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1 Colorado Water Conservation Board Department of Natural Resources 721 Centennial Building 1 31 3 Sherman Street Denver, Colorado 80203 Phone: (303) 866-3441 FAX: (303) 866-4474 STATE OF COLORADO . Bill Owens Governor February 17, 1999 Greg E. Walcher Executive Director, DNR Mr. Steve Sims First Assistant Attomey General 1525 Sherman Street, Fifth Floor Denver, CO 80203 Peter H. Evans Acting Director, CWCB Dear Mr. Sims: As you are aware, the Board reviews and designates 100-year floodplains under the provisions of its "Floodplain Designation and Approval Rules" (2CCR 408-1), dated December 11, 1987. See attached. An issue has surfaced regarding "future conditions" flood hydrology, as contrasted with "existing conditions" flood hydrology. This issue discussed as part of the Water Conservation Board's discussion under Agenda Item # 4h during the November 23 - 24, 1998 Board meeting. One of the questions raised during the discussion was, "What options, if any, does the Water Conservation Board have for designating and approving, or otherwise formally acknowledging, two floodplain studies for the same stream reach, one study based on "existing conditions" flood hydrology and the other study based on "future conditions" flood hydrology?" The CWCB' s Rules and Regulations for the Designation and Approval of Floodplains and of Storm or Floodwater Runoff Channels in Colorado state in Rule 5.B.3. regarding drainage conditions: "The hydrologic analysis shall be based on drainage basin conditions that will exist in the community at least 12 months following completion of the report, except if future urbanization of a basin is likely to occur, the hydrologic analysis shall be based on foreseeable development in the basin. " The first part of that particular rule (through the words, "at least 12 months following completion of the report,") relates to "existing conditions" hydrologic analyses, and the second part relates to "future conditions" hydrologic analyses. The Rules and Regulations go on to define "foreseeable development", which is the basis for a "future conditions" hydrologic analysis, in Rule 4.A.17.