11.2 The proposals to deal with the leachate within the closed Lodmoor North Landfill site are assessed in Chapter 10 Geology and Soils.

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1 Chapter Eleven Water Quality, Drainage and Hydrogeology Introduction 11.1 This Chapter considers the impact of the Relief Road on the hydrology of the area and considers both surface and groundwater. Runoff from the new road construction can affect both the amount and quality of the water in local watercourses. Deep road cuttings can potentially affect the groundwater regime and affect both springs and water abstraction boreholes The proposals to deal with the leachate within the closed Lodmoor North Landfill site are assessed in Chapter 10 Geology and Soils. Methodology Overall Approach 11.3 The potential impacts are divided into three geographical areas based on the watercourse catchments of Lodmoor and Lorton Valley, Littlemoor and Preston, and Bincombe and Ridgeway (Figure 11.1). Both surface and groundwater are discussed together although their catchment areas are slightly different Information about the area has been collected from a wide range of sources including current and historical Ordnance Survey maps, geological maps, site investigations and site visits. Information has also been obtained from the Environment Agency, Weymouth and Portland Borough Council, West Dorset District Council, and from liaison with landowners Rainfall and surface water runoff predictions for the area have been primarily based on the Flood Studies Report 1.The design and assessment of balancing ponds and soakaways has been undertaken using specialist computer software 2. The risk of an accidental pollution spillage and the prediction of water quality has been calculated using the methodology described in the Design Manual for Roads and Bridges, Volume 11, Section 3, Part 10 (including the November 2002 corrections), supplemented with advice given in Ciria 609 Sustainable Drainage Systems The performance of the proposed balancing ponds and detention basins has been assessed for three scenarios :- 1. Current rainfalls/runoff with all outlets fully functioning. 2. Current rainfalls/runoff with the outlets half blocked. 3. Current rainfalls/runoff x 1.2 with outlets half blocked. The third scenario acts as a sensitivity test and is considered to represent the potential impact of global warming Full details of the assessment are in given in Volume 2 Chapter 11. Baseline Conditions Area 1 - Lorton Valley and Lodmoor 11.8 This rainfall catchment area extends from the Preston Beach Road to the top of Southdown Ridge. The geology of the area comprises the Corallian Group of Southdown Ridge, the Oxford Clay Formation in the Lorton Valley and the Kellaways Formation to the south. Despite the excavation of ditches and land drains, the area is generally poorly drained with the Lodmoor valley floor supporting reed beds which form part of the Lodmoor SSSI. The area drains to the sea via two sluice gates. Typically the water level within the reed beds is below 1m AOD which is lower than the High Spring Tides of 1.5m AOD The most significant water course flowing into the reed beds is the Preston Brook which drains a large part of Preston, Coombe Valley and the eastern end of Littlemoor Road (see Area 2). Surface water drains discharge water from the Redlands and Greenway areas into ditches through Two Mile Coppice. The water from these areas flows through the Lodmoor nature reserve managed by the RSPB as indicated on Figure 11.2 and discharges via the sluice gate mid-way along Preston Beach Road For ecological reasons, it is important that the RSPB are able to control the water levels in their reserve to enable a good balance of mud flats, reed beds and open waters to be achieved The Weymouth Bay Estate (the housing area to the south of Manor Roundabout and east of Looking south east across the reed beds within the Lodmoor nature reserve. Dorchester Road) drains towards the reed beds but is intercepted by a ditch along the western side of Lodmoor South Landfill site and discharges into the sea via the second sluice gate outfall Since the Second World War a number of landfill sites have been developed which have reduced the area of wetlands. The most recent two are known as Lodmoor North and Lodmoor South. Both sites are land raise areas and were created by constructing a perimeter earth bund and infilling with waste directly onto the low lying ground. Parts of Lodmoor South have been capped and there are some measures to control leachate levels. Lodmoor North has not been capped. In addition to these two sites, large areas to the west of them were previously tipped on. Water sampling at various points around these sites has been undertaken and typical results are included in Volume 2 of Chapter 11. Whilst these show elevated levels of some contaminants, there is no evidence that the contaminants are causing ecological harm. The remediation of the WeymouthReliefRoad Environmental Statement 114

