Submitted to: American Society of Civil Engineers Water Resources Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: August 8, 1999
Publication Date: August 8, 1999
Interpretive Summary: Vertical drops or headcuts move upstream in a channel, causing large sediment loads and channel bank stability problems. These headcuts cause channel widening that results in the loss of valuable land and presents a threat to bridges, roadways, and other improvements. This research was done to develop a low-drop structure that will halt headcut movement and stabilize the channel. The hydraulic performance of this drop structure wa evaluated with sloping sidewalls. Sloping sidewalls are better suited for construction with alternative materials such as gabions, roller compacted concrete, or cable-tied concrete. The sloping sidewalls were found to perform as well as the vertical walls in the original design criteria. These results will be of value to water resource managers and designers interested in cost effective alternative structures. Numerous applications exist for reliable and cost effective structures that make use of innovative and alternative materials.
Technical Abstract: A generalized physical model study was conducted to develop design criteria for a low-drop grade control structure. This design was modified to examine sloping as well as vertical sidewalls, so alternative materials could be more widely used for construction. The hydraulic performance of the drop structure was evaluated for 1 to 1 and 1 to 2 (vert. to horiz.) side slopes. Velocity measurements exiting the basin were used to quantify performance characteristics, as were the depth and length of observed scour. Tailwater to critical depth ratios of 0.50, 0.75, 1.0, and 1.25 were examined to simulate a wide range of flow conditions. The maximum observed flow velocity exiting the drop structure was typically largest for the 1 to 2 side slopes and smallest for the vertical walls. At the highest tailwater level, velocities were similar for all sidewall conditions. The sloping sidewalls caused a larger recirculation zone within the basin where flow reentered the basin along the sides of the sloping walls. These circulatio zones influenced the location of maximum scour. The sloping sidewalls typically caused increased scour depth and a decreased scour length when compared with vertical walls. At tailwater to critical depth ratios of 1.0 and larger, the sloping sidewalls performed as well as or better than the vertical sidewalls. Adding sloping sidewalls to this structure is considered a viable alternative.