Technical Memorandum Report No. 58
Estimating Peak Discharges in Small Urban Hawaiian Watersheds for Selected Rainfall Frequencies, Kane’ohe Watershed, O’ahu, Hawai’i
Nancy C. Lopez and Gordon L. Dugan
Since its establishment in the spring of 1971, the Hawaii Environmental Simulation Laboratory (HESL) has attempted to simulate some of the consequences of alternative land use-economic decisions. The Kaneohe region on O’ahu Island, Hawaii was selected as a study area. Flooding, which has historically created hazard areas in Kaneohe, appears to be significantly altered by the rapid urbanization of the region. The problem of predicting flooding patterns in the Kaneohe region, as well as Hawaii in general, is complicated by the rather small area of the individual watersheds, the abrupt changes in terrain, and the short times of concentration, generally less than 1 hr. Using existing technology, a planning-oriented tool for predicting peak discharges resulting from various patterns of urbanization has been developed by HESL for Hawaiian conditions. The tool utilizes the U.S. Soil Conservation Service Runoff Curves, a unique time of concentration formula, and the U.S. Weather Bureau Rainfall-Frequency Atlas of the Hawaiian Islands. Areas within the watershed were segregated by ranges of slope into response zones. Input data include soil class and cover, hydraulic length, and average slope. The model was applied to ten individual watersheds within the Kaneohe region, and estimates of peak discharge for the watersheds were made for selected rainfall return intervals. Peak discharge values were determined for existing land use and for three different scenarios (hypothetical patterns of urban growth) for the year 1995. Tests of the model using the rather limited existing peak discharge records have been very encouraging.