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<br />Slope of the Trench
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<br />6-lnch Sand Layer
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<br />Exfillration
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<br />Stonmoater rWlOfffrom low-density residential development call be managed fhrough the use of grassy
<br />swales and infiltration trenches. Swales are linear, grass-lined depressions (with a slope 110 greater than
<br />5 percent) that collect stOr/l1water, filter pollutants, and direct it towards trenches. Trenches help to
<br />recharge groundwater, slmv peak flows of stormwater, and reduce streambank erosioll.
<br />
<br />In coastal areas, NFIP criteria require that all structures
<br />be "located land\vard of the reach of mean high tide."
<br />North Carolina, among other states, has established
<br />setback requirements more stringent than the federal
<br />standard. Its four-tiered system relates the setback to the
<br />type and size of the structure and the annual erosion rate
<br />in the area. For example, residential structures must be set
<br />back beyond the 30-vear erosion line, and nonresidential
<br />structu;es must be s~t back beyond the 60-year erosion
<br />line (National Research Council 1990, 104-7).
<br />Delineating the appropriate boundary of a buffer
<br />zone or setback is part of the planning process to
<br />protect the river, stream, floodplain, or coastline. In
<br />riverine areas, the setback distance can be adjusted
<br />according to the particular site conditions, such as the
<br />presence of slopes and the location of natural
<br />drainage\vays. Seasonal water levels, the nature of
<br />the vegetation, and the wildlife values of adjacent
<br />
<br />32
<br />
<br />lands also need to be taken into account \vhen
<br />dra\,ring the setback line.
<br />In coastal areas, development setbacks are generally
<br />established by multiplying the annual erosion rate (in
<br />feet) by a certain number of years. Therefore a 3D-year
<br />erosion line setback is the point at which the coastline
<br />\\fill have moved inland after 30 years, given the
<br />existing rate of erosion.
<br />Drawing this line in the sand is not an easy matter for
<br />a local government. Tt \vill depend on what resources
<br />are being protected, the level of protection deemed
<br />necessary for those resources, the existing land uses
<br />adjacent to the water, and the legal and political
<br />feasibility of prohibiting future development in the
<br />zone. So while the erosion rate is established through
<br />scientific analysis, the process of establishing a
<br />development setback line is a policy decision that must
<br />weigh broad economic and political factors.
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