change in the orientation of the shoreline to face more to the west, therefore is the most exposed area to high water and waves being driven either through or over the narrow southern end of the ebb tide delta in westerly wind/sea conditions. Hence the local geomorphology is having a strong influence on the erosional components of the shoreline fluctuations. The two rock outcrops near the middle and at the eastern end of this section of beach respectively (1 st and 2 nd rock, see Appendix B), appear to provide very localised stability of the shoreline as shown in Figure 15 (next page). This is most probably due to the diffraction of energy around the submerged parts of the outcrops during high water level conditions. For all sections of shoreline on the Omaui side of the mouth, the shoreline positions on the most recent aerial photographs are also very similar to those shown on the 1947 photographs. Environment Southland 51

57 Figure 14: Area of Greatest Fluctuations in Shoreline Position, immediately to the East of Omaui Village. (Photo Dallas Bradley, 1 st April 2007) Figure 15: The influence of exposed rock outcrops on shoreline stability of the Holocene dunes to the east of Omaui. In this report, the rock outcrop in the centre of the picture is referred to as 1st rock and the outcrop in the distance is referred to as 2 nd rock. (Photo Dallas Bradley, 27 th April 2007) Environment Southland 52

58 On the Oreti side of the mouth, shoreline movements on the south-west facing sections have generally been the opposite of those to the east of Omaui on the south side of the mouth, with retreat in the order of m during the 1960 s-70 s period, then rapid advance in the order of 90 m from the mid 1980 s to mid 1990 s. It is felt that there may be two possible reasons for this pattern: 1) The presence/absence of the blind channel adjacent to the beach on the northern side, and/or 2) medium term shifts in the wind/wave climate, with more northerly conditions dominating in the 1960 s and 1970 s causing erosion on the Oreti side, then a switch to more dominant westerly conditions in the 1980 s and 1990 s, which would erode the Omaui coast. The first of these reasons can not to substantiated due to gaps in the aerial photograph coverage of the northern (Oreti) shore and channel, however the comparison of wind data from Figure 2 (p12) for the period with the wave data from the 20 year WAM hindcast in the 1980 s and 1990 s suggests that there may be some basis to the second reason. The pattern of shoreline movements along the throat of the estuary towards Sandy Point, is different from at the spit tip and to the west. Along this section of coast there has been a slow general retreat of around 25 m since 1956, without the rapid accretion experienced further to the west since For the majority of the shoreline on both sides of the mouth, the magnitude of the fluctuations in position of the vegetation line appears to have decreased over the last 10 years. It is possible that this indicates that the shorelines and mouth configuration is reaching a degree of dynamic equilibrium, however this can not be confirmed Beach Width Changes in shoreline position as mapped by the vegetation position can be masked or accentuated by changes in the beach width. This is particularly the case at estuary mouths, where beach widths can alter dramatically with shifts in channel location, hence result in large changes in the amount of natural protection the beach provides against erosion of the shoreline position. The interviews with long-term local residents clearly shows this variability in beach width has been the case for Omaui, with accounts of being able to walk around Bombay Rock on occasions and reference to regular cycles of occurrence of the swimming lagoon which formed in the beach adjacent to the village when a wide beach was present. The aerial photographs analysis confirmed this large variably in beach width, with the beach being over 500 m wide at the Omaui village in the 1962 photographs, the swimming lagoon being Environment Southland 53

