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In October 2004, a flood caused extensive damage to the University of Hawaii (UH) campus and neighboring residential areas in Manoa Valley, Oahu, Hawaii. 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 Hawaii 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 flood water 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 Manoa Watershed) that contains the university campus.

Hawaii’s climate is characterized by high rainfall rates. However, due to the high permeability of the rocks and soils, most streams do not flow continuously throughout the year (U.S. Army Corps of Engineers 1998). On the other hand, the steep slope nature of watersheds in Hawaii creates conditions of high peak flows with a sharp rise and recession, increasing the chance of flash floods occurring during storm events. More than 12 major floods have occurred in Manoa Valley, Oahu, causing damage and fatalities in some cases (HDLNR 1995). On October 30, 2004, the area received about 25 cm of rain in a 10-hour period. According to the National Weather Service Forecast Office, Honolulu, Hawaii (NWS 2006), the unstable atmosphere allowed showers to rapidly develop into a thunderstorm and remain focused over a small area of southeast Oahu (see http://www.prh.noaa.gov/hnl/pages/events/ ManoaFlood20041030/). The thunderstorm was locked in place due to the terrain. At the height of the heavy rainfall, around 7 pm, rainfall rates recorded by the gage at the Lyon Arboretum, in the upper portion of Manoa Valley, were over 12 cm/hr. Maximum rainfall accumulations at that site with the respective times were 3.3 cm (15 min), 9.4 cm (1 hr), 11.1 cm (2 hr), 14.5 cm (3 hr), and 22.1 cm (6 hr). These large rainfall rates are estimated to occur with a return rate of almost 50 years. In other words, in any given year, there is only a 2% probability of such a heavy rainfall event like this occurring in upper Manoa Valley.

The storm washed trees and debris into Manoa Stream, creating a dam under the Woodlawn bridge. Flood waters flowed onto the University of Hawaii (UH) campus (Figure), causing damage to buildings. Several Manoa Valley neighborhoods also sustained damage  

Simulated area for the 2004 Manoa flood and buildings damaged by the flood