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03/07/9b v:4& QJUJ LL4 IGLU • invert to allow sediment and debris accumulation and removal and a baffle at the outlet to prevent floating materials from entering the storm drain pipe. • Sedimentation manholes — simple variations on the traditional manhole and placed upstream from infiltration sumps and other water quality facilities. Similar to trapped catch basins, the majority of the manhole is located below the pipe invert to allow sediment and debris accumulation and removal. • Underground detention vaults /tanks —large underground detention facilities that store runoff. Wet tanks and vaults maintain a permanent pool of water to provide quiescent settling conditions so particulates (and attached pollutants) will settle and collect at the bottom. These facilities are usually constructed from reinforced concrete (vaults) or corrugated metal pipe (tanks). Prior to recommending guidance references for these facilities, it is important to inform staff about the current school of thought related to performance. These devices involve limited, if any, biological processes and rely on gravity settling to reduce sediment loads in stonnwater runoff. Sedimentation allows insoluble particulates and their attached pollutants to separate from the runoff. Structural devices in this category have been declining in popularity for use as stormwater quality devices as more information is learned about actual pollutant removal efficiencies and performance. Some agencies are recommending that designers consider other options such as filters and infiltration, before selecting structural control devices. Eugene will need to make a decision about applicability of the structural devices outlined above to the local conditions, in light of recent findings in other locales. Oil/water separators have proven applications in treating industrial process waters (provided they are properly maintained); however, their track record at stormwater runoff pollutant removal has been generally poor due to the highly variable flow with high discharge rates, turbulent flow regimes, and high suspended solids content of typical urban runoff. Sedimentation manholes continue to be used in the Portland area with little known about their actual effectiveness. A study conducted by the City in 1994 found that removals of phosphorus and total suspended solids were not significant. Similarly, no studies have been performed to test the effectiveness of vaults/tanks, although, in theory, one should be able to design the volume of these devices to achieve reasonable pollutant removals through settling. In general, the effectiveness of all the devices discussed in this chapter depends on routine maintenance. Without maintenance, devices that accumulate large volumes of sediment and oil will clog or release those pollutants during high flows. For this reason it is now commonly recommended that some of these devices, such as oil /water separators, be constructed off -line, thereby allowing high flows to be bypassed. Also, S \945042na \task0200 \onsite conrrols\techmem.ck 13