EVALUATING MECHANICS OF EMBANKMENT EROSION DURING OVERTOPPING

Authors: Hanson, Gregory; Temple, Darrel

Submitted to: Federal Interagency Sedimentation Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 3/25/2001
Publication Date: 3/27/2001
Citation: Hanson, G.J., Temple, D.M. 2001. Evaluating mechanics of embankment erosion during overtopping. Proceedings of the 7th Federal Interagency Sedimentation Conference. 2:V-24-V-30.

Interpretive Summary: Interest in the occurrence and effects of water flowing over the top of earthen embankments by rising floodwaters has existed for years. Even though dam overtopping failures are rare, the impact to people and property downstream of a reservoir is important to engineers and planners alike, who must evaluate the hazards of local flooding. Research is presently underway ywith the goal of providing a means of evaluating the performance of overtopped earth embankment dams. Recent outdoor laboratory studies on steep-sloped channels have been conducted to contrast the difference in erosion between vegetated embankment slopes and non-vegetated slopes during overtopping. Tests were conducted for up to 75 hours of continuous flow. Maximum erosion rates were observed to be 25 to 50 times greater in the non-vegetated channels. Maximum erosion occurred at the downstream end of the slope in the vegetated channels and at the upstream end of the slope in nthe non-vegetated channels. The erosion pattern in the non-vegetated channels consisted of multiple waterfalls and pools cascading down the embankment slope. These studies add insight into the nature of embankment overtopping erosion and failure in the initial phases of erosion. These observations will be essential in updating earthen embankment failure models and adding to the safety and well being of people and property.

Technical Abstract: Interest in the occurrence and effects of water flowing over the top of earthen embankments by rising floodwaters has existed for years. Even though dam overtopping failures are rare, the impact to people and property downstream of a reservoir is important to engineers and planners alike, who must evaluate the hazards of local flooding. Research is presently underway ywith the goal of providing a means of evaluating the performance of overtopped earth embankment dams protected only by a vegetal cover. Recent outdoor laboratory studies on steep sloped (33%) trapezoidal channels have been conducted on a 3 m high constructed earthen embankment. The embankment was constructed of a silty sand material. The channels are 6 m long, and 1 meter wide with 1 to 1 side slopes. Contrasting tests, two vegetated and two non-vegetated, were conducted. The tests were conducted for 75 hours of continuous flow in the vegetated channels and for 52 hours and 73 hours in the non-vegetated channels. Maximum erosion rates were observed to be 25 t 50 times greater in the non-vegetated channels. The maximum observed erosion rate was 0.030 m/h in the non-vegetated state. Maximum erosion occurred at the toe of the slope in the vegetated channels and at the upstream end of the slope in the non-vegetated channels. The non-vegetated channels eroded in a classical stair-stepped fashion with overfalls and plunge pools. These studies add insight into the nature of embankment overtopping erosion and failure in the initial phases of erosion. These observations will be essential in updating earthen embankment failure models and adding to the safety and well being of people and property.