Flood Control Structure
Construction of the original Kawainui Marsh Flood Control Project was authorized by the Flood Control Act of 1950 and completed in August 1966 [Editors Note: this date from the ACOE may be for completion of the entire project; the channel is seen under construction in aerial photographs from the mid-1950s and is believed to have been completed by 1956]. Project features include a 9,470-foot-long trapezoidal channel (Oneawa Canal); a 6,850-foot-long earth levee with a maximum crest elevation of 9.5 feet; a 50-foot-long stub groin and a 50-foot-long revetment at the outlet of Oneawa Channel; a 370-foot-long and 10-foot-deep silt basin; and drainage outlets.1
Kawainui Marsh functions as a flood storage basin to protect Kailua. Over the years, vegetation within the marsh has created a dense mat which has affected the hydraulics of the marsh. On December 31, 1987 through January 1, 1988, the water level in the marsh exceeded the crest of the existing levee due to changed conditions in the marsh and the community of Coconut Grove sustained severe flood damages. Following the 1988 New Year's storm, an emergency ditch was excavated alongside the levee to increase outflow from the marsh. However, the hydraulic capacity of the ditch (1,700 cfs) and Oneawa Channel (6,750 cfs) was much smaller than the 100-year flow (21,300 cfs) into the marsh.1
As a result of the 1988 flooding,the Corps and the City and County of Honolulu modified the project under the authority of Section 205 of the Flood Control Act of 1948, as amended. The modifications were completed in 1997 and include a concrete floodwall on a raised levee. The levee was raised up to 4-1/2 feet and the floodwall averages 4 feet in height. The total structure raise is about 6,300 feet long and varies from 8-1/2 feet at Kailua Road on the south to a gradual tapering into the existing levee at the Oneawa outlet channel on the north. A flood warning system is also included in the project. The flood warning system consists of two real time reporting stations. The stations monitor rainfall and stream level in the Makawao Stream area of Maunawili Valley and monitor marsh water elevation near the levee.1
Oneawa Channel is tidally influenced and effectively blocked from intruding on the marsh by a weir of vegetation. The US Geological Service (USGS) maintains two WL gages at the upper end of the channel [view tide at Oneawa Street Bridge]. In all, USGS monitors water levels at three stations along the levee and Oneawa Channel with data displayed on the internet: to view go to USGS index page and choose a Kawai Nui or "Kawainui Canal" station. Realize that the water level elevations are not correct for at least two of the instruments. Swain and Huxel (1971) showed that the water in the marsh normally remains higher than high tide levels in the canal, maintaining a head against saltwater intrusion. This being the case, it is unlikely that Oneawa Channel exercises any tidal influence over water height in the marsh or that salinity intrusion is very great.2 During the wet season, outflow from the marsh can be seen and heard as numerous small waterfalls flowing over the vegetation mat into the drainage channel inside of and running the length of the levee. Penetration of salt water into the marsh beyond Oneawa Channel seems not to occur, even during prolonged dry periods.
A short distance upstream from the point where Oneawa Channel turns behind the flood-control levee there occurs a linear series of small islets constructed at the time of levee improvements in the 1990's as mitigation for wetlands lost when the levee structure was enlarged. These islets are made of material dredged from the channel (mostly coral rubble). Unfortunately, the original design (shape and elevation) of the islets did not result in their becoming wetland habitat. As shown in the photograph, the ebbing tide leaves a steep shore, and the flat surface at an elevation above the reach of the tide simply promotes their being covered with non-aquatic vegetation. These features make the islets unattractive to wetland birds.
"Mitigation islets" at the upper end of Oneawa Channel as seen at a low tide from the levee. Note the steep, bare banks, except for an area of bulrush on the right. Vegetation on the islets typically consists of a narrow rim of seashore paspalum grass (Paspalum vaginatum) and dense cover of sourbush (Pluchea indica).
See Honolulu Advertiser article.
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2 (modified from) Linda L. Smith. 1978. Development of emergent vegetation in a tropical marsh. MS Thesis, U.H., Botany. p. 19-20.
Swain, L.A. and C.J. Huxel. 1971. Relation of drainage problems to high ground-water levels, Coconut Grove area, Oahu, Hawaii. U.S. Geological Survey, Honolulu.