GEOMORPHOLOGICAL ADJUSTMENT OF A NEWLY ENGINEERED UPLAND SINUOUS GRAVEL-BED RIVER DIVERSION - EVAN WATER, SCOTLAND

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 geom orphological adjustment to the channel. The floods resulted in bank er osion, point bar formation, scour on the outside of meander bends and particle sorting, but the overall stability of the river diversion, wh ich 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 h abitat, although spawning gravels placed at the head of riffles in the river diversion were scoured. The extent and nature of the geomorphol ogical changes relate principally to the high stream power and bedload transport capacity of the river. Geomorphological principles, critica l stream powers and bedload transport rates can account for the size a nd redistribution of bed material transport. Other factors accounting for the adjustment include limited vegetation colonization of geotexti le-covered river banks and construction of overly narrow and high chan nel constrictions to promote flow convergence and acceleration at the head of downstream pools. The case study provides lessons for future c onstruction of river diversions on high energy gravel-bed rivers. (C) by 1997 John Wiley & Sons, Ltd.