River Pinn, Park Wood: Slow the Flow

Flooding poses a threat to people and property across the UK and this risk is increasing due to the effects of climate change. This is no different in Greater London where thousands of us live or work in areas threatened by flooding.

Park Wood is part of Ruislip Woods SSSI and National Nature Reserve in Hillingdon. Within this section of woodland there are several flow paths and natural springs that join at various points to form a single channel and tributary of the River Pinn. Flood risk to the local community around the convergence of watercourses is evident with homes impacted as recently as 1 October 2019.


The Park Wood Natural Flood Management Project

Natural Flood Management (NFM) is the practice of using and enhancing the landscape’s ability to store and slow flood waters using natural measures. Depending on the local area, NFM could involve creating semi-natural water brakes with large wood, such as leaky dams; wetlands; planting trees; or reducing soil compaction to make it easier for rainwater to infiltrate and be stored in the ground. Hydrogeological modelling is being carried out by Thames21 to help us understand the best places in the catchment to install these features to have maximum impact on reducing flood risk.

The Park Wood Community NFM pilot project continues to work with the local community. This helps to increase the awareness of how flooding happens and the measures that can be implemented to slow the flow and create more flood water storage within Park Wood, Hillingdon. Through this mechanism we can also improve the biodiversity of habitats by creating new wetlands habitats which weren’t previously present in this area of the SSSI/NNR. Gathering physical habitat data using the MoRPh Modular River Survey will tell us what changes are taking place over time in response to new measures.

Working closely with local residents from the community, Ruislip Woods Management Advisory Group (RWMAG), North Ruislip Flood Action Group (NRFLAG), and Hillingdon Council, we identified numerous locations suitable to slow down the flow and store water using large wood and materials found within each reserve. These measures are being carefully designed and delivered with local groups to increase habitat diversity and help deliver existing woodland management plans. This project is working closely with the River Pinn Project​ and the Environment Agency.

A leaky dam otherwise known as large woody debris dam or leaky barrier: just one of a range of measures used by Natural Flood Management (NFM).


Monitoring and Reporting

This project (alongside similar projects in Harrow, Havering and Enfield that Thames21 is working on) is being intensively monitored and evaluated to enable the results to be fed back to DEFRA in March 2021. We aim to demonstrate the costs, benefits and lessons learnt of carrying out NFM in the urban fringe – and this will inform best practice nationally.

At agreed locations we have been able to use some of the large wood (up to 15cm diameter) generated by previous habitat management to create naturalised flow check points otherwise known as leaky dams. This work will continue over the next year utilising larger wood (up to 50cm diameter) removed as part of the Woodland management plan to create more substantial and longer lasting features. Adjustments are also being made to those interventions previously built to increase the benefits and effectiveness.

Using specially designed visual surveys, project partners and volunteers are able to record how the features are preforming in different weather and flow conditions as well as report any changes observed in the ecology and soil moisture holding capacity. The MoRPh citizen science survey method is monitoring changes in geomorphology over a longer period of time to reveal how the streams habitats are changing.

Digital monitoring devices called Freestations, located in the channels up and downstream of the NFM interventions are also providing vital data on how the installations are performing. These work by measuring how water level changes during rainfall sending data via telemetry to online databases. This is building on research at Kings College London. More information can be found here.


Volunteering opportunities

There are many ways that you could get involved in this project or one of our other NFM projects across North London. These include helping to install leaky dams, monitor their effectiveness at storing water and tracking changes in habitat, soil moisture and ecology that result from their installation. We post all volunteering events on our events calendar and we will be running the second NFM training course this summer. There are also ad hoc volunteering opportunities for monitoring that plays a vital part in the project. For more information please contact Stephen Haywood.

Email: Stephen.Haywood@thames21.org.uk

Meanwhile, we encourage you to take photos of our leaky dams and NFM installations in action and send them to us using our Slow Flow NFM app, by email or social media to @Thames21 with hashtag #Thames21NFM.

This will enhance our understanding of their performance and be a visual check to compare with our Freestation data.