|Tour Stop #6: Local Scour in the Vicinity of Spur Dikes|
A spur dike can be defined as an elongated structure having one end on the bank of a stream and the other end projecting into the current. Spur dikes have been widely used to protect eroding stream banks. They have also been used to enhance aquatic habitats by causing stable pools in unstable, disturbed streams. In general, spurs are more beneficial to aquatic habitat resources than other types of bank protection, primarily because their presence causes pool habitats to be created and maintained. The depth and volume of local scour caused by a spur dike is difficult to estimate accurately. The complex physics of the 3-D unsteady flow associated with the scouring process are poorly understood and difficult to characterize. As a result, most scour prediction algorithms are empirically based and only predict the maximum depth of scour.
The goal of this research is to provide information toward optimizing the design of spur dikes for aquatic habitats as well as for bank stabilization. This is being accomplished by relating upstream flow conditions to the maximum depth of scour, geometry, planar area, and volume of the local scour associated with spur dikes of different length and angle.
Experiments with spur dikes under steady flows have shown that the contraction ratio and flow depth are positively correlated with the volume of the scour for a given elapsed time. The maximum depth of scour measured in this study and other published works was adequately predicted using an equation proposed by Garde et al.(l 961). These findings are important for designing spur dikes for the maximum benefit to aquatic habitats as well as providing adequate bank protection. The goal of designers should be to select spur geometry which stabilizes the bank and provides the largest scour volume subject to cost constraints. A large area of scour of moderate depth would likely be superior habitat to a similar area of greater depth.
Further experiments are planned which will consider the effect of the angle of the spur dike on the volume and geometry of the scour hole. Detailed flow-velocity fields will also be measured in a subset of representative scour holes.
Key Words:Local scour, Spur dikes, Aquatic habitat Volume of scour