POOLE HARBOUR APPROACH CHANNEL DEEPENING EIA: SEDIMENT MANAGEMENT PLAN 1. Introduction The purpose of the Poole Harbour Sediment Management Plan (SedMP) is twofold: to mitigate the predicted impact of the Poole Harbour Approach Channel Deepening and to put in place an approach to the management of sediment in Poole Harbour that will limit the effects of maintenance dredging in the future on the intertidal and hence the various European sites within and adjacent to Poole Harbour. With respect to the first of these, HR Wallingford predicts that there will be two one-off effects associated with the construction works (a. and b. below) and two year-on-year effects associated with the operational phase (c. and d.): a. An increase in the area of exposed intertidal due to a slight increase in the tidal range, potentially amounting to 2.9 to 11.7ha above Chart Datum (CD). b. An increase in the area of intertidal due to the release of fines from the dredging process, amounting to 0.4 to 2.6ha above CD. c. An increase in the natural export of sediment out of Poole Harbour, equating to the loss of 0.15 to 0.6ha of intertidal (above CD) per annum (i.e. between 4,800 and 6,800m 3 /year). d. An increase in the deposition of fine material in the Turning Basin. However, it is proposed that this material will be kept in the system through the use of agitation dredging methods in this area in the future and, consequently, that there would be a gain of intertidal area equating to 0.04 to 0.26ha (above CD) per annum compared to the present situation 1. The ranges quoted above reflect uncertainty in the gradient of the slope of the intertidal around CD. Thus for the scenario of the slope being 1:50, the low end of the ranges can be taken to lead to a one-off increase of 3.3ha and a year-on-year increase in erosion of 0.11ha/year. In the scenario where the slopes are shallower (1:200), the high end of the ranges applies, leading to a one-off increase of 14.3ha and a year-on-year increase in erosion of 0.34ha/year. Overall, therefore, the year-on-year net increase in erosion of intertidal area is predicted to be between 2.3% and 3.3% of the one-off gains (2.3% for the 1:200 slopes and 3.3% for the 1:50 slopes). Given this, and without further mitigation, the one-off increases in intertidal area are expected to be balanced by the increase in the underlying rate of erosion over a period of 30 to 45 years after the completion of the capital works. It is recognised that a range of possible outcomes exists in terms of the mitigation balance, due to the variability in the slopes of the intertidal area of Poole Harbour. Therefore, it is acknowledged that a best practice approach 1 Note that in calculating this area of gain the assumption has been made that agitation from the Turning Basin will return an amount of material to the intertidal areas comparable to the losses associated with the capital dredging of silty material from Middle Channel. This is a precautionary approach and a greater benefit may occur.
would be to offset the predicted increase in the year-on-year export of material from the Harbour from the project inception. The objective of the Sediment Management Plan, therefore, is to mitigate this potential increase in the rate of future loss of intertidal area due to the channel deepening (i.e. 0.11 to 0.34ha above CD per annum) through the management of maintenance dredging operations undertaken throughout Poole Harbour in order to retain sediment. The achievement of this objective will then allow an approach to be put in place that will be controlled by the Poole Harbour Commissioners (PHC) in order to retain as much fine material as possible within the Harbour (as opposed to its disposal offshore), thereby avoiding any potential long term effects on the relevant European sites due to maintenance dredging. 2. Background to maintenance dredging in Poole Harbour There are two types of maintenance dredging currently undertaken within Poole Harbour. The major bi-annual maintenance dredge undertaken by Contractor on behalf of PHC in the main navigation areas utilising a trailer suction hopper dredger and smaller dredging operations undertaken generally at the various boatyards and marinas utilising mechanical dredging plant loading into barges. Traditionally, all dredged material unsuitable for beach nourishment has been placed offshore at the licensed site in Swanage Bay. The main PHC maintenance dredge is typically undertaken every two years and involves dredging of about 110,000m 3 of material. Of this amount an average of about 42,000m 3 of sandy material is removed respectively from the Swash Channel and the Middle Channel. The remainder, about 26,000m 3, which is predominantly silty is removed from the Turning Basin. These volumes are predicted to increase as a result of the proposed channel deepening to a total of about 132,000m 3. Of this amount about 46,000m 3 of sandy material is to be removed both from the Swash Channel and Middle Channel and about 40,000m 3 of predominantly silty material from the Turning Basin. Typically, the maintenance dredging is undertaken by a trailer suction hopper dredger with a capacity of 1,500m 3 working 24 hours a day for a period of up to 3 weeks. The volumes of maintenance dredging undertaken by smaller scale mechanical plant at the marinas and boatyards throughout the Harbour (including Holes Bay) vary from year to year. Over the last ten years the minimum amount of dredging was about 3,000m 3 (in 1997) and the maximum was about 38,000m 3 (in 1996). The average volume dredged annually has been about 18,000m 3, of this approximately half is carried out in Holes Bay. The production rates for the mechanical dredging are lower than those of the main maintenance dredging contract with barge capacity, typically in the range 200m 3 to 800m 3. In addition to the maintenance dredging of the marinas and boatyards PHC also use their own grab hopper dredger the C.H.Horn to undertake small scale maintenance of the main navigation areas: Middle Channel, Little Channel, North Channel and Turning Basin. The amount of such dredging has declined in recent years but over the last ten years the average annual quantity was about 13,000m 3. The majority of the dredging with mechanical plant yields silty material. However, some locations, particularly those closest to the Harbour mouth, generate sandy material. 16 June 2005 9P0171/N/SAJ/PBor 2/20
