Pacific Islands Water Science Center
Waikele Stream at Waipahu, Oahu
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Suspended-Sediment Monitoring, Waikele Watershed, Oahu
Project Chief: Scot K. Izuka
Channel cross-section surveying on Waikele Stream
Runoff from areas in the Waikele watershed, Oahu, Hawaii yields sediment to Waikele Stream and Pearl Harbor. Both water bodies are listed as impaired and the State of Hawaii Department of Health (DOH) plans to establish a suspended-sediment Total Maximum Daily Load (TMDL). To ensure that the allocation of TMDL sediment loadings is in accord with the actual proportions of sediment contributed by individual land users, the Department of Environmental Services (DES) of the City and County of Honolulu seeks an independent assessment of the relative contribution of sediment from all of the major land-use types (forested conservation, urban, military, and agriculture) within the watershed.
The objective of this study is to determine the total suspended-sediment load for Waikele Stream and the proportion of the load contributed by the following categories of land use: (1) urban residential areas under the control of the City and County of Honolulu, (2) forested conservation, (3) military, and (4) agriculture.
Relevance and Benefits
The study responds directly to the concerns of the City and County of Honolulu, yet provides information that is useful to anyone concerned with the water quality of Waikele Stream and Pearl Harbor. By providing information that is needed for the accurate assessment of the sources of sediment that are contributing to the impairment of Waikele Stream and Pearl Harbor, the study will help the local and State government comply with the mandates of the Federal Clean Water Act. The study is, therefore, consistent with the USGS mission to serve the Nation by providing reliable scientific information to manage natural resources, and enhance and protect our quality of life.
The objectives of this study will be met by (1) using stream discharge and sediment gaging stations to monitor suspended-sediment loads at selected locations along Waikele Stream and its tributaries and (2) using the difference in annual measurements of stream-channel cross sections to determine changes in sediment storage. Data collection and monitoring will be conducted over a three-year period, October 1, 2007 to September 30, 2010. The study and its results will be published in the USGS Scientific Investigations Report series and made available on the internet.
Real-time streamflow and suspended-sediment monitoring stations were operated at four sites (Waikele Stream at Wheeler Field, Mililani Storm Drain A at Mililani, Kipapa Stream near Wahiawa, and Waikele Stream at Waipahu) in 2007-2010. Twenty eight stream-channel cross sections in seven different stream reaches were surveyed between 2007 and 2010. The selected stream reaches were located throughout the Waikele watershed between Wheeler Air Force Base and Waipahu. Initial surveys of the channel cross sections were made in 2007. The cross sections were resurveyed annually. Data were analyzed from the streamflow/sediment gages and the channel cross sections to determine the sediment budgets from the drainage areas encompassing the different land uses in the watershed.
Izuka, S.K., 2012, Sources of suspended sediment in the Waikele watershed, O'ahu, Hawai'i: U.S. Geological Survey Scientific Investigations Report 2012-5085, 28 p.