Patterns of Soil Instability and Sediment Delivery from the peri-urban Red House Gill catchment, County Durham

Abstract

Catchment urbanisation raises important concerns over surface water management and potential negative impacts on erosion and the ecological status of rivers. Red House Gill is a small peri-urban sub-catchment of the River Wear, Northern England, which over the past 50 years has undergone extensive urban growth. This research assesses the effects of urbanisation on sediment delivery from this catchment. A sediment budget approach is used to investigate spatial patterns of fine sediment delivery from three main sub-catchments (East, Middle and West streams). Processes were monitored over a six month period using direct stream flow gauging and spatial sampling (with a network of nine Time-Integrated Mass Samplers- TIMS) together with fixed point photography of sediment sources and terrestrial laser scanning. Historic patterns of soil movement were indirectly assessed by measuring the basal trunk angle of 340 trees across the wooded catchment and a detailed dendrochronologic analysis of eight trees.
Results demonstrate that the steep catchment is highly sensitive to high rainfall which leads to rapid discharge (peaks up to 4 m3 s-1) and large suspended sediment loads. Over the study period 80% of suspended sediment load was transported in 2% of the time. Peaks in erosion observed from the static photography and high suspended sediment flux from the TIMS show good temporal agreement. The network of TIMS demonstrates that the majority of sediment is delivered from the Middle stream (55%), which has the second largest catchment area, steepest channel and valley side slopes. In contrast the West stream has the lowest channel gradient and least steep side slopes and consequently delivers the least suspended sediment (3%). Trees growing on steeper slopes generally show greater tilt but the rate of tilt is not significantly correlated with local slope. Results from the dendrochronology demonstrate that soil creep is continually occurring in the catchment, with only a few brief periods where tree rings suggest reduced rates. The preliminary sediment budget demonstrates that over the course of the monitoring period, 482 t of sediment was eroded from the hillslope and delivered to the stream, while 493 t of sediment has been transported by the stream. The 11 t (3%) discrepancy is likely to be caused by in-channel erosion. Assuming a similar annual sediment yield, this equates to a specific annual sediment load of 386 t km-2 yr-1, which is close to the maximum reported sediment yields from other UK catchments. Erosion can be potentially managed by reducing peak flows and limiting sediment supply to the stream.