Analysis of Sediment Transport Following Removal of the Sandy

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					Proposed Restoration of Original Sandy River Channel
 Remove dike over original Sandy River channel.  Replace access via a new bridge or other means.

Channel after dike removal.

In 1938 a dike was placed across the original (east) channel of the Sandy River, and the flow redirected to the west channel (originally called the Little Sandy River). The purpose was to concentrate the flow into one channel to improve fish passage. The east channel has since partially filled with sediment and dense riparian vegetation, including mature cottonwoods. Although once the main channel, the east channel now typically flows under high flows on the Sandy or Columbia Rivers. The channel dries to isolated pools in the summer. The Forest Service wants to investigate removal of this dike in order to restore a more natural hydrologic condition, improve fish habitat and overall aquatic habitat. Conclusions of Hydrologic Analysis East Channel: The pre-dike east channel is not likely to ever be restored without active human restoration such as dredging and tree removal. I-84 and Railroad Bridges: Flow hydraulics and local scour at the bridges are not expected to change from dike removal. Downstream Erosion along Right Bank of the West Channel: Dike removal is not likely to accelerate bank erosion, and may marginally allay erosion. Bank erosion without protection is likely to continue, but at a reduced rate. Percentage Flow Split to the East and West Channels Existing Condition Sandy River Flow West Channel East Channel 2-Year Flow Event 89% 11% 5-Year Flow Event 85% 15% 10-Year Flow Event 82% 18%
Some Time (10 Years) After Dike Removal West Channel East Channel 73% 27% 72% 28% 71% 29%