Thresholds for Gravel and Cobble Motion

by Katherine J. Chase, (A.M.ASCE), Taggart Engineering Associates, Inc, Denver, United States,

Document Type: Proceeding Paper

Part of: Hydraulic Engineering


Flows necessary to initiate motion of rocks on a lateral bar along the Gunnison River in Black Canyon of the Gunnison National Monument are observed. Gravel and cobbles are painted and measured in situ. Velocities and water surface elevations in the vicinities of the rocks are observed during four increasing flood releases form an upstream dam. Of the 110 rocks exposed to flow, 43 move. Three predictive equations are used to calculate the velocity or shear ranges required to move each rock. These theoretical velocities and shears are compared with those observed during the flow releases. The equations overpredicted flows necessary to move many of the rocks set in motion during the first releases, and underpredicted flows necessary to displace some rocks that moved during the last (highest) releases. To improve predictive accuracy, ways to estimate velocity profiles, bed pocket angles, and turbulence from field measurements should be studied further.

Subject Headings: Rocks | Shear stress | Gravels | Sediment transport | Rivers and streams | Flow measurement | Field tests

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