Rock Ramp Interstitial Flow and Design Guidelines
* Over the range of potential rock ramp design situations, can stable rock ramps be constructed that provide enough surface flow to provide for fish passage during critical low flow periods without the use of artificial materials to control seepage (grout, geomembranes, soil cement, etc)?
* For various ramp configurations, what is the recommended stable rock size gradation for a given discharge hydrograph?
* For various ramp configurations, what is the depth to which fines are removed?
* For various ramp configurations and riprap gradations, what methods should be applied to predict the transport of fine particles through the interstitial spaces in the rock substrate?
Need and Benefit
Reclamation has a growing need to understand and predict the impacts of fish passage requirements on water delivery facilities. Fish passage criteria must be considered in many rock ramp/roughened channel designs, often dictating minimum river flow requirements at water diversion structures. The current state of the art does not provide the designer with sufficient guidelines to establish a low flow limit for operating rock ramps. In many previous designs, uncertainty of interstitial flow capacity led to grouting to ensure sufficient surface flow to support fish passage during periods of low river flow. Grouting is costly and often not favored by resource agencies. Surface grouting can also create structural problems over time by reducing the structure's ability to adjust to foundation settlement or river degradation.
Several Reclamation regions and other agencies are using increasing numbers of rock ramps and/or roughened channels to maintain stable water diversion and provide fish passage. As more rock ramps/roughened channels are constructed, predicting the volume of surface water available for fish passage and the effect of interstitial flow on structural stability becomes more critical. Three projects, in three different Reclamation regions, with possible interstitial flow or stability concerns are described below.
On the Chewuch River in northern Washington, Reclamation's Pacific Northwest Regional Office is redesigning Fulton Dam as a rock ramp to improve water diversion and enable fish passage. The Washington Department of Fish and Wildlife requires that a sufficient percentage of fine sediment be included in the rock ramp material to prevent dewatering of the low flow channel through infiltration of surface water to groundwater. Removal of fines from the sediment mixture comprising the ramp could potentially increase the seepage of Chewuch River water through it. Reclamation's Sedimentation and River Hydraulics Group of the Technical Service Center in Denver used a one-dimensional, numerical sediment transport model to evaluate the stability and simulate the removal and transport of fine sediment contained in the proposed rock ramp. Simulation results indicated that fine sediment would be transported from the ramp, but did not indicate the depth below the ramp surface to which the fines would be removed, preventing accurate estimation of the interstitial flow through the ramp. The proposed research project would build on the results of this study to address some of the questions raised.
The Grand Junction Area Office of Reclamation's Upper Colorado Region is considering the modification of the Price-Stubb Diversion Dam on the Colorado River by adding a rock ramp and separate fish passage channel downstream of the existing dam crest. During design of the ramp, the impacts of interstitial flow on ramp stability and fish passage were examined. To minimize the effects of interstitial flow engineers proposed the use of grout on the ramp surface and designed a separate low flow fishway along the ramp. A longer, flatter ramp would be required for all migrating Colorado River fish species to pass upstream, but the amount of flow that would remain on the surface is unknown. Computational tools for estimation of interstitial flow on rock ramps are needed for effective design.
The Omaha District of the U.S. Army Corps of Engineers and Reclamation's Montana Area Office (in the Great Plains Region) are evaluating replacement alternatives for Intake Diversion Dam on the Yellowstone River. Rock ramps capable of passing fish upstream at 2 percent and 5 percent slopes are being considered. Currently, high flows transport rocks from the dam crest, requiring replacement on a frequent basis. Design guidelines for a stable gradation capable of retaining surface flows sufficient for fish passage are needed for a successful implementation of the chosen alternative.
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This information was last updated on April 1, 2015
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