|Grass-lined Channels and design|
The 1930’s were a time of extreme erosion brought about by drought, poor farming techniques, and severe floods. It was during this time that the nation began to work on conservation programs. The earliest research at the hydraulic laboratory was conducted on grassed waterways. This research resulted in the 1946 publication, SCS-TP-61, handbook of Channel Design for Soil and Water Conservation. Over a half million miles of grassed waterways have been built around the world based upon this criteria. The laboratory was designated in 1990 as a historic landmark of Agricultural engineering for its work in this area.
An open channel is defined as any conduit in which water flows with a free-water surface. Rivers, canals, and uncovered flumes are open channels. Pipes, drains, sewers, etc., act as open channels when flowing partially full. Following are definitions of a number of common terms describing flow in open channels:
Steady flow - When the discharge or rate of flow remains constant at a cross section, the flow is described as steady. Flows in drainage channels and rivers tend to be steady except during surface-runoff periods.
Unsteady flow - when the discharge is changing at a cross section, the flow is unsteady. Intermittent runoff into terrace outlets, meadow strips, diversion channels, and pond spillways is generally unsteady. Uniform flow - When the flow is steady and the mean velocity is the same at each succeeding cross section, the flow is uniform. The channel must have a constant cross section and slope. In designing ordinary vegetation-lined channels the flow is considered to be uniform at the maximum discharge.
Nonuniform flow - when the mean velocity changes from cross section to cross section, the flow is nonuniform. Flow in the upper portions of steep channels near the entrance is nonuniform. In a channel of changing-cross section the flow is nonuniform.
For more information and how to design a vegetative spillway-Handbook of Channel Design for Soil and Water Conservation-SCS-TP-61