3. Understanding the sediment regime and sediment budget of Poole Harbour The hydraulic studies undertaken as part of the Environmental Impact Assessment for the proposed Channel Deepening (Royal Haskoning, 2004) have improved the understanding of the sediment regime of Poole Harbour. A brief summary of which is provided here (the reader is referred to Section 3 and Appendix 3 of the ES for full details). On the basis of available information relating to the bathymetry and area of saltmarsh of Poole Harbour, it has been shown that the Harbour is losing sediment at a rate of between 56,000 and 76,000m 3 /year. These rates of loss of fine sediment are comparable to, but greater than, the average rate of removal to the offshore disposal site of fine silty material through maintenance dredging. In simple terms the area of shallow subtidal waters is increasing, the area of intertidal flats is staying constant and the area of saltmarsh is reducing. For the purposes of the EIA it has been assumed that there are no supplies of fine silty material to Poole Harbour from offshore, fluvial or cliff erosion within the Harbour. The combined actions of waves, rainfall and the tides are thus redistributing material within the Harbour, some of which naturally leaves the harbour as a result of an ebb dominant sediment regime at the Harbour Entrance and some as a result of offshore disposal of material from maintenance dredging. Sands (on the other hand) are readily able to enter the harbour from offshore. In a situation where there is a net import of sand into the Harbour the total losses of silty material from the Harbour could thus be greater than the estimate of 56,000 to 76,000m 3 /year given above. The picture illustrated above is simplistic, relating to the Harbour as a whole. In reality parts of the Harbour are more or less stable than others, responding to different energy regimes. The area to the south of Brownsea Island has been particularly dynamic in recent years. The studies undertaken as part of the EIA have shown that the last deepening of the Swash Channel in 1988-90 would have had an impact on the natural ebb dominant sediment regime at the Harbour Entrance, increasing the natural rate of loss of material from the Harbour. Similarly the proposed deepening of the Swash Channel will further increase this effect by some 5,000 to 7,000m 3 /year. The studies also show that the effects of the proposed deepening of Middle Channel are similar to those of the last deepening in 1990. As noted above, the proposed channel deepening is predicted to increase the requirement for maintenance dredging in the main navigation channels by about 20%. However, there is not predicted to be an impact on the requirement for maintenance dredging at any of the small boatyards and marinas. In summary, the studies undertaken for the EIA have demonstrated that Poole Harbour is gradually losing silty sediment. In the long term this effect will be enhanced by the proposed channel deepening. In the short to medium term (~50 years) there will be some offsetting effects associated with release of fines during the capital dredge and a minor effect associated with a very small reduction in low water levels which will slightly increase the amount of intertidal area exposed. English Nature and other conservation bodies have requested that consideration is given to mitigating this effect through a Sediment Management Plan for Poole Harbour. It is intended that this plan will evolve over time. It will also require the co-operation of all parties with a need to undertake routine maintenance dredging within the Harbour. However, with the exception of dredging activities undertaken by The Crown, all other dredging activities undertaken within Poole Harbour (amounting to around 99% of the dredging undertaken) are subject to the Commissioners licensing regime. Through this mechanism, therefore, the PHC can exercise 16 June 2005 9P0171/N/SAJ/PBor 3/20
control over future maintenance dredging operations. The remainder of this document outlines the PHC s proposals for the sediment management. 