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" Federal Emergency Management Agency Washington, D,C. 20472 CERTIFIED MAIL RETURN RECEIPT REQUESTED IN REPLY REFER TO: Case No.: 95-08-110P The Honorable Mark Williams Mayor, Town of Castle Rock 680 North Wilcox Street Castle Rock, COlorado 80104 Community: Town of Castle Rock, Colorado Community No.: 080050 Panel Affected: 0188 C Effective Date of This Revision: APR 051995 102-D Dear Mayor Williams: This is in response to a request for a revision to the effective Flood Insurance Study and National Flood Insurance Program (NFIP) map for your community. Specifically, this responds to a letter dated February 28, 1995, from Mr. Daniel W. Bare II, P.E., Engineer III, Town of Castle Rock, regarding the effective Flood Insurance Rate Map (FIRM) for the Town of Castle Rock, Colorado. Mr. Bare requested that we revise the effective FIRM to show the effects of the construction of the Village North Diversion Channel, which comprises a trapezoidal grass-lined channel and associated drop structures, and more detailed hydrologic and hydraulic analyses along an unnamed tributary to East Plum Creek. The channel improvements extend from the Atchison, Topeka, and Santa Fe Railway to the Omni Drainageway. All data required to complete our review of this request were submitted with Mr. Bare's February 28 letter, and with a letter dated March 21, 1995, from Mr. Richard N, Wray, P.E., Principal, Kiowa Engineering COl'poration. Because this Letter of Map Revision (LOMR) shows the effects of a publicly sponsored flood-control project that reduces flooding to existing development, fees were not assessed for the review. We have completed our review of the submitted data and the flood data shown on the effective FIRM, and have revised the FIRM to modify the floodplain boundary delineations and zone designations of a flood having a I-percent probability of being equaled or exceeded in any given year (base flood) along the unnamed tributary to East Plum Creek from the Atchison, Topeka, and Santa Fe Railway to the Omni Drainageway. The construction ofthe Village North Diversion Channel, which lies within the Town of Castle Rock and Douglas County, diverts the lOO-year flood discharge from its present confluence with East Plum Creek to the Omni Drainageway. The lOO-year flood is contained within the channel. Because of map scale limitations, the IOO,year floodplain with a maximum topwidth of 23 feet cannot be shown on the FIRM. The diversion of the lOO-year flood discharge removes the effective floodplain designated Zone X (shaded), which was created by shallow flooding from the lOO-year flooding event from downstream of the Atchison, Topeka, and Santa Fe Railway to approximately 800 feet upstream of its past confluence with East Plum Creek. The impact of the diversion on flood hazards from the Omni Drainageway were summarized in a LOMR issued on March 10, 1994. The data submitted in support of this revision also affect the FIRM for the unincorporated areas of Douglas County, Colorado. A separate LOMR was issued to Douglas County with the same date as this LOMR to describe the changes to the flood hazard information for that community.