Improved flood estimation model for bridge and culvert design in Zimbabwe

Abstract

Economics and risk minimization in the design of infrastructure that is vulnerable to flood damage often prove to be non-commensurate objectives. To economically minimize the risk of hydraulic failure of this type of infrastructure is a noble design objective. The hydraulic design of bridges and culverts in Zimbabwe is currently hinged on empirical algorithms of flood estimation. In a changing climate and changing land use environment, the current flood estimation method seems to under-estimate the design floods as evidenced by the increasing number of bridges and culverts being overtopped by floods. This paper discusses the several shortcomings of the current method in use and proposes a new consistent but robust computerized method of flood routing based on historical flow data and statistical analysis techniques. The methodological praxis and resultant design software are intended to substantively improve the flood estimation process, producing a more precisely estimated design flood. From the results obtained, the proposed new model is more conservative than the current method used by the Ministry of Transport (MoT) by an average factor of 1.4. At ungauged sites, or at sites where the stream flow data is inadequate in quantity and quality for Flood Frequency Analysis (FFA), it is recommended that a factor of safety of 1.4 be applied to MoT current flood estimates.