Fish Movement and Weir Drown-Out

For small weir structures, fish often have to rely on drowned conditions to move upstream. A structure is considered drowned when the tail water level rises above the height of the barrier and the barrier becomes submerged. There is surprisingly little data on upstream fish movement across barriers under drowned conditions Important parameters affecting the success of an individual fish moving across a weir include the depth and velocity above the weir under drowned conditions, the head difference across the weir, and the surface roughness and roughness around the edges of the weir. For fish populations, other critical factors are the frequency, duration and timing of drowned flows. There are, thus, two requirements to address the adequacy of a given or proposed weir for upstream fish passage under drowned conditions. Firstly, the hydraulic properties of the drowned weir with respect to the requirements of the target fish species and, secondly, the duration and timing of drowning flows with respect to the timing of migration movements. In this paper a computer program, incorporated in a simple-to-operate spreadsheet, for the determination of the hydraulic characteristics of a drowned weir is described. The program is based on a theoretical analysis of drowned weirs and subsequent extensive verification in laboratory experiments. Some discussion is also provided on the frequency, duration and timing of drowned flows. It is shown that the program is an extremely useful tool in the design and/or assessment of a weir structure for fish passage under drowning conditions.