A headcut is the sudden change in elevation or knickpoint at the leading edge of a gully. Headcuts can range from less than an inch to several feet in height, depending on several factors. The formation and movement of a gully headcut are often the dominant form of damage observed in an earth spillway. Failure of an earth emergency spillway poses a threat to the people and property downstream of a dam. To evaluate the risk of a spillway failure or dam breach, the rate of headcut advance or gully movement must be predicted. This prediction requires knowledge of the physical processes and erosion mechanics acting at the overfall. Soil properties, such as density, moisture content and erodibility, as well as factors affecting the flow hydraulics, such as flow rate, overfall height, and tailwater conditions can have a large impact on the headcut advance rate.
Beginning in 1992, researchers at our lab began construction of a large scale headcut testing facility to examine these erosion processes and to measure headcut advance. Completed in 1993, the facility features a 96-ft long, 6-ft wide concrete test flume with 8-ft high sidewalls. Other conveniences of the system include a modified Parshall flume for flow measurement, an oulet control structure to manipulate tailwater elevations, and two viewing windows in one wall of the flume.
Since its completion, over fifty headcut erosion tests have been conducted in this facility. In preparation for a test, the flume is filled by placing soil in horizontal layers until the desired fill height is achieved. Compaction is performed after the placement of each layer with a self-propelled vibratory roller. After filling, samples are taken in a specified area to measure soil properties such as density, moisture content, and soil strength. A vertical headcut is then pre-formed upstream of the sampling area, and flow is introduced from the upstream end of the flume. Water falling over the headcut causes erosion on the face of the overfall, moving the knickpoint upstream in the fill. Depending on the depth of fill in the flume, as much as 130 cfs can be directed through the flume.
The tests completed to date have been to compare the effects of differing soil and hydraulic properties on the rate of headcut advance. The results from this study have enhanced our knowledge of earth spillways and gully headcut advance.
For additional information contact our office.