Hebden Bridge A case study considering the contribution of overland flow processes to flood risk in...
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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.
LeedsBradford
Halifax
HebdenBridge
Flood Risk IssuesRelatively 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 - 2006
Focus on overland flow processesWhy 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 BridgeExercise:-
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