Slow the Flow and Natural Flood Management
Llangattock Community Council have been exploring the possibility of a Slow the Flow scheme on the Onneu Fach initially, but also could be considered for other local streams that feed into the Usk River. We have been working alongside Powys County Council and are awaiting news on funding for this work.
What is Slow the Flow?
Natural flood management aims to reduce the downstream maximum water height of a flood (the flood peak) or to delay the arrival of the flood peak downstream, increasing the time available to prepare for floods. The myriad Slow the Flow items work together, helping to reduce flow rate in the valley, under storm events. This is achieved by restricting the progress of water through a catchment in 3 ways:
- Increasing soil infiltration and allowing water to soak away.
- Storing water by using natural features such as ponds, ditches or low lying land or by creating new ponds and areas to store water.
- Slowing water by increasing resistance to flow. For example, by planting trees on the floodplain or constructing “leaky dams” in channels.
What is a “leaky dam”?
Leaky dams form naturally when large sections of trees fall into and across the channel. These large pieces of wood start to gather smaller sticks and leaves which allows some water through, but holds back some of the water in the stream during high flows.
We can mimic nature by building leaky dams using locally sourced wood. The leaky dams can be pinned in place or dug into the bank to ensure they don't move around in high flows. Building a series of dams along a section of stream increases the effectiveness of the dams.
Woody debris helps to create pools and riffles, providing a variety of habitats for fish and aquatic insects and attracts mammals and birds. Importantly leaky dams can slow the movement of silt and sediment downstream. Silt can increase flood risk by reducing the amount of space for water in a channel.
A new type of river management: https://www.youtube.com/watch?v=21YAP8RF_sw&t=210s
USEFUL WEBSITES
Calderdale Slow the Flow Charity: https://slowtheflow.net/about-us-2/
Slow the Flow in the Calder Valley video on YouTube: https://www.youtube.com/watch?v=GUsS-gQFTJc&t=64s
Pickering Slow the Flow scheme:
https://www.forestresearch.gov.uk/research/slowing-the-flow-at-pickering/
Susdrain: The community for sustainable drainage, more info and ideas: https://www.susdrain.org/delivering-suds/using-suds/suds-principles/suds-principals.html
Yorkshire Water: tips on water saving: https://www.yorkshirewater.com/your-water/save-water/
Lowland Natural Flood Management Measures – a practical guide for farmers
A detailed guide to different levels of NFM interventions, their costs and benefits:
https://www.ydrt.org.uk/wp-content/uploads/2021/04/DVRN_lowland_NFM.pdf
Yorkshire Dales: Natural Flood Management Measures – a practical guide for farmers Similar to the above but with examples of Upland NFM:
https://www.ydrt.org.uk/wp-content/uploads/2021/04/YDNP-NFM-handbook.pdf
West and North Northamptonshire Councils: Flood Toolkit
An impressive resource of information by a pioneering Lead Local Flood Authority (LLFA), including the ‘Flood Library’ of guidance documents.
https://www.floodtoolkit.com/pdf-library/
