Poole Harbour Channel Deepening and Beneficial Use Review of Physical monitoring in Poole Harbour Technical Note 2 WITHOUT PREJUDICE

Transcription

1 Poole Harbour Channel Deepening and Beneficial Use Review of Physical monitoring in Poole Harbour Technical Note WITHOUT PREJUDICE. Introduction This technical note reviews the physical data that was collected as part of the monitoring of the dredging activities that took place in Poole Harbour over the period October 5 to March 6.. Description of dredging activity There were three elements to the dredging activity within Poole Harbour. The main activity was the removal and disposal offshore of material that was considered too silty to be used for beach replenishment. The second activity was the dredging of sandy material from the seaward part of the Harbour approach channel that was suitable for beach replenishment. The third activity that was undertaken was a short trial of overflowing of silty material in the Turning Basin at the Ro-Ro berths. This trial was undertaken to inform the preparation of the sediment management plan which is being developed to assist in the retention of fine sediment within Poole Harbour. The dredging works were undertaken by a combination of three trailer suction hopper dredgers (TSHD). The dredging commenced on 3 th November with the,5m 3 hopper capacity TSHD Volvox Scaldia. On 3 th November the TSHD HAM 3 joined the dredging. The HAM 3 has a hopper capacity of about 3,5m 3. On 5 th December the Volvox Scaldia was replaced by the TSHD Waterway which has a hopper capacity of about 4,9m 3. On th January the Waterway completed her work on the contract. On 9 th March the HAM 3 completed the contract with the final pump ashore at Bournemouth. A diary of the beach replenishment works can be found at The bulk of the in harbour dredging was completed by early January 6. The remainder of the works were undertaken at convenient times within the programme of dredging sands from the Swash Channel. Over this latter period monitoring was undertaken at the offshore disposal site (see Technical Note ). The dredging contractor, Van Oord, who undertook the dredging works maintained records of the volumes of material being placed offshore at the disposal site. These records are presented in terms of a volume in the hopper the method for measurement of this volume is not known. It is understood that the bulk density of the material loaded into the hopper during the dredging of the silty material varied considerable between 5 and 5kg/m 3 (R N Appleton pers comm.). The mass of material dredged from the harbour is unknown. A requirement of the license for offshore disposal of material at the Swanage disposal site was that no more than,m 3 of material was placed offshore in a 4 hour period and that there was no disposal of material in the last hour of the

2 south west flowing period of the tide (ebb). It is assumed that it is these constraints, combined with other issues such as crew changes etc. that resulted in the considerable variability in the reported volume (and mass) of material from one hopper load to the next. Records of the mass of material placed at the site were also required as part of the licence. The approach adopted to providing this mass estimate were consistent with the licence application whereby the volume dredged was multiplied by. to convert hopper volume to a representative average mass dredged. The records of volumes disposed from Van Oord provide a basis for examining the intensity of dredging activity of silty material within Poole Harbour. This is illustrated in Figure.

3 Figure Reported volumes of offshore disposal of material arising from in harbour dredging 3

4 The disposal record shows that initially dredging was at a rate of about,m 3 per day and increased when the two larger dredgers were operating together. On occasions the reported volumes of dredging slightly exceeded,m 3 per day. The total volume reported by Van Oord as being disposed offshore at the licensed disposal site was about 7,m 3. A significant proportion of the placed material would be expected to be sandy. This volume is consistent with the EIA. Sandy material was also removed from within the Harbour from the seaward stretch of the approach channel to the north and east of Brownsea Island. Based on the dredging records an estimate of the quantities and timing of this dredging activity is presented in Figure. 4

5 Figure Estimated volumes of in harbour dredging of sandy material Poole Harbour Commissioners (PHC) undertook bathymetric surveys pre- and post- the capital dredging operation. From the PHC surveys the volume of 5

