Identifying peak-imperviousness-recurrence relationships on a growing-impervious watershed, Taiwan

This study focuses mainly on increases in peak discharges with reductions in recurrence intervals for design hydrographs due to growing imperviousness. Rainfall–runoff simulation is a major basis for analyzing the hydrological effects of urbanization. Available recordings of 50 rainfall–runoff events during 1966–2002 were used as the study sample. Forty events were calibrated to determine the relationships between impervious areas and hydrological parameters in the Nash and SCS models. Block Kriging and non-linear programming methods were used to estimate the mean rainfall and its hourly excess value, respectively. The remaining 10 cases were used to test the established relationships. Calibration and verification results confirm that the methods used in this study effectively illustrate the hydrological and geomorphic conditions in complex urbanization processes. The rainfall–runoff routings, by using the design storm approach and the established relationships, demonstrated the following: (1) peak flows of the design flood hydrographs increased by about 127, 266, 375, 440, 515, 564, 593, and 629 m3/s for eight return periods, respectively; (2) peak times were individually shortened from 9 to 6 h due to urbanization; (3) the recurrence intervals of 200, 100, 50, and 25 years before urbanization were reduced to about 88, 33, 16, and 8 years also due to urbanization if they would have occurred at the present time within the Wu–Tu watershed. Finally, these analytical results can be obtained easily from the designed diagram that shows the relationships among peak discharges, impervious areas and return periods. This research can be used to prevent disasters, loss of life and property damage.