Flood Discharge Changes by Urbanization from Farmland Based on the Unit Flood Concept at the Kurabe River, Japan

Flood runoff models of urbanization from farmland based on the physical characteristics of a basin have been minimally used in previous research until today. Consequently, the runoff analysis has not been performed that is based on physical basis. Therefore, this research undertook flood discharge analysis from urbanization using the unit flood discharge concept that is enhanced the previous research. The study area was selected at the Kurabe River basin, which is 17.5 km² in area having a very steep landscape. Twenty-one rainfall events at 10-minute intervals were selected, and five urbanized years were tested. From 1976 to 2009 during 35 years, the flood discharge increased approximately 2.0 times, in which residential areas increased from 23% to 48%; the maximum specific discharge was 21.7 m³·s^-1·km^-2 in a some block, which is a remarkably large amount. Furthermore, following issues investigated: changes in the hydrograph were associated with urbanization, the effect of a small reservoir aiming to cut down the peak discharge and the relationship between the unit discharge, and the relationship between our method and the discharge estimated by a “Rational Formula”. In particular, the effect of the small reservoir for flood control was found to be remarkably efficient. Finally, the validity of our method was confirmed at the study area in the observed discharge. This result is very useful for estimating runoff discharge changes by urbanization from farmland.