Hebden Bridge A case study considering the contribution of overland flow processes to flood risk in Hebden Bridge, West Yorkshire

Location Calder Valley, West Yorks. Leeds Bradford Halifax Hebden Bridge

Flood Risk Issues Relatively small, steep catchment Peat moorland Reservoirs Confluence of two rivers Heavily modified river channel Tightly confined valley with railway, road and canal links Old mill structures (development opportunities?) Informal flood defences Physical, cultural and environmental constraints

Flood sources River Calder and Hebden Water Rochdale Canal Combined sewers Highway drainage Overland flow Groundwater

Hebden Water

Calder/ Hebden Water Confluence

Flow velocities, scour potential and bridge afflux

Shoal and vegetation accumulation

Rochdale Canal

Rochdale canal - July 2006

Historic Flooding Market Street at night, 2000

Extreme rainfall intensities A Case Study

Debris deposited by highway run-off

Pluvial flooding

Focus on overland flow processes Why is understanding overland flow processes important for flood risk managers? “Flood risk, especially in built up areas, can be managed most effectively if there is an understanding of the way the floods arise and have an impact on the various drainage systems. Such an understanding should enable better use to be made of above ground pathways and storage for extreme events”. Making Space for Water, Defra, 2004

bedrock soil infiltration percolation throughflow saturated Infiltration-excess overland flow* Saturation-excess overland flow precipitation Evapotranspiration Hydrological processes *Hortonian

Types of overland flow Hortonian - infiltration excess Saturation - saturation return flow - saturation excess

Modelling overland flow processes Key parameters:- Source area Slope gradient Infiltration capacity Storage potential, soil type/depth Vegetation coverage Antecedent conditions Rainfall intensity and duration Flowpaths

Factors affecting overland flowpaths Moorland and valley side slopes Converging and diverging contours Gullies, natural drainage channels Artificial drainage, highways and paved areas Physical barriers - Buildings - Walls - Other infrastructure (eg railways/canals)

Flood Risk in Hebden Bridge Exercise:- How would you assess the relative contribution of overland flow processes to property flooding in Hebden Bridge? How might run-off processes in the Calder valley change in the future? How might planners, scientists and engineers avoid or mitigate the negative impacts of such changes? Why is it important that these processes are considered when designing river flood alleviation works?

Possible mitigation – integrated urban drainage Pathway – overland flow is intercepted and concentrated by walled roads cutting across contours. Provide alternate flow routes for overland flow to and from roads, reducing the extent to which the flow becomes concentrated/channelled towards areas containing vulnerable properties Pathway – overland flow occurs due to failure of flow to enter drainage systems Retrofit larger gulleys to highway drainage system or up-grade the drainage system Implement sustainable drainage measures to reduce run-off from new developments Receptor - Overland flow becomes a problem at the location that it ponds. This is a particular issue for flood defence schemes because new river-side floodwalls can block the drainage pathway to the river. Provide flapped gravity outfalls with pumps or storage areas behind the new walls to over- pump or store water during flood-locked periods Reduce the vulnerability of exposed properties – by implementing flood resilience or resistance measures

Future changes in run-off processes Climate change Increased rainfall intensities Different vegetation patterns Dryer soils Land Use Increased urbanisation Alternative agricultural/rural land use management practices Impacts of these changes Increased run-off, leading to increased flood flows Reduced time to peak, leading to shorter flood warning times