First-Year Downstream Sediment Budget Following the Marmot Dam Removal from the Sandy River, Oregon
Abstract
The October 2007 removal of the Marmot Dam, from the Sandy River, OR, provides an opportunity to assess the impact of increased sediment flux on a river channel. The Sandy River drains the west flank of Mt Hood and typically carries a large load of sand and gravel. The 14-meter-tall dam impounded over 750,000 m3 of sediment, only a small amount of which was removed during the decommissioning. Using a one- dimensional modeling approach, it was assessed that the river could transport the accumulated sediment without large adverse impacts downstream of the dam (Cui et al, 2008 - abstract submitted). In order to observe the actual changes to the river due to the dam removal and to test the modeled predictions, a significant monitoring effort has be in place on the Sandy River including bedload and suspended load measurements, discharge measurements, high-fidelity topographic surveys, repeat photography, multiple airborne LIDAR flights, long profile surveys, as well as mapping and characterizing the grain sizes throughout several reaches downstream of the dam. A key step in the quest to describe and predict the spatial distribution of the sediment throughout the downstream reach is to first account for all the sediment (both stored in the reservoir and supplied from upstream). Here, we examine the transport and deposition downstream of the dam through a 2-fraction sediment budget approach using the former dam as the upstream limit of the reach and choosing a the mouth of a bedrock gorge 7 km below the dam site as the downstream limit. Suspended sediment and bedload measurements taken by the USGS just below the dam site (Major et al, 2008 - abstract submitted) are combined with suspended sediment and bedload measurements collected just below the mouth of the gorge and the annual hydrograph to define the sediment fluxes into and out of the reach. Repeat surveys in the reach below the dam (Wallick et al, 2008 - abstract submitted) provide the measure of change in storage of the reach. This work combines the flux measurements and the deposition measurements to describe the fate of both the fine (sand and finer) and the coarse (gravel and larger) sediment from the reservoir deposit. During the initial cofferdam breach, the sediment flux into the reach was nearly two orders of magnitude larger than the sediment flux leaving the reach, however as the water year progressed the sediment outflux was generally an order of magnitude less than the influx. This divergence in sediment flux demands an increase in storage within the reach. The storage primarily took the form of a sediment wedge decreasing from a thickness of 4 m over 1.5 km. Much of the sand from the deposit was transported out of the budget reach. The outflux of sand was more than 20 times the flux of gravel (260,000 m3 of sand, 11,000 m3 of gravel). This accounting for sediment over the first year integrates the work of several research groups and provides a description of the behavior of both fine and coarse sediment released from the Marmot Dam into a highly energetic gravel-bed river.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.H43B0996P
- Keywords:
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- 1808 Dams;
- 1825 Geomorphology: fluvial (1625);
- 1862 Sediment transport (4558)