2 Lodmoor North Landfill site is discussed in more detail in Chapter 10 Geology and Soils There are a number of minor seepages which support two small ponds in the area, most of which dry up in the summer. These are considered to be the result of shallow, localised seepages in head deposits and more permeable bands of limited storage near the surface in the Kellaways Formation. Area 2 - Littlemoor Valley and Preston This catchment area includes the Littlemoor housing estate and the rural area to the north of Littlemoor Road. The steep northfacing Corallian slopes of Southdown Ridge give way to Kimmeridge Clay in the valley bottom. The western half of this catchment drains westward to the River Wey, whilst the eastern half drains east to Preston Brook which discharges into the Lodmoor nature reserve (Area 1) To compensate for the increased surface water runoff from the housing developments, a number of detention basins have been constructed. The outlet controls of these basins have not been well designed and are prone to blocking. Debris that has built up on the outlet grill of the basin adjacent to the railway culvert has recently resulted in water levels that were close to the adjacent housing threshold levels. Looking north west from Littlemoor Road across the existing Littlemoor Balancing Ponds towards the rear of Nightingale Drive In the winter of 1993/4, the Chalbury and Preston Detention Basins over-topped which resulted in water flowing through the downstream gardens and along Chalbury Close. Further downstream properties were flooded. The return period of the event is not known but computer modelling indicates that the existing basin has sufficient capacity to cope with a storm that is predicted to occur on average once every 50 years (i.e. 1 in 50 year storm which has an annual probability of over topping of 2%). Currently the Chalbury Basin outlet control is being improved to reduce the risk of blockage as this can significantly reduce its effectiveness. Figure 11.1 Catchment Areas Floods in 1993 and 1994 also caused flooding of properties adjacent to the Preston Brook. The Preston Downs and Chalbury Detention Basins contribute to about a fifth of the total catchment area for the Preston Brook (at it s outfall into Lodmoor reserve). Works are currently underway to improve the channel and provide a flood relief culvert. It is estimated that these works will protect properties against flooding for a 1 in 25 year event (i.e. an annual probability of 4% of being exceeded). WeymouthReliefRoad Environmental Statement 115

3 Figure 11.2 Lodmoor Drainage WeymouthReliefRoad Environmental Statement 116

4 Area3-LowerBincombeandRidgeway This catchment area is principally underlain by Portland Beds in the south and chalk to the north. The hydrogeology of the area is complicated by the Ridgeway Fault which runs E-W just to the north of the A354 hairpin bend There is a small stream in the Bincombe valley that flows under the railway embankment east of Upwey and westwards to the River Wey. The base flow in this stream is very low but it is important as it flows through the Ridgeway SNCI just west of the railway There are no surface streams on the chalk and the hydrogeological maps indicate that the groundwater flows northwards. The chalk is a major aquifer and is classified as a groundwater protection area with a number of abstraction points in the locality (see Figure 11.1) Mitigation Strategy Sustainable drainage systems concepts, referred to as SUDS, have been used throughout the Scheme. These mimic as far as possible the natural drainage of the undeveloped catchment areas to minimise the impact of any increased rate of surface water runoff or pollution. The techniques utilise vegetation features to treat pollution and slow down and reduce flows. SUDS can provide wildlife habitats and enhance the landscape The principal techniques that are appropriate for this Scheme to reduce the rate of runoff and improve water quality are :- Swales Stone-filled trenches and french drains Balancing ponds, detention basins and wetlands. Infiltration basins. The locations where these would be used are shown on Figure Positive drainage involves the use of a sealed pipe system Where practical, more than one technique is used to maximise the benefits and to provide greater protection in the event of an accidental spillage It is proposed to construct a bypass oil interceptor immediately prior to the discharge of surface water run-off to any balancing pond or infiltration area Should groundwater become polluted by a major spillage, it would be difficult to remediate and the water quality could be affected for a long time. Consequently the water from these oil interceptors would be passed through an impermeable holding bay (forebay) prior to discharge to the infiltration area. The holding bay would have an outlet valve that the emergency services could close to contain the spillage. The infiltration area would be soiled and seeded to filter the surface water runoff and allow biological action within the top soil zone. Figure 11.3 Drainage Strategy WeymouthReliefRoad Environmental Statement 117

