Responses of the sediment system of a regulated river to a scour valve release: Llyn Clywedog,mid-Wales,U.K.

In March 1985 engineering tests of scour valves at the base of the Llyn Clywedog regulating reservoir resulted in a controlled peak release discharge of 53m³ s^?1 into regulated reaches of the upper Severn. To exploit this unusual opportunity, simple monitoring systems were set up within the first 50 km downstream to assess the morphological effects on bed and banks and the movement of bed and suspended sediments. The interpretation of suspended sediment transport patterns became the major analytical task following the release, since bedload and morphological impacts were minimal. In comparison with natural floods, the scour valve release exhibited suspended sediment concentrations usually associated with a five-year event, but with a complex time and space pattern resulting from the release strategy. Wet antecedent conditions and the short duration of the scour valve release help to explain why impacts on the downstream channel were, otherwise, much more restricted than those of a water resource test release in 1975. Since scour releases can be planned with greater flexibility than major water resource releases, indications are that they should be made ‘on the back of’ natural high flows, although sensitive sites may require protection.