6 material dredged from areas where silty material was expected to arise was about 77,m 3. The volume of silty material reported to have been placed at the offshore disposal site by Van Oord was about 7,m 3. As stated above the insitu density of the silty material and the in-hopper density of the dredged material are unknown. If there was no bulking up of the material on dredging (i.e. density in hopper after dredging was the same as density in-situ) it can be calculated that about 8% of the material removed was lost from the dredged areas. If the material bulked up by % then the loss would be about 6%. If the bulking was % then the loss would be proportionately larger at 3%. The loss of material would be expected to be fines from overflow and disturbance by the dredger. In the hydraulic studies of dispersion undertaken for the EIA it was assumed that 5% of the silty material dredged was released into the Harbour during the dredging operation. This is not inconsistent with the assumption of a bulking factor of about % for the dredging of the silty material. For the EIA studies an in-situ bulk density for the silty material being dredged of,9kg/m 3 was assumed and a proportion of fines in the material of 5-3%. 3. Monitoring programmes Two physical monitoring programmes were undertaken in the Harbour during the course of the dredging operations. Both monitoring programmes were undertaken in response to an agreement with the environmental regulators (Royal Haskoning, 5). The Environment Agency required a real-time monitoring programme utilising surface buoy mounted instrumentation to be underway throughout the dredging period. The purpose of this monitoring was to provide feedback on suspended sediment concentrations and thereby provide a mechanism to control the dredging operations if suspended sediment concentrations in the vicinity of the dredging operation approached levels that the Environment Agency considered would have the potential to impact the shellfishery areas of the Harbour. The locations for this real-time monitoring were selected in conjunction with the Environment Agency (subject to approval by the PHC Harbour Master) and were to the south of the main channel opposite the Ro-Ro terminal (eastern monitoring location SSC) and south south west of Moriconium Quay (western monitoring location SSC). The monitoring was to provide continuous information to the Dredging Contractor on suspended sediment concentrations. If the concentrations exceeded,mg/l the Contractor was to stop and modify the dredging activity in order to reduce concentrations. Given the tidal streams and excursions in Poole Harbour the real time monitoring can be considered to have represented a net through which any adverse impact of the dredging would have had to have passed if it were to affect the shell fish beds to the south. The objective of the real time monitoring programme was to detect any potentially adverse affects of the dredging and, if such an effect occurred and was considered to be associated with the dredging, to control the dredging so that the adverse effect ceased. The real-time monitoring was undertaken by Van Oord the Dredging Contractor. The data collection commenced with a two week deployment of instrumentation starting in late October 5 and, after a short break, continued through into 6

7 February 6. The complete data record is shown in the third graphic of Figure 3 (blue for the eastern monitoring point and green for the western monitoring point). It should be noted that this real-time monitoring was undertaken for a distinct purpose to rapidly identify elevated levels of suspended sediment concentration at the monitoring positions and, if the levels exceeded a particular threshold, to take appropriate action. The monitoring was not supposed to provide a rigorous scientific data base of small scale variations in suspended sediment concentrations at the monitoring locations. The monitors were calibrated on 8 th November and 3 rd January. From the available records of this monitoring, and in the absence of any detailed instrumentation configuration information, it appears as though the instrumentation was reconfigured between the initial baseline (pre-dredge) period and the main monitoring period and that during the pre-dredge monitoring period the maximum detectable concentrations were set low (close examination of the initial blue and green curves in the third graphic of Figure 3 suggests that recorded suspended sediment concentration is cut-off at about 5mg/l). 7

8 Figure 3 Summary data for tidal range, wind speed and direction, suspended solids concentrations and in harbour dredging activity (after Partrac 6). 8

