Vegetated Auxiliary Spillways

Vegetated Auxiliary Spillways
-Improving erosion prediction tools for vegetated auxiliary spillways

Auxiliary spillways convey extreme flood flow events around dams to the valley downstream. The majority of USDA-assisted embankment dams constructed under the authority of the USDA Small Watershed Program are equipped with a vegetated auxiliary spillway. Erosion problems of auxiliary spillways are unique because flow is usually infrequent and occur for short duration. Research evaluating the performance of vegetated auxiliary spillways for flood control dams and other hydraulic structures began to emerge as part of the USDA-Agricultural (ARS) Hydraulic Engineering Research Unit (HERU) research program by the 1980s.

Erosion of auxiliary vegetated spillways is considered permissible if the spillway does not breach during the passage of the flood flow. Erosion is a complex process in earth spillways, and it wasn’t completely understood. In 1983, scientists from the USDA-ARS HERU and engineers from the USDA-Natural Resources Conservation Service (NRCS) joined forces to form the Emergency Spillway Flow Study Task Group. Their objective was to observe and gather data from field spillways that had experienced gather than 0.9 m depth of flow or had sustained major damage during flood flows. The specific aim of the group was to create a databased and develop a better understanding of the erosion processes that occurred during vegetated earth spillways flows. Research was simultaneously conducted in the laboratory by ARS scientists on related erosion processes. Specifically, research focused on the development of a stress based rather than a velocity based grass-lined channel design method. These efforts led to improved design procedures for vegetated channels and published in Ag Handbook 667 Stability Design of Grass-lined Open Channels.

By 1991, USDA-ARS HERU and USDA-NRCS created a new partnership, known as the Design and Analysis of Earth Spillways team. The intention of this team was to develop new technology for the design of vegetated earth spillways. To assist in this effort, scientists expanded research to examine the underlying soil materials and headcut erosion processes as they relate to spillway performance. Large-scale flume tests were conducted on headcut erosion processes to examine cohesive soils with different geometries, compaction energies, moisture contents and densities. Data from both field and laboratory was compiled for the development of computational algorithms for use in the design and analysis of earth spillways. The computational algorithms were incorporated into the Water Resources Site Analysis computer program, Sites. The dam safety community worldwide has accepted this engineering tool to aid in the design of flood control embankment dams. To learn more about vegetated earth spillways, visit our publication index as technical publications are available upon request. Sites is available for download.