COM 0176.041 2006-2008 - Laserfiche WebLink

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DESCRIPTION OF STUDY AREA The island of Hawaii, designated by the U.S. Water Resources Council as Subregion I of the Hawaii Region, lies at the southeastern end of the Hawaiian archipelago, near the northern limit of the tropics. The study area encompasses approximately 137,600 acres in the South Kona district of the island of Hawaii (see map). Located on the western slopes of Mauna Loa, the study area consists of a series of narrow drainage areas with underdeveloped watercourses that drain into the Pacific Ocean. Twenty-five such watercourses were identified as having flood hazard potential. As development and clearing occur in both agriculture and urban lands, other watercourses may also become problem areas. Because the topography is steep, existing undefined watercourses could easily be diverted to form new watercourses creating hazard areas not identified in this study. All watercourses within the study area are intermittent. Kiilae Stream, which has the longest sustained flow, goes dry during the winter months. Much of the flood flows percolate into lava formations and reach the ocean as surface flow only after periods of intense and sustained rainfall. The drainage azea above 6,000 feet elevation appears not to contribute to surface runoff. This area includes nearly barren lava, volcanic ash, pumice, and cinders. The drainage azea is blocked from the tradewinds by the mountain masses of Mauna Loa, Mauna Kea, and Hualalai. As a result the pre- v~iling wind circulation is an alternating land-sea system resulting from the differential heating of the land and water masses. During the day, the land heats faster than the ocean resulting in a sea breeze; during the night, the land cools faster than the ocean resulting in a land breeze. Rainfall is mostly produced by the daytime sea breeze that push moist air over the land mass. This moist air yields regular, and sometimes heavy, showers as it ascends the steep mountain Slopes. These showers are usually of short duration and high intensity. Seasonal distribution of the rainfall is unique in this area. Summer months are wet while the winter months are dry. This is con- trary to distribution in other parts of the state. This u~hique pattern is largely due to greater intensity of the onshore breeze during the summer months.