9 The measurements from the surface mounted buoys show a strong correlation with tidal range (tidal range is shown in the first graphic of Figure 3), as would be expected in any shallow muddy tidal system. Suspended sediment concentrations are greater at the western monitoring location than the eastern. From the middle of December to early January there is an exception to this with apparent elevated concentrations at the eastern monitoring location at a time of neap tides. Data from the eastern monitoring buoy at this time has a much higher variability than other periods of the data record and this, combined with the apparently high concentrations measured at a time of neap tides, would suggest that this data is erroneous. This is considered further below. Towards the end of January the eastern monitoring buoy was damaged by an unknown third party and a fully operational buoy was not reinstated until mid February. After this instrumentation was removed it appears that there may also have been periods of erroneous data from the buoy at the western monitoring site. The second graphic in Figure 3 shows the magnitude of the wind speed as recorded by PHC (blue line) and the direction of the wind speed (red vector with winds from a southerly direction being shown as a downward pointing arrow). The fourth graphic shows the daily volume of in harbour dredging throughout the monitoring period and is a combination of information presented in Figures (shown in blue in Figure 3) and Figure (shown in brown in Figure 3). The second monitoring programme was undertaken by Partrac Ltd on behalf of PHC (Partrac, 6a). The Partrac monitoring involved the deployment of a bedframe at various locations throughout the Harbour. The bed frame was instrumented to measure near bed suspended sediment concentrations, tidal current speed and direction, wave height and bed level. The Partrac deployments were to provide rigorous quality controlled data for scientific analysis. The instruments on the frames were rigorously maintained and calibrated. Data return from the Partrac deployments was of particularly high quality. The suspended sediment concentration measured during the bed frame deployments is also shown in the third graphic on Figure 3 (in red). The bed frame was initially deployed at a location close to the eastern of the two Van Oord surface mounted monitors (9 th November to 6 th December BF3). The bed frame was then relocated to the east of the Harbour to be close to the area where the capital dredging of sands from within the Harbour was occurring (6 th December to 4 th January BF4a and BF4b). The bed frame was then relocated close to the western of the two Van Oord surface mounted buoys between 4 th January and 9 th February (BF) before being returned to the initial monitoring position for the period 9 th February to nd February (BF3). The locations of all monitoring points are indicated in the top most graphic of Figure 4. The concentrations measured by the surface mounted instrumentation show typical concentrations during the first period of dredging in mid November over a spring tide period of between to 7mg/l at the eastern monitoring position and between and mg/l at the western monitoring position. Over the following neap tide period towards the end of November concentrations at both locations were between about 5mg/l and 5mg/l. A fairly clear tidal variation in suspended sediment concentration was observed at this time. The bed mounted instrumentation (closer to the eastern monitoring location) displayed very 9

10 comparable behaviour with concentrations elevated compared to the eastern location providing evidence of consistency between the two monitoring programmes and evidence of a surface to bed gradient in suspended sediment concentration (perhaps double the concentration near bed compared to near surface). At the beginning of December a significant storm/tidal surge event occurred ( st to 3 rd of December). This event elevated concentrations at all the monitoring positions and also appears to have raised bed levels at the position of the bed frame by between 5 and mm over a three day period. Peak concentrations of 5mg/l were recorded at the eastern monitoring location, 35mg/l at the western and 4mg/l at the bed frame. Minimum concentrations at all locations rarely dropped below 5mg/l over a two day period. Following this event, and over a spring tide period, suspended sediment concentrations were elevated compared to the previous spring tide period. Concentrations remained elevated through the neap tide period and again into the following spring tide period. From the time of commencement of dredging activity to this point in mid December dredging of silty material from within the Harbour had been undertaken at a relatively constant rate of about,m 3 /day (albeit that there is a gap in dredging of silty material towards the end of November). Interestingly after the major storm, less intense windy periods on 8 th December and 6 th December also appear to influence suspended sediment concentrations significantly. It appears that the instrumentation registered the natural variations in suspended sediment concentrations at the monitoring locations and that following the major storm at the beginning of December the bed sediments of the Harbour were reworked and made more susceptible to resuspension by lesser wind events. On 6 th December the bed frame was moved to the west of the Harbour. From the st December the eastern surface mounted monitor (blue line in third graphic of Figure 3) appears to give erroneous data with elevated concentrations averaging about mg/l for about a week over a period of neap tides. At this time the western monitor indicates a near constant level of about 3mg/l which is slightly higher than during previous periods of calm neap tides. The timing of the apparent elevation of near surface concentrations at the eastern monitoring position occurs about five days after commencement of dredging of sands from the seawards part of the approach channel in Poole Harbour. At this location the in-situ material was understood to be clean sand (R N Appleton pers comm.) and it would appear unlikely that this dredging activity led to the release of significant fine material that might have then elevated concentrations at the eastern monitoring position. At the time the sand dredging within Poole Harbour commenced the bed frame was relocated to the western side of the channel adjacent to the sand dredging area (BF4a). For the short period of the deployment at this location there is no evidence of elevated concentrations at the bed frame indeed concentrations near bed rarely exceeded mg/l. This is further evidence that the elevated readings at the eastern monitor during the period st December to 9 th December are not associated with dredging activity.

