2001-05-22_REVISION - M2000158 (3)

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i14VIndNMiNT, INO. MAY em, iqq 1 C~ P.~aC 7 In the areas where armoring is noeoasary, the gradation of the soil and subsoil (nand end gravel) sill be examined along with vegeta- tion characterletiae to det~rmine if the erosive power of tho flood will cut into the vegetation and null. Yn those nrena were it is determi.nod that erosion Will occur the operator will place riprap materi.a.l, that cannot be lifted and moved by floodwater'. Tllis will prevent erosion and down cutting on those elopes subject Lo n~our. As notod in the Division's review, we will.iiaP a n„ = 24 to PulPill thin raquiremont. For Chia wino piakett 8nginaoring optod to us9 a 1.nf1_yPar Avant. as tl~u doaign oriteria for the plan. This is oonaiatent with the standard national proeeduros adopted by the Federal Rmergency Manage- ment Agency an the base flood for purposes of floodplain management. PSfiG rncuynlcva Lltat the erosion eoatrole installed are only intended to be effective up to bhn Ilvw rats associated with the dBaign avcnt. 'Pltere may be rnnldual losses from extreme flood events, above and beyond those for whioll mlClyation ie coat effective. Ae noted in the original Iload plain information the 100-year floodwater velocity at the south end of this mine will be from 1.5 Co Z.0 feet per sanond up to 3.6 feet per second at the north end of l'.11H site. Tha inoreasad flows and vo.locity on the upper end are north oP the confluonae of the South Dlatte River and St. Vrai.n (:reek. ~l~heee velocity eetimatcs aro on a macroscopic araln, and were the avera4e velocity of the largo volume of orator flowing through the minor reach during a 100-ycer flood. einoa no spocifie veloci.tioa arP known, the operator will follow the general guidelines and nnnarrvative formulae outlined in the guidancs document. The map in the Pickett study has bean revised to include the ardan that may be nYfaeted by a 100-year flood. These areas incorpo- rate the srsaa cutllna~ in the division letter, in nddition to those shown previously. IAtliln tl~e orlyinal plans were based on a gualita~- tive analysis of the rivnr and floodplain, the operator will begin to develop the qualitative analysts as miniag progresses. Uoiny the information provided in the review letter, Lhe map was revised to extend the armoring around the south side of the alupn !n Meadow Lake. Pit slopes that intersect with natural ewalea, abandoned fail water ditches, et cetera, will also be armored. These areas do not readily show on the map so ae the site is developed, they will be identified and armored using th• design criteria outlined i.n this plan. The last arose of concern aro the gaswell pads and across road alignments in 3hnrkey'a Lake whore they aro in oloee proximity to the River. COd Wd`S:C TOOZ Z2 'FE61 66~L C2t• CDC '^N 3FDHd 'BFI] '1N3WP4]?J Il1N3 urn ~d