Application Note: Hydraulic Structures

Design and evaluation: Hydraulic structures are typically used for flood control, flood conveyance, irrigation purposes, fish passage, banks protection, navigation, recreation and ecological restoration. A hydraulic structure must meet the safety, functional and aesthetic goals for its purpose. Thus, valuation studies must be carried out before and after the construction of the structure to assess its impacts. The structure must be of sufficient size that natural flooding is not worsened and to ensure that the structure can withstand the design flood and remain traversable. This is required in order to protect the property and residents upstream and downstream of a structure. In the hydraulic design, one main thing to remember is that water is dynamic.

*Fish screen:* The hydraulic characteristics of a V-shaped fish screen were investigated to be used for irrigation diversion purposes on the Salmon River in Idaho. The system consists of a pair of rotating drums which screen the trash from the irrigation system, and prevent the fish from passing as well. The trash rises to the surface, but the fish must pass through an upstream orifice into a pool; from this pool, a downstream orifice leads to a bypass conduit which returns the fish to the river. Since this passageway has not been used previously, a 45 % scale model was constructed with variable orifice size and placement to study the characteristics of the flow through the structure. The objective of the study was to determine the optimal position for the downstream orifice, which can be placed in one of three slots in the pool. It was desired that the fish exit the passageway as quickly as possible, and this can be achieved by circulating the flow through the pool so that the fish do not have an opportunity to rest within the structure. Four orifice configurations were studied: small orifices were used upstream and downstream, but the downstream orifice was tested in two different locations; large orifices were used upstream and downstream, and the downstream orifice was again tested at two locations. The turbulent velocities in the pool were mapped using a three-dimensional Acoustic Doppler Velocimeter (ADV). The velocity measurements were taken across a section of the pool, at four depths; this was done at each of the three downstream slots. Measurements were also made along the centerline of the pool, using the same depths for each orifice pair as in the cross-section profiles. From this procedure, the average longitudinal velocity was obtained at different depths and horizontal locations. Results of the study showed the recirculation pattern in the tank, and demonstrated overall a drastic decay in the velocities of the submerged jet from the upper orifice.