7. Construction and Setting of Standard Fully Contracted Submerged Orifices

Submerged-orifice structures should be substantially constructed of concrete or, under certain circumstances, of wood. The structure should extend several conduit widths downstream from the orifice wall to provide erosion protection in unlined channels. The floor and sides of the box conduit downstream from the orifice opening should be set outward from the opening a distance of at least twice the smallest orifice opening dimension. A flashboard may be placed at the downstream end of the orifice box conduit to assure submergence of the orifice; however, the flashboard should be sufficiently downstream to prevent disturbing the water issuing from the orifice. The orifice wall should be set truly vertical, and the top of the wall should reach only to the maximum expected water level, so the wall can act as an overflow weir in the event of operational difficulties. Wingwalls or cutoff walls should be provided at the upstream and downstream end of the orifice box conduit to prevent undermining the orifice structure and leakage around the structure. Figure 9-1 and table 9­1 provide recommended dimensions for a concrete or wood submerged-orifice structure.