South Essex Surface Water Option Scoring Matrix
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SOUTH ESSEX SURFACE WATER OPTION SCORING MATRIX Stage 1 - Identify Potential Measures Critical Drainage Area ID: CAS 1 South Benfleet • Surface water tends to flow from the northwest to the southwest of the CDA, towards Benfleet Marsh where the surface water flood depths are greatest. There is one PSWFH area within the CDA. There is a extensive network of drainage ditches that will convey surface water runoff across the catchment. The steep topography to the Description: east of the CDA generates the relative high flow velocities and flood hazards. • A large proportion of the PSWFH coincides with the Benfleet Hall Sewer tidal and fluvial flood zones 2 and 3. • Surface water ponds within the South Benfleet storage area. Flood Risk Surface Water, Ordinary Watercourse and Sewer Categorisation: 639 buildings of which 542 are residential properties >0.1m Property Count: 39 buildings of which 25 are residential properties >0.3m 6 buildings of which 2 are residential properties >0.5m • South Benfleet Foundation School, Jotmans Hall School, The Appleton School, Critical Kents School, Holy Family Catholic School. • 5 Pumping Stations Infrastructure: • Police Station • Nursing Home • Flood records have been recorded at Clifton Way, Grove Road and Avondale Road. The latter two flood locations are in proximity to a drainage ditch. Validation: • Sewer flooding records are at Fernlea Road, Greenwood Avenue and at the junction between Vicarage Hill and High Road. Pluvial modelling indicates that surface water flows are generated from Thundersley Glen (to the northeast of CDA) and Hill Top Farm (to the southeast of the CDA). Surface water from these areas flows towards the Benfleet Hall Sewer which then PSWFH: converges with a tributary of the Benfleet Brook. These main flow paths follow the topography and fluvial channels through the CDA. The PSWFH encompasses the areas of Vicarage Hill, Grove Road and Saxon Way. Figure C 19 – Surface Water Flood Depth (1% AEP) Figures: Figure C 20 – Surface Water Flood Hazard (1% AEP) SOUTH ESSEX SURFACE WATER OPTION SCORING MATRIX Stage 1 - Identify Potential Measures Critical Drainage Area ID: CAS 1 South Benfleet Measure Opportunity Assessment Description Location / Specific Details Comments To be identified on site-by-site basis when opportunities arise but likely to be limited Green Roof opportunity for implementation of measure within a Generic Measure Throughout CDA the CDA. Further investigation is needed to assess the Soakaways a Generic Measure Throughout CDA infiltration potential due to geology. Further investigation is needed to assess the Swales a Generic Measure Throughout CDA infiltration potential due to geology. Further investigation is needed to assess the Permeable Paving a Generic Measure Throughout CDA infiltration potential due to geology. SOURCE Rainwater Harvesting g Generic Measure Northern residential properties To be identified on site-by-site basis. Create and enhance water Modification of opens spaces to temporarily retain Detention Basins storage spaces in existing green Western extent of Benfleet Marsh, stormwater, either through creation of bunds or g and open spaces Boyce Hill Golf Course excavation. Ponds and wetlands with Would require a permanent water supply, but Ponds and Wetlands additional capacity to could also provide additional social and a accommodate runoff Hope Green environmental benefits. Other 'Source' Measures N/A Increasing Capacity in Drainage Systems G PSWFH Separation of Foul and Surface Water Sewers r Foul and surface sewers already separate Improved Maintenance Regimes G Generic Measure Throughout CDA To be identified on site-by-site basis. Provide additional capacity for flood water along Managing Overland Flows (Online Storage) Increase channel width along water courses. Could be developed to support a sections of channel Hopes Green riparian habitats. Alteration of road structure (increased pavement height and Managing Overland Flows (Preferential Flowpaths) lower road depth) to define flow Divert water to flow down Grove g path of surface water Road and Thundersley Park Road. PATHWAY Aeration of compact ground on In the north and west of the CDA sports fields. Increased (Thundersley Glen, Badger Hall, Increasing infiltration within the topsoils of the Land Management Practices vegetation coverage. Farming Shipwrights Wood and Boyce Hill sports ground at the head of the catchment will g practices Golf Club). reduce the volume of surface water generated Deculverting Watercourse(s) r Watercourses are already open where possible Other 'Pathway' Measures N/A Employ Extremer Weather Alert Improved Weather Warning service provided by Mett Office Combine with SWMP mapped outputs to inform a and