Dispersal of augmented gravel in a steep, boulder-bedded reach: early implications for spawning habitat (#73)
Gravel augmentation is for partially offsetting deleterious impacts of dams on downstream salmonid spawning habitats. Here, gravel was added to a steep (0.0236 slope), boulder-bedded reach of the River Avon, England, downstream of a dam. The granitic headwaters provide naturally low rates of sediment supply, but the dam is implicated in lowering fish populations. Unlike gravel augmentation in alluvial reaches where the dispersal path of gravels is generally understood to intersect with riffle-tail spawning habitats, the benefits of gravel dispersing through a hydraulically-chaotic reach with ‘pocket redd’ spawning is far from certain. Here, monitoring using seismic impact plates and RFID-tagged particles is attempting to determine particle mobility rates, dispersal distances and settling locations in relation to high flows events, and in determining appropriate volumes, frequencies and locations for future augmentation. 150 tagged particles of augmented (median 42 mm) and native (55 mm) sediments were added to a pilot augmentation in autumn 2014. Ten small floods were capable of mobilising a majority of the augmented load with possibly more than 50% of the material passing over an impact plate situated 50 m downstream. Tracer recovery is challenging in the reach the 55% recovery rate indicated that the augmented gravels travelled a mean of 47 m (maximum 91 m) whereas the native particles travelled an average of 35 m (maximum 78 m). Mapping of the recovered particles appear to indicate a preferred track for long-distance material but also settling of particles in elevated marginal positions which may limit salmonid habitat benefits.