7000 HARDESON RD 2018-01-02 MF Import

JUL 24'96 17:Oe FR W a H PACIFIC 206 822 5341 TO I2062598829 P.06/09 Mr. Michael Crowson, P.E. June 11, 1996 Page Two Most of the testing results the City has is for the hori Still new and has not yet bees field tested. However, lahoratory research with resaect to the radial flow fill jib -lG C�- al bed filter. The radial flow system is have provided the City fooias of our rTm- of the media in Uie cs ttidges which have an aM xbkQts 6" flo r Thi9specin"OZIRt!L, eda_�Sa a higher tocttuosity factor that helps compensate for n nnner section. B experiments and field observations it is our opinion that the amount of soluble pollutaed by the radial flow filter will decrease on the order of 10 to 20% depending on th However, we expect the filter to operate for a longer period of time and be mono consistent sire it is controlled by an orifice (see detailed discussions in the BMP technical memoranda). The net result should result In a ast Increase In the total mum of pollataate removed from *as period to another. C We have no information with regard to the City's data on wet ponds they have tested• If they ni 7 i� provide us with their data sets we will be happy to provide them with a compa*ison. However, Iuy �r.tl�5 data vomperiaons to published wet pond data have ahowa the CSF to provide equal pollutant 1 �„ai temovallevcls. There arc two types of sytaetn failures_ The fast and most common is a complete plugging of the system due to heavy sediment loads and the second is leakage (bypass) resulting from A mechanical or structural problem. in the first can the unit must be maintained to restore it to a 11 functional level. This is described In further detail in the O&M Manual. in the second case the " o )5 particular cause of the failure is identified and repaired accordingly. td 5/ ^grab The radial flow filters are designed to handle 15 gpm per filter. You will need to provide the City the hydrologic calculations ham wh..'e the treatment flow rate of 3.0 CFS (92 cartridges)(15 gpm/csrtridge)j(449 M eo,-fs) was determined. �;) l beGK' We have not prepared a monitoring plan. It is difficult to determine the time that a 6 month 24 1 hour storm will occur. Our experience has been that the most oc onomic method to have a designated onsite staff person, take grab samples at the first flush of a storm which generates enough mnofFto make the system work but not generate a significant amount of overflow. I would also recommend that you follow the standard monitoring requitement set by the City for other BMP'S. Flow measurement from each bay is difficult since you would treed to put a monitoring device at each pipe outlet while accounting for backwater influence. The beat method would be to measure the height of the water at the outlet and modeling it as a culvert in inlet conditions, i.e. 07/24/96 16:52 TX/RX N0.0327 P.006