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BMP Descriptions <br />Iron Enhanced Sand Filter Check Dam <br />Permeable check dams provide additional <br />treatment for pollutants within ditches and <br />grassed waterways through two processes. <br />First, the dams act as a barrier to flow <br />through the channel, allowing sediment and <br />particulate pollutants to drop out of solution <br />upstream of the dam. This promotes <br />infiltration and evaporation of stormwater <br />as well. Second, any water retained behind <br />the dam can seep through a sand filter <br />located within the rock dam. The sand, <br />mixed with iron filings (similar to an IESF <br />pond bench), creates an opportunity for <br />dissolved pollutant species to be filtered out <br />of the stormwater runoff. <br />These practices are often installed in a <br />series, from two to a dozen practices <br />depending on the length and slope of the <br />ditch or waterway (Figure 9). For short ditch <br />lengths a single check dam is often sufficient. The dams include an inner sand filter mixed with iron <br />filings. The ratio of iron filings to sand should be between 5-8% by weight and these should be mixed <br />thoroughly prior to installation. The sand -iron mix should be encased within a permeable membrane <br />allowing for flow in and out of the filter. This filter is surrounded by rocks to promote settling and <br />inhibit clogging of the filter. <br />Figure 9: Rock check dams in a small ditch <br />(www.casfm.org/stormwater_committee/LID-Summary.htm) <br />It is recommended that these dams are installed such that the buried rock toe of the upstream dam is at <br />the same elevation as the top of any downstream dams (Figure 10). This reduces the likelihood of <br />scouring downstream of dams as water flowing over the dam intercepts ponded water rather than <br />erodible soil. Also, the top of the most upstream dam should be installed below the outlet elevation of <br />any pipe draining to the practice to ensure water does not back up into the upstream storm sewer <br />infrastructure. <br />L = The distance such that points <br />A and B of equal elevation <br />Figure 10: Check dam schematic (MPCA 2000) <br />City of Ramsey Stormwater Retrofit Analysis <br />