EA Throughout CDA emergency planners of areas to focus resources. Planning Policies to Influence Development G Generic Measure Throughout CDA For all new development. Temporary or Demountable Flood Defences g PSWFH Will be dependent on engagement opportunities with community. Social Change, Education and Awareness g Generic Measure Throughout CDA In areas with a large migration of population it will be difficult to undertake / pass on information from RECEPTOR one property owner to other This could potentially be implemented across a Improved Resilience and Resistance Measures Throughout CDA large proportion of buildings in the CDA and is a unlikely to be cost beneficial. Other 'Receptor' Measures N/A SOUTH ESSEX SURFACE WATER OPTION SCORING MATRIX Stage 2 and 3 - Optons Identification and Short Listing Critical Drainage Area ID: CAS 1 South Benfleet Standard Measures Shortlisting Options SOURCE PATHWAY RECEPTOR ssessment? Option No. Option (Scheme Category) Comments easures Available? Social Overall Technical Economic Objectives Environmental Forward Option to Detailed A Detailed to Option Forward Appropriate M Appropriate Take Green Roof Soakaways Swales Permeable Paving Rainwater Harvesting Detention Basins Ponds and Wetlands Other 'Source' Measures Increasing Capacity in Drainage Systems Separation of Foul and Surface Water Sewers Improved Maintenance Regimes Managing Overland Flows (Online Storage) Managing Overland Flows (Preferential Flowpaths) Land Management Practices Deculverting Watercourse(s) Other 'Pathway' Measures Improved Weather Warning Planning Policies to Influence Development Temporary or Demountable Flood Defences Social Change, Education and Awareness Improved Resilience and Resistance Measures Other 'Receptor' Measures 1 Do Nothing 3 2 -1 -2 0 -2 -3 3 In line with PAG the 'do nothing' option (no intervention and no maintenance) and 'do minimum' (continuation of current practise) 2 Do Minimum 3 20-10-10 3 should be taken forward to the detailed options assessment. This option is relatively easy to implement through the revision of the 3 Improved Maintenance G N/A 3 211015 3 existing maintenance schedule. However this will only have localised benefits To implement this option into new developments would be relatively 4 Planning Policy G 3 220116 3 simple through planning policy. There are limited opportunities for the use of source control systems as the CDA is already heavily developed and there are no planned 5 Source Control, Attenuation and SUDS a aaag N/A g 3 101114 2 future developments. Retrofitting source controls can be expensive and are unlikely to have a significant impact as individual units. Minimal modification of current land structure could provide a 6 Flood Storage / Permeability g a N/A a g N/A 3 101125 3 significant flood storage area. The use of infiltration systems is potentially limited by the geology of the area. Foul and surface water system already separate 7 Separate Surface Water and Foul Water Sewer Systems r 2 8 De-culvert / Increase Conveyance r N/A 2 0 Designating roads as flow paths would reduce flood risk from surrounding buildings. However this will need to be coupled with attenuation or increased drainage at the end of the flow path. Roads 9 Preferential / Designated Overland Flow Routes g N/A 3 210025 3 that channel water to some extent in present conditions are main access roads, so reducing access under flooding conditions may be undesirable. 10 Community Resilience a gga N/A 3 201014 2 The benefits will not outweigh the costs of such a measure South Benfleet School and two pumping stations are in the PSWFH. 11 Infrastructure Resilience g a N/A 3 201014 2 The benefits of resilience measures will outweigh the costs. This is technically possible but the cost-benefit ratio is likely to be 12 Other - Improvement to Drainage Infrastructure G N/A 3 1-10011 2 negative. A combination of flood storage (Boyce Hill Golf Course and Benfelet Marsh) with land management will retain surface water runoff at 13 Other or Combination of Above gga 3 210025 3 desired points within the CDA. Community awareness will ensure people understand the risk. SOUTH ESSEX SURFACE WATER MANAGEMENT PLAN CDA DESCRIPTION Critical Drainage Area ID: CAS 2 New Thundersley Surface water generally flows from the east to the west of the CDA. There are two distinct flow paths. Surface water accumulates in the flood storage area to the west Description: of the CDA and behind the embankment of the A130. This coincides with a Tributary of the Rawreth Brook which has been diverted under the road. Flood Risk Surface Water, Ordinary Watercourse and Sewer Categorisation: 491 buildings of which 438 are residential properties >0.1m Property Count: 39 buildings of which and 25 are residential properties >0.3m 6 buildings of which 2 are residential properties