Bromley, Chris
(2008)
The morphodynamics of sediment movement through a reservoir during dam removal.
PhD thesis, University of Nottingham.
Abstract
Dam removal has recently emerged as a growing trend in river rehabilitation in the United States. The rate of dam removal has been increasing rapidly since 2000, but is doing so with large gaps in our understanding of how the fluvial system will respond to this disturbance. Most of the structures removed to date have been relatively small and, in the vast majority of cases, have not received any pre- or post-removal monitoring. Very few large structures have been removed but, when such removals occur or are proposed, they tend to attract more monitoring activity because of the generally larger volumes of water and sediment involved. It is thus important to understand the form-process-response interactions that occur during the removal of large dams and the extent to which these may be applicable to other removals of varying sizes.
The proposed removal of the Glines Canyon Dam from the Elwha River in Washington, USA provides such an opportunity. The 67-m high dam is due to be incrementally removed in 2011 but its reservoir, Lake Mills, contains 80 years-worth of uncontaminated sediment that has the potential to adversely impact the aquatic and human environment once released into the channel downstream from the dam. In order to better understand the dynamics that control how sediment might be transported into the downstream channel, a series of scaled physical model experiments was performed in which the principle variable investigated was the magnitude of the drop in reservoir water surface elevation.
Four main findings emerged from the research. First, the hypothesised relationship between increasing magnitudes of baselevel drop and increasing delta erosion volumes is only weakly developed. Furthermore, the small increases in additional erosion volume for very large increases in magnitude of drop suggest that there may be an upper limit beyond which the volume of sediment eroded does not increase substantially, irrespective of the magnitude of drop. The reasons for this are explored.
Second, the volume of delta sediment eroded was greatly affected by the channel’s position on the delta surface at the start of each experimental run. The erosion volumes were greatly modulated when the channel was close to the basin boundary (marginal runs), because the boundary inhibited lateral channel movements and the formation of meander bends. When the channel ran through the middle of the delta surface (central runs) the erosion volumes were much larger, because meander bends were able to more fully develop and the channel had a greater overall freedom to adjust laterally over the entire delta surface. In contrast, the marginal runs generally incised slightly more along the full length of the delta than the central runs, despite a more extensively developed armour layer.
Related to the channel’s starting position, a strong element of topographical steering of the incising channel in the original delta area was observed during the marginal runs. When the channels started close to the left basin boundary (left marginal runs), the left hand curvature of this boundary tended to direct the majority of the flow’s erosive power away from the main body of the delta. In contrast, during the right marginal runs, the cross-basin downwards slope from basin right to basin left allowed the incising channel to move downslope and into the main body of the delta, particularly during the post-dam removal flood flows.
Third, the armour layer in the central runs generally extended less far downstream through the original delta area than during the marginal runs and this may be because of a frequently observed mobility reversal in sediment transport, which meant that gravels were more effectively flushed out of the original delta area than during the marginal runs.
Finally, a phenomenon hitherto unreported in the literature was observed on many occasions over the course of the experimental runs. Bed Elevation Lowering Without Armour Layer Break-up (BELWALB) occurred when the finer sub-armour sediment was eroded, thus undermining the armour layer and allowing the coarser grains to roll forward by a distance equivalent to a few grain diameters. This undermining action was able to migrate a certain distance upstream, either at the onset or end point of more extensive armour layer disturbance, thus causing subtle changes to bed morphology which are important in understanding how the system approached thresholds of stability and how it responded once these thresholds were exceeded.
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