Rapid channel widening following weir removal due to bed-material wave dispersion on the River Monnow, Wales

Kentchurch Weir, a low-head weir on the river Monnow, Wales, was demolished in August 2011, releasing a sediment wave that had formed behind the structure for at least a century. We surveyed channel topography and bed-material composition through a 1.5-km long reach prior to weir removal and then periodically over a 2-year period. The fill material was finer than the ambient bed material with all particles mobilized by bankfull flows. Rapid degradation of the 1460-m3 sediment fill in the previously impounded reach occurred as bed material appeared to disperse downstream, consistent with other studies of sediment waves in gravel-bed rivers. The riverbed profile was gradually smoothed through the study reach by degrading the elevated fill as a migrating knickpoint and aggrading the channel bed and bars immediately downstream of the former weir location. Extensive bank erosion was evident in the previously impounded reach with up to 10m of widening following a single flow event, increasing channel width by more than 20%. Mitigation measures to enforce the riverbanks have been required. as the gradual dispersion of the sediment wave continues to force flow diversion towards the riverbanks. The evolution of sediment stores behind flow obstructions follows that of sediment waves and theory available to describe wave evolution should do much to improve management efforts that seek to minimize channel widening following weir removal