Prototype Spillway Performance Verification

The flow performance of a prototype closed conduit spillway which could not be explained analytically was verified by a physical model. During the falling stage of the outflow from the reservoir, full conduit flow was observed in the Marsh Creek spillway even though the pool level was below the spillway crest. Engineers inspecting the spillway could not agreeably explain the flow performance. A physical model of the prototype was built and tested to verify the prototype performance. Tested with open and closed air vents, the model demonstrated the hysteresis shown in the falling limb of the prototype head-discharge relationship and the siphon-regulated flow observed in the prototype. Model observations indicate that the prototype flow control problem is the result of inadequate venting of the covered-top drop inlet.