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08%22/95 TUE :50 FAX WOODWARD CLYDE (21016 <br /> 1 <br /> veg tati n• The sch edule of maintenance will depend on factors such as stormwater influent <br /> l� <br /> q . `t d vegetation type. Sediment should be removed before the accumulated layer alters <br /> the l y' aulic characteristics of the basin, typically when the layer is 6 inches thick <br /> (W: gton Dept. of Ecology, 1992). The surrounding area should be mowed as often as <br /> n - o e o maintain aesthetics and access to the basin. _ These maintenance activities require <br /> s : d equipment such as mowers, trimmers, and 4x4 scrapers. <br /> St ' rna ater Filters. Stormwater filters include controls such as vegetated swales, vegetated <br /> fill r I 'ps, and sand filters. The primary benefit of filters is water quality improvement <br /> t • • u the filtration of pollutants, but they also may reduce peak runoff rates. Vegetated <br /> s ales and filter strips should be inspected three times a year and sand filters should be <br /> in petted at least twice annually. Accumulated sediment should be removed from vegetated <br /> fil a and strips before the filtration characteristics are significantly altered, typically when <br /> t of Ecology, 1992). For sand filters, sediment <br /> . • debris should be removed from the top Y <br /> th � Dyer is 6 inches deep (Washington Dept. <br /> the sand layer twice a year or as often as <br /> n- • ed (Portland, 1995). It is important not to damage the vegetation or sand layer of a filter. <br /> ii a maintenance may require manual equipment such as shovels and rakes or possibly vactor <br /> cks. Mowing and reseeding may also be necessary. Even with regular maintenance, the <br /> s • d layer of a sand filter will eventually need to be replaced as fines clog the pores and . <br /> r- uce its permeability. The replacement frequency will depend on the influent quality. <br /> S i ctural Storm Sewer Controls. Structural storm sewer controls include structures such <br /> . ' trapped catch basins, manhole sediment traps, oil /water separators, and underground <br /> • . tention tanks. These structures are generally used in urban areas to improve water quality. <br /> apped catch basins, manhole sediment traps, and detention tanks remove suspended <br /> s ent through settling. Oil /water separators remove buoyant pollutants from the <br /> o tflowing water with baffles or other separators. Structural controls should be inspected at <br /> 1 ast twice annually and more often if the influent has particularly high levels of oil and <br /> grease or suspended sediments (Portland, 1995). The primary maintenance activity is <br /> removing accumulated sediment and oil and cleaning the inlet and outlet to maintain flow <br /> Capacity. Vactor trucks are commonly used for cleaning structural controls, although manual <br /> ¶iuiPment may also be useful. <br /> Cost of Maintenance. The cost of maintaining a stormwater control system depends on the <br /> number and type of facilities involved. Snohomish County, Washington estimates that its <br /> annual cost for maintaining a single detention facility is $2000. In Kitsap County, <br /> Washington, maintaining 400 residential stormwater facilities is estimated to cost <br /> t pproxiI ately $400,00 It is important to note that the maintenance activitiado not occur <br /> hroughout the year on a continual basis. For example, dead vegetation should be removed <br /> from wet detention ponds prior to the rainy season. Similarly, mowing will generally need to <br /> be done only during the summer. <br /> Droj94/ 945 042nnhssk0200 /standud/mdnmc.doc 2 <br />