West Maui Watershed Study

Vitally important coral reefs in West Maui have declined by as much as 50% in the recent past, and have been targeted by Federal, State, and private entities for watershed planning efforts. The intent of the West Maui Watershed Study was to contribute to the restoration, enhancement and resiliency of West Maui coral reefs and nearshore waters through the reduction of land-based pollution threats. Sediment has been identified as one of the two major land-based pollutants in West Maui. This sediment is coming from a variety of sources, the largest contributor being the in-stream erosion of historic agricultural fill terraces within stream gulches. There are several dams and basins functioning as debris and flood risk reduction structures in West Maui that capture some sediment (to varying degrees of efficiency) as flood runoff travels to the coastal waters. Despite this, sediment plumes are often seen in West Maui after small and large storm events. Flow frequency relationships and corresponding sediment loads were roughly estimated for each watershed in the study area, supported by new field investigations and monitoring data. Several alternatives were developed to reduce the amount and frequency of riverine sediment being discharged into the ocean. The initial array of alternatives considered includes modification of the existing detention basins, use of a flocculant, flow diversion, manual excavation of highly erodible material from the stream banks, construction of lo‘i terraces, and construction of small detention basins. Ultimately, modification of the existing detention basins and construction of smaller detention basins throughout the study area had the greatest potential for efficient sediment capture.