11 At the end of the year wind speeds picked up to over 6m/s for 36 hours and this, combined with rising spring tides, again resulted in elevated suspended sediment concentrations at all monitoring locations. Over this period there was again an observed increase in bed level of some 5-mm over a period of two days at the location of the monitoring frame (now in the western part of the Harbour BF). By 5 th January the bulk of the in-harbour dredging had been completed. There was a short trial of overflow dredging in the Turning Area and monitoring of this event is reported separately by Partrac Ltd (Partrac, 6b). Following the trial dredge in the Turning Basin peak suspended sediment concentrations at the western monitoring position were at some of the highest levels they attained throughout the whole monitoring period. At this time peak concentrations may have been elevated as a result of the trial overflow but the period also followed a period of some of the largest tidal ranges throughout the monitoring period (the winds however were calm). Further influence of wind on suspended solids concentrations in the Harbour is evidenced by elevated concentrations around th and th January, 3 th to 4 th January and again on th January. The eastern monitoring buoy was removed by 6 th January and it is possible that the instrumentation had not functioned properly since th January. From 5 th February it appears that the instrumentation on the western monitoring buoy was also malfunctioning and it is possible that this problem developed earlier in the monitoring period. The Partrac monitoring of the trial dredge provides useful information about the nature of the overflow plume from the dredging activity (Partrac 6b). With dredging and overflow occurring in the Turning Basin the maximum extent of the plume from the dredging was found to extend (on the flooding tide) some 9 to,m to the west. The plume was constrained to an extent by the deep water channel axis and transported dominantly along this axis. High turbidity waters were recorded to the north of the plume, but the plume was not judged to have reached m distance south from the dredge vessel. The thickness of the surface plume from the overflow was between 3. to 3.5m. It was concluded that the plume did not directly interact (i.e in the short term) with either of the Van Oord surface monitoring positions. This is an important result that demonstrates that the surface mounted monitoring positions are unlikely to have responded immediately to dredging activity. They would tend to pick up the subsequent effects of any dredging activity after tidal dispersion. The peakiness of any influence of dredging activity would thus be reduced and consequently harder to identify. The surface monitors did however effectively provide a net through which fines resulting from the dredging activity would have had to pass to influence the southern part of the Harbour where the majority of the shellfish beds are. The general conclusion from the monitoring of suspended sediment concentrations in the Harbour is that when the recorded data appears to be of good

12 quality the data broadly represents the general background levels of suspended sediment in the Harbour. The suspended sediment regime responds, as would be expected, to tidal forcing (the spring neap cycle) and the influence of wind waves which, in the shallow waters of Poole Harbour, are considered likely to be able to enhance the resuspension of bed material and maintain such material in suspension in the shallower areas. Some consideration has been given to the significance of the elevated water levels and strong winds that occurred prior to and during the period of the dredging activity. An approximately 5 year data set was provided by PHC of wind speed and direction and measured tide levels at the Ro-Ro. This data set has been examined to identify events when water levels consistently exceeded m CD (i.e. water levels would have had to have been influenced by a positive surge except over a period of neap tides) the average water level for the period exceeded.m CD and average wind speeds at some point during the period of elevated water levels were in excess of m/s (i.e. wind speeds at times would have been high and water levels would have been sufficient for the wind waves generated to have assisted in agitating the bed of the shallow areas of Poole Harbour). On the basis of these criteria it is found that the storms that centred on the nd December 5 and the 3 rd November 5 were the largest and fourth largest resuspension events of the 5 year record each lasting for about days. On the face of this analysis and the other monitoring results it is suggested that the natural forcing conditions influencing the suspended sediment regime of Poole Harbour were atypical in respect of storm activity (winds and surge) during the period of the capital dredging. This in turn is likely to have meant that background conditions within the Harbour, in terms of the potential for large scale mobilisation and redeposition of sediment, were also atypical during the period of the dredging. Against this background the influence of the dredging itself appears to have been localised to the proximity of the dredged channel. 4. Comparison of monitoring and EIA modelling results The hydraulic studies for the EIA (HR Wallingford, 4) included predictions of baseline suspended sediment conditions, conditions following completion of the channel deepening and predictions of the effects of the capital dredging itself. A supplement to the EIA addressing the proposed Poole Harbour Sediment Management Plan included the results of a simulation of overflowing of muddy material in the Turning Basin. In this section consideration is given to the results of these simulations against the results of the monitoring described above. The baseline mud transport modelling was calibrated against field data collected in January and March 99 in the north and eastern part of the Harbour under calm and windy spring tide conditions. Highest concentrations were observed and predicted in the western part of the Harbour just upstream of the port area. This is consistent with the results of the recent monitoring. The mud transport model model was run for spring tide conditions with no waves, 5% exceedence winds, % exceedence winds and % exceedence winds to demonstrate the variability of the suspended sediment regime in the harbour. The

