Underestimation of flood quantiles from parallel drainage areas

A drainage area is often divided into sub-areas for estimating the area's time of concentration. If runoff from the sub-areas concentrates in parallel to the drainage outlet, the time of concentration of the drainage area is usually estimated as the longest time of travel through the individual sub-areas. Findings reported here show that this conventionally used method of estimating time of concentration for parallel drainage areas is inappropriate and may result in underestimation of flood quantiles. A more suitable method of estimating the time of concentration for areas comprised of parallel sub-areas is to use a runoff-weighted average of the times of travel through the parallel sub-areas. The runoff weight of each sub-area can be calculated as the product of its runoff coefficient and area. This proposed method was tested and shown to provide more accurate design flood estimates. Also compared in this study are design flood estimates obtained through continuous simulation and design storm modelling.