Old Slack Farm – Natural Flood Management Project

The topography and geology of the Calder Valley renders it susceptible to flooding, compacted subsoils overlying impermeable clay soils result in rapid run off during extended periods of rainfall which is funnelled quickly to the valley floor through an extensive system of headwater streams and old culverts originally built to supply water to the mills in the valley. Flood alleviation has traditionally relied upon engineered in channel solutions, for example raising river walls and widening the river channel, these techniques are expensive and disruptive to implement and are carbon intensive. Natural Flood Management (NFM) is an emerging approach intended to assist with excessive run off generated during flood events and seeks to store and slow the flow of water allowing it to infiltrate the subsoils if possible on its journey to the river system.

The land in the video above contains a number of NFM features as follows:

Cross Slope Woodland: In the bottom corner of the far field 200 trees have been planted around a stream which has been de-culverted, the trees which are a mixture of English broadleaf include alder, sessile oak, crab apple, hawthorn, birch and rowan are intended to slow the flow of water to the watercourse and by concentrating alder around the stream an alder carr will be created when the trees eventually mature. 

Studies have found woodland soils to be characterised by high infiltration rates, tree cover protects soils from physical disturbance and, together with leaf fall and tree rooting helps build up organic matter creating good soil structure with interconnected soil pores or ‘macroporisity’, encouraging rainwater to enter the soils following deeper longer pathways to streams. In addition to slowing the flow in this way, woodland creation offers multiple benefits such as habitat creation and improvement in water quality through removal of sediment and diffuse pollutants.

Contour placed hedgerows: hedgerows are a type of cross slope woodland their main purpose is to intercept and reduce rapid run off from upslope land. The root systems increase infiltration rates and the hydraulic roughness of the landscape reducing the rate of run off, or slowing the flow. 

Run-Off Attenuation Features: Also known as attenuation or flood storage ponds, detention basins and washlands, these work best when many clusters of these features are included throughout the landscape to slow, store and filter water, reducing flood risk locally for small events and disrupting and attenuating overland flow on its way to the valley floor. There are two here, a smaller one with a capacity of 150 m3, is situated on a field drain which flows through it in the near field, a swale or interception channel which crosses the field also supplies water to the pond. A larger pond in the corner of the bottom field has a capacity of 350 m3, when in spate an inlet channel diverts water from an adjacent stream which runs along the field boundary into the pond, another swale also intercepts run off from the sloping field above. The ponds are normally empty allowing the land to return to pasture and only fill during periods of extended heavy rainfall.

There are now around 10 completed attenuation ponds in Calderdale constructed as part of the Calderdale NFM Grant Scheme, collectively, these ponds hold around 9 million litres of stormwater and were constructed without disruption to everyday life as has been the case for the last three years with the Mytholmroyd Flood Alleviation Scheme.  A further 7 ponds are planned under the current round of funding however to continue this good work we need to identify many more landowners who would be willing to allow their land to be used for purposes such as this, see our webpage if you think you can help: http://slowtheflow.net/you-can-volunteer-your-land/

It is clear that these low impact interventions can store and slow down large volumes of water at a fraction of the cost of a hard-engineered scheme.  Our Blog here http://slowtheflow.net/attenuation-ponds/ compares the cost of storing water in the uplands against dealing with it once it makes its way to the valley bottom.  The cost savings are significant, a further blog looks at the carbon savings and benefits to the environment http://slowtheflow.net/slowing-the-flow-and-carbon-reduction-through-floodwater-storage/For further information on other NFM solutions being used to mitigate flood risk the Slow The Flow website is really useful resource for ideas that anyone can implement .

The natural flood management (NFM) measures on this site are funded by the NFM grant scheme as part of the Calderdale flood action plan.

Funding is provided by Calderdale Council and DEFRA/Environment Agency. The Grant Scheme was developed by the SOURCE partnership with support from the Woodland Trust to help farmers and landowners deliver NFM and contribute to flood mitigation and alleviation.

More information on NFM in Calderdale and current and upcoming funding opportunities can be found at www.eyeoncalderdale.com/natural-flood-managemen