59 present in the 1947 and 1962 photographs, and Bombay Rock being surrounded by sand in the 1956 photograph (see Appendix A). Beach widths were measured by the distance from the wetted line (used to reduce the influence of different tide levels on the result) to the vegetation line. The wetted line digitised from each aerial photographs is plotted on a 1998 photo base in Figure 16 (next page), with dashed lines indicating uncertainty in position due to tides. The resulting beach widths for each of the eight sections of shoreline are presented in Table 4 (p55), and plotted in Figure 17 (p56). It is recognised that the accuracy of these widths is limited by the uncertainty in the location of the wetted line in some photographs. Nevertheless, it is considered that they do provide suitable indications of the patterns and magnitudes of change in beach widths. The location of the ebb channels relative to the shoreline can also be inferred from the beach width. The following observations can be made from data presented in Table 4 and Figure 17. Beach widths at Omaui were at their maximum in the last 50 years during the 1940 s to early 1960 s, with widths up to 500 m in front of the village and east to the 1 st rock, and 300 m further east between the 1 st and 2 nd rocks on the dune foreland. While the 1947, 1952 and 1962 photographs show the beach to have a similar triangular plan shape across the front of the Omaui embayment, the plan shape is considerably different in the 1956 photograph (see Appendix A), including Bombay Rock being high and dry surrounded by beach, and the absence from either erosion or infilling of the swimming lagoon. This suggests that a wide beach was not necessarily constantly present during this period, which is in line with the observations of cycles of behaviour observed by long-term residents, and with measurements of vegetation line erosion between 1947 and At some time between 1962 and 1978, the mouth channel migrated around 300 m southward toward the Omaui shore removing the large volumes of sand from the beach areas and reducing beach widths to under 100 m for the 1.5 km stretch of shoreline from west of the village to the 2 nd rocks on the dune foreland. The channel has remaining relatively fixed in this southerly position since this time, resulting in the sand material not having the opportunity to return to the beach. This has lead to the perception that the former cycles of beach growth and decay have been terminated. Environment Southland 54

60 Figure 16: Wetted Lines and Ebb Tide Delta Positions From Aerial Photographs Environment Southland 55

61 Table 4: Mean Beach Widths as Measured on Aerial Photographs. (All distances in metres) Oreti West Beach Oreti Spit Tip Oreti-Sandy Point West Omaui Village Omaui Village Village to 1 st Rock Between 1 st & 2 nd Rocks East of 2 nd Rock Oreti West Beach Oreti Spit Tip Oreti-Sandy Point West Omaui Village Omaui Village Village to 1 st Rock Between 1 st & 2 nd Rocks East of 2 nd Rock Beach Width Changes in Consecutive Photographs Environment Southland 56

62 Beach Width (m) Average Beach Widths at Oreti and Omaui since 1947 Oreti West Beach Oreti spit tip Oreti-Sandy Point West of Omaui Village Omaui village Village to 1st Rock 1st-2nd Rock East of 2nd Rock Jan-45 Jan-50 Jan-55 Jan-60 Jan-65 Jan-70 Jan-75 Jan-80 Jan-85 Jan-90 Jan-95 Jan-00 Jan-05 Time Figure 17: Mean Beach Widths Measured From Aerial Photographs Since 1947 Environment Southland 57

63 On the Oreti shore, beach widths on the more exposed western shore and spit tip have experienced more irregular fluctuations, but as a general pattern they increased m from the mid 1940 s to the mid 1950 s, then fluctuated in the order of 100 m through to the mid 1980 s, and have been in general decrease since, although some of this is in response to vegetation advance. Periods of reduced beach width also generally correlate to when the blind ebb channel is more developed along this northern shore (e.g. 1947, 1998, 2004). Beach widths along both sides of the throat of the estuary have changed far less than those to the west along the edges of the ebb delta channels. This reflects the dramatic reduction in the influence of sea conditions in the throat, due to both the blocking by the ebb tide delta, and the difference in shoreline orientation. Two further key observations can be made from the aerial photographs and the comparison of the temporal trends in Figures 13 and 17: The beaches on both sides of the ebb tide delta experienced growth during the 1950 s and early 1960 s, indicating that there was a net influx of sand to the beaches during this time, rather than transfers of sand from one side of the mouth to the other by barby-passing. On the north (Oreti) side of the mouth this growth appears to be due to a welding of the ebb tide bar to the shore in the absence of the blind ebb channel. On the south (Omaui) side, the increased beach widths clearly reflect a straighter approach of the main ebb channel. Changes in beach width at Omaui have a large influence on changes in the position of the shoreline vegetation, with a time lag in response. This is particularly pronounced for the section of coast immediately east of Omaui village, where the steady erosion of the vegetation line by around 60 m since the mid 1980 s is a lagged response to the 400 m reduction in beach width from the mid 1950 s to the early 1980 s. The discussions with long-term residents and field observations suggest that in recent years the beach width has become insufficient to perform a natural buffering function against erosion during events that combine strong westerly conditions with high water levels, hence erosion of the shoreline vegetation has been on-going since the mid 1980 s. As indicated in the previous section, this erosion is most pronounced to the east of the village due to this area having the greatest exposure to westerly conditions, the lack of more resistant rock material, and the landform presence having the least resistance to erosion being low sand dunes which only developed since the early 1950 s erosion episode. Environment Southland 58