13 model predicted a deposition zone between the port area and Brownsea Island under periods of high winds. Peak deposition in this zone was predicted to be between and 5mm per tide under % exceedence winds (see Figure 4.9 of HR Wallingford, 4). This result is broadly consistent with the bedframe observations of changes in bed level under the December storm. The effect of the completed channel deepening on this zone of accretion was predicted to be insignificant. The in-harbour plume modelling undertaken as part of the hydraulic studies considered the scenario of a single TSHD of 5m 3 capacity operating continuously on a three hour dredging cycle. It was assumed that 5% of the dry mass of material dredged was released in a one hour overflow period during the course of the dredging. No direct evidence in the form of a mass balance for insitu material vs in hopper material is available against which to consider this assumption. On the basis of the analysis of volumes the assumption of 5% loss of material assumed in the EIA broadly equates to a scenario where the in-situ muddy material bulked by about % during the course of the dredging. The production rate simulated is consistent with the first period of dredging of silty material when rates of removal offshore were reported to be about,m 3 per day. The results of the plume simulations are supported by the observations of the plume in the vicinity of the trial overflow which took place during spring tides. The plume from the dredger was strongly advected along the axis of the channel in line with the tidal stream and weakly dispersed and diffused across the tidal flow. The plume modelling predicted that peak increases above background levels away from the dredged area itself would be in the range of -5mg/l and that tidally averaged increases above background levels would be in the range - 5mg/l. The results of the plume monitoring presented in the EIA (Figures 8.5 and 8.7 of HR Wallingford (4)) have been reprocessed to output depth averaged suspended sediment concentrations at the locations of the monitoring (Figure 4). The results presented in Figure 4 show that the plume modelling predicted that over neap tides no influence of the dredging operations was expected to be observed at the two surface mounted monitoring positions (SSC and SSC). This is consistent with the observations over the first period of neap tide dredging in November (see Figure 3). On spring tides depth averaged suspended sediment concentrations at these locations were predicted to be increased by up to 5mg/l over short periods. The extent of increases above background levels was predicted to be greater at the western monitoring location (SSC) than at the eastern location (SSC). The observations as presented in Figure 3 appear to be more strongly influenced by wind wave activity than by the dredging activity although it is possible that on spring tides during periods of dredging there is some overall increase in peak suspended sediment concentrations (but not by as much as 5mg/l). From this comparison it appears that the source terms utilised in the plume modelling for the EIA studies were precautionary and that any increases in suspended sediment concentrations above background levels associated with the dredging activity were less than those predicted in the EIA. 3

14 The hydraulic studies to support the EIA were based upon flow and mud transport models that were calibrated for the northern and eastern part of the Harbour. In this respect they provide a good basis from which to consider the baseline conditions in this area and the potential for the channel deepening to have modified those conditions. In this area for which the models have been well calibrated the models compare well with the results of the recent monitoring. It was always accepted that in other areas of Poole Harbour less was known about natural variability in the sediment regime. One of the purposes of the bed frame monitoring undertaken by PHC was to extend the knowledge of physical processes throughout the Harbour. The Environment Agency required that a programme of real time monitoring was undertaken to provide a basis for controlling the release of fines from the dredging operation. The Environment Agency specified the thresholds to be used in the real-time monitoring. The fact that concentrations at the monitoring locations did not approach the upper (stop) threshold of,mg/l proposed by the Environment Agency demonstrates that the influence of the dredging activity on suspended solids concentrations in the northern part of Poole Harbour was broadly within (or less than) the predicted effects presented in the EIA. If the suspended sediment concentrations associated with the dredging activity in the vicinity of the monitors were generally less than the predictions in the EIA then the associated deposition would also be expected to be comparable or less than that predicted in the EIA. 4

15 5

16 SSC BF BF3 SSC BF4a BF4b 5 BF3 5 BF4a Concentration (mg/l) Concentration (mg/l) BF4b BF SSC 5 SSC Concentration (mg/l) Time (days) Time (days) /HR_projects/ddr48/model/temp_poole/dredge_res.RUB/conc_comp.i Figure 4 Results of in harbour plume dispersion modelling presented in EIA to provide output at the various monitoring locations 6

