Laserfiche WebLink
JNDS <br />are variable in density and composi� <br />d'�fiicult to predict accurately a levej <br />design storm data. It may, however; <br />a effici�ncies given an assumed freque <br />�e. The distribution of stormwater settl <br />a log•normal distribution (U.S. Envir <br />precipitation intensity, volume, duraW <br />-formance during runoff evenu (Ri) � <br />ital Protection Agency (1986). The e <br />i of removal at mean runoff (RM) and i <br />olume, C�. The tesults are shown <br />ed as maximum removal at very low }H <br />:ntly assumed equal to 1.0 for stormw� <br />mean runON f�ow (RA�IZ) <br />�val at �ery low flow <br />ed as percent) <br />ievice where removal mcchanism is sensn� <br />,vironmrntal Protcction Agency, 1986J ' <br />82 MODELING CONCEPTS FOR WET•DEfENTION �`JNDS 279 <br />d����g studies. The average removal at mean runoff flow is estimated <br />assuminB a� �xponential relationship as a function of overflow rate or <br />detention time, or , = <br />: / �C.-�VP <br />� 1 _ et��,/oa� � �.� �•t,• (8.10) <br />RM "% : .'•�� �. <br />RM 3 1 _ e-k�. � (8.11) <br />•here R,k — efficiency at mean tunoff conditions (fraction) <br />pR � overflow rate, ft/h <br />�a = detentian time, h <br />k a rate coe(ficient � u,/ha; ho a settling depth <br />v, a sedimentation rate, ft/h <br />The two equations give equivalent results. Using them requires data for the <br />ph�sical dimensions of a pond with estimates of settling velocities. <br />Captured stormwaters, after runoff ceases, also undergo removal. These <br />ponded volumes also follow a gamma distribution whose integration resuls <br />ue shown in Figure 8.6. The average long-term performance of a sedimenta- <br />�ion pond is estimated based on "effective" pond volume (VE). DiToro and <br />0 <br />¢ <br />0 <br />c <br />� <br />e <br />� <br />� <br />0 <br />N <br />) <br />O <br />E <br />Y <br />C <br />u <br />u <br />Y <br />a <br />Ratio: EMective Dasin vdume � <br />Mean runoll volume Vq <br />LO <br />F`�ure 8.6 Average long•term performance during nonrunoff. (From U.S. Environ- <br />mcntal Protection Agency, 1986.) <br />