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T---~ -.~TA~9A~J (- , 2480 West 26m Avenu. . Suit. 340-8 . Denver. COlOraclO 80211 . (303) 4"-3eOO . FAX (:)00) 455-9929 MEMORANDUM TO: John Liou, FEMA Peter Johnson, City of Lafayette David Sell, Town of Erie Curt Parker, Boulder County Mac McKee, COOT Kathy Chase, Taggart Engineering Associates, Inc. ~ FROM: RE: Bullhead Gulch Flood Insurance Study - 287 potential changes DATE: March 4, 1997 I' " As TEA and FEMA discussed at our November, 1996 meeting, Peter Johnson, of Lafayette brought to TEA's attention that the newest plans for Hwy 287 appeared to raise the profile of the highway in the vicinity of the Bullhead Gulch crossing. Peter's concern was that the raised profile would result in higher 100 year water surface elevations. In this area near the intersection of Isabelle and Hwy 287, the Bullhead Gulch channel disappears, and the floodplain is described in the Bullhead Gulch FHAD as shallow flooding about 2000 ft wide. (See floodplain map from FHAD), Last week, Bob Chadderton of COOT gave TEA 287 plans, profiles and cross sections. The proposed profile does appear to be higher than the existing profile, but that is misleading because the new road's alignment is different, so the new centerline is not in the same location as the old centerline. The cross sections indicate that the old and new high points on the highway are equal at station 33 + 300 to 33 + 250, and south to Isabelle the new highway is 1.2 to 2,6 feet higher than the existing 287. (See attached spreadsheet and chart). TEA has done no hydraulic analysis of this area. FEMA has directed TEA to use J ~ the FHAD HEC-2, According to the FHAD HEC-2 ("bullmult,dat") the control section for flows crossing highway 287 is 140.2, which is 1200 ft, (365 m) north of Isabelle (COOT station 33 + 315). Downstream (north) of section 140.2, the HEC-2 model shows most of the flood flows to be east of 287; upstream (south), the major portion of the flow is west of 287. (Selected cross sections are enclosed). The existing and proposed highway elevations in this area are the same. . (. The highway profile will be raised at the 287/lsabelle intersection south of the FHAD HEC-2 control section. However, the FHAD HEC-2 modeling for this area is simplified; no split flow analysis was done to determine locations and quantities of spills from the west side to the east side of 287. It appears that this raise Hydrology .. Hydraulics. Sediment TrartSQOrt .. Cillll engineering for Flood Contl'tll, Waterways, Water Supply and ParkS .. envIronmental MitigatiOn .. ForenSIC ServICes ..