Kenwith Valley National Flood Risk Management

Background

The Kenwith Valley has been identified as a catchment that would benefit from NFM measures, and has the advantage of being analogous to other rapid response catchment in uplands. The project will work with landowners to develop measures identified through desk studies completed, that include the EA NFM mapping products. The project will deliver NFM solutions that will reduce the risk to 11 properties upstream of the defended community and improve the resilience of engineered defences (Kenwith Dam) at the bottom of the valley.

The NFM project will quantify these benefits. A priority of the NFM project will be the assessment of challenges such as value for money and maintenance liabilities.

Natural Flood Risk Management (NFM) strategies harness environmental processes and work with the local landscape to mitigate catchment runoff and consequently flooding downstream.

As these various techniques are driven by natural processes, they are considered sustainable and ecosystem supporting alternatives to more common hard engineering methods (Environment Agency, 2010 and The Parliamentary Office of Science and Technology, 2011). This project seeks to propose key NFM measures that can be adopted in Kenwith Valley to reduce the risk of downstream flooding, implement and monitor them.

Study Area

The area that will be focused on for this NFM project is the Kenwith Valley, Bideford and its River Torridge tributary, with a catchment area of 6.6 km²(see Figure 1). Located within the North Devon Biosphere Reserve, and partly within the North Devon AONB, this particular area is of great conservational interest, both to environmentalists and the local community through various citizen scientist projects (Environment Agency, 1996) (see Figure 2).  It is also on the northern edge of Bideford where the developing new local plan foresees a large expansion of the town that could impinge on the natural flow attentuation of the catchment. The main soil type in this area is Denbigh 1 which is consistent with poor and naturally impermeable soils that are difficult to manage and farm (Fish, et al., 2016). Coupled with the Torridge catchment having a maritime climate, with high rainfall rates, this can lead to serious episodes of flooding as soil infiltration capacity is limited.

At the base of the catchment is the the estuarine parts of Bideford which is in a tidal and fluvial risk zone. The main set of properties and large supermarket are protected by a small reservoir for flood storage volume, a reduced flood plain and a pump system that operates at high water when fluvial and surface water is tide locked. A scheme has been proposed to raise the reservoir level by 1m, which will bring current protection to the 1800 properties to an acceptable standard. While the NFM option was included in the appraisal, it was not the preferred option because NFM alone would not provide the required benefit. However they do indicate that there could be a benefit in reducing the volume of flood flows and the frequency in operation of the sluice gates and high volume pumps. Further, there are 11 homes upstream of the reservoir that are not supported by this measure.

The project proposes to use NFM to support the flood risk management for the 11 homes not currently afforded protection and identify the measures needed to improve the protection for the rest of the catchment in the scenario of expansion of the town as highlighted in the forthcoming local plan.

The EA Area PSO Team with the Catchment Partnership considers that even with the programmed capital project NFM would have a role in providing resilience to climate change and reducing the frequency of asset operation. This work will be important in learning how NFM contributes to the mitigation provided by highly engineered asset systems.The catchment benefits from existing monitoring networks for flow and water quality and has a significant data set allowing more specific monitoring of NFM measures. The monitoring strategy will be finalised with the continued contribution of the EA, this will be consistent with projects such as the funded Dartmoor NFM project to allow comparison.

This include the statutory WFD monitoring network in the catchment. The present WFD classification is moderate and the pressure is nutrients from agriculture. The project will complement this network with specific monitoring or NFM measures to understand the benefit to water quality. The project will also report on the creation of habitat and any benefit to hydromorphology with before and after surveys. The is already a link with the community group managing the Local Nature reserve at the base of the catchment that are interested in contributing to the project.

The Biosphere Reserve, in partnership with the EA and Devon County Council, is successfully leading the Braunton NFM Project and there are many lessons which can be transferred to the Kenwith project. The project has also benefited from site visits and support from other projects such as those in Stroud, who will continue to provide input.

The Catchment Partnership will continue to align sources of funding to apply broader scale land management and land use changes to match project outcomes.

We will explore the use of biodiversity offsets to help implement the biodiversity rich measures in the area. Support will be available from the partners in the area (EA, Devon County Council, Torridge District Council)

This project is to be funded from the community section of the NFM fund that is mandated to experiment with measures to apply low technical, soft engineering methods to reduce flood risk in small catchments.

Objectives

The study will:

1. Support the delivery of the Torridge District Council Green Infrastructure Strategy that will maximize natural SuDs, and enhance the flow attenuation in the catchment.
2. Apply the newly developed Working With Natural Processes knowledge base to identify the interventiosn and their locations to reduce the frequenct of flood events on the properties at the base of the Kenwith Valley
3. Implement the non-development parts of the measures to test their efficacy
4. Monitor the impact of the measures put in place
5. Test the approaches and the use of developer contributions, through biodiversity offsetting, as a tool to finance NFM