Geomorphological Study
Sediment Delivery Into Bassenthwaite Lake: A Catchment-scale geomorphological assessment of contemporary and historical sediment dynamics
EXECUTIVE SUMMARY
Bassenthwaite Lake is a candidate Special Area of Conservation, a National Nature Reserve and a Grade 1 Site of Special Scientific Interest (SSSI). The later designation was partly due to the presence of vendace fish in the lake, a species rare in Britain. This species is threatened, amongst other factors, by siltation and this report addresses fine sediment dynamics within the 350 km2 catchment area of the lake.
Field studies using Fluvial Audit were used to identify: sources of fine sediment from erosion of riparian margins; channel management issues; and the fate of eroded sediments. Digital aerial photography was used to identify areas of bare ground over the wider land area. Individual erosion features were examined and crude sediment budgets derived that indicate the relative importance of different types of erosion within the catchment.
The largest areas of potential sediment sources within the catchment are eroded ground on the high fells (15.5 % of unenclosed fell is experiencing some erosion – estimated volume of sediment 4,000,000 m3). However, delivery of this sediment to the channel network is largely dependent on the degree of channel connectivity (the processes by which sediments enter the channel network), which in turn is largely dependent on slope and the presence and trapping efficiency of vegetation.
Erosion of steeply incised channel margins and bank erosion of main rivers are a significant source of fine and coarse sediment. Of the 110 km of surveyed channels 46 km were given a low fine sediment delivery ratio, 34 km were given high/low (i.e. sediment supply highly dependent on local conditions), 15 km were moderate and 8 km high. The key target areas are the high and high/low where there is considerable scope to make improvements. Full height bank erosion was observed on 27 km of channel banks surveyed (12 % of all banks – estimated volume of sediment 14,000 m3).
Other important sources that remain unquantified are: the poaching of field drains by livestock; erosion of sub-surface drains; and mine spoil. It seems very likely that fine sediment delivery through sub-surface drainage is a particular issue on in-bye land, which accounts for approximately 30 % of the catchment. The contribution of fine sediment through drainage is a poorly understood area.
Linked to erosion of agricultural land is the issue of phosphorus attachment and delivery, this is likely to be a significant issue of improved and drained or heavily poached land (30 % of the catchment area); the amount of phosphorus from upland open grazing is an unstudied area.
Sub catchments have been identified where it seems likely that the greatest amount of sediment is being derived from, based on the presence of bare ground, density and structure of vegetation, slope angle and stream order. These may be used to target changes in land use or intensity of land use.
Priority management recommendations are identified, along with medium term issues and longer-term research needs. These range from catchment to reach scale and are targeted to specific areas.
Given the volume of material estimated to have been lost from erosion on the high fells, this must be the major, short, medium and long-term issue. The state of the high fells has significant impacts on: sediment supply; runoff; ecology; and indicates that these sensitive areas are in a poor state to adapt to the impacts of predicted climate change for this region.
Since Derwent Water and Thirlmere both act as natural sinks, priority areas in which to address sediment supply to Bassenthwaite are recommended as those downstream of these two major sinks. The Floodplain area between Derwent and Bassenthwaite is probably the 1st priority due to extensive bank erosion, soil poaching, and extensive drainage. More detailed recommendations are presented, together with channel typologies and recommended management advice to be used in combination with data contained in the GIS project (BassGIS).
A second priority area is the Newlands valley, which is identified as high risk for erosion, delivery and % runoff, has major mining issues and floodplain/channel connectivity issues. The Skiddaw Massive, including Blencathra is the other main priority area due to extensive erosion of the soil resource directly into Bassenthwaite tributaries.
Medium to long term priorities are focussed on the lowland areas surrounding the Glendermackin, St John’s Beck and the Greta, which are the most intensively farmed and have soils vulnerable to erosion and structural damage (medium risk). Farm scale fluvial audits to assess runoff from the farm unit are recommended to lead to best practice guidance for the catchment.
The major recommendations from this study are spatially targeted and include:
- reduction of grazing pressure on sensitive uplands
- restoration of channel/floodplain connectivity
- an increase in wetland storage
- improved riparian vegetation cover
- encouraging woodland, particularly on steeply incised slopes and in ghylls
- managing mine waste
- considering options to stabilise areas where the soil resource has been lost.
For a copy of the full report please contact John Pinder at the address below.
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