Downstream change in channel hydraulics along the River Severn, UK

Abstract

An understanding of the longstream distribution of hydraulic processes is important for evaluating the mechanisms which control the catchment-scale variation of channel stability, sediment transport and siltation, flood generation and aquatic habitats. However, little attention has been given to quantifying the detailed spatial and temporal variability of channel hydraulic parameters, despite considerable attention to reach-scale processes. This study uses a combined field and modelling approach to define the distributions of channel hydraulic parameters between the source and the near-tidal limit of the River Severn, UK. Field measurements were made at 25 logarithmically-spaced sites along the Severn, under 3 flow conditions (low, medium and high) defined by exceedence frequencies. A flow event, occurring between 15-24 February 1989, was simulated by the 1-D hydraulic model, MIKE11. The simulated reach consists of cross sections spaced at 1 km intervals between 4 km and 254 km downstream from the source. Channel hydraulic parameters showed considerable variability in both space and time, reflecting cross section geometry variation downstream. Mean velocity increased with distance downstream from 0.23 m s$^{-1}$ to 1.72 m s$^{-1}$ under steady, bankfull flow conditions. However, unsteady flows simulated by MIKE11 demonstrated a longitudinal decline in the mean velocity of the wave peak associated with the rapid movement and minimal attenuation of the flood wave through the unconfined upper Severn. Flow resistance (Manning's n and Darcy-Weisbach f) decreased downstream from the source (0.32 - 0.06), although under low flow conditions it increased from 0.3 to 1.1 downstream to the non-alluvial - alluvial transition at Llanidloes; thereafter it exhibited a steady downstream decline. Reach mean shear stress and unit stream power peaked near the source (5-10 km downstream; drainage area < 50 km$^2$) at 120 N m$^{-2}$ and 290 W m-2 and further downstream at the Ironbridge Gorge (170 km) (38 N m$^{-2}$; 40 W m$^{-2}$) in response to the lithological controls on channel slope and valley width