Saturday, 15 July 2006

Using Vetiver Technology to Mitigate Sediment Transport for Erosion Control and Water Quality Improvement at a Typical Watershed in Southern Guam.

Mohammad H. Golabi, Univ of Guam, College of Natural & Applied Sciences, 5395 UOG Station, Mangilao, 96923, Guam

Sedimentation as a result of runoff is the principal human-caused threat to the environment in general and to water quality in particular on the Pacific island of Guam. Runoff takes the form of flash floods of high velocity but short duration. The rapid flow is attributed to low soil infiltration, a high proportion of rain converted to overland flow, and scanty or absent vegetation cover due to wildfires. In the areas where protective vegetation cover is lowest, the soil is subjected to high shearing forces by such overland flow. Erosion damage is a serious problem to the environmental ecosystem of the island. Sediment lost to erosion clogs rivers, lakes, and waterways. Erosion and sedimentation loss are also a major source of water-quality problems in Guam. Sedimentation provides a vehicle for the transport of agricultural chemical residues into canals, streams, rivers, and eventually near-shore ecosystems, where it damages coral reefs. The objective of the project reported here was to assess the sediment-loading rate to the near-shore coral reef originating from the upland watershed. The effectiveness of vetiver grass as a sediment trap and its effect on quality of the water leaving the upland watershed were evaluated. Four plots (each 72 by 5.5 ft) were laid out on a uniformly sloped (12%) watershed for estimation of sedimentation rates. Each plot was equipped with an 8 inch high flume wall that separated its surface from those of the other plots and their surroundings. Flumes were equipped with cone-shaped weirs that directed the runoff and sediments into a collecting tank beneath the weirs.

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