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2B0 WET•RETENTION/DEfENT10NPONDS <br />Small (1979) presented an empirical equation (or approximating effectiv� <br />pond volume based on an emprying rate ratio (E): <br />sn. <br />E� V <br />R <br />where d� average inten�al between the midpoints of two storms, h <br />fl — rate at which pond empties, ft�/h <br />VR � runoff volume from mean storm, ft; <br />and <br />na�,A <br />(8.12) <br />(8.13) <br />n <br />7 <br />J� <br />O J <br />:'o <br />u> <br />�_ <br />5= <br />�, <br />: <br />=x <br />.a <br />u. <br />•s <br />w <br />u <br />where A is the sudace area (ft�). ��' <br />The volume of water removed between storms on the average is the <br />product bl1. In cases where the volume that can be removed between stotms <br />is stnall on the average relative to the storm volume that enters much ot the <br />volume may be occupied with carrywer from prior storms. Thus effecti�ro <br />volume may be smaller. Figure 8.7 indicates the effect that emprying rate hu <br />on effective storage volume. The fraction removals during runoff and durin` <br />nonrunoff are assumed independent; thus the removals can be multiplied by� <br />each other or the combined percent removals can be estimated by <br />overali percent removal �'00 — i� <br />where Fr � percentage not removed during runoff conditions <br />tQ - percentage not removed during quiescent conditians <br />(8.14) <br />Driscoll (1989) estimated the overall percent solids removal using a mean <br />rainfall volume of 0.40 in. (C„ — 1.50) for all storm events and compared the <br />results to solids removal for a mean rainfall volume of 0.52 in. (and no runofl <br />for rainfall less than 0.1 in.) with a C„ = 1.28 and found that the o��erall <br />(long-tertn) performance efiiciency was about 75% tor both calculations. For <br />this efficiency, the pond surface area was about 2% of watershed. <br />Ewmpk Probkm 8.3 A sedimentation pond with a storage volume below <br />the outlet invert elevation is used for a 5-acre commercial site with a runoH <br />coefi'icient af 0.5. 'li�e distribution of solids is given by Table 7.5. The physica� <br />B.2 MODELING <br />�,�L <br />y� MI <br />Fgurc 8.7 Eftect ot previous storm<. <br />U.S. Environmental Protection AQenc <br />srorage volume is 5000 ft; with <br />rainfall statistics for the area wec <br />States. <br />Average volume of precipitation: <br />Average rainfall intensity: <br />Average duralion: <br />Average interval between: <br />is an estimate of solids rerc <br />