Capacity of Box Inlet Drop Spillways Under Free and Submerged Flow Conditions

The box inlet drop spillway is defined as a rectangular box open at the top and at the downstream end. The spillway is shown in Figure 1. Storm runoff is directed to the box by dikes and headwalls, enters over the upstream end and two sides, and leaves through the open downstream end. An outlet structure is attached to the downstream end of the box. The long crest of the box inlet permits large flows to pass over it with relatively low heads, yet the width of the spillway need be no greater than that of the exit channel. the drop spillway has been extensively used as a gully control structure where it is necessary to drop water from as short a distance as 2 feet to as much as 12 feet. In more recent years it has also been used in drainage ditches where it functions as a title outlet and means of dropping excess surface water into the ditch. For the sake of economy, auxiliary vegetated spillways are sometimes provided to pass part of the runoff from the larger storms and to permit the use of smaller mechanical spillway. In order to prevent scour of the drainage ditch banks the elevations of the vegetated spillways are adjusted so no water will pass over them until the downstream drainage ditch flows back full. In other words, the mechanical spillway must have sufficient capacity to fill the ditch completely before any flow passes over the vegetated spillway. Under these conditions the high downstream water level will likely submerge the spillway and reduce its flow. After the vegetated spillways come into operation, the downstream level rises still further and submergence of the mechanical spillway becomes greater. Spillways designed in this manner are known as the "island dam" type because they can be completely surrounded by water during flood periods. The necessity for these studies to determine the capacity of box inlet drop spillways under submerged flow conditions thus becomes apparent.