Merkinch Greenspace. Merkinch Local Nature Reserve: Hydrological Assessment

Transcription

1 Merkinch Local Nature Reserve: Hydrological Assessment June 2011 EnviroCentre Craighall Business Park Eagle Street Glasgow G4 9XA t f w e info@envirocentre.co.uk Project Manager K. MacDougall Offices Glasgow Belfast Stonehaven Report No 4593 Status: Final Project No: 12998j Copy No: 0 Rev. No: 00 This Document is of UK Origin June 2011

2 Table of Contents 1. Introduction Terms of Reference Scope of Report Site Details Site Description Historic Review Baseline Hydrological Conditions Hydrology Tidal Regime Hydrological Assessment Site Walk Over Site Investigation and Monitoring Installation Conceptual Hydrological Regime at Merkinch Key Findings and Recommendations Key Findings Recommendations...11 Appendix A: Photographic record of site visit Appendices List of Figures Figure 1: Site location... 2 Figure 2: Key features of Merkinch Local Natural Reserve... 3 Figure 3: 1832 Map of Merkinch... 4 Figure 4: Comparison of change in tidal extents Figure 5: Location of sections to demonstrate conceptual hydrological regime... 9 Figure 6: Conceptual hydrological section through Merkinch (south west to north east)... 9 Figure 7: Conceptual hydrological section through Merkinch (south west to north east)... 9 List of Tables Table 1: Tidal range at Inverness... 6 Table 2: Extreme water level predictions at Inverness... 6 Table 3: Monitoring installation details... 8 Table 4: Salinity monitoring locations... 8 Table 5: Approximate salinity values... 8 i

3 1. INTRODUCTION 1.1 Terms of Reference EnviroCentre were commissioned by to undertake a hydrological assessment of the Merkinch Local Nature Reserve (LNR) and initiate a hydrological monitoring programme for the reserve. 1.2 Scope of Report The scope of the assessment includes the following: Baseline review of available information; Walkover site visit and inspection of relevant archive material; Develop conceptual hydrological model for the area; Production of draft monitoring plan and agreement of locations; Installation of up to 3 dipwells/water level monitoring points following agreement of locations (one day on site installation); and Report on above assessment. 1

4 2. SITE DETAILS 2.1 Site Description The Merkinch LNR is located in the north west of the wider area of Inverness known as Merkinch (Figure 1). This area was historically an island at the mouth of the River Ness as it flowed into the Beauly Firth. Over time as Inverness has grown, it no longer has the appearance of an island and now forms part of the urban area of Inverness. The bulk of the LNR lies below the Mean High Water Spring (MHWS) tide level which is constrained by the sea wall running along the shore. A previous hydrological map of the LNR (Figure 2) named the lagoons within the LNR as Kingfisher, Westfield and Witches Coffin, and these names have been retained within this report. On the landward side of the sea wall is low lying ground with a series of lagoons in the south west which have pipes with flap valves connecting them to the tidal area. Beyond the LNR to the south east the ground rises and the land use changes to housing and industrial units. The Caledonian Canal and the Muirtown Basin form the south eastern boundary of the LNR, while the Mean Low Water Spring (MLWS) tide level forms the northern boundary. Figure 1: Site location 2

5 Figure 2: Key features of Merkinch Local Natural Reserve 2.2 Historic Review A review of historical maps has been undertaken to examine how the area in and around Merkinch has changed over time as Inverness has grown. An extract from an 1832 map is shown in Figure 3, while Figure 4 compares the change in tidal extents between 1832 and present day. The main change has been the construction of the sea wall and railway. The relative extents may be exaggerated around South Kessock Pier due to differences in mapping scales, however the trends can be clearly observed. The key observations from the wider map review are as follows: Timothy Pont Map (1590) shows Inverness at mouth of River Ness, but not sufficient detail to show Merkinch. The Military map of Inverness (1725) predates construction of canal. Showing Merkinch almost cut off from mainland by an old channel of the River Ness. Merkinch shown divided into fields with only a few houses located on southern margin. No visible drainage from west. Town plan of Inverness (1821), showing Merkinch, post construction of the canal. Predates construction of the seawall and railway. 3

6 Great Reform Act Plans (1832), show Inverness with greater detail in terms of watercourses than the 1821 town plan. Merkinch shown still almost cut off by old channel of River Ness. Canal shown, map predates construction of seawall and railway. Muirtown basin on canal noted as canal basin with old channel of River Ness shown running along the east bank of the canal. The previous river channel is shown to north of Telford Street. The Ordnance Survey large scale town plan (1867), shows part of Merkinch after construction of the railway and canal. Old channel of River Ness not shown. Seawall possibly shown, but not annotated. Ordnance Survey 25 inch to the mile map of Inverness (1875), shows the canal, railway and seawall. Muirtown basin shown with small body of water located adjacent within Merkinch. Old river channel now reduced to small ditch extending from end of small water body east to the edge of buildings. Shows seawall with benchmarks ranging between 12 4 and 12 6 ( m) and to the south, Merkinch has spot heights from 10 to 14 and 16 2 ( and 4.93m). Embankment also marked to the east of area marked liable to floods between Muirtown Basin and railway. Muirtown is shown on other side of canal from Merkinch and a small stream is shown running off hillside near Muirtown House, disappearing near to a well marked near to Muirtown. Bartholomew survey atlas (1912) shows Inverness with railway, canal, and seawall all present. Muirtown Basin on canal shown with small body of water adjacent within Merkinch. The Ordnance Survey (1920s) map showing Inverness with Muirtown Basin marked. Most recent Ordnance Survey 1:25,000 map shows drainage from hillside through Scorguie towards Muirtown Basin, and well featured in 1875 map. Also shows stream flow in alongside the Muirtown Basin towards the sea wall, and from the lagoon into stream. Figure 3: 1832 Map of Merkinch 4