4. Proposal for the management of future maintenance dredging in Poole Harbour The proposals set out below relate to the four main types of maintenance dredging undertaken in Poole Harbour: dredging in the main Approach Channel and the Turning Basin; independent dredging by small-scale operators; small-scale dredging undertaken by PHC; and dredging within Holes Bay. They represent the proposed initial approach to the increased retention of fine material within the Harbour. However, it is intended that an iterative approach will be adopted to the management of sediment in Poole Harbour. That is, monitored trials will be undertaken in the first instance in order help determine the optimum sediment placement techniques, locations, quantities and timing for encouraging the retention of sediment on the intertidals (with due respect to the benthic resource and fishery). From these, preferred approaches will be selected and implemented. The results of monitoring will then be used to refine the approach taken to sediment management over time as and when required. Regardless of the final nature of the strategy progressed, all future maintenance dredging in Poole Harbour will remain subject to the full consents regime (as at present and this will consider potential levels of contamination in the material to be dredged, amongst other things). Changes to present practice, therefore, will be discussed formally with the Regulators and other interested parties. 4.1 Main maintenance dredging contract and agitation dredging in the Turning Basin In the future, PHC will carry out dredging in the Turning Basin using agitation techniques. The majority of the material to be removed from the Swash Channel and Middle Channel will be sand and this will continue to be used beneficially on beaches within Poole Bay. A trailer suction hopper dredger overflowing for extended periods during the loading of silty sands from the Turning Basin will undertake the proposed agitation dredging. This will generate the highest release rates of silty material into the water column compared to other methods of agitation and, therefore, represents the scenario with the maximum rates of release and maximum potential short term effects on areas of the bed of the Harbour adjacent to the Turning Basin (see Figure 6 of Annex 1). The total volume of overflow of fine material from this activity could be up to 10,000m 3 to 20,000m 3 (equivalent in-situ volume) per campaign. The resultant dredged cargo will contain some silts and is unlikely to be suitable for placement on the beaches within Poole Bay. It is therefore proposed that this material is placed offshore at the licensed disposal site. Overall, however, it is expected that the total mass of silty material placed offshore will be reduced compared to present levels by between 10,000m 3 to 20,000m 3 per campaign. In addition, as techniques improve for handling material in the hopper (to decrease the silt content), it is acknowledged that more material could be retained for use in beach nourishment schemes (should this be sought by the coastal protection authorities) in the future. 4.2 Dredging by small operators and methods for retaining sediment in Poole Harbour The small operators in Poole Harbour that require routine maintenance dredging will be actively encouraged (through PHC s licensing process) to consider the practicality of agitation methods 16 June 2005 9P0171/N/SAJ/PBor 4/20
for removal of this material 2. Agitation dredging methods utilise the natural tidal flows in the Harbour to disperse material from the dredging site. Methods include ploughing, harrowing, water injection and pumped discharge. The applicability of a particular technique will be site dependent and PHC will participate in discussions with the operators over practical means of undertaking agitation dredging in each case. Over time, it is envisaged that the best working methods for a particular site will evolve. At some sites, for example, it may prove impractical to attempt to remove all sediment build up by agitation dredging methods and some mechanical dredging may be required. Where ploughing is applicable, material will be ploughed to adjacent deeper water areas. The process of ploughing would agitate and disperse some of the material. Furthermore the locations to which material would be ploughed would be likely to exhibit some tendency for scour of the ploughed material (which would have less strength than the in-situ sediments) dispersing more of the material. It is proposed that any significant build up of material that occurs in the navigation channels is removed by PHC s dredger and deposited in naturally scouring areas of the Harbour (see below). 4.3 PHC dredging and potential disposal in naturally scouring areas of the Harbour PHC presently undertake small-scale maintenance dredging of some of the marinas, boatyards and navigation channels within Poole Harbour on a contract basis, using their own vessel (C.H.Horn). It is proposed that, where practical, agitation methods will be employed to redistribute fine sediment within the Harbour (as described above), either with the boatyards employing their own contractors or undertaking the work through PHC (as appropriate). However, in some locations this will not be possible. It may also be necessary to remove material that builds up as a result of the change in dredging practice from locations where maintenance dredging is not presently undertaken. This could include the locations to which it is proposed that material is ploughed, that is, around the edge of the North Channel and within the downstream parts of the Wareham Channel. It may also include channels such as the Wych and Wareham Channels which are not presently maintained by PHC but where a build up of sediment could occur in places in the future as a result of the proposed changes to the maintenance strategy (this will not affect maintenance dredging in the main channels). Fundamentally it is proposed that the mechanically dredged material is placed in deeper water locations within the Harbour rather than being deposited offshore. Potential sites for placement include naturally scouring areas of the Harbour, such as some of the deep water in the vicinity of Brownsea Road, parts of the Turning Basin and the entrance to Lytchett Bay off Rockley Point. Simulations