222 W MARINE VIEW DR 2016-01-01 MF Import

278 WET•pETEN7'ION/DETENTIONPONOS 8.2.6 Probability Modeis Since the particles in stormwater are variable in density and compositiop from one storm to another, it is difficult to predict accurately a level a sediment removal for a year using design storm data. It may, however, y� more accurate to estimate long-term efficiencies given an assumed frequenq distribution on solids densiry and sizc. The distribution of stormwater settliuj velocities is usually well defined by a log-nortnal distribution (iJ,S. Enviroo- mrntal Protection Agency, 1986). ! Using frequency distribution for precipitation intensiry, volume, duratioy and interevent times, long-term performance during runoff events (RL) M.� estimated by the U.S. Environmental Protection Agenty (1986), '1'lu ef�, ciencies were shown to be a function of remo��al at mean runoff (RM) and the coefficicnt of variation of runoff volume, C�.�. The results are show�n �y Figure 8.5. The variable (Z) is defined as maximum removal at very Iow flor, expressed as a fraction and is freyuently assumed equal to 1.0 for stormwater Y 1 Removal at mean runoH 11ow �RM2) Ma:imum rertroval at very Iow flow (lKpressed as percenp FiQurc 8.5 Long-term performance of a device w here removal mcchanism is sensiii�ro �o flow rate during runoH. (From U.S. Enviranmental Protection Agenq, 1986.) 8.2 MODEUNG CONC studies. The average remov� an exponential relationship time, or RM=1— RMal— �e RM a efficiency at mean runofl OR a a�erflow rate, ft/h re = detention time, h k = rate coefl9cient = u,/ho; �� = sedimentation rate, ft/6 two equations give equivalent re ;ical dimer.sions of a pond with e: aptured stormwaters, after runo: ded volumes also follow'a gsmR shown in Figure 8.6. The average i pond is estimated based an "efl a � � 7 Cq L � w � a c .� a n Ratio: E� �Nrc B.6 Average long•term perfoi mcmal Protection Agency, 1986.)