\nIn recent years, the push towards more sustainable developments and\u00a0environmental initiatives such as “LEED”, have forced engineers to look for cost effective, innovative solutions\u00a0to achieve stringent stormwater quality and quantity targets, and to reduce all storm flows to the municipal storm systems by retaining or re-using stormwater on-site.<\/p>\n
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The customary approach to stormwater management\u00a0has been to\u00a0store excess runoff on sites within parking lots, rooftops or the underground\u00a0storm system. The quality component in most instances\u00a0was typically provided by an oil grit separator installed at the outlet from the storm system prior to discharging to the municipal system.<\/p>\n
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Recently, an increasing number of municipalities and conservation authorities\u00a0have imposed a water balance requirement\u00a0to their stormwater management\u00a0objectives.\u00a0This new requirement typically requires that a minimum of 5mm of stormwater over the entire site area be retained on site and “used”, infiltrated, or evaporated\u00a0within a 72 hour time frame.\u00a0On a\u00a0one hectare site a minimum of 50 cubic meters of water is required to be retained on the site and either used, infiltrated,\u00a0or evaporated within\u00a0a 72 hour timeframe.<\/p>\n
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Some methods of achieving the required water balance targets are more practical and cost effective than others.\u00a0On small sites,\u00a0irrigation and transpiration will provide only a small percentage of the overall required volume to be dealt with. Grey water systems do contribute to achieving the objective, but can be cost prohibitive and it is often\u00a0difficult to utilize the quantities of water within a 72 hour timeframe needed to meet the target.<\/p>\n
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The ground water recharge alternatives typically employ infiltration trenches or pits that allow for stormwater to infiltrate into the ground. These features can not only assist in meeting the water balance objectives, but they also provide additional site storage, and serve to trap sediments assisting in achieving stormwater quality objectives. Bio-swale\u00a0planting areas, constructed overtop of infiltration areas provide an additional level of treatment as required to meet the most stringent quality control targets.<\/p>\n
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Typically, bioswales are designed to accept sheet flow from parking lots or landscaped areas. During frequent storm events,\u00a0drainage is directed toward the bioswales\u00a0and absorbed into the planting medium with a percentage being used\u00a0by the plants. During heavier storms, the excess runoff will drain into the lower stone pit\u00a0where it will be\u00a0drawn down\u00a0into the water table over\u00a0a\u00a072 hour period.\u00a0\u00a0While there\u00a0are certain constraints such as native soil percolation rates, existing topography etc., they are often the most cost effective solution to achieving the water balance, and water quality criteria.<\/p>\n
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MGM has recently designed several institutional, commercial and industrial sites employing a hybrid bioswale\/infiltration pit design. To find out more information regarding stormwater management alternatives please contact\u00a0MGM<\/a> directly.<\/p>\n