Geomorphological adjustment of a newly engineered upland sinuous gravel-bed river diversion: Evan Water, Scotland

Abstract

Following construction of a sinuous diversion on an upland gravel-bed river two flood events within three days of each other, with estimated recurrence intervals of between 1·5 and 2·0 years, caused marked geomorphological adjustment to the channel. The floods resulted in bank erosion, point bar formation, scour on the outside of meander bends and particle sorting, but the overall stability of the river diversion, which is to flow adjacent to a new motorway embankment was not affected. Moreover, the channel adjustment brought about beneficial changes in terms of increasing morphological diversity: probably enhancing fish habitat, although spawning gravels placed at the head of riffles in the river diversion were scoured. The extent and nature of the geomorphological changes relate principally to the high stream power and bedload transport capacity of the river. Geomorphological principles, critical stream powers and bedload transport rates can account for the size and redistribution of bed material transport. Other factors accounting for the adjustment include limited vegetation colonization of geotextile-covered river banks and construction of overly narrow and high channel constrictions to promote flow convergence and acceleration at the head of downstream pools. The case study provides lessons for future construction of river diversions on high energy gravel-bed rivers. © 1997 John Wiley & Sons, Ltd.