The study was 1 of 5 pilot catchment investigation studies aimed at developing processes for Yorkshire Water and providing indicative costs for PR19. RES Environmental Ltd were commissioned to provide specialist modelling service to develop a) a fit for purpose hydraulic model of Todmorden, b) notional solutions that reduce or remove the risk of flooding to properties and other areas on a catchment wide basis and c) a method for quantifying solution benefits.

Solutions were aimed at non-traditional methods including removal of flow at source and separation. The project involved working collaboratively with catchment stakeholders including the Environment Agency and Calderdale Metropolitan Borough Council.

Todmorden is situated in the bottom of a steep sided valley with overland runoff and high river levels impacting on the performance of the sewer system. An integrated catchment model using InfoWorks ICM was created to investigate the interaction between overland flow, river flow and the sewer network.

The existing 1D DAP model was integrated with a mesh and converted to a 2D format to allow overland 2D flow and in sewer 1D flow to interact.  Rainfall was applied to the mesh and the DAP model reverified across the catchment against the DAP flow survey.  Predicted overland flow paths compared well with the EA ‘Flood Risk from Surface Water map’.

The 2D mesh was created from a mixture of 50cm to 2m LiDAR using LiDAR of the highest resolution in the urbanised regions and the lowest in the rural outskirts. New 50cm LiDAR was commissioned in urban areas where there was no coverage.

To assess the impact of the river for high return periods, peak river level data supplied by the EA was applied as level files at outfall locations using a cumulative distribution function.

Intervention Zones were identified and non-traditional solutions developed where they were considered to provide the greatest benefit to the sewer network. The impact of solutions were assessed using the bespoke DREAM2D software developed by RES which categorised flood risk for each of the 8400 properties across the catchment based on internal or external flood risk due to overland flow or surcharge.   The return period for first flood at each property was also determined. The results were reported in the form of a set of Performance Commitments for each Intervention Zone.