In the Medway Catchment, the South East Rivers Trust is delivering natural flood management (NFM) with a range of partners in the Medway Flood Partnership and EU Interreg North Sea FRAMES Project. This blog introduces the Alder Stream – one of the two sub-catchments we are focusing our efforts on in the Medway (the other being the Hogg Stream which you can read about here).
Meet The Alder Stream
The Alder Stream flows for approximately 6 km from its headwaters, through predominantly agricultural land before flowing through the village of Five Oak Green. Downstream of Five Oak Green, the Alder Stream flows for 6 km further before joining the River Medway.
The upper catchment is relatively steep, falling from approximately 150 metres above sea level down to 20 metres above sea level through Five Oak Green. Over the next 6 km the watercourse falls roughly only 10 m.
The geology of the catchment upstream of Five Oak Green varies between Tunbridge Wells Sand Formation and Wadhurst Clay Formation. In the headwaters of the catchment, the geology consists of a mix of Lower Tunbridge Wells Sand and Ardingly Sandstone Member.
The mix of bedrock types suggest the response of the catchment to rainfall varies in different areas. Runoff from areas of Wadhurst Clay – which correspond closely to the path of the Alder Stream and its main watercourses – will typically be more rapid. This is because clay is largely impermeable to water and so water rushes along the surfaces instead of being absorbed. Areas of sandstone will typically respond more slowly to rainfall events. However, in general the Alder Stream is known to be flashy in nature, with a rapid response to rainfall. This can pose 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.
Upstream of the village of Five Oak Green the Alder Stream passes through a culvert which is known to cause flooding issues when the capacity of the culvert is exceeded. The railway embankment to the north of Five Oak Green can also increase flood risk by restricting the flow of water from the Alder Stream northwards and away from the village.
Areas of Five Oak Green are known to have experienced flooding in 1960, 1968 and on a number of occasions since 2000. Flooding as a result of high flows in the Alder Stream can also be exacerbated by flooding from the River Medway itself. Capel Parish Council manages the Capel Flooding Committee which meets bi-annually and aims to encourage action to alleviate flood risk and increase resilience within the community.
Environment Agency flood modelling estimates that over 100 properties could be at risk of flooding from a 1 in 20-year flood event from the Alder 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 Alder 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 Capel Parish Council, meeting with a local Flood Warden, as well as members of the National Flood Forum and the Environment Agency to gain an understanding of the flood risk issues in the catchment from the people who experience it first-hand. This local knowledge has been combined with a desktop assessment of the Alder Stream catchment to identify key flow paths which could impact areas properties and other land, particularly in Five Oak Green.
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 Alder Stream, shown below, has been used to target walkovers, engagement with landowners and the 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.
We have carried out watercourse and ecological surveys to inform development of the proposed options within the catchment and ensure that our proposals are sensitive to local settings and maximise their benefits.
A number of walkovers have been carried out across the Alder Stream catchment – with more planned in the future.
As with the Hogg Stream, a number of landowners are enthusiastic to install NFM measures on their land in order to benefit those at risk downstream.