Influences on Sediment Transportation and Deposition in a Lowland UK Heathland Catchment: Natural Flood and Sediment Management as Tools for Promoting Floodplain Deposition, Carbon Sequestration and Habitat Restoration.

Abstract:

Fleet Pond is the largest freshwater lake (21 hectares [ha]) in Hampshire, United Kingdom, and forms a key component of the nationally important Fleet Pond Site of Special Scientific Interest (SSSI) and Local Nature Reserve (LNR), which includes other habitats such as heathland and woodland etc. Fleet Pond is located on the eastern fringe of the town of Fleet and forms an extremely valuable historical, social, recreational and biodiversity resource for the community. Over the years, the ecological and physical condition of Fleet Pond has deteriorated due to a number of reasons that include sediment deposition, reduction in water quality and reduction in lake depth, originating from upstream erosion and transportation of sand, silt and clay sized particles through modified drainage channels that typically are overdeepened and disconnected with the floodplain.

The upstream largely heathland catchment has a long history of being used for military vehicle testing and driver training and a substantial area of bare ground (forming the vehicle tracks) exists within the surrounding heathland and woodland. Runoff from this area has, in the past, contributed large volumes of sediment to Fleet Pond; although this input is now significantly reduced as a result of previous and on-going management works that are reported on in this paper. Historic realignment and re-sectioning of the main watercourse, the Gelvert Stream, has also resulted in the overdeepening, vertical and lateral erosion of the stream channel resulting in downstream transport of sediment to Fleet Pond. In addition to sediment erosion, the associated limited connectivity with the floodplain and focus of sediment transport in the fluvial channel has been a key factor in the shallowing and deterioration in the condition of the lake SSSI.

Over the last 13 years a wide range of investigative, monitoring and management work has been undertaken within the catchment by a partnership between Hart District Council, the UK Ministry of Defence, Natural England, Environment Agency, Johns Associates and the Fleet Pond Society. This work has evaluated the causes and effects associated with this erosion and transportation, tested and defined viable practical solutions (the delivery of natural sediment and flood management solutions and habitat restoration) and delivered a series of sustainable management interventions to reduce erosion, promote sediment deposition and to reconnect the fluvial pathway to the floodplain – supporting the restoration of Fleet Pond SSSI.

These works have resulted in the reduction in erosion at source and increased deposition through the floodplain, ultimately contributing to the improvement in condition of the lake and associated wetland habitats.

Works in the headwaters of the catchment focused on defining the existing distribution, status and significance of areas of sediment generation, transport and deposition to the Gelvert Stream and Fleet Pond, facilitating sustainable sediment management within this area. Works in the lower reaches focused on slowing flow velocities and diverting higher velocity sediment rich flows into new channels to reconnect with the floodplain and promote deposition.

Management measures included the use of small diversion channels through woodland with the creation of glades to increase understory recovery and sediment deposition; use of geotextile cells filled with sand, gravel or stone to increase the flow path, reduce velocity and promote out of channel flooding and deposition of sediment; use of scrub and woody material to form leaky dams and increase channel roughness promoting out of bank flooding and deposition; use of online ponds, backwaters and embayments; blanking off channels to promote overland flow through woodland to reduce flow depth and velocity and promote deposition; use of leaky dams to promote higher flows transporting sediment into new sinuous channels and allowing out of bank flooding to promote sediment deposition.

Monitoring methods included use of continuous flow, electrical conductivity and turbidity monitoring, spot sampling of water temperature, dissolved oxygen electrical conductivity, pH turbidity and depth, suspended sediment analysis, use of erosion/deposition pins, repeat photographs, habitat surveys, repeat walkover surveys during different weather conditions.

In general, the measures implemented have been shown to be successful, slowing contributing runoff and channel flows, reducing erosion, promoting deposition of sediment out of channel (in a range of localised and more extensive situations), with re-vegetation in many circumstances aiding sediment stability and habitat restoration. There is clear deposition of sediment away from Fleet Pond SSSI, although the potential for further sediment mobillisation, transport and deposition away from the floodplain remains.

Further investigations in this catchment are proposed as part of ongoing catchment management works led by the project partners and also a PhD being undertaken by the author at the University of Manchester UK. This PhD will investigate the geochemical and geomicrobiological aspects of these channel and floodplain management works over a 5-6 year period commencing in 2020. In particular, the PhD will seek to confirm whether there are wider benefits from natural sediment and flood management methods and reconnecting with the floodplain (in particular the sequestering of carbon and the increase in ecosystem services), and how these can best be optimised. It is anticipated that these will be reported on in future submissions to the AGU.