RESEARCH RESULTS OF LARGE-SCALE EMBANKMENT OVERTOPPING BREACH TESTS

Authors: Hanson, Gregory; Cook, Kevin; Temple, Darrel

Submitted to: State Dam Safety Officials Association Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 9/8/2002
Publication Date: 9/25/2002
Citation: HANSON, G.J., COOK, K.R., TEMPLE, D.M. RESEARCH RESULTS OF LARGE-SCALE EMBANKMENT OVERTOPPING BREACH TESTS. IN DAM SAFETY 2002. PROCEEDINGS OF THE ASSOCIATION OF STATE DAM SAFETY OFFICIALS, SEPTEMBER 2003, TAMPA, FL.

Interpretive Summary: Even though the occurrence and effects of water flowing over the top of earthen embankments by rising floodwaters are rare, the impact to people and property downstream of such an embankment is important to engineers, and planners alike, who must evaluate the hazards of local flooding. There are about 57,000 dams on the national dam inventory that have the potential for overtopping. Recent outdoor laboratory overtopping studies have been conducted on relatively large-scale models to help assess these impacts. The erosion process during overtopping and breaching tests was observed to be multi-phase: initial downstream surface erosion progressing into stair-stepped multiple overfalls, ultimately merging into a single upstream-migrating headcut. The rate and type of erosion during these tests is evaluated based on measured material properties, flow measurements during overtopping, and physically based mathematical procedures. Prediction of the rate and type of erosion for varying conditions will play a key role in evaluating the timing and magnitude of the embankment failure and flooding downstream from an overtopping embankment.

Technical Abstract: Dam and embankment breaching from overtopping is important to both engineers and planners. The processes observed during overtopping and breach tests conducted on large-scale models as well as preliminary analysis of the results are described in this paper. Three large-scale embankments, two at 2.3 m and one at 1.5m in height have been tested. Two of the embankments, 2.3 m high and 1.5 m high, had three test sections, 7.3 m and 4.9 m wide with 2 m and 1.5 m trapezoidal notch overflow sections respectively, and 3H:1V slopes on both the upstream and downstream sides, consisting of three materials varying from a non-plastic SM silty sand to a CL lean clay. The other embankment, 2.3 m high, had a single 12 m wide test section of non-plastic SM silty sand with an 8.2 m wide trapezoidal overflow notch. The erosion process during overtopping and breaching was observed to be multi-phase: initial downstream surface erosion progressing into stair-stepped multiple overfalls, ultimately merging into a single upstream-migrating headcut. The rate of upstream headcut migration from these tests is evaluated based on measured material properties, hydraulic parameters during overtopping, and physically based analytic procedures. Headcut migration rates ranged from 0.15 to 0.003 m/min for the 2.3-m high embankment and 0.15 to 0.001 m/min for the 1.5-m high embankment depending on material type. Measured soil properties help explain the difference in migration rates of the headcuts. The rate of headcut migration is an important part of the rate of breach development. Prediction of headcut advance rate for varying conditions will play a key role in evaluating the timing and magnitude of the outflow hydrograph from an overtopping failure event.