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NOV-09-1999 TUE 04:39 PM FAX N0. P, 04/10 From.JrnldS.FMek,Ph-Itydro-ynmlesk~eoryar~hdFre.~11&1B6Uake:(,io3IB1109i1To:LadPetlerttVtepyMtphmdlGtm~n Prpe7ol9Tuetday,NoYtmberp8,19fl2:W:1BPM Ms. Lori Potter Project No, 9974-0] Page 2 November 8, 1999 Tha A1vIIvIA report to DMG inchules information about tba volume of a eedimrnt pond that is dao inwrrect. It appeen the calouladoa of volume assumes 3:] aids slopes along the 136 foot lengrh and vertical aide slopes along the 54 foot width. Ia+tcad of a Wengulu eonteinmeet volume, it is our opialon the interact shape of the pond will be an inverted truncated prism. Tha following table illustrates projected volumes for different depths ns illustrated in the DMG submittal far a truncated prism. Depth fleet Volume cuhlc feet 0 0 3 ! 836 6 6 234 9 1H 115 We do not ague the proposed pond will have an ".. ,adequate volume and design to sustain the storm flow volume during a 2-yr/24•hr precipitation event" Only when water fills the pond to a depth of sat feet will ell runoff waters be retained assuming little seepage through the riprap, At boat (l.e., when CN ~ 86), rho pond w111 otdy totally contain about 9,3% of the volume of runoff anticipated from iB.S mined ecru due to a 2-year, 24-hour storm event. The pond will fiurction se a detention structure when water is nine feet deep. Undo these oondidotu, only about 27,tN/° of the runoff will be detained for the beat aoenarfo wher< Z-year, 2a• hour storm eveatt occur. Whop considering DMO requirements of a sediment pond fora 10• year, 24-hour alerts event, we estimate that only about 5.6% end 16.2% of the projected runoff will bs retained or detained, respectively. Unfortunately, little information e~dsts in the submittal that allows for a detailed evaluation on the effectiveness of this structure to redoes sediment in runoff waters. However, it I9 our opinion the pond w!ll be ineffective in removing suspended particles (l.e., eodlmeot) when runoff watus dischagic over the spillway. We have concerns that the sedlateN wntaltunent system is not an effective design. Specific items that mutt accompany a properly designed system include the following: • Diverdon of offeite flows around the system, • A langth;width ratio of at least 5:1. • Aa outlet device that drains the structure. • Development of a aplllway that disohuges runoff from a 10-year, 24•hour atone event. Since downeueam properties can be (and have been) impacted, we recommend the spillway be designed to ueoauaodate dltclrarge from a 100-year, 24-hour stoma event, • Proper embankment oompaetlon to minimize the potential of tisilure. + Proper erosion protection of ttl embankments, We have serious wncamt about how sediment from ails quarry will bo minimized ea the mining operation expands. As the operation becomes larger, more bare ground condltloas will exist and be susceptible to erosion. Unless the DMG requires continue! erasion control, updating, and