5 11.26 The permanently wet ponds have a constrained outlet so that they only discharge water slowly. This gives the silts time to settle out and ensures that the existing downstream flows are not increased The balancing ponds and infiltration areas have been designed to attenuate flows up a 1 in 100 year event. Impact Assessment Area 1 - Lorton Valley and Lodmoor The section of Relief Road on the south side of Southdown Ridge would be drained using a combination of swales and french drains. There would be very little infiltration as the road would be founded on the Oxford Clays. At various points the swales would discharge to a pipe system, to limit the depth flow and erosion in the swale. The water would be conveyed to the Two Mile Balancing Pond where it would pass through a forebay before being retained in the main pond. The ponds are designed to allow silts to settle out as well as to attenuate the flows. The water would then discharge via a pipe under the Mercury Roundabout to the ditch on the west side of the Lodmoor North Landfill site The proposed Mercury Road link road would drain to the existing surface water drainage system The Park and Ride facility would drain via by a series of swales and stone filled trenches to a small balancing pond on the west side of the landfill site. The pond s controlled outlet would discharge into the adjacent ditch. The treated water from both ponds would flow via the large existing hydobrake (flow control device), through the existing large concrete oil skimmer, into the Lodmoor nature reserve The balancing ponds would ensure that the flow rate of water into the reserve is not increased and that the water quality of the reserve is not affected. The proposed works to the Lodmoor North Landfill site would stem the leachate leaks, resulting in an overall improvement in water quality The two small streams that flow through Two Mile Coppice would be piped under the Relief Road and would not be adversely affected by the Scheme. No evidence of any springs has been found in the Coppice and the boggy nature of some areas is attributed to the impermeable nature of the underlying clays. The site investigation in the area established artesian groundwater (in BH 401) to the south of Two Mile Coppice where the road would be on embankment. Only slight seepages were detected in other boreholes and window samples in the area The road cutting through Southdown Ridge would intercept the water table. A hydrological assessment has identified that the groundwater flow is northwards and so the Lorton Valley area would not be affected. The intercepted groundwater would be piped northwards to the Broadwey Brook. Area 2 - Littlemoor Valley and Preston The section of the Scheme between Southdown Ridge and Ridgeway would primarily drain to a dedicated highway balancing pond adjacent to Icen Roundabout. The pond would attenuate the flow and enable silts to settle out. The pond would discharge to a culvert under Littlemoor Road. The culvert would be extended to the main Broadwey Stream. The proposed road levels have been kept as low as possible to reduce the visual impact of the road and consequently it is not possible to drain the lowest section of road adjacent to the Broadwey Brook to the balancing pond. This section would drain directly to the Brook via an oil interceptor. The potential impact of increased peak flows would be compensated by re-routing an area of Littlemoor Road that currently drains directly to the ditch, through the dedicated highway balancing pond The existing balancing ponds that attenuate the natural runoff would be relocated clear of the proposed works. The west pond would be re-established slightly to the west, whilst the existing east pond would be relocated to the east of the link to Icen Lane. To provide additional protection in the event of a blocked Looking west along Broadwey Stream through the Littlemoor estate. The Relief Road would cross from left to right beyond the building on the right. outlet and to take into account the effects of global warming, an additional pond would be located in the centre of the Littlemoor Junction. This pond would only rarely impound water Overall, the proposed ponds would reduce the peak flows to about 2/3 rds of their current level, thereby reducing the risk of flooding downstream. The ponds would have superior outlet control grilles to reduce the probability of them becoming blocked. The ponds would have sufficient capacity to handle storms with a return period of 1 in 100 years (chance of being exceeded of less than 1% ),even allowing for global warming and a partially blocked outlet control The central section of the Littlemoor Road Improvement would drain to a small balancing pond opposite the Littlemoor shopping complex, the Bincombe Marsh Balancing Pond. The pond would attenuate the maximum flows from the road to that of the existing flows from the undeveloped site The eastern end of the Littlemoor Road Improvement would drain to the Chalbury Detention Basin. The existing basin s capacity would be increased by raising the earth dam by 1.5 m and by widening the valley floor. As part of the works to the dam, the existing riser shaft would be removed and the outlet pipe size reduced slightly. The objective being to ensure that the outflow from the basin does not exceed the capacity of the Chalbury Close culvert. The proposal would give protection against a 1 in 100 year event (annual probability of being Looking east from Littlemoor Road across the existing Chalbury Detention Basin towards Preston. exceeded of 1%), even allowing for global warming and a partially blocked outlet control. This would not only benefit the properties in Chalbury Close, but would provide a little benefit for properties adjacent to the Preston Brook Where practical swales would be used to collect