17 The models used in the EIA studies to simulate baseline conditions are well calibrated for the northern part of the Harbour. They are, however, not calibrated for baseline conditions away from this area. The models therefore, may not fully represent the effects of storm wave agitation of the bed and the associated sediment fluxes and deposition outside of this area. The EIA studies therefore do not demonstrate the significance of the range of naturally occurring conditions that can influence the full extent of the Harbour. As noted above it is considered that the storm of st to 3 rd December was likely to have been a particularly significant sediment transporting event in the context of the whole of Poole Harbour. 5. Tidal currents in the Harbour Entrance Spring tidal currents in the entrance to Poole Harbour were measured using Acoustic Doppler Current Meter (ADCP) before the capital dredge. These baseline measurements were used to validate a local 3D flow modelling exercise undertaken as part of the EIA. The local 3D model of Poole Harbour Entrance was set up in response to consideration of the high currents (5-6 knots) in the area and the curvature of the deepwater channel in the entrance to the channel. This model was calibrated against specifically commissioned current observations. The locations of the comparisons are shown in Figure 5. The model comparison is reproduced from Appendix 3 of the Environmental Statement for the deepening (HR Wallingford, 4). Figure 6 shows an example of the comparison of the model against data extracted from the ADCP Transects at mid-depth. The model comparisons were generally good although some difficulty in reproducing some of the observed currents was encountered due to the difference in the simulated and observed tide shape with the tide on the day of the observations having a greater change in water levels for the secondary LW and HW, and the difference of the phasing of the stronger ebb tide. These differences in the tidal conditions between the simulated and observed periods make any more objective analysis of the model s accuracy in predicting the effect of the deepening difficult. One objective measure of model accuracy is the Mean Absolute Error (MAE). This statistic, outlined in Southerland and Soulsby (3), is calculated from the average difference between the observed and simulated values. Comparing the observed currents with those simulated gives an average MAE of.5 m/s across the 9 locations. This value represents a general error of the order of % of the peak currents observed. The same statistical calculation of the tidal comparison gives a value of cm (approximately % of the observed tidal range,.m at Poole Ro-Ro). 7

18 8775 Model Bathymetry (mcd).. Line Line Line Figure 5. Locations of 4 ADCP Transects 8

19 Poole Harbour HAT output at mid-depth measured speed measured direction model speed model direction 36 Model run: 3D run, vvz at boundaries (_vvz).5 Line 3 point Line 3 point Line 3 point hrs after LW Line point Line point Line point Line point Line point Line point Figure 6. Comparison of observed and simulated mid depth currents, predredge conditions Following completion of the dredging the currents were remeasured with ADCP. The two sets of measured currents have been extensively compared with each other and with the model predictions (Compass Hydrographic, 6). The Compass report identified a number of drawbacks to a straightforward comparison. 9

20 deviations from the planned survey line as a result of the difficulties associated in navigating the vessel in the strong currents and with a high volume of small craft traffic (as evidenced by differences in the bathymetric profiles from pass to pass) variations in the timing of the survey passes within the tidal cycle (which can differ by order minutes) variations in the nature of the tidal curves of the tides measured on each survey (identical tides having a repeat frequency of approximately 9 years) the impact of any residual currents arising from wind driven circulations in Poole Bay itself or from the discharge of fresh water into the harbour. The collected observations were extracted at points as close as possible to the 4 data points and compared to the model simulation of the post-dredge situation (This prediction was carried out for the same tidal range and form as the pre-dredge baseline condition). Figure 7 shows the locations of the 9 post dredge comparison points. Figures 8- show a sample of the comparisons of the observed and simulated currents at the central point extracted from each ADCP transect. This product has been derived in part from material obtained from the UKHO Crown Copyright and/or database rights. Reproduced by permission of the Controller of Her Majesty s Stationery Office and the UK Hydrographic Office. Licence Number 7/4. Figure 7. Locations of 6 ADCP Transects (from Compass report)

21 Surface Direction (on) Speed (m/s) Direction (model) Direction (obs) Speed (model) Speed (obs) Time (s) (HW=) Mid-depth Direction (on) Speed (m/s) Direction (model) Direction (obs) Speed (model) Speed (obs) Time (s) (HW=) Bed Direction (on) Speed (m/s) Direction (model) Direction (obs) Speed (model) Speed (obs) Time (s) (HW=) Figure 8. Comparison of observed and simulated currents, post dredge conditions, Transect, central point.

