Hydrology and sediment transport, Moanalua Valley, Oahu, Hawaii

Abstract

The first 2 years of intensive data collection in Moanalua Valley have resulted in some observations concerning the rainfall- runoff and rainfall-sedimentation characteristics of the basin. This initial study period has been concerned primarily with establishing a reliable hydrologic data-collection network. However, enough data have been collected to determine that rainfall within the valley shows extreme areal variations from one storm to another. An initial run on the U.S. Geological Survey small watershed model indicates some differences in basin characteristics in the two upper subregions. Results of model studies to date are encouraging. Flood routing with the Corps of Engineers' HEC-1 flood hydrograph package seems to be feasible. By preliminary estimates, the 50-year flood at gage 2282 would probably be between 5,500 and 6,000 cubic feet per second. The narrow, steep nature of Moanalua Valley results in very rapid concentration of runoff, with extremely sharp hydrograph peaks. Stream channel width ranges from about 20 feet in the headwaters to 35 feet near the downstream limit of the study area. Mean slope of the main stem is about 0.027 foot/foot.1/ Where not identified, units for slope are ft/ft. Observations indicate that the stream transports very large particles (in excess of 2 feet in diameter) quite frequently. Detached fragments from old stone-arch bridges have been marked in the channel fill for use as tracers. Only one storm has been analyzed for suspended-sediment discharge, but rough calculations indicate a possible suspended-sediment load of 12,000 tons for a storm similar to the design flow at gage 2282. This compares with a computed bedload discharge of 2,600 tons for the same flow. Accumulation in the debris basin between October 2, 1969 and July 26, 1971, as measured by level survey, was 48,000 cubic feet or about 2,900 tons.