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
A catchment in southern England, UK, included a substantial area of bare
ground within the surrounding heathland and woodland. Runoff from this
area has, in the past, contributed large volumes of sediment to a large
lake; 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,
has also resulted in the overdeepening, vertical and lateral erosion of
the stream channel resulting in downstream transport of sediment to the
lake. 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. Over the last 15 years a
wide range of investigative, monitoring and management work has been
undertaken within the catchment by a partnership between UK Government
organisations, a local authority and a charity, with continuous
involment by the author throughout this period. 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 stage zero and
larger fluvial pathways to the floodplain – supporting the restoration
of the lake. These works have resulted in the reduction in erosion at
source and increased deposition through the catchment system, 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 stream and lake,
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.