- March 31st, 2010
- in Yachting
Perhaps the greatest assets these islands have to offer are their crystal clear waters and abundant reef life. However, these assets are under threat from the impacts of human development.
Whenever we make a cut in the hillside, we disturb the natural systems which protect our waters from the harmful effects of erosion and sedimentation. Erosion is the process where soil and rock are loosened and removed from their original position, primarily by the affects of rainfall and wind. Surface rainwater runoff carries away sediment and deposits it in ghuts andwaterways that lead to bays, harbours and coral reefs.
Without human influence, the rate of erosion is slow enough for nature to compensate for the adverse affects. But human activities dramatically accelerate the erosion process until nature struggles to absorb the negative impacts: terrestrial and aquatic habitats are smothered, degraded and ultimately destroyed. Look down at many of the bays of the BVI after a heavy rainstorm, and you’ll see the usually clear water turned brown and cloudy. Drive along the coast road, and you’re likely to see a heavy flow of brown runoff crossing the road as you pass through each bay.
Sediment runoff in Brewers Bay. Photo courtesy of OBMI.
Sediment eroded from dirt roads, construction sites and other cleared areas is by far the most significant pollutant responsible for harming water quality in the BVI. All of us involved in development activity are therefore responsible for minimising, controlling and managing erosion and soil loss in order to protect our coastal waters. Although most of the damage is caused by road building, every construction site has the potential to have a harmful impact, so all individual home builders should be mindful of these issues and should ensure that their design and construction team employ appropriate mitigation measures set out in an effective Erosion and Sedimentation Control (ESC) plan.
Sediment control is not as effective as erosion control, because it does not remove most clay particles (most of the soils in the BVI are clay), which can pass through silt fence material and can take a very long time to settle out of runoff in holding ponds. It’s much easier and more cost-effective to keep the soil in place than it is to attempt to remove soil from stormwater, and the best way to keep soil in place is to preserve existing vegetation and replant cleared or bare areas as soon as possible.
An effective ESC plan should:
• Minimise the disturbance and clearance of vegetation.
• Prevent upper watershed rainfall runoff from flowing across bare soils, using perimeter dikes, swales and other diversions.
• Minimise quantities of loose soil, and stabilise bare soils on the site, using erosion control matting, temporary seeding, mulching and planting.
• Remove sediment from runoff before it leaves the site, using stabilized site road entrances, silt fences, sediment traps, gravel or stone berms and check dams.
• Plan construction so that, if possible, soil disturbance activities take place during the dry season.
After construction has been completed, the site should be permanently stabilized and landscaped. Retaining structures should be built where slopes are too steep for planting. Pervious surfaces can be used in low-traffic areas such as parking areas and walkways to encourage absorption of rainwater, and driveways can be built as tyre-strips with a grassed central strip. Rain gardens, known as bioretention systems, can be used in drainage areas on shallow slopes to absorb excess stormwater and filter sediment. These require good loamy planting soils to ensure adequate absorption and filtration.
Mimimising erosion and controlling the transport of sediment is not difficult to achieve. What is required is a firm commitment to the issue, an understanding of the basic principles and proper thought and planning to prioritise efforts which are too easily overlooked or forgotten. As with most aspects of “green” design and construction, the end results will be beautiful, natural-looking, cost effective and durable.