Sediment Processes Associated with the Removal of the Glines Canyon Dam

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.