22 Surface Direction (on).5 Speed (m/s) Direction (model) Direction (obs) Speed (model) Speed (obs) Time (s) (HW=) Mid-depth Direction (on).5 Speed (m/s) Direction (model) Direction (obs) Speed (model) Speed (obs) Time (s) (HW=) Bed Direction (on) Speed (m/s) Direction (model) Direction (obs) Speed (model) Speed (obs) Time (s) (HW=) Figure 9. Comparison of observed and simulated currents, post dredge conditions, Transect, central point.

23 Surface Direction (on) 5..8 Speed (m/s) Direction (model) Direction (obs) Speed (model) Speed (obs) Time (s) (HW=) Mid-depth Direction (on) 5.8 Speed (m/s) Direction (model) Direction (obs) Speed (model) Speed (obs) Time (s) (HW=) Bed Direction (on) Speed (m/s) Direction (model) Direction (obs) Speed (model) Speed (obs) Time (s) (HW=) Figure. Comparison of observed and simulated currents, post dredge conditions, Transect 3, central point. 3

24 The statistical analysis of the model accuracy as calculated for the pre-dredge comparison of the observed and simulated currents gives an average Mean Absolute Error (MAE) of.38 m/s across the analysed locations, which is not as good as that for the pre-dredge case as might be expected from comparison of tides of different range and shape. Indications of the source of this slightly increased average error comes from the MAE for the simulated tidal levels compared with those observed which gives a value of.8 m, a result of the difference in the tide range and shape of the 6 observations with that of the simulated conditions. Not withstanding differences between the actual tidal range and form on the days of measurements the measured data is consistent with the pre and post dredge model predictions presented in the EIA. 6. Conclusions The physical monitoring within Poole Harbour has provided results which are generally consistent with the baseline sediment transport studies undertaken to support the EIA. No evidence of increased suspended sediment concentrations associated with capital dredging of silty material above those predicted in the EIA is apparent. If anything the evidence from the monitoring is that the EIA predictions were precautionary. There are a number of periods in the latter part of the monitoring record when one or other of the near surface monitors operated by Van Oord appear to have been providing erroneous data. These periods of poor data occurred after the bulk of the dredging of silty material in the Harbour had been completed. The near surface monitoring was undertaken to provide a basis for controlling the dredging activity within limits (in terms of absolute suspended sediment concentrations) which were never approached. There is no data available from which to construct a mass balance for the capital dredging operation which would enable an improved estimate of fine sediment losses during the dredging works to be made. From the analysis of the monitoring results it is suggested that the natural forcing conditions influencing the suspended sediment regime of Poole Harbour were atypical in respect of storm activity (winds and surge) during the period of the capital dredging. This in turn is likely to have meant that background conditions within the Harbour, in terms of the potential for large scale mobilisation and redeposition of sediment, were also atypical during the period of the dredging. Against this background the influence of the dredging itself appears to have been localised to the proximity of the dredged channel and within the predictions provided in the EIA. Spring tidal currents in the entrance to Poole Harbour were measured using ADCP before the capital dredge. These baseline measurements were used to validate the 4

25 D and 3D flow modelling undertaken as part of the EIA. Following completion of the dredging the currents were remeasured with ADCP. The two sets of measured currents have been compared with each other and with the model predictions. Not withstanding differences between the actual tidal range and form on the days of measurements the measured data is consistent with the model predictions presented in the EIA. 7. References Compass Hydrographic Services Ltd (6). Measurement of the changes in flow regime at Poole harbour Entrance following Channel Deepening, Report No. 6/9/47/, October 6. Partrac (6a) Measurement of Metocean & Other Parameters within Poole Harbour, Dorset, During Dredge Operations (9//5 //6), April 6. Partrac (6b) Measurement of Suspended Sediment Concentration Profiles During a Dredge Overflow Pumping Exercise, Poole Harbour, Dorset, June 6. HR Wallingford (4) Poole Harbour Approach Channel Deepening, Hydrodynamic and sedimentations studies, Report EX 4945, November 4, Technical Annex to EIA. Royal Haskoning (5) Note to MCEU dated 6 June 5, Reference 9P7/N/MAS/Exet. Sutherland and Soulsby, (3). Use of model performance statistics in modelling coastal morphodynamics, Proceedings of the International Conference on Coastal Sediments 3. HR Wallingford 7 th May 7 5