7 Note: Tidal extents at South Kessock Pier may be exaggerated due to mapping differences rather than entirely absolute change. Figure 4: Comparison of change in tidal extents

8 3. BASELINE HYDROLOGICAL CONDITIONS 3.1 Hydrology There is only one major surface water drain flowing into the Merkinch LNR. This drains surface water runoff from the local urban area (predominantly industrial units) and collects seepage and possibly under drains from the Caledonian Canal to the west, and referred to as the Canal Drain. The surface water runoff to the west from Scorguie does not have any defined channels, but is likely to drain under the canal to the Canal Drain. The extensive residential and industrial development in the local area will have modified the local drainage paths, and the natural drainage catchment may not be reflective of where the runoff now flows. Further assessment of this catchment would require a walk over survey and review of drainage plans held by Highland Council and Scottish Water. 3.2 Tidal Regime The closest standard port to Inverness is Invergordon and from the Admiralty Tide Tables (2010) the predicted tidal range is shown in Table 1. The levels are presented relative to Chart Datum (CD) and above Ordnance Datum (AOD) using a datum correction of 2.25m for Inverness. Recent flood studies undertaken for Inverness have assessed more extreme flood event levels, which are presented in Table 2. These show that the sea wall ( mAOD) can protect up to the 1 in 200 year return period event. These levels do not take into account the effects of climate change or wave action, however wave action will be limited by the relatively short distances of open water. Table 1: Tidal range at Inverness Chart Datum (m) Ordnance Datum (m)* Highest Astronomical Tide (HAT) Mean High Water Springs (MHWS) Mean High Water Neaps (MHWN) Mean Low Water Neaps (MLWN) Mean Low Water Springs (MLWS) * Chart Datum correction for Ordnance Datum (Newlyn) is 2.25m Source: 2010 Admiralty Tide Tables Table 2: Extreme water level predictions at Inverness Return Period (years) Still Water Level (maod) Location: All locations downstream of Celt Street (node 35) 482C 4C87 9FC2 4F506CEBBEE3/0/invernessfloodingreport.pdf. 6

9 4. HYDROLOGICAL ASSESSMENT 4.1 Site Walk Over A site walk over was undertaken in March 2011, with the key features observed being as follows: The sea wall forms the limit of the MHWS and prevents the tide from flowing further inland towards the south east. Landward of the sea wall, the north eastern area is the driest part of the LNR and it becomes progressively wetter towards the south west where the lagoons are located. There are two drainage pipes through the sea wall which allow water from behind the sea wall to drain to the tidal area. These have flap valves to prevent inflow of tidal water through these pipes when the tide is high, however these do not have a tight seal, and the weed growth does not allow the valves to close properly with sea water flowing in behind the sea wall during high tides. The tide flows through the railway causeway where is passes to the seaward side of the sea wall under an open bridge. There is an inflow of fresh water from the canalside to the Kingfisher Lagoon. This is referred to in this report as the Canal Drain. It is open for alongside the Muirtown Basin, and is culverted for a short distance under the old railway embankment before discharging into the Kingfisher Lagoon. The flow in the Canal Drain increases along its length which indicates that it is receiving flows from the canal. These are likely to be from a combination of seepage from the canal and underdrains constructed at the time of the canal. The upper reaches of the open Canal Drain are fed by a piped discharge and also receive piped discharges from the adjacent road drainage and industrial units. The upper reach appears to have been widened and lowered in level, presumably to allow more recent drains to tie into this drain. A new Sustainable Drainage System (SuDS) for industrial units is being constructed with a discharge to the Kingfisher Lagoon. Within the tidal area north west of the sea wall, areas of groundwater seepage can be observed where water can be seen at the surface during low tide. These are relatively fresh or brackish water and there is limited seaweed growth in these areas. 4.2 Site Investigation and Monitoring Installation Two monitoring points were installed at Merkinch in March 2011, a dipwell (Dip 1) for providing water samples and recoding water depths, and a walrag which records the maximum, minimum and actual groundwater levels. The details and locations of these are summarised in Table 3. 7

