Spatial and temporal variations in the erosion threshold of fine riverbed sediments

Abstract

Lowland chalk streams in the UK are experiencing increased deposition of fine sediment due to changes in land-use practices, channel modifications, and groundwater abstraction. The excessive fine sediment deposits have been linked to benthic habitat degradation, the obstruction of surface–groundwater flow, and the storage of contaminants, such as nutrients and pesticides. While research has been conducted on the provenance, transport, deposition, and storage of fine sediment in chalk streams, none has expressly investigated the erosion of fine sediment deposits. A year-long field survey was conducted in two reaches of the Frome-Piddle catchment (Dorset, UK) to quantify spatial and temporal variations in the erosion thresholds of surficial fine sediment deposits. Erosion thresholds were measured at randomly located points within areas of sediment accumulation using a cohesive strength meter (CSM). The threshold measurements were paired with sediment cores for analysis of the physical, chemical, and biological properties of the sediment. Spatial and temporal patterns in the erosion thresholds of fine sediment were analyzed using nonparametric statistical tests and visualized with GIS. The sediment properties underlying the variations in erosion thresholds were examined through correlation and linear regression analyses. Erosion thresholds varied significantly over space and time within the stream reaches. Erosion thresholds were greater for fine sediment deposits found in the center of the channel than in the margins. Thresholds were highest in September 2008 and declined substantially to a minimum in May 2009, with a small peak in March 2009, indicating an annual cycle in erosion thresholds. Effective particle size was identified statistically as the most important sediment property influencing erosion thresholds and was probably underlying much of the spatial variation within the reaches. None of the measured sediment properties adequately characterized the temporal variation in erosion thresholds, however, the results suggest that biological sediment properties and water geochemistry (i.e., cation content) may play a role. By identifying significant spatial and temporal variations in erosion thresholds, this study provides valuable information on the stability of fine sediment deposits, and sediment-bound contaminants, in lowland river systems. This is a crucial step in assessing their local environmental impacts and developing models of fine sediment transport for the effective management of catchment sediment budgets and water resources.