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Pacific Islands Water Science Center
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Flood Frequency Estimates for the State of Hawaii (StreamStats)
This project proposes to implement StreamStats for flood-frequency estimates in Hawaii and will provide a user-friendly tool for estimating peak-flow magnitudes using regional-regression equations that are being developed as part of a separate ongoing flood-frequency project. Reliable estimates of peak-flow magnitudes are needed for the safe and efficient design of roads, bridges, water-conveyance structures, and flood control projects, and management of flood plains and flood-prone areas. Reliable flood-frequency estimates are critically important for the preservation of human lives and property, and are needed by federal, state, and county agencies. Some of the existing flood-frequency studies for the State of Hawaii are about 30 years old. About 30 years of additional peak-flow data are now available to update flood-frequency studies for the islands of Kauai, Maui, Molokai, and Maui, and more than 10 years of additional data are available to update the most recent flood-frequency study for Oahu. The USGS is currently developing updated regional-regression equations to provide improved estimates of the magnitudes of the 2-, 5-, 10-, 25-, 50-, 100-, and 500-year floods in Hawaii.
The objective of this study is to develop a tool that (1) readily and accurately quantifies drainage-basin characteristics needed as input to regional flood-frequency regression equations, and (2) can be used to estimate the magnitudes of the 2-, 5-, 10-, 25-, 50-, 100-, and 500-year floods in a manner consistent with the assumptions of the regional-regression equations.
Relevance and Benefits
This study is consistent with the mission of the USGS Strategic Plan to provide scientific information to manage water resources and to protect our quality of life. This study is also consistent with the need to increase emphasis on understanding hydrologic hazards, which is a priority water-resource issue identified by the USGS Water Resources Discipline. This study will generate improved peak-flow magnitude and frequency estimates at gaged sites that can be used to estimate flood frequencies at ungaged sites. Geographic information system (GIS) tools and new statistical techniques that were unavailable 30 years ago will be used in this study.
Available annual peak-discharge data from both continuous and crest-stage gaging stations in Hawaii will be used to estimate peak-flow magnitude and frequency at these gaged sites. Equations will be developed, using generalized least squares regression techniques, relating the 2-, 5-, 10-, 25-, 50-, 100-, and 500-year peak discharges to significant basin characteristics and these equations can be used to estimate peak discharges at ungaged sites on unregulated streams. Furthermore, simplified equations relating the 2-, 5-, 10-, 25-, 50-, 100-, and 500-year peak discharges to drainage-basin area will be developed, and these estimates can be compared to equations and nomographs that previously have been developed for use in the State.
The regional-regression equations for Hawaii will use drainage-basin characteristics as independent variables to estimate peak flows. Unfortunately, drainage-basin characteristics are not readily quantified without sophisticated GIS tools. Furthermore, the method of quantifying drainage-basin characteristics may not be readily replicated by potential users of the equations. Thus, a tool is needed to (1) quantify drainage-basin characteristics readily and accurately, (2) estimate the magnitudes of peak flows in a manner consistent with the assumptions of the regional-regression equations, and (3) generate consistent and reproducible flood-magnitude estimates among different users.
To meet the objective of this study, GIS datasets and regional-regression equations developed by the USGS will be incorporated into StreamStats and made readily available to the public.
Work began on this study in April 2002 and since then we have delineated drainage basins and quantified drainage-basin characteristics, checked the existing peak-flow data for accuracy and usability, estimated the generalized skew for the state, and estimated the magnitude and frequency of peak discharges at gaged sites.
Oki, D.S., Rosa, S.N., and Yeung, C.W., 2010, Flood-frequency estimates for streams on Kaua'i, O'ahu, Moloka'i, Maui, and Hawai'i, State of Hawai'i: U.S. Geological Survey Scientific Investigations Report 2010-5035, 121 p.
Rosa, S.N., and Oki, D.S., 2010, Hawaii StreamStats: A web application for defining drainage-basin characteristics and estimating peak-streamflow statistics: U.S. Geological Survey Fact Sheet 2010-3052, 4 p.
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