River or fluvial flooding occurs when too much water arrives at any given point on the river system at one time overcoming the available in bank volume at that point. The following diagram illustrates this point:
At a given point the river channel has a known width, gradient and bed type (rock, concrete, gravel, cobbles and/or boulders). If the depth of the river is measured then the volume of water can be calculated using these parameters. This is known as discharge and is given the units metres cubed per second (Cumecs). This volume varies with time during the storm as the water takes time to arrive at the main river channel from the catchment where it falls. This is called basin lag or lag time.
The above graph called a hydrograph is a plot of discharge against time and it also shows rainfall on the vertical axis. The horizontal red line shows the bank full discharge. In other words if the flood peak breaks above this point, the river will flood the surrounding areas. By increasing the time for the water to arrive at this point the curve will flatten or “squash” below the red line and the river will remain in bank.
Ideally we want the river to flood in areas that are not critical to our towns and villages and if we can do that we can slow the arrival of that water at the main channel. We can “squash” the curve by introducing interventions higher up the catchment to achieve the desired results downstream and this is what we mean by “slow the flow”.
Slow The Flow Calderdale is committed to seeking solutions to the flooding problem in the Calder Valley by slowing the flow by various means such as the following:
- Introducing leaky woody dams into the river channel
- Introducing plate weirs into brooks and ditches
- Tree planting on the higher catchment
- Re-establishing our peat uplands
- Promoting the use of Sustainable Urban Drainage systems (SuDS)
- Constructing new flood attenuation ponds
- Investigating old infrastructure such as mill ponds for re-use as attenuation ponds
- Constructing turnbyers on tracks and footpaths to turn water into pasture or woodland
These measures can, given careful thought, planning and implementation reduce the amount of expensive “hard” civil engineering more often used to deal with the effects of flooding. Raising river walls is so often deemed the only answer to this problem, yet time and time again it has been shown to merely shift the problem further downstream. This is aptly illustrated in the Calder Valley by the works carried out in recent years in Todmorden and their effects on Boxing Day 2015 further downstream at Eastwood, Charlestown, Hebden Bridge, Mytholmroyd, Luddendenfoot and Sowerby Bridge.
How can you help?
We are looking for sites in the catchment, particularly the upper catchment areas where we can introduce these interventions. If you know of land containing brooks or ditches, old mill ponds or any type of watercourse, and that could mean a track that is normally dry but runs like a torrent during heavy rain, then please get in touch via our contacts page and let us know where these areas are and preferably who owns them.
If you can help by providing scientific and technical support or you have land which might be suitable, please contact us.
Watch this short film on why ‘Slowing the Flow’ works HERE