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<br />POTENTIAL HYDROLOGIC STRUCTURES ~"O <br />MINIMIZE EFFECTS OF MINING METHODS <br />There are a number of measures that can be applied [o areas where in-stream aggregate extraction <br />is being conducted that will minimize potential effects. The following sections describe some of <br />the measures that can be used by themselves or in combination. <br />BUFFER ZONES <br />~ Low flow buffers may be established to protect the integrity of the channel shape, and to reduce <br />~ effects of mining on aquatic habitats. Figure D-1 shows a schematic of a low-flow buffer, and <br />+ Figure D-2 provides a plan view with sizing specifications. The upper limit of a low flow buffer <br />is the lesser of an elevation of one to two feet above the low summer flow elevation, or, the <br />elevation intersected by half the channel width from the low summer flow elevation at bankfull <br />conditions. <br />Undisturbed buffer zones can be established between the active channel and the material <br />~ processing and stockpile site to provide protection for the extracted resource during floods (Fiewre <br />D-3). This distance should take into consideration historic migration patterns while continuing <br />to provide the desired buffer. Operators can determine migration patterns from a review of annual <br />flows through the site and a comparison of aerial photos acquired over several decades. The <br />1 elevation of the material storage area should take into account the flood risk an operator is willing <br />to assume. The mining method, size of the river, channel configuration, type of vegetation, and <br />soil composition will influence the buffer width. <br />The CDMG recommends that dredging operations protect their stockpiles by locating them at a <br />minimum floodplain elevation above that of a 25-year 24-hour storm (a storm with a 4 percent <br />annual probability of occurrence). This will result in minimum horizontal distances from the bank <br />of 500 to 800 feet depending on the size of the river. Scraper operations should consider <br />placement of material stockpiles a[ an elevation equivalent to the 5-year 24-hour flood (a storm <br />with a 20 percent annual probability of occurrence), and buffer widths ranging from 100 to 150 <br />feet. Ldwer elevations or smaller buffer widths should plan for stockpile armoring or temporary <br />diking for flood stabilization (Figure D-4). <br />Braided rivers exhibit maximum lateral migration on an annual and seasonal basis. Lateral stability <br />improves when progressing through an evaluation of split channel rivers, meandering rivers, and <br />sinuous rivers. Not unexpectedly, straight rivers display the leas[ lateral migration. <br />Vegetation can provide substantial protection from lateral migration within a buffer zone. Deeply <br />rooted plants, or dense ground cover can reduce the potential for erosion in the buffer zone. Soil <br />composition also influences the erosion rate. Fine-grained materials such as clays and sands are <br />more erosive than coarse-grained gravels or cobbles. If the vegetation or soils vary within the <br />proposed buffer zone, the width of the buffer should change correspondingly. <br />r1.658~AC;mi ]. 1998 D-2 <br />