Reducing sedimentation and water quality impacts in the tropics

摘要

Implementation of the United States Department of Agriculture Natural Resources Conservation Service PL-566 Small Watershed Program reduced floodwater and sediment damage in the flood plain and reduced turbidity of the ocean along the coast. Eight desilting basins and two lined channels were installed with financial and technical assistance through the watershed program. Land treatment measures consisted of soil conservation practices installed to reduce runoff and sediment production. The 24,800 acre Honolua watershed is located on the western end of the island of Maui, Hawaii. Land use includes forest, pineapple, sugarcane, grassland and the tourist resort area along the coast also known as the "gold coast." The runoff is very flashy due to theintense rainfall and the steep gradient of the watershed. The flashy nature of the runoff contributed to flooding, damage to residences and resort areas, loss of crops, and degradation of the marine life. Turbidity of the ocean created less than desirable water quality conditions for recreational activities. Planning for the Honolua watershed was authorized in July 1966. In March of 1976, the plan, Watershed Work Plan, Honolua Watershed was completed. Environmental studies identified archeological sitesand endangered plants and species. Construction of the first structure, Kahana Dam, began in July 1983. The final structures, Honokeana and Kaopala basins, were completed in July 1998. Two concrete lined channels with a total length of 3,000 feet, one 2,000 feet long reinforced concrete box culvert, four basins and three dams with a total storage of 239 acre feet were installed. Structures in the resort areas were "blended in" with surrounding infrastructures. Concrete channels were routed between, underground, and through the resort hotel area. One of the basins became a part of the resort hotel golf course hazard area. Conservation practices were installed on the undulating, rolling pineapple fields. Pineapples were planted on the contour to reducerunoff. Terraces were installed to reduce sedimentation. Twenty-eight infield sediment basins were constructed. Prior to the watershed project, the pineapple plants were disked then burned immediately after the last harvest. As part of the watershed project, cultural practices were changed to leave the plant residue on the soil surface and incorporation of the residue occurred just prior to the next planting. Screens were installed to trap plastic debris from the cultivation practices.