of the predicted dispersion of material from these locations (including concentrations and deposition patterns) are described in Annex 1 to this note. Placement sites will be selected based on their potential to disperse material, while limiting the area of the seabed potentially directly affected by the placement and avoiding any obviously sensitive locations and receptors. Placement of material at any of these locations will only proceed on the basis of discussion with the Regulators and appropriate consents. All placements will be subject to initial monitored trials. The trials will be co-ordinated by PHC and funded variously by PHC and/or the relevant 2 Discussions with the marina and boatyard operators that have already taken place have indicated that they are generally happy with the proposals, given that the potential exists for significant cost savings to arise if they do not need to take material to the Swanage offshore disposal ground. In the detail of the proposals, however, they need to retain a degree of flexibility in the timing of their maintenance dredging operations. 16 June 2005 9P0171/N/SAJ/PBor 5/20
operators. Operators benefiting from the use of a given site would be expected to fund the trials. Monitoring during the trial phase and into the future will be co-ordinated by PHC and conditions set down as part of the dredging licences issued by them. The objective of the trials will be to establish the behaviour of the material at the site once placed. If the material is shown to rapidly disperse from a site after placement (i.e. within a few minutes to hours) then further consideration will be given to the precise timing (i.e. the use of ebb or flood tide windows) for the placement of material at the site to improve retention within the Harbour and/or avoid effects on the benthic resource and fishery. If the placed material is found to accumulate on the bed and then to be more gradually eroded over the medium term (a few weeks to months), then the phasing of placements throughout the tide is unlikely to be significant in terms of the fate of material placed at the site. This is because the release rates will be insignificant compared to natural fluxes. In either case, it is not proposed to undertake monitoring at locations distant from the actual placement sites themselves; although such monitoring is proposed as part of the monitoring of the capital dredge itself and, as part of this, monitoring of trailer dredger overflow whilst removing silts from the Turning Basin will be undertaken (see Note on Monitoring). The processing of material that has been mechanically dredged (to reduce its strength and increase its speed of dispersion) is unlikely to be practical within the Harbour because the majority of the small scale dredging will involve mechanical plant. In the long term it is proposed that PHC will explore further options with other parties for the retention of dredged material within the Harbour as part of the application of a best practice approach to the management of sediment within the Harbour, particularly in the context of its designated status. This could include practical options for direct intertidal placement of some of the material arising from the maintenance dredging. However, potential effects on designated areas would need to be considered in detail before any such proposals are progressed. It is significant to note, however, that the short and medium term actions (i.e. the trials and the implementation of a sediment retention strategy) are expected to mitigate the predicted effect of the channel deepening on the intertidal areas of the Harbour. The long term strategy represents management options beyond this. PHC will continue to contribute their experience regarding the environmental management of port and dredging operations to the appropriate National and International fora and will, where appropriate, look to apply the results of relevant experience from elsewhere to the Poole Harbour situation. 4.4 Maintenance dredging within Holes Bay Presently about 9,000m 3 /year of maintenance dredging is undertaken within Holes Bay. During the initial stages of the implementation of the Sediment Management Plan it is proposed that Holes Bay maintenance dredging continues as at present. There is no strong evidence to suggest that the removal of muddy material by maintenance dredging within Holes Bay has an impact on the sediment budget or regime of Poole Harbour (its effects are likely to be more localised). However, in the future it likely that, on the basis of ongoing monitoring of Holes Bay and recording of dredging activity therein, PHC (with the operators in Holes Bay) would promote options for the retention of material arising from maintenance dredging within Holes Bay, within the Bay itself. 16 June 2005 9P0171/N/SAJ/PBor 6/20