64 4.1.3 Channel Width and Ebb Tide Delta Location Indications of the width of the main southern ebb channel, the location of the ebb tide delta and the presence or absence of the northern blind ebb channel can be made from the digitised information presented in Figure 16. Unfortunately due to limitations of the photograph coverage on many of the dates, insufficient information is available to undertake a full analysis or to determine changes in the size on the delta. For the main ebb channel, more detailed information on temporal and spatial changes is presented in Figure 18 (next page), along with information on the location of Bombay Rock relative to the southern edge of the channel. It is recognised that the accuracy of this data is limited by the uncertainty in the location of the wetted line in some photographs. Nevertheless, it is considered that they do provide suitable indications of the patterns and magnitudes of change in channel widths. Unfortunately depth information is not available to determine changes in the cross-section area of the channel. The key observations from Figure 18 and the photographs in Appendix A include: There is a general reduction in the size of the main ebb channel downstream from the throat of the estuary, with channel to the west of Omaui village generally being only 30-50% of the width at the throat. Although water depths are likely to be greater along the Omaui section of the channel, there is also likely to be a net downstream reduction in flow below the throat, primarily due to the flood tide entering over the northern end of the ebb delta, and on occasions assisted by larger ebb tide discharges through the blind channel such as in 1947, 1978 and 1998, and a well developed central channel in Channel widths did not response to the large fluctuations in the wetted line position at Omaui village, and between the village and the 1 st rock in the 1950 s and 1960 s, indicating that the southern position of the ebb tide moved largely in unison with the wetted line movements, and transfers of sediment between the beach and delta were occurring. In the 1956 photograph the delta is clearly bisected by the ebb channel located around 300 m north of its normal position at Bombay Rock, and up to 700 m north to the west of the village. The large southward shift in the channel in the period, and relative stability since is clearly shown by the changes in distance from Bombay Rock to the channel edge (wetted line). There was a corresponding increase in channel width at the throat and over the Omaui sections of the channel, but a reduction in width to the west of village due to encroachment of the ebb delta. Environment Southland 59

65 Channel Width (m) Changes in Main Ebb Channel Widths Since E west of village E village E Bombay/1st rock Throat entrance Bombay Rock from Wetted edge Jan-45 Jan-50 Jan-55 Jan-60 Jan-65 Jan-70 Jan-75 Jan-80 Jan-85 Jan-90 Jan-95 Jan-00 Jan Time Figure 18: Changes in the Widths of the Main Ebb Channel Since 1947 Environment Southland 60

66 The 1979 photograph (Appendix A) shows a split channel formation with two ebb channels discharges; the normal southern channel, and second one of similar size (around 450 m wide) splitting off between Bombay Rock and Omaui village. The blind channel along the Oreti shoreline is also evident. Based on the tide levels at the time of the photographs (see Appendix A), the delta also appears to be considerably lower in the 1979 photo than in the pervious 1978 photograph. This channel split had migrated downstream by the time of the 1981 photograph Appendix A) to be located well west of the village, and the ebb delta reformed across the entrance to the blind channel and the former central channel. As a result main ebb channel had increased widths pass Omaui, particularly to the west of the village where the split in the channel was located. Although it is difficult to be definitive, it appears from the 1985 photo that the lower secondary channel had also closed. This channel also does not show on the 1996 photograph, but does appear to be present as a minor channel in the 1998 and 2004 photographs (Appendix A). A central channel through the middle of the ebb delta developed between the 1998 and 2000 photographs, with the opening opposite the 2 nd rock being shown in the 2000 photograph. This channel is clearly shown in an oblique aerial photograph from October 2002 (Figure 19), however does not appear to be present on the 2004 photograph (appendix A). Figure 19: New River Ebb Tide Delta 8 th October (Photo Environment Southland) Environment Southland 61