Modeling Streamflows and Flood Delineation of the 2004 Flood Disaster, Mānoa, O‘ahu, Hawai‘i1

Abstract

In October 2004 a flood caused extensive damage to the University of Hawai‘i (UH) campus and neighboring residential areas in Mānoa Valley, O‘ahu, Hawai‘i. This modeling study was aimed at streamflow evaluation and flood delineation for the area impacted by the flood. The study concluded that the HEC-1 model of the U.S. Army Corps of Engineers is suitable for simulating storm runoff response for the study area, considering the nature of small Hawai‘i watersheds, which generate hydrographs with steep rising and falling limbs. The curve-number method of the U.S. Soil Conservation Service is also suitable because it predicts reasonably well the main features of streamflow hydrographs, including runoff duration and time of peaks. To improve on accuracy, however, there is a need for better characterization of spatial rainfall distribution through measurements. A flood delineation model, which treats the flood as a hypothetical dam break, was used to predict the floodwater pathway, flood zone extent, maximum flood depth, and the time to reach that depth. The model predicted an upper value for storm total flow volume that would not cause flooding on the UH campus. Although not fully validated, the developed models can guide data-collection and decision-making processes. For example, the models demonstrated that it is possible to mitigate the flood through streamflow diversion and stream dredging, realignment, and lining. For efficient management, we recommend defining a new subwatershed of the Ala Wai basin (to be called the West Mānoa Watershed) that contains the university campus.