Breach Flow Modeled as Flow over a Weir

Abstract

The main objective of the study is to understand the effects of breach properties (e.g. the top width, the depth and the bottom width of the breach) on the breach flow. The breach flow was modeled as flow over a compound broad-crested weir. A laboratory experiment was carried out with a fixed weir model in a flume. Five cases with different breach properties were tested. Firstly, the discharge coefficients for all cases were quantitatively determined. The calibrations were done under two flow conditions, namely emerged flow conditions (flow only through the breach) and overtopping flow conditions (flow through the compound cross-section of weir). In perfect weir situations, it turned out that the values of the discharge coefficient were below 1 and rather different in the two flow conditions. Particularly in the case of overtopping flow, the linear combination of traditional discharge equations was verified for predicting the discharge and the discharge distribution over the weir. In imperfect weir situations, submergence coefficient was introduced in emerged condition. Secondly, the dependence of the energy head loss caused by the weir on the upstream discharge and the downstream water depth was discussed in imperfect weir situations. The Form Drag model for estimating the energy head loss was proved to be applicable for the modeled breach flow. Thirdly, the local hydraulic characteristics of breach flow were described by means of velocity distribution, water level elevation and flow patterns appearing behind the weir. At last, the numerical model (Delft3D) was verified on the modeling of the present experiment by comparing with the experimental data. The report provides the information for the breach flow which can be of use in the development of breach models, inundation models and compound weir design.