Whitchurch Brook

The Challenge

Runoff from the urban environment is a significant factor in the response of rivers within all towns and cities in the UK. Traditionally, lateral inflows (calculated using the Flood Estimation Handbook (FEH)) are used to represent the movement of surface water into an urban river. This approach is valid in many circumstances but has limitations particularly for smaller rivers where:

• Surface water run-off provides a significant proportion of the total flow in the watercourse.
• There are discrete storm sewer catchments which discharge to specific locations to the watercourse
• Source control measures such as SuDS are proposed to alleviate flooding

To overcome these limitations for the Whitchurch Brook study, the existing FMP-TUFLOW model was converted to an ESTRY-TUFLOW DHM model using a rainfall boundary. This has the advantage that the urban environment including roads, building, the storm sewer network and infiltration through permeable surfaces are explicitly represented. DHM models reflect the complexity of the urban environment and give a combined representation of surface water and fluvial flooding which is important in Whitchurch. It should be noted that DHM models represent a significant advance on direct rainfall modelling which have been used for surface water flood mapping.