Influence of Fine Sediment Introduced to an Armored Bed Downstream from a Dam
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Dam removal continues to emerge as a viable option in river management, yet little is known about the effects that a release of associated reservoir sediment may have on the riverine system downstream. An armored riverbed, commonly seen downstream from dams may have a complicating effect on the response of the channel to high inputs of fine sediment. It is hypothesized that an influx of fine sediment can affect the downstream bed by enhancing the transport of gravel fractions or in-filling the pore spaces in the surface gravel of the bed. When reservoir sediment is flushed downstream as part of a dam removal, the sediment is input to an armored bed condition. Upon the sudden input of fines, does the armored bed break or remain intact? If broken, then engineers must account for not only the reservoir sediment flushing downstream, but also the newly broken armor layer and associated substrate.
A sediment feed flume was used to examine the response of an armored bed to a sudden influx of sand. Four separate runs were conducted, each consisting of three phases: the first to obtain a dynamic equilibrium, the second to establish an armor layer, and the third phase to simulate a sudden flush of fine grained sediment, defined as less than or equal to 2 mm, onto an armored bed surface downstream from a dam. The four runs tested two sediment transport rates against two flow rates. Results show the armor layer being broken and mobilized during the third phase of each run. An increase in the total sediment transport rate is recorded and both sand and gravel are transported out of the flume. The remaining bed significantly fills in with sand regardless of flow rate or sediment feed rate. Distinctive patterns between Runs 1 and 3 (lower flow rate) and between Runs 2 and 4 (higher flow rate) exist when the bed both armors and breaks. The results indicate that the flow rate has more control over the amount of sediment being mobilized and transported downstream while the sediment feed rate controls the level of sand deposition on the surface. Microtopography, imbrication, and clusters are observed to varying degrees in the armored bed, which may prove, with further research, to have a greater influence on the response of how the armor bed breaks.