Performance of Bareearth and Vegetated Steep Channels Under Longduration Flows

Thousands of dams built in the U.S. have a potential to be overtopped by flood flows. It is therefore necessary tounderstand the mechanics of embankment erosion in order to evaluate the potential erosion damage to an embankment andthe safety risk that overtopping represents. This article describes tests conducted at the USDAARS Hydraulic EngineeringResearch Laboratory in Stillwater, Oklahoma, using an embankment 3 m high and 37 m long with 3:1 slopes. Measured watersurface, bed, and velocity profiles and predicted hydraulic stress for four 1m wide trapezoidal channels, two vegetated andtwo nonvegetated, are compared. The tests were conducted at discharges of 0.2 m 3 /s and 0.6 m 3 /s. The tests in the vegetatedcondition were run for 75 hours, and the tests in the nonvegetated condition for 73 hours and 52 hours, respectively.The maximum average erosion rate in the nonvegetated channels was 25 and 50 times greater than that in the vegetatedchannels for the two flows. The erosion in the nonvegetated channels was not uniform spatially along the length of thechannel or temporally over the duration of the tests. The erosion progressed into stairstepped overfalls. Jet test analysis oflaboratory soil samples indicated that erosion resistance varied up to 3 orders of magnitude due to variations in compaction,percent saturation, and density. The excess stress parameters, erodibility coefficient (kd) and critical stress (.... c), based onposttest samples, were predicted to be 0.00029 m/Pah and 0.55 Pa, respectively. Hydraulic stress evaluation of the twobareearth soil channels over the duration of the tests resulted in kd values of 0.00026 and 0.00029 m/Pah for the testsconducted at discharges of 0.2 m 3 /s and 0.6 m 3 /s, respectively. Evaluation of the two vegetated channels resulted in kd valuesof 0.00070 and 0.00004 m/Pah. It is anticipated that the results of these studies will provide the basis for improved evaluationof the erosion of embankments subjected to overtopping. This study will provide a tool for evaluating dam rehabilitation needs,as well as for assessing the risk to human and livestock life and property loss downstream.