Derivation of annual reach-scale sediment transfers in the River Coquet, Northumberland, UK

摘要

Measurement of three-dimensional morphological change in a river channel provides a useful means of assessing reach-scale rates and patterns of fluvial sediment erosion, transfer and deposition. The morphological sediment budgeting techniques used to generate these estimates of sediment flux may be particularly valuable in unstable gravel bed rivers, owing to the inherent difficulties associated with direct measurement of bedload transport. This paper compares two approaches used to derive a measure of annual sediment transfers within a 1 km long piedmont reach of the gravel-bed River Coquet in Northumberland, northern England, which has a locally braided channel planform and has experienced lateral instability over the past 150 yr. The first technique utilized channel planform and cross-profile surveys based on theodolite-EDM survey of (i) 21 monumented channel cross-profiles and (ii) channel and gravel bar margins. The second method used theodolite-EDM surveys to generate a series of xyz coordinates for the channel and bars within the reach, from which digital elevation models (DEMs) can be constructed. Calculating the difference between two DEM surfaces provides a measure of volumetric change between surveys. The compatibility between the two exercises, carried out during the springs of 1999 and 2000, was assessed by an error analysis comparing the surveyed cross-profiles with sections abstracted from the DEMs. This indicates a mean gross error between surveyed and DEM profiles of approximately twice the value of the D_(50) of the surface sediment in the reach. The accuracy of the DEMs as a representation of the terrain surface is quantified using residual analysis, which indicates that more than 96.3% of the interpolated surface is accurate to ±5 cm (equivalent to the surface sediment D50) for both the 1999 and 2000 DEM surfaces. Comparison of sediment volumes derived from the two approaches suggests that, relative to the higher resolution DEM survey, estimation of sediment transfers using monumented profiles and planform underestimates the magnitude of volumetric changes that occur within the reach. The degree of underestimation is reach and time dependent. The DEM method using a carefully designed sampling strategy based on morphological units provides a rigorous identification of spatial patterns of erosion and deposition, which cross-section-based approaches may fail to include.