Pinning Force during Closure Process at Blocked Pipe Entrance

The high design standards for the preservation of the urban water environment result in more detention basins in residential areas. The steep channel and outfall pipe from a detention basin can be hazardous to children and small animals. During an event, a trapped person may flow with water toward the outfall entrance. When the outfall pipe is gradually blocked, the flow force acting on the blocking body can be pinning at the beginning to eventually deadly. It is not clear as to how to quantify the pinning force on the block during the closure of flow because the number of unknown forces is greater than the number of equations for force balance. This paper presents a new approach using the method of superposition to calculate the pinning force with and without a blockage at the culvert entrance. The analysis conducted in this paper verifies that the pinning force on the clogging block is dominated by the flow dynamic force until the flow becomes discontinued. As soon as the hydrostatic force is developed, the pinning force can be lethal. This study confirms that an outfall pipe shall be protected by a trash rack with its surface area 4 times the culvert opening area. As long as the continuity of flow is sustained by the blocked rack, the hydrostatic force will not be developed or a chance for survival remains.