Technical Report No. 64
Studies conducted during Phase I of the “Flood Hydrology and Urban Water Resources of the Island of Oahu, Hawaii” project, were divided into four major tasks. The first is an examination of the causes of flooding and flood damages on Oahu. This task has been completed and data obtained reveals that historical floods in Oahu happened most frequently within the city limits of Honolulu and the Waialua area. Watersheds in Honolulu such as Kalihi, Manoa, Makiki, Moanalua, Nuuanu, Palolo and Pauoani were found to have frequent floods up to the 1940’s; however, they have not been subjected to severe frequent floods for the past two decades. Watersheds such as Kailua, Kaneohe, and Waimanalo in Windward Oahu and Aina Haina, Niu Valley, and Pearl City on the fringes of the city limits of Honolulu were reported to have more frequent floods in the past two decades. Some of the reason for the patterns of flooding are: 1.The Honolulu district has the highest concentration of people and housing on the island. 2.The Waialua area is located downstream of many large streams of the island. 3.Kalihi., Manoa, Makiki, Moanalua watersheds have been urbanized in the early part of the century; therefore, the flood prevention system in these watersheds have been improved over these years. The second task was to evaluate the effect of urbanization upon flood hydrographs from selected watersheds in Oahu. In general, the effect of progress of urbanization on a watershed is evident in the increase in the flood peak and reduction in the time to peak. However, it is felt that this finding cannot be generalized. More hydrological data is needed to substantiate, such a generalization. The third task was to establish the rainfall-runoff data collection devices in the adjoined urban (St. Louis Heights) and natural (Waahila) watersheds. The installation and operation of two auto-recording raingages and two auto-recording water stage recorders were completed. These rainfall and runoff recording instruments will provide the required data for the hydrologic simulation modeling studies and for the evaluation of the effect of urbanization upon watershed hydrology. The fourth task was to initiate the studies of watershed simulation models. Thus far, efforts have been concentrated on two watershed simulation models. one model utilizes an auto-optimal technique in determining the instantaneous unit hydrograph. The other model is a modification of the Kentucky Watershed Model to better fit Hawaiian conditions. Additional modifications are required before a good fit is achieved by both models.