Sources of suspended sediment in the Waikele watershed, Oʻahu, Hawaiʻi

Scientific Investigations Report 2012-5085
Prepared in cooperation with the City and County of Honolulu Department of Environmental Services



Data from streamflow/sediment gages and measurements of changes in channel-bed sediment storage were gathered between October 1, 2007, and September 30, 2010, to assess the sources of suspended sediment in the Waikele watershed, Oʻahu, Hawaiʻi. Streamflow from the watershed averaged 33 cubic feet per second during the study period, with interannual variations corresponding with variations in the frequency and magnitude of storm-flow peaks. Average streamflow during the study period was lower than the long-term average, but the study period included a storm on December 11, 2008, that caused record-high streamflows in parts of the watershed. Suspended-sediment yield from the Waikele watershed during the study period averaged 82,500 tons per year, which is 2.7 times higher than the long-term average. More than 90 percent of the yield during the study period was discharged during the December 11, 2008, storm. The study-period results are consistent with long-term records that show that the vast majority of suspended-sediment transport occurs during a few large storms. Results of this study also show that all but a small percentage of the suspended-sediment yield came from hillslopes. Only a small fraction of bed sediments is fine enough to be transported as suspended load; most bed sediments in the watershed are coarse. Silt and clay constitute less than 3 percent of the bed-sediment volume on average. Some larger clasts, however, can disintegrate during transport and contribute to the suspended load downstream. During the study period, suspended-sediment yield from the urbanized Mililani subbasin averaged 25 tons per year per square mile (tons/yr/mi2), which was much smaller than the yield from any other subbasin; these results indicate that urban land use yields much less sediment than other land uses. The wet, forested Kipapa subbasin had an average normalized hillslope suspended-sediment yield of 386 tons/yr/mi2; the average yield for forested areas in the watershed may be lower. Suspended-sediment yield from agricultural land use in the watershed is estimated to range between 5,590 and 6,440 tons/yr/mi2 during the study period; the long-term average is estimated to be 2,070 to 2,390 tons/yr/mi2. Of the three land uses considered, agriculture had by far the highest normalized suspended-sediment yield during this study - about an order of magnitude higher than forests and two orders of magnitude higher than urban areas.

Study Area

Publication type Report
Publication Subtype USGS Numbered Series
Title Sources of suspended sediment in the Waikele watershed, Oʻahu, Hawaiʻi
Series title Scientific Investigations Report
Series number 2012-5085
DOI 10.3133/sir20125085
Year Published 2012
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Pacific Islands Water Science Center
Description x, 28 p.
Time Range Start 2007-10-01
Time Range End 2010-09-30
Country United States
State Hawai'i
Other Geospatial OʻAhu;Waikele Watershed
Datum North American Datum of 1983
Projection Universal Transverse Mercator projection, Zone 4
Online Only (Y/N) Y
Google Analytic Metrics Metrics page
Additional publication details