10 The installation works confirmed that the ground conditions are alluvial sand and gravels similar to those which extend across the majority of the area landward of the sea wall. These deposits will have a productive groundwater flow regime and likely to be in hydraulic conductivity with the nearby freshwater River Ness and saltwater Beauly Firth. A series of electrical conductivity readings of the water within the LNR were taken using a water quality sensor calibrated for fresh water. The readings are provided in Table 4 in units of micro Siemens per centimetre, which can be correlated to salinity as shown in Table 5. The water was found to be across the range from suitable for drinking water to brackish. Additional salinity monitoring was undertaken across the tidal seepage area, however the threshold of the sensor was too low to refine the extent of the brackish water above 10,000 µs/cm, although water at this threshold level is still relatively fresh and can be used for agricultural use in some areas. Table 3: Monitoring installation details Monitoring Point Location Depth Ground Conditions (National Grid Ref) (m) Dip 1 NH Damp, organic material with coarse sand Alluvial sand and gravels Walrag NH Damp, organic material with coarse sand Alluvial sand and gravels Table 4: Salinity monitoring locations Monitoring Location Position (National Grid Ref) Electrical Conductivity (µs/cm) Dip 1 NH NH >10, NH , NH , NH , NH ,162 Table 5: Approximate salinity values Water Description Electrical Conductivity (µs/cm) Approximate Salinity (ppt) Typical drinking water 200 1, Brackish 1,000 60, Sea water 60, Conceptual Hydrological Regime at Merkinch A conceptual model of the hydrological regime at Merkinch has been developed and is shown in Figures 5 7. Figure 5 contains a plan showing two cross sections through Merkinch, while Figures 6 and 7 show the hydrological and hydrogeological conditions expected. The key features to note are: 8

11 Figure 5: Location of sections to demonstrate conceptual hydrological regime Figure 6: Conceptual hydrological section through Merkinch (south west to north east) Figure 7: Conceptual hydrological section through Merkinch (south west to north east) 9

12 Surface water inflows are limited to direct rainfall/precipitation over Merkinch and the Canal Drain. Runoff from Scorguie flows down towards Merkinch and some will flow under the canal. There will be some leakage from the canal towards Merkinch. Merkinch is formed from alluvial sand and gravel which is in close hydraulic connectivity with the River Ness. The groundwater under Merkinch will be predominantly fresh, especially near the surface given the influence of the River Ness. There will be zones of higher groundwater flow through larger gravels associated with old river channels in and through Merkinch. The tidal influence on groundwater will create a brackish zone between the less density freshwater and the more dense sea water. Fresh and brackish groundwater will discharge to the tidal zone from around the high to mid tide area. 10

13 5. KEY FINDINGS AND RECOMMENDATIONS 5.1 Key Findings The shallow groundwater is fresh or slightly brackish due to the inputs from rainfall, the Canal Drain and groundwater flow from the River Ness. This fresh water lies on top of more dense saline water. Ground conditions are well compacted alluvial sand and gravel overlain by thin soils and made ground. The only surface water flow into the area is from the Canal Drain which flows into the Kingfisher Lagoon and then into the Witch s Coffin Lagoon via a pipe with flap valve through the sea wall. The Canal Drain collects seepage and underdrains from the canal to the west and surface water drainage from the roads and industrial units to the east. The three lagoons have increasing levels of salinity, with Westfield being the least saline, followed by Kingfisher and finally the Witch s Coffin which is entirely tidal. The extent of saline influence in the Westfield and Kingfisher lagoons is determined by the inflow of water through the flap valves, and Kingfisher also has a freshwater inflow from the Canal Drain. The tidal area of the LNR has extensive areas of groundwater seepage comprising of fresh to slightly brackish water around the mid tide area. 5.2 Recommendations Management 1. Maintain and clean the flap valves on the drainage pipes through the sea wall to allow them to function as designed. 2. Construct a stop log control on the outlet pipe from the Westfield Lagoon to provide a control for water levels and salinity. 3. Consider incorporating the Canal drain into the reserve and enhancing the riparian zone for one of the few open, flowing waterbodies within Inverness. (Could benefit from further investigation). Monitoring 1. Monitor the groundwater levels recorded by the walrag (present, maximum and minimum) on a regular weekly or fortnightly basis, while also noting the approximate stage of the tide. This can be increased in frequency to daily or hourly during one spring and one neap tide to establish shorter term variations, especially for the larger spring high tide. 2. The salinity in the dipwell and the lagoons can be routinely monitored. This can be on a weekly basis, noting the approximate stage of the tide. Again this can be increased in frequency over a spring tide to hourly to establish the extent of tidal influence on the near surface groundwater. 3. The salinity meter used should be capable of detecting from fresh to saline water. 11

14 Appendix A Photographic record of site visit Plate 1: Tidal flap valve on drainage pipe from Westfield Lagoon 12

15 Plate 2: Tidal flap valve on drainage pipe from Kingfisher Lagoon Plate 3: Sea wall showing area of tidal seepage to north west (left) and Westfield Lagoon to south east (right) 13

16 Plate 4: Tidal area showing area of tidal seepage Plate 5: Upstream inflow to Canal Drain adjacent to Muirtown Basin 14

17 Plate 6: Upstream reach of Canal Drain with original drain in background Plate 7: Outflow from short reach of culverted Canal drain discharging to Kingfisher Lagoon 15