WRRCTR No.117 An Automatic Hydraulic Structure to Open Sand Plugs at Stream Mouths on O’ahu.

Abstract

Numerous stream mouths on O'ahu are affected by sand berm blockages which prevent free flow of storm waters to the ocean and cause the stream water level to rise rapidly; often overflowing of the stream banks and flooding of adjacent land areas result. To avoid this hazard, removal of the sand blockage prior to the arrival of peak flood flows is necessary. An automatic hydraulic structure which utilizes early arriving storm waters to hydraulically erode the sand berm has been developed during this study. A hydraulic model study of a 1- to 15-scale Froundian model based on a generalized O'ahu prototype stream channel was conducted to test physically the conceptual structure design. The hydraulic model structure performed effectively, causing rapid breaching of the sand berm under severe storm flow conditions. In all cases, overtopping of the sand berm, which would result in high stream stages and possible flooding, was avoided. Design recommendations and modifications have been developed to permit proper application of the structure to various stream mouth conditions.