Surface Water Management Plan - Hampshire County Council

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1 Best Possible - No better available; not possible to improve in the nearfuture2 Data with known deficiencies - Best replaced as soon as new data areavailable3 Gross assumptions - Based on experience and judgement4 Heroic assumptions - An educated guessEach dataset obtained for the use of this SWMP is listed within Appendix Atogether with its data quality score.Information must only be used in ways appropriate to the quality of data, scaleof mapping etc and each partner has specific conditions of use for dataoutside each individual organisation. These conditions must be complied withand no third party information will be distributed or reproduced without eitherthe express consent of that data owner or confirmation that the conditions foruse of the data is adhered to. When considering Freedom of Information (FoI)or Environmental Information Regulations (EIR) requests, Hampshire CountyCouncil is unable to release information which is owned by a third party. Datalicensing requirements are identified within Appendix A alongside the dataquality scoring.6.1. Existing and Historic Flooding IncidentsData relating to historic flooding incidents was available from all keystakeholders and was generally assessed as being of good quality in terms offlood location although the extent and consequences of these incidents werenot as well recorded.Information on known flooding has been incorporated into a flood databaseincluding as much information as is known about each flood site. Thisdatabase has been ratified by the SWMP partners and it has been agreed thatthe localised flooding incidents database constitutes the best availableinformation on existing and historic flooding incidents. However this data mustnot be used to identify individual properties at risk of flooding or with basemapping of more than 1:25,000.This database includes sites which have experienced flooding as a result ofextreme weather conditions or rainfall which has exceeded the designcapacity of the existing drainage system as well as sites where blockages orfailures in the system has led to flooding.Historic flooding data can be subjective as often few formal records exist andmuch of the information comes from local knowledge gathered over a periodof time. The Borough Council, Environment Agency and water companieswere asked to comment on all historic flood data, corroborating the details aswell as providing additional local flood risk sites, to ensure that the data is asaccurate and up to date as possible allowing a high level of confidence to begiven to these records. It is a living dataset and will be expanded and updatedas new data is obtained.8
6.2. Future / Potential Surface Water Flood RiskThere are two key datasets available from the Environment Agency whichassess the potential for surface water flooding, namely the Areas Susceptibleto Surface Water Flooding and the Flood Map for Surface Water.The Areas Susceptible to Surface Water Flooding map was produced in July2009. It was produced using a simplistic modelling technique based on asingle rainfall event, namely a storm with a 1 in 200 chance of occurring in anyyear, highlighting areas that could be affected by surface water flooding withinthree different bandings; less, intermediate and more. It does not take intoaccount buildings, drainage or ground permeability. This map should not beused with base mapping of more than 1:50,000 and is not designed to be adetailed representation of areas that could flood.The Agency produced a more detailed Flood Map for Surface Water, issued inNovember 2010. This uses a more sophisticated modelling technique basedon a number of different assumptions for ground permeability, drainagecapacity and flow routing around structures. It was created using both a 1:30and 1:200 annual probability and separates the level of flood risk into areasthat could experience flooding in excess of 100mm and 300mm. It should notbe used with base mapping of more than 1:25,000 and is not designed toidentify individual properties at risk.Environment Agency guidance suggests that the Areas Susceptible to SurfaceWater Flooding (2009) map would be more representative of flooding in areaswhere there is minimal drainage capacity and flat areas with a longer stormduration with the Flood Map for Surface Water (2010) being more suitable forother areas. However, the Environment Agency has stated that localinformation on flooding should be used to identify the most suitable mappingfor specific locations.Upon analysis of the available flood risk data, the SWMP partners haveagreed that the dataset providing the best representation of potential flood riskin Eastleigh is the 2010 Flood Map for Surface Water.The 1 in 30 year annual probability surface water flood mapping has beenchosen to assess flood risk as the majority of drainage infrastructure andproperty resilience measures are currently designed to accommodate thisscale of rainfall event. Sites at risk of flooding from 1 in 30 year rainfall eventor less will therefore be prioritised for investigatory work so that those areas atthe highest risk can be considered for future works.Surface water flooding arising from 1 in 200 year rainfall event is consideredvery extreme rainfall which exceeds that which is generally designed to becontained within drainage systems. However, it should be modelled in termsof exceedance so that overland flows do not cause property flooding and arerouted away from properties where feasible. This dataset is therefore of mostuse for land-use planning purposes.9
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Page 9 and 10: 3. Aims & ObjectivesWithin the over
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Page 17 and 18: However, for those sites where the
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Page 25 and 26: is also linked to the Southampton I
Page 27 and 28: significant issue. However, given t
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Page 31 and 32: Figure 3: Location of Sites Recomme
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Page 37 and 38: 10. Review & MonitoringIn order for
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Page 41 and 42: EssentialInfrastructureExceedance F
Page 43 and 44: Multi-Agency FloodPlans (MAFP)Natio
Page 45: UK Climate ImpactsProgramme (UKCIP)

Surface Water Management Plan - Hampshire County Council

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