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Mapped outputs displaying properties at risk from potential flooding

Aston Cantlow is a small village in Warwickshire (England) that has suffered from flooding from both the nearby River Alne and surface water ponding. Numerous properties have previously been flooded and surface water flooding remains a problem. Using the Flood Modeller 1D solver and TUFLOW, CH2M HILL specialists undertook a flood study of Aston Cantlow which included flood modelling, flood mapping, flood impact calculations and options analysis.

The causes of flooding vary throughout Aston Cantlow - including under capacity in the drainage network, flooding from the main river and blockages at key structures.

Existing modelling for the area only included a flood mapping model of the main River Alne. This model was insufficient to map past flood routes or to show the high frequency surface water flooding that was perceived to cause the most damage and inconvenience.

Therefore, an integrated mapping model for surface water and fluvial flooding was constructed using our 1D solver and TUFLOW to determine likely flooding from combined sources. This existing 1D-2D river model was updated to include the area covering Aston Cantlow, direct rainfall and the surface water drainage network.

This leveraged the strengths of both our 1D solver and TUFLOW, including our software's integrated TUFLOW link, to ensure that fluvial, pluvial and pipe drainage issues were included. To avoid calculating flooding unnecessarily for rural areas outside the area of interest, catchments contributing overland flow routes to Aston Cantlow were derived in GIS and point inflows were applied at the boundary of the new 2D domain. Additionally, new tributaries were added to the model that had suffered from flooding in the past.

This integrated model was then run for the 2007 calibration event and successfully reproduced the pattern of property flooding seen at that time. Mapping of multiple return periods confirmed surface water flooding as the most dominant risk to the village. Despite significant out-of-bank flooding from the river, this was not found to affect a large number of properties although higher river levels prevented effective drainage of the surface water system. A number of the properties were also found to be at dual risk of flooding from the River Alne and its nearest tributary (the Little Alne).

The model demonstrated that the disperse nature of flooding was not conducive to a single cost effective solution. Flood storage, weir changes and pipe enlargement were ruled out on the basis that they would not be cost-effective in relation to such a small number of properties and would only potentially deal with one source of flooding.

Therefore, from the mapped outputs, the most at-risk properties were identified and individual property level protection was proposed as the best solution to deal with the varied sources of flooding.

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