Design and Control of Interstitial Flow for Steep Rock Ramps

Rock ramps are riffle like structures constructed in a stream to provide passage of aquatic species over low barriers.  Ramps are usually constructed using stone placed on a gravel or synthetic filter overlying a foundation of native material.  The size and gradation of the stone is based on achieving stability of the material under maximum design flow conditions.  Ramp slope is generally selected based on achieving flow criteria required for aquatic species passage.  Smooth ramps are typically constructed at slopes less than about three percent, however slopes up to five percent are used on streams with steep natural slopes.  To determine if fish passage flow conditions are suitable for the range of desired river flows, a designer must be able to predict the amount of surface flow, flow depth and average flow velocity on the rock ramp.  These parameters require determining the percent of flow that passes through the rock material as interstitial flow, the flow resistance of the rock material and insight on how sediment transported by the river can alter the initial assumptions with time.

These issues are of particular importance during periods of low stream flow when interstitial flow could result in little or no surface flow.  This study investigates the ratio of interstitial flow to surface flow passing down five percent slope rock ramps. Secondly, the study investigates the effectiveness of treating the finished rock ramp surface with “choke material” designed to reduce surface permeability.