Method of Superimposition for Suction Force on Trash Rack

As recommended, culvert entrances in urban areas should be protected with a rack or a grate because urban flood flows are quick, concentrated, and fast. Safety around storm-water facilities is an increasing concern for the public. Many forensic cases indicate that a trash rack at the entrance can prevent a human body from being washed into the culvert pipe, but on the other hand, a trash rack increases the flow velocity and results in a pinning force on the human body landed on the rack. While having a trash rack at a culvert entrance has the potential to result in a pinning force and to accumulate trash/debris, the public safety benefit of a trash rack in preventing a person from being drawn into an underground conduit outweigh the risks of the potential pinning force and/or trash/debris blockage. The conventional approach can only provide the total external force acting on a culvert-rack system, including the reaction forces from the wing walls and the rack with or without blockage. This study presents a new method of superimposition that can solve the external forces one by one progressively. Results from the case study indicate that the hydrostatic force due to the high headwater in front of the culvert entrance is mostly balanced by the reaction force from the wing walls. The pinning force on the submerged trash rack is mainly the response to the change in the flow momentum force. In comparison, the pinning force is much smaller than the total external force. A pinning force is a normal force on the rack surface. The effort to escape from being pinned on the trash rack is to overcome the friction along the rack surface.