5. Implementation of the proposals Given consent for the proposed works, the approach detailed above will be delivered through the Poole Harbour Commissioners Maintenance Dredging Policy; which is provided in Annex 2 to this note. This has been agreed at board level and sets out their formal commitment to the future management of maintenance dredging within Poole Harbour. This Policy addresses both actions to be undertaken directly by the PHC with respect to sediment management in the Harbour and actions to be undertaken by other operators under the control of the Commissioners. Much of the dredging proposed will be undertaken by third parties (see 4.2 above) under licence to the PHC and these operators will be expected to promote proposals that fall in line with the requirements of the Sediment Management Plan for retaining fine material within the Harbour. Moreover, during the trial phase (see below), the Commissioners will provide assistance with technical advice and monitoring until new processes are satisfactorily established. It is proposed that an iterative, monitored approach to sediment retention in the Harbour is adopted; with individual operations being carried out under the Commissioners supervision and co-ordination. The first stage, to be undertaken by the PHC, involves monitoring the overflow of silts from the Turning Basin during the early stages of the Approach Channel Deepening. The second stage involves commencing wider trials of agitation dredging and sediment placement within the Harbour by relevant operators. It is proposed that these trials are undertaken over the two years following the completion of the channel deepening. Trials are not expected to commence until the first autumn/winter period after completion of the channel deepening. A schedule and timescale for the trials will be developed and agreed by the Commissioners with the Regulators ahead of this. Towards the end of the trail phase, similarly, a schedule and timescale for the full scale application of sediment management will be developed and agreed with the Regulators before works proceed, based on the outcome of the trials (this will include formal agreement of the locations for placement, quantities of material to be introduced and the timing of the works, particularly with respect to agitation dredging). This will be fine tuned with time in order to ensure that the placements are effective. Both trials and full scale application will need to take account of the different operational requirements and material types throughout the Harbour. The example placement scenarios provided in Annex 1 illustrate schematically the use of placement sites in different parts of the Harbour and the issue of whether material disperses rapidly at the time of placement or resides on the bed for some time after placement. This type of modelling, and existing knowledge of tidal streams in the Harbour, can be used to predict the footprints of impact of any placement operations and thus address the sensitivity of existing benthic and fishery resources to placement. The influence of both the works and the mitigation (sediment management) programme on the Harbour (and, in particular, intertidal habitats) will then be monitored and reported to the Regulators and stakeholders through a detailed programme of measures; set out in the Monitoring Programme that accompanies this paper. 6. Conclusion The primary objective of the Sediment Management Plan is to mitigate any increase in the rate of future loss of intertidal area due to the proposed channel deepening (i.e. 0.11 to 0.34ha above CD per annum) through the management of maintenance dredging operations undertaken 16 June 2005 9P0171/N/SAJ/PBor 7/20
throughout Poole Harbour. Assuming that sediment could be returned to the intertidal areas via a method that was 100% efficient, this would require a volume of muddy material to be returned each year in the range of 2,700m 3 to 3,900m 3. The total volume of sediment available within Poole Harbour for management from mechanical dredging (i.e. not including the agitation dredging in the Turning Basin) is about 22,000m 3 /year (the average quantity dredged over the last ten years). This does not include the maintenance dredging undertaken in Holes Bay. Thus the required target for sediment retention, in order to offset the predicted year-on-year loss of intertidal area attributed to the proposed channel deepening, is to achieve about 15% efficiency in terms of the retention of maintenance dredged silts within the intertidal areas of the Harbour. The predictions in Annex 1 indicate that placement in the vicinity of Brownsea Island leads to a return of in excess of 30% of the material dredged to the intertidal areas of the Harbour. Given the volumes of fine sediment available, it can thus be concluded that practical methods exist for retaining at least the target figure within the intertidal areas of Poole Harbour through a programme of sediment management. Moreover, greater volumes of fine sediment retention are likely to be achievable, particularly through a programme of activities which is fine tuned over time. It is therefore concluded that the sediment management proposals set out herein (based on trials in the short term and followed by the implementation of a sediment retention strategy) are expected to mitigate the predicted effect of the channel deepening on the intertidal areas of the Harbour. In the short to medium term (i.e. around 30 to 45 years hence), the one-off increases in intertidal area due to the capital dredging will offset the predicted increase in the underlying rate of erosion. In parallel with this, and in the first two to five years following the deepening, the sediment retention strategy will be refined so as to ensure that any future increase in the rate of intertidal erosion due to the channel deepening is mitigated. To this end, and as set out in their Maintenance Dredging Policy, the Poole Harbour Commissioners will commit to minimising the amount of maintenance dredged material that is taken offshore in the future from Poole Harbour and to using this sediment, as far as possible, to benefit the intertidal areas of the Harbour. 16 June 2005 9P0171/N/SAJ/PBor 8/20
