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1 <br /> 4.1 Source Control BMPs <br /> BMP C101: Preserving Natural Vegetation <br /> Purpose The purpose of preserving natural vegetation is to reduce erosion wherever <br /> practicable. Limiting site disturbance is the single most effective method <br /> for reducing erosion. For example, conifers can hold up to about 50 <br /> percent of all rain that falls during a storm. Up to 20-30 percent of this rain <br /> may never reach the ground but is taken up by the tree or evaporates. <br /> Another benefit is that the rain held in the tree can be released slowly to the <br /> ground after the storm. <br /> rConditions of Use • Natural vegetation should be preserved on steep slopes,near <br /> perennial and intermittent watercourses or swales, and on building <br /> sites in wooded areas. <br /> • As required by local governments. <br /> 1 Design and Installation Natural vegetation can be preserved in natural clumps or as individual <br /> trees, shrubs and vines. <br /> Specifications <br /> The preservation of individual plants is more difficult because heavy <br /> equipment is generally used to remove unwanted vegetation. The points <br /> to remember when attempting to save individual plants are: <br /> • Is the plant worth saving? Consider the location, species, size, age, <br /> vigor, and the work involved. Local governments may also have <br /> ordinances to save natural vegetation and trees. <br /> ' Fence or clearly mark areas around trees that are to be saved. It is <br /> preferable to keep ground disturbance away from the trees at least as <br /> far out as the dripline. <br /> Plants need protection from three kinds of injuries: <br /> • Construction Equipment- This injury can be above or below the <br /> ground level. Damage results from scarring, cutting of roots, and <br /> compaction of the soil. Placing a fenced buffer zone around plants to <br /> be saved prior to construction can prevent construction equipment <br /> injuries. <br /> • Grade Changes - Changing the natural ground level will alter grades, <br /> which affects the plant's ability to obtain the necessary air, water, and <br /> minerals. Minor fills usually do not cause problems although <br /> sensitivity between species does vary and should be checked. Trees <br /> can tolerate fill of 6 inches or less. For shrubs and other plants, the fill <br /> should be less. <br /> ' When there are major changes in grade, it may become necessary to <br /> supply air to the roots of plants. This can be done by placing a layer of <br /> gravel and a tile system over the roots before the fill is made. A tile <br /> 4-2 Volume ll— Construction Stormwater Pollution Prevention February 2005 <br />