FLOOD02476 - Laserfiche WebLink

Home Browse Search

,~ ~~ & Associates, Inc, ~je' '.sou,e. eon,""onj, June 14. 1994 2995 CBntergreen CCl,;rt Soutn. 5uJte C Bcu'cer. Colorado 30301-5421 Prel'''' (303) 4~o-J-..:3Q ,~', c''" '~>"\").:40- -&2~ :\ _" I , .r--^,~,^, "" V. '~l: ,. " RfCElVED \ JUl : 6 1994 CWC~ DIA SC Af!C F I L E Mr. William R. Locke, Division Director Hazard Identification & Risk Assessment Division Mitigation Directorate Federal Emergency Mam,gement Agency Washington D,C. 20472 REF: 8924D - Limited Map Maintenance Program Project; Oak Creek; City of Florence and Fremont County, Colorado Dear Mr, Locke: This letter is in response to your comments stated in your memorandum to Douglass A, Gore, Division Director, Mitigation Division, FEMA Region VIII dated March 31,1994 on the above referenced project. The following text responds to your analysis and concerns: Hydraulic Modeling We strongly disagree with the suggestion that the left overbank and main channel should be modeled together for both the natural and floodway HEC-2 runs. Careful analysis of the contours on our work map in the left overbank area show that once the flood traverses the railroad tracks, the water will be conveyed down valley to the north in a direction parallel to the contours. This water cannot physically rejoin the main channel since the down-valley gradient is so steep. The HEC-2 cross sections developed for the left overbank are bent somewhat upstream at the outside edge of the floodplain to hydraulically model the lateral extent of the floodplain. Previous HEC-2 modeling attempts undertaken by our company during the development of this project did, in fact, model the left overbank hydraulically connected to the main channel though this reach of Oak Creek. These modeling attempts were proven to be indefensible because the total flow would be contained in the main channel at one section, then the flows would spread out to a wide floodplain in the next section and then be contained again in the main channel at yet another section. The addition of extra cross sections as suggested in your letter is not warranted due to the physical hydraulic properties of the floodplain. We have reviewed our reasoning for our delineation of the floodplain for Oak Creek for this project with the Regional Project Officer in FEMA Region VIII and he agrees with our analysis. As such, in our professional opinion additional analysis of the left overbank flows is not warranted. Recent Fill In the Floodplain The recent fill in the left overbank of Oak Creek (by a local developer) near the Arkansas River was not discovered by our firm or by FEMA until after all work was completed and submitted for review to FEMA Central. The Regional Project Officer for FEMA was recently in Florence and has informed us that the City had realigned and excavated the Oak Creek channel since the date that the fill was placed in the left overbank, He also told us that a new dike is planned to be constructed