Poole Harbour Sediment Management Plan Annex 1 HR Wallingford Dispersion Modelling of Sediment Recycling in Poole Harbour POOLE HARBOUR SEDIMENT MANAGEMENT PLAN This Annex summarises the results of further sediment dispersion modelling undertaken to help illustrate the principles of the Sediment Management Plan in Poole Harbour. Simulations were carried out using the HR Wallingford RW-PLUME model using the same methodologies as for the studies for the EIA of the proposed channel deepening. A total of five simulations were undertaken. The first four representing the disposal of a single barge load of material (200 dry tonnes of fine silt) at 3 differing locations (Figure 1). The simulations assumed that the material rapidly settled to the bed of the Harbour and was then dispersed from the bed by the tidal currents. At the most seaward placement site to the east of Brownsea Island this test was also run for the situation where upon release all the material was retained within the water column and then dispersed with the tidal currents. The final simulation is a schematic of the situation where the bi-annual maintenance dredging of the Turning Basin is undertaken in such a way that a large proportion of the silty material that has accumulated is released by overflow into the water column. For the scenario it is assumed that this activity is maintained for a period of nine days. This is a highly conservative scenario. In reality any such agitation dredging would be likely to occur for a short duration and would not need to be a continuous activity. For all the simulations the material was placed (or release commenced) during the flood tide on rising spring tides. Since the majority of sediment transport within Poole Harbour occurs during the spring tides, an 9-day simulation of the hydrodynamic regime (approaching and including the spring tides) was used as input into RW-PLUME. Wave conditions were not included, hence the studies represent calm conditions. The bathymetry used in the simulations represents that after the proposed channel deepening. For the last simulation of overflow from trailer dredging in the Turning Basin the dredger is simulated as moving back and forth along a 1km long dredge path, with a 2 hour cycle time for the duration of the model run (9 days). All of the fine sediment in the overflow was assumed to be immediately released into the water column as a passive plume. Table 1 below summarises the mass releases and disposal site locations of the model runs. 30/08/05 Commercial in confidence copyright HR Wallingford Ltd. Not to be copied, transmitted, extracted or divulged to any third parties, or in any way exploited without the express prior written agreement of HR Wallingford Ltd.
Poole Harbour Sediment Management Plan Run Disposal site Mass release (kg) Easting (UTM) Northing (UTM) 1 Site 1-200,000 at bed 397216 91123 Lytchett Bay 2 Site 2-200,000 at bed 401311 89601 Turning Basin 3 Site 3-200,000 at bed 403240 87655 Brownsea Island 4 Site 3 Brownsea Island 200,000 through water column 403240 87655 5 Site 2 Turning Basin Table 1 Dredger activity: Overflowing occurs for 2 hours at a rate of 25kg/s. Dredging cycle time is 4 hours for duration of simulation (9 days). 400433 to 401430 constant at 89640 Showing mass releases and disposal site locations of the model runs The results are presented as Figures 2 to 6. Figure 2 shows that the deposition resulting from the placement at Lytchett Bay is localised and peaks at 50mm, reducing to sub-millimetre levels 500m away. Peak concentrations are high at the disposal site, but fall rapidly with distance. After 4 days of hydrodynamic (spring tide) flow, 99.9% of the total (deposited and suspended) sediment is within the intertidal area of the Harbour. After this time, the tidal flows are not strong enough to redistribute the sediment further. Note that the flow model resolution in this part of the Harbour is not adequate to fully represent the detail of the flows in the narrow subtidal channels and therefore dispersion from this site is likely to be underestimated. Peak deposition and concentrations are lower at the Turning Basin site (Figure 3) than at Lytchett Bay. Maximum deposition is 13mm, with peak concentrations reaching 1220mg/l in the vicinity of the placement itself. It is evident that the tidal flows act to trap the sediment plume within the Turning Basin, extending the plume along the deep water towards the west. 13% of the total released sediment was transported to the intertidal regions after 5 days. Figure 4 shows the peak deposition and concentration patterns at a placement site to the east of Brownsea Island. Peak deposition is very low within Poole Harbour, reaching a maximum of 1.4mm; peak concentrations reach 1120mg/l in the vicinity of the placement itself, but fall away rapidly with distance away from the placement location. Similar to the Turning Basin simulation, the sediment plume extends within the channel towards the west at lower concentrations. 91% of the total released sediment remained in Poole Harbour after 5 days, of which 45% was transported to the intertidal regions. The results for material placement through the water column at Disposal Site 3 are shown in Figure 5 and show a similar distribution pattern to sea bed placement (Figure 4). Peak deposition is slightly lower (1.1mm) and is confined to the placement site location. Peak concentrations reach approximately 400mg/l. 90% of the released material remains within Poole Bay, of which 35% was transported to the intertidal regions. This equilibrium is reached in 5 30/08/05 Commercial in confidence copyright HR Wallingford Ltd. Not to be copied, transmitted, extracted or divulged to any third parties, or in any way exploited without the express prior written agreement of HR Wallingford Ltd.