and convey the water, although a positive drainage system would be required at junctions where the roads are kerbed. The drainage gradients north of Littlemoor Junction are not well suited for swales and so it is proposed to use french drains. Area3-LowerBincombeandRidgeway The very small stream in the Bincombe Valley would be culverted under the Relief Road. Bincombe Lane currently drains to this stream but as the realigned Bincombe Lane would be longer, it would result in an increase in runoff. However, the percentage increase would be very small and consequently the impact would be insignificant North of Bincombe Lane, the area through which the road would run is classified as an Inner Zone of a Source Protection Area by the Environmental Agency as it is an aquifer area close to a water abstraction point. As a result of this highway runoff on the southern slopes of the Ridgeway would be piped to the Littlemoor Balancing ponds. The road on the north side of the Ridgeway would be drained to an infiltration area just outside the Inner Zone identified on the EA maps. Where the Scheme is within the Source Protection Area, the low side of the road would be kerbed with gullies to prevent surface water run-off from infiltrating the aquifer. WeymouthReliefRoad Environmental Statement 118

6 Water Quality The impact of the road on water quality has been assessed as negligible in accordance with the Design Manual for Roads and Bridges Volume 11, Section 3, Part 10 (including the Feb Amendment) and Ciria C The risk of an accident resulting in a pollution spillage has been assessed as low in accordance with DMRB and found to be:- Area draining to Pond(s) at : Two Mile Balancing Littlemoor Bincombe Marsh Preston/ Chalbury Ridgeway area Impacts during Construction Calculated Return Period: 280 years 180 years >1000 years >1000 years >1000 years There would be three principal potential impacts during the construction of this Scheme namely the contamination of watercourses with silts, the pollution of watercourses with chemicals/ fuels and oils, and flooding. Control of Silts The risk of silts reaching the watercourse can be greatly reduced by good practice. The contractor would be required to submit detailed proposals to demonstrate how he would minimise the risks. These would include the requirement to ensure that (a) side slopes are soiled and seeded as quickly as possible, (b) the construction of the permanent drainage and balancing ponds is carried out early in the works so that site water runoff does not drain directly to a watercourse, (c) any pumped water is treated prior to discharge and (d) silt fences are provided in sensitive locations. Accidental Spillages The contractor would not be permitted to store any fuels or chemicals in sensitive locations i.e. close to watercourses or within the Source Protection Area. All fuel storage would comply with the control of Pollution (Oil storage) (England) Regulations Generally refuelling of site plant would not be permitted in sensitive locations. The contractor would be required to have available on site emergency cleanup equipment for dealing with spillages and a suitable absorbent boom for placing across a watercourse. Flooding The programme of works would ensure that the effective storage capacity of the existing balancing ponds/basins is retained until replacement capacity is provided. The storage of materials, plant and equipment would not be permitted in these areas. All works in a watercourse would be subjected to the conditions laid down in the land drainage consents. Summary The Lodmoor Area is a low-lying poorly drained area which supports extensive reeds beds and an important bird reserve. The area is underlain by Oxford Clay and the principal water sources are the Preston Brook and the streams from the Redlands area. Water quality monitoring has identified that leachate is seeping from the various landfill sites into the nature reserve. Remediation of the Lodmoor North Landfill site would control the leachate within the site and improve the water quality. The Two Mile and Mount Pleasant Balancing Ponds would ensure that the surface water runoff from the road would be attenuated and treated. Consequently, the Scheme would have a negligible impact upon the area The Lorton Valley and Two Mile Coppice Area is also an important ecological area founded on Oxford Clay. Site investigations and desk studies have shown that there would be negligible impacts on the hydrology of the area Modifications to the balancing ponds at Littlemoor and Preston would reduce the flood risks to the downstream residential areas The proposed road drainage system would be based on the Sustainable Urban Drainage Systems principles and assessments show that the water quality of the highway runoff can be satisfactorily treated The probability of an accident resulting in a spillage has been assessed as less than 1in 100 year for the entire Scheme. The road has been designed with oil interceptors and forebays prior to balancing ponds and infiltration areas, to enable any pollution spillages to be dealt with satisfactorily. References: 1 Flood Study Report. Natural Environment Research Council. 2 Flood Estimation Handbook. Instute Of Hydrology Wallingford and Microdrainage Program Storage and Source. WeymouthReliefRoad Environmental Statement 119