Variation of Critical Rainfall Duration upon Its Magnitude in Middle and Lower Yom Basin, Thailand

M. Lu; S. Chotpantarat; P. Klongvessa
In many flood simulations, critical rainfall duration, defined as duration of rainfall which causes a critical flood, is assumed to be fixed. However, in some area, the critical rainfall duration varies upon its magnitude and assuming the fixed critical rainfall duration may underestimate an extreme peak discharge. The middle and lower Yom basin, Thailand, is one of the areas where a larger flood appears to coincide with longer critical rainfall duration. In this study, we have investigated the variation of critical rainfall duration in that area. The conceptual model was developed from HEC-HMS and HEC-RAS models. The upstream and downstream parts of the area were calibrated separately. Then, uniform 2-, 5-, and 10-year rainfalls with 24-, 48-, and 72-hour durations were simulated. The model successfully reproduced the variation of critical rainfall duration as 48 hours for 2- and 5-year rainfalls and 72 hours for 10-year rainfall. This variation is caused by a high percolation rate over the upstream area. With this percolation, some portion of rainfall over the upstream part is loss. Hence, most of the flood water comes from the downstream part which has short travel time. However, when the rainfall is larger, the influence of percolation is less.
Critical Rainfall Duration; Flood; Yom Basin; Percolation
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