Poole Harbour Sediment Management Plan days. It can be seen that sediment release into the water column results in a slightly wider dispersion than that from the bed and (in this case) a slightly greater loss of material out of the Harbour. Overflow from dredging activity in the vicinity of Disposal Site 2 were simulated and are presented in Figure 6. The total amount of sediment released over the simulation is about 10,000 tonnes, which is a significantly higher release rate than the discrete mass releases from single barge loads. The simulation is at Disposal Sites 1 to 3. Peak deposition within Poole Harbour reach about 30mm, with associated peak concentrations of about 200mg/l. It is clear from the figure that the tidal currents act to advect the sediment plume towards the west, as well as dispersing some outside of the harbour during the ebb tide. 85% of the released material remains within Poole Bay, of which 3% was transported to the intertidal regions. Conclusions The majority of the placement of material on the seabed at each of the 3 placement sites remains within Poole Harbour. Sediment transport occurs during the spring tides, and reaches an equilibrium after 4 to 5 days. For the material release through the water column, deposition rates and concentrations are lower than for sea bed releases. A proportion of the material released is predicted to disperse to intertidal areas. For the simulations undertaken the greatest proportion of material accumulating on intertidal areas is for subtidal release from the Brownsea Island site. HR Wallingford 24 February 2005 30/08/05 Commercial in confidence copyright HR Wallingford Ltd. Not to be copied, transmitted, extracted or divulged to any third parties, or in any way exploited without the express prior written agreement of HR Wallingford Ltd.
Poole Harbour Sediment Management Plan Lytchett Bay (Disposal Site 1) Bottom (mcd) 1-1 -3-5 Dredger activity Turning Basin (Disposal Site 2) Brownsea Island (Disposal Site 3) -7-9 -11-13 -15-17 -19 sprnpres.rub/siteloc.i Figure 1 Showing location of simulated disposal sites and dredger path 30/08/05 Commercial in confidence copyright HR Wallingford Ltd. Not to be copied, transmitted, extracted or divulged to any third parties, or in any way exploited without the express prior written agreement of HR Wallingford Ltd.
Poole Harbour Sediment Management Plan Peak deposition (mm) 100.00 50.00 10.00 5.00 2.50 0.50 0.20 0.10 0.05 0.04 0.03 Bottom (mcd) 0.02 (contours) 0.01 0 0.00-4 Peak concentration (mg/l) 50000 2000 1000 500 100 50 10 5 2 1 0 plumeres1.rub/peaks1a.i Figure 2 Peak deposition and concentration patterns at Lytchett Bay (material released at bed) 30/08/05 Commercial in confidence copyright HR Wallingford Ltd. Not to be copied, transmitted, extracted or divulged to any third parties, or in any way exploited without the express prior written agreement of HR Wallingford Ltd.
Poole Harbour Sediment Management Plan Peak deposition (mm) 100.00 50.00 10.00 5.00 2.50 0.50 0.20 0.10 0.05 0.04 0.03 Bottom (mcd) 0.02 (contours) 0.01 0 0.00-4 Peak concentration (mg/l) 50000 2000 1000 500 100 50 10 5 2 1 0 plumeres2.rub/peaks2a.i Figure 3 Peak deposition and concentration patterns at the turning basin (material released at bed) 30/08/05 Commercial in confidence copyright HR Wallingford Ltd. Not to be copied, transmitted, extracted or divulged to any third parties, or in any way exploited without the express prior written agreement of HR Wallingford Ltd.
Poole Harbour Sediment Management Plan Peak deposition (mm) 100.00 50.00 10.00 5.00 2.50 0.50 0.20 0.10 0.05 0.04 0.03 Bottom (mcd) 0.02 (contours) 0.01 0 0.00-4 Peak concentration (mg/l) 50000 2000 1000 500 100 50 10 5 2 1 0 plumeres3.rub/peaks3a.i Figure 4 Peak deposition and concentration patterns at Brownsea Island (material released at bed) 30/08/05 Commercial in confidence copyright HR Wallingford Ltd. Not to be copied, transmitted, extracted or divulged to any third parties, or in any way exploited without the express prior written agreement of HR Wallingford Ltd.
