Sediment budget for a North Pennine upland reservoir catchment, UK

Abstract

Sediment delivery from upland fluvial systems in the UK is of considerable importance in catchment management. However, scarcity of detailed data on sediment sources, storage and linkages between geomorphic processes inhibits current understanding of such systems. A sediment budget for an un-gauged, upland catchment at Burnhope Reservoir (North Pennines, UK) has been developed that couples catchment sediment sources and suspended sediment dynamics to reservoir sedimentation and; quantifies historic and contemporary sediment yields. Stream-side scars and cut banks are the dominant catchment sediment sources with greatest connectivity in first order tributaries during high discharge events. The catchment sediment system is supply-limited and sediment exhaustion occurs on an intra and inter-storm event basis. Bathymetric surveys, core transects and aerial photographs were used to assess spatial variability in sediment accumulation in the reservoir. Physical and radiometric analysis ((^137)Cs) of core sediments provided estimates of dry bulk density, particle size variations and a sedimentation chronology. Total reservoir sedimentation over the 67 year period has been estimated at 592 t yr(^1) ± 10% (33.3 t km(^-2) yr(^1)) with average sedimentation rates of 1.24 and 0.77cm yr(^-1)calculated from the distal and proximal areas of the reservoir respectively. Inputs of fine suspended sediment from direct catchwater streams accounts for 54% of sediment supply to the reservoir (best estimate yield of 318 t yr(^-1) ± 129%), while inputs from the actively eroding reservoir slopes and shorelines contribute a gross yield of 328 t yr(^-1) + 92%. However, 70% of sediment from shoreline erosion is >2 mm diameter and is stored on the shoreline and toe slopes. The remaining 30% (98.41) of fine sediment is transferred to deep-water reservoir storage. This highlights the importance of shoreline erosion and sediment storage in the overall budget. Error analysis of the sediment balance equation enabled the residual sediment inputs from ungauged tributary streams to be estimated (232.6 t yr(^-1) ± 394.9%). The specific sediment yield of 33.3 t km(^-2) yr(^-1) to Burnhope Reservoir is relatively low. It is 40% lower than the average yield of 84 t km(^-2) yr(^-1) estimated from British storage reservoirs (DETR, 2001) and an order of magnitude lower than estimates from South Pennine reservoirs. Analysis of the particle size of core sediments showed abrupt increases in sand-sized particles in the top 20 cm of the cores (late 1970s onwards). This is related to the diverging trends in winter and summer-centred rainfall records and rapidly fluctuating reservoir levels. The sediment budget approach together with the chronology of reservoir sedimentation identifies the main sediment transfer pathways in the Burnhope catchment, and provides evidence of both extrinsic and intrinsic controls on sediment transfer and deposition.