Stories From The Field

Modeling Sediment Yield in Hawaii

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Issue

Water quality and the coral reef ecosystem in Pelekane Bay on the island of Hawaii are threatened by high sediment yield from the adjacent watershed. Large-scale land use changes increased the rates of erosion over the last two centuries, and efforts to decrease sediment yield from the watershed through alternative land use management practices are necessary to protect the health of Pelekane Bay.

Process

The Nonpoint Source Pollution and Erosion Comparison Tool (N-SPECT) was used to help local planners obtain data about the origin and volume of sediment transported to Pelekane Bay. Using information such as rainfall, terrain characteristics, soil type, and land cover, the tool estimated average annual sediment yield and sediment yield from historical storms in the Pelekane Bay watershed. Several different historical storms were modeled during the analysis. Results indicated that the largest storm (3.99 inches of rain) caused exponentially more erosion than the smallest storm (0.67 inches of rain) and that most of the sediment was generated in areas of moderate to steep slope with sparse to no vegetation cover. An analysis of smaller areas within the watershed was also employed to better show rainfall intensity in specific areas and runoff characteristics across the watershed.

Impact

The results of this sediment yield assessment were included in a final project report, and they provide the Mauna Kea Soil and Water Conservation District and other stakeholders with important baseline data about where the sediment comes from and how much sediment is transported to the bay. This information helped determine effective watershed management practices that would reduce sediment yields from the most significant sediment sources.

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Graphic representation of high erosion areas (dark brown) within the Pelekane Bay watershed for a 1-year, 24-hour storm event

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