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Title: EVALUATING HEADCUT MIGRATION RATES OF EARTHEN EMBANKMENT BREACH TESTS

Author
item Hanson, Gregory
item Cook, Kevin
item HAHN, WILL - HDR CONSULTANTS

Submitted to: American Society of Agricultural Engineers Meetings Papers
Publication Type: Proceedings
Publication Acceptance Date: 7/29/2001
Publication Date: 7/30/2001
Citation: Hanson, G.J., Cook, K.R., Hahn, W. 2001. Evaluating headcut migration rates of earthen embankment breach tests. American Society of Agricultural Engineers Meetings Paper No. 01012080.

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 or are being conducted on relatively large-scale models, earthen embankments 2.3 m and 1.5 m high to help assess these impacts. Each embankment has three test sections with each test section made up of a different soil. The test sections are 7.3 m wide for the 2.3 m high embankment and 4.9 m wide for the 1.5 m high embankment. The soils tested vary from a silty sand to a lean clay. Evaluation of these tests has shown that soil type and embankment size have a significant effect on the timing and rate of erosion of the embankment. One type of erosion measurement, headcut migration rate, varied some 400 times depending on soil type and embankment size. Evaluation of measured soil properties and energy dissipation help account for differences in this type of erosion. Results from these tests will impact the modeling efforts for determining embankment performance during extreme events.

Technical Abstract: There are a number of recognized technology advancement needs associated with watershed structure re-evaluation and re-habilitation. One of those needs, addressed by embankment breach research, is technology for predicting overtopping performance during extreme events. 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. Recent outdoor laboratory overtopping studies have been and are being conducted on relatively large- scale models, earthen embankments 2.3 m and 1.5 m high. Each embankment tested to date has three test sections with each section made up of a different soil. The test sections are 7.3 m wide for the 2.3 m high embankment and 4.9 m wide for the 1.5 m high embankment. The test soils vary from a silty-sand to a lean-clay. Preliminary evaluation of the overtopping and breaching tests indicate that soil type and embankment size have a significant effect on headcut migration rates. Headcut migration is an important erosion process during overtopping and rates ranged from 0.15 to 0.003 m/min for the 2.3-m high embankment and from 0.15 to 0.001 m/min for the 1.5-m high embankment depending on material type. Erodibility varied 200 fold and unconfined compressive strength varied 4 fold for the soils used in the study. Evaluation of the measured soil properties and energy dissipation help account for differences in headcut migration rates between tests. These results will impact the modeling efforts for predicting embankment performance during extreme events.