Poole Harbour Sediment Management Plan Peak deposition (mm) 100.00 50.00 10.00 5.00 2.50 0.50 0.20 0.10 0.05 0.04 0.03 Bottom (mcd) 0.02 (contours) 0.01 0 0.00-4 Peak concentration (mg/l) 50000 2000 1000 500 100 50 10 5 2 1 0 plumeres4b.rub/peaks4b.i Figure 5 Peak deposition and concentration patterns at Brownsea Island (material released through water column) 30/08/05 Commercial in confidence copyright HR Wallingford Ltd. Not to be copied, transmitted, extracted or divulged to any third parties, or in any way exploited without the express prior written agreement of HR Wallingford Ltd.
Poole Harbour Sediment Management Plan Peak deposition (mm) 100.00 50.00 10.00 5.00 2.50 0.50 0.20 0.10 0.05 0.04 0.03 Bottom (mcd) 0.02 (contours) 0.01 0 0.00-4 Peak concentration (mg/l) 50000 2000 1000 500 100 50 10 5 2 1 0 plumeres5.rub/peaks5a.i Figure 6 Peak deposition and concentration patterns arising from dredger overflow in the turning basin 30/08/05 Commercial in confidence copyright HR Wallingford Ltd. Not to be copied, transmitted, extracted or divulged to any third parties, or in any way exploited without the express prior written agreement of HR Wallingford Ltd.
Annex 2 Poole Harbour Commissioners Maintenance Dredging Policy POOLE HARBOUR COMMISSIONERS MAINTENANCE DREDGING POLICY 1. This policy will be implemented by the Poole Harbour Commissioners subsequent to the carrying out of the Poole Harbour Approach Channel Deepening and Beneficial Use of Dredged Material Project. This project is described in the Environmental Statement dated November 2004 (ES). 2. It is anticipated that the Sediment Management Plan (SedMP) and Monitoring Plan already complete will be used by the consenting Authorities to condition the works and the subsequent maintenance dredging of the harbour. 3. Subject to continued compliance with the above Plans and compliance with relevant legislation present and future, the Commissioners will:- a) Continue the policy of hydrographic monitoring of the whole harbour, using this information to inform adjustments and revisions to harbour marks for the safety and convenience of all users. b) Continue maintenance dredging of the commercial channels using processes and procedures generally as set out in the Environmental Statement and the SedMP. The following channels are considered to fall into this category:- Swash Channel Middle Ship Channel Turning Basin Little Channel c) Continue to grant Licences to operators of marinas, havens, and other facilities, to carry out necessary maintenance dredging of their basins and approach channels from time to time. These operators will be expected to promote proposals which fall in line with the requirements of the SedMP for retaining suitable fine material within the harbour. These Licences following, consideration against existing criteria, will be granted on the basis that dredging remains the entire responsibility of the operator and the operator will obtain any other approvals required as at present. As these proposals will initially be conducted on a trial basis in accordance with Sediment Management Plan, the Commissioners will provide assistance with technical advice and monitoring until new processes are satisfactorily established. d) Ensure as far as possible that minor channels are maintained in a manner to allow their continued safe use by vessels of a similar nature and size to those at present using them. Any loss of depth in these channels that is caused as a result of actions under the SedMP will be managed to avoid constraint to existing
navigation as far as is reasonable. This will be achieved by increasing the frequency of monitoring and adjustment if appropriate and by minor dredging or clearance subject to any necessary statutory approvals (which will have to include consideration of affects on conservation interests) being obtained and within the framework of the SedMP. The following channels are considered the most likely to fall into this category, but all minor channels including those in the southern part of the harbour will continue be monitored:- North Channel whole length Wych Channel to Pottery Pier Wareham Channel to 82 Buoy Rockley Channel to Rockley Point Backwater Channel upstream of the site of the Second Harbour Crossing works e) There are farmed shellfish beds within certain of the channels highlighted in d above. It is likely that they are located in areas not susceptible to accretion. However the sites will be monitored and appropriate measures taken by adjustment to the detail of the SedMP, in the event of adverse changes being detected that are associated with the actions taken under the SedMP. 4. This policy to be reviewed in consultation with relevant harbour user groups on a 6 yearly cycle following completion of the Channel Deepening Project. RNA/JPL/432-0501 03.06.05