Entec and FRM > Project experience: Southern
Isle of Wight Strategic Flood Risk Assessment
Client: Isle of Wight Council
The project involved a strategic assessment of all flood risks facing the Isle of Wight, the communication of these risks to the council planners and the provision of flood risk management advice to inform safe development. The project involved the re-mapping of tidal flood zones and sea level rise scenarios and the mapping of Functional Floodplain for the River Western Yar.
This information along with all the existing hydraulic modelling was used to produce flood risk maps for the island. The colour coding of land was considered by the Council and the Environment Agency’s Development Control Officer to represent an effective means of communicating the identified levels of flood risk.
Shapefiles of the modelled risk zones and the fully attributed potential development sites were issued to the council in a bundled ArcGIS project which facilitated the easy access to all the flood risk data and enabled efficient processing of planning applications.
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Groundwater flood warning tools Client: Environment Agency As the body responsible for monitoring the hydrological situation in major catchments, particularly those prone to flooding, the Environment Agency (Southern Region) commissioned Entec to design and validate flood forecasting tools for flood warning in the Upper River Itchen. These tools predict both groundwater levels and river flows in the upper catchment for potential implementation in the National Flood Forecasting System (NFFS), and to provide input to a rainfall-runoff model for the lower catchment. During flood events in the Upper Itchen, groundwater is a dominant and complicating influence on river response to rainfall. This makes it difficult to forecast both the time to flood peak, and peak levels using ‘traditional’ rainfall-runoff models. For the City of Winchester, this type of information is especially significant as both police and fire stations are located in the immediate floodplain. River flow in the Upper Itchen is primarily derived from groundwater in the chalk. Even in flood events, the proportion of surface runoff entering the river is relatively minor. Therefore, any flood forecasting tool needs to incorporate prediction of groundwater levels and river flows in the upper catchment. Simple relationships were successfully derived, despite catchment complexities, to produce five-day forecasts of groundwater levels and river flows utilising Met Office rainfall forecasts. These models are to be implemented in the NFFS to predict flooding and aid in the provision of flood warning services. |
Strategic flood risk assessment
Client: Thanet District Council
This Entec project involved a strategic assessment of all flood risks facing the District of Thanet, the communication of these risks to the Council Planners and the provision of flood risk management advise to inform safe development. The project involved the re-mapping of tidal flood zones and sea level rise scenarios. This information along with all the existing hydraulic modelling was used to produce flood risk maps for the island. The purpose of which was to inform the PPS25 Sequential Test and site allocation process.
Two types of tidal modelling were undertaken in the production of the SFRA:
Horizontal Projection Modelling - The 1 in 200 year (flood zone 3) and 1 in 1000 year (flood zone 2) outlines for the years 2026, 2070 and 2115 were modelled. The base sea level data (year 2000) used in this modelling was drawn from the Environment Agency’s ‘Extreme Sea Levels – Kent, Sussex, Hampshire and Isle of Wight’ (Dec 2004) report. Using the sea level rise rates defined by Defra (which are presented in table B1 of PPS25), the year 2000 levels were extrapolated up to year 2115. The 0.5% AEP and 0.1% AEP tide levels for years 2026, 2070 and 2115 were extracted and used for the modelling. The calculated flat sea level values were applied to discrete areas of coastline and flood extents and depths were generated using the ArcGIS Spatial AND 3-D Analyst tools.
TuFLOW Overtopping Modelling, 1-D Drainage Modelling and Establishment of ABDs - Overtopping of the flood defences in Margate and Brichington was undertaken to define the areas of residual flood risk. Areas benefiting from flood defences (ABDs) were established by comparing model results from ‘with’ and ‘without’ flood defence scenarios. Historic records indicated that there was a flood risk associated with a culverted stream that flowed through part of Margate (Tivoli Brook). The impact of sea water backing up the drainage network was simulated by building in a 1-D pipe network into the TuFLOW model using the Estury 1-D modelling component of TuFLOW. The pipe network configuration was derived from Environment Agency survey plans. Entec produced the extreme tidal hydrographs for Margate and Birchington. The peak 1:200 and 1:1000 year tide levels for the years 2010 and 2115 were calculated using table B1 in PPS25). These peak values had to be converted into tide hydrographs. This process involved the assessment of the nearest three tidal gauges which resulted in the production of a normal/predicted tidal curve. The November 2007 storm surge shape was extracted from the tide gauge records and applied to the normal tidal cycle. The product was tidal wave shape representative of recorded surge events at this location on the North Sea coast. This curve was then scaled to the required maximum peak levels and run through the TuFLOW model.
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