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MP•.h1O: Snyder Amendment hydrologic Supplement J1rnC 20, 19A9 ri4 9:10 A.M. Project No. 89002 7. The silt retention facility sizinC includes art efflclertey factor (709b was Used) and a particle s17a, lit mlrrons (40 was used). What was rite source of these flgttres? R1~:SPUNSE: The particle size, wltett originally Considered was somewhat larger than the size used for t}te design based ort Lttc operation and specific gravity of the material. upon further consideration we feel there could be some smaller particles of a nine slit var[ety created. We selected r.he 40 micron size as the design particle to assure maximum removal. 'fhc 7095 efficiency is felt to be a reasonable performance level based on the s}tapo of Ltte pored (L=J x width). R. The grading plan indicates some disturbance will occur below rite silt retention facility. A silt fence and possibly a berm is recorntnertded fot areas below Lire slit pond. 1tESYUN51;: A silt fence }tas been shown below Lire pond along contour 7225. IP disturbance occurs below this elevation, Il,e s[iL fence would be moved to the interface. A berm to direr. r. runoff from the West boundary area Lo the silt retention fac111Ly will be installed. Tills 1s indicated in Stormwater Runoff Improvements, Site llermirtg subsection ott Page 11/12 of the Suppletncnt. 9. 'J'he silt retention faculty indicates the outlet. pipe will go through the embatrkmertt w}tlch will actually •be created by excavaLlon into the limestone material. How is the pipe proposed Lo be Installed? RESYONSI;: We would propose Lo develop ibis construction dulall as part of the. Yinal design plans based on recommendations from ttte Operator, Castlo Concrete. Orre po)nt of reminder, water will not be retained in this facility, the maximum drain down time 1s 5 hours. If some seepage should occur, the result would Urobably be Otte of rf lta•atinn. !0. The standpipe calculations as submitted are acceptable. It }tas been recommended that the riser be wrapped in a fflter fabric (at least. the lower yortlurt), thus preverttlrtg arty suspended rnatorlal from Icavlrtg Ihruugh the attsndplpe holes, itESI.ONSE: The pond design allows for adequate particle settlement and the slit reaching the standpipe will be very minor. The fabric would have a tendency to clog the outlet holes daring a major event catrslttg the orifice (standpipe toP) t.n overflow and possibly flow over the spillway. '1'ttls may result In a poorer quality effluent discharge than if the holes operate. We do not propose use of a filter fabric around the standpipe holes. ~. ,, :f .~ ~y;~.