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Drainage System Maintenance SC-44 <br />■ Methods used for illicit connection detection (smoke testing, dye testing, visual inspection, <br />and flow monitoring) can be costly and time-consuming. Site -specific factors, such as the <br />level of impervious area, the density and ages of buildings, and type of land use will <br />determine the level of investigation necessary. <br />Maintenance <br />■ Two -person teams may be required to clean catch basins with vactor trucks. <br />■ Teams of at least two people plus administrative personnel are required to identify illicit <br />discharges, depending on the complexity of the storm sewer system. <br />■ Arrangements must be made for proper disposal of collected wastes. <br />■ Technical staff are required to detect and investigate illegal dumping violations. <br />Supplemental Information <br />Further Detail of the BMP <br />Storm Drain Flushing <br />Flushing is a common maintenance activity used to improve pipe hydraulics and to remove <br />pollutants in storm drainage systems. Flushing may be designed to hydraulically convey <br />accumulated material to strategic locations, such as an open channel, another point where <br />flushing will be initiated, or the sanitary sewer and the treatment facilities, thus preventing <br />resuspension and overflow of a portion of the solids during storm events. Flushing prevents <br />"plug flow" discharges of concentrated pollutant loadings and sediments. Deposits can hinder <br />the designed conveyance capacity of the storm drain system and potentially cause backwater <br />conditions in severe cases of clogging. <br />Storm drain flushing usually takes place along segments of pipe with grades that are too flat to <br />maintain adequate velocity to keep particles in suspension. An upstream manhole is selected to <br />place an inflatable device that temporarily plugs the pipe. Further upstream, water is pumped <br />into the line to create a flushing wave. When the upstream reach of pipe is sufficiently full to <br />cause a flushing wave, the inflated device is rapidly deflated with the assistance of a vacuum <br />pump, thereby releasing the backed up water and resulting in the cleaning of the storm drain <br />segment. <br />To further reduce impacts of stormwater pollution, a second inflatable device placed well <br />downstream may be used to recollect the water after the force of the flushing wave has <br />dissipated. A pump may then be used to transfer the water and accumulated material to the <br />sanitary sewer for treatment. In some cases, an interceptor structure may be more practical or <br />required to recollect the flushed waters. <br />It has been found that cleansing efficiency of periodic flush waves is dependent upon flush <br />volume, flush discharge rate, sewer slope, sewer length, sewer flow rate, sewer diameter, and <br />population density. As a rule of thumb, the length of line to be flushed should not exceed'7oo <br />feet. At this maximum recommended length, the percent removal efficiency ranges between 65- <br />75% for organics and 55-65% for dry weather grit/inorganic material. The percent removal <br />efficiency drops rapidly beyond that. Water is commonly supplied by a water truck, but fire <br />hydrants can also supply water. To make the best use of water, it is recommended that <br />reclaimed water be used or that fire hydrant line flushing coincide with storm sewer flushing. <br />January 2003 California Stormwater BMP Handbook 5 of 6 <br />Industrial and Commercial <br />www.cabmphandbooks.com <br />