We have previously introduced the concept of Natural Flood Management (NFM) in an earlier blog. In the Medway Catchment (below), the South East Rivers Trust is delivering NFM with a range of partners in the Medway Flood Partnership and EU Interreg North Sea FRAMES Project.
The entire Medway catchment, upstream of the tidal limit at Allington Lock, is approximately 1,450 km2 in area. As explained in Part 2 of our “NFM: how does it work?” blog, in order to focus our efforts, the decision was made with our project partners to prioritise two sub-catchments for delivery of NFM. These catchments are the Hogg Stream and Alder Stream. Check out the map below to see where these are.
In this blog, we will provide a bit more background on the Hogg Stream catchment, with a second blog describing the Alder Stream to follow. We’ll look at the characteristics of the catchment, and how we have gone about targeting NFM to help protect communities from flood risk.
The Hogg Stream
The Hogg Stream flows for approximately 8 km from its headwaters, through predominantly agricultural land, flowing through the village of Headcorn before finally connecting into the River Beult. The catchment is relatively steep, falling from approximately 160 metres above sea level down to 20 metres above sea level through Headcorn.
The bedrock geology is dominated by Weald Clay, which is typified by low infiltration rates, leading to rapid surface runoff during heavy rainfall events. This encourages the Hogg Stream to swell, posing a risk to properties and other land, particularly towards the downstream end of the catchment. Rapid runoff over fields can also lead to erosion and loss of productive topsoil.
The Hogg Stream has a long history of overtopping its banks and flooding adjacent land, as evidenced by recent flood records. Stretching back much further, the presence of river terrace deposits and alluvium, as shown by the British Geological Survey, indicates flooding has occurred for hundreds and thousands of years.
In particular, the village of Headcorn has experienced flooding in past years, with high flows causing flooding of areas as recently as 2018. As well as the risk of flooding from the Hogg Stream, the village is also threatened by the River Beult and another of its tributaries, the River Sherway. The Hogg Stream poses a risk to properties in the west, as well as Headcorn Primary School. Reports of flooding from the Hogg Stream extend back to 1960. As a result of flooding, the Headcorn Flood Action Group has been active since April 2017 to encourage action to alleviate flood risk and increase resilience within the community.
Environment Agency flood modelling estimates that up to 14 properties could be at risk of flooding from a 1 in 20 year flood event from the Hogg Stream alone. Natural Flood Management is unlikely to prevent flooding during extreme rainfall events which we may only see once in a hundred years or so. However, it can be well suited to trying to attenuate the smaller, but more frequent, flood events which are a scourge to communities. This is why the Hogg Stream is one of the catchments we are currently focusing our work in.
Scoping Opportunities for Natural Flood Management
Since the beginning of our work on Natural Flood Management, we have engaged with Headcorn Parish Council to gain an understanding of the flood risk issues in the village from the people who experience it first-hand. This local knowledge has been combined with a desktop assessment of the Hogg Stream catchment to identify key flow paths which impact could Headcorn.
The desktop study used SCIMAP – a mapping tool originally developed to map diffuse pollution risk within a catchment. It uses data on topography and land use to identify source areas and their connectivity to a waterbody such as a river. For NFM, this hydrological connectivity can be used to identify potential flow paths within a river catchment.
The SCIMAP data for the Hogg Stream, shown below, has been used to target walkovers, engage with landowners and decide on placement of NFM interventions such as woody dams and field bunds to intercept the key flow paths, and reduce the flood peak downstream. Maps of indicative flow paths have been shown to landowners to ensure they match what they see with their own eyes.
A number of walkovers have been carried out across the Hogg Stream catchment – with more planned in the future. This is vital to ground-truth the desktop assessment and SCIMAP.
A number of landowners are enthusiastic to install NFM measures on their land in order to benefit those at risk downstream. Options to install Large Woody Structures, create attenuation areas and field corner bunds have been identified and are currently being developed to final designs. The South East Rivers Trust is due to start work installing Large Woody Structures along a long section of the Hogg Stream in the immediate future.