FLOOD02044 - Laserfiche WebLink

Home Browse Search

ENGINEERING MKTHODS HIdrolol!ic Analvsis Table 1 lists all streams and stream reaches for which approximate or detailed f100dplain boundary information is contained in the 1995 FIS, The scope of the 1995 study was not expanded from the original 1981 FIS which covered exactiy the same streams and reaches. Hydrologic analysis for streams studied by detailed methods in unincorporated La Plata County is based 0:0 one of three approaches: . Discharge.frequency analysis of stream gage records, . Discharge.frequency analysis of stream gag(~ records for similar streams, . Hydrologic modeling using synthetic: unit hydro graph methodologies, Di~;charge rates for streams studied by detailed methocs a,^(~ presented in the FIS for the 10" 50., 100. and ~;OO.year eVI~nts. Hydrologic analyses and discharge rates for streams studied by approximate methods are not document,~d by FEMA in either the 1981 or 1995 FrS'I;, As previously discussed, Colorado st,nutes r~quire that this information be provided in order for floodplain studies to be "designat(:d" and thus made legally usable for floodplain regulation, Table 1 presents the list of flooding sources in unincorporated La Plata County studied by approximate methods in the 1995 FIS, For the purposes of this HA, 100.year flow rates for streams in La Plata County studied by approximate methods in the 1995 HS have been detenn.ined using one of three infoffilation sources: . CWCB regional regression equations. . CWCB regional regression equations + dam outlet capacity . 1999lJSACE Floodplain Study for Los Pinos River. Table 2 presents 100.year discharge rates determined I' Dr approximate streams in the 1995 FIS and sources of these computed values. Details of these in formation sources and their application to approximate streams in La Plata County is provided in the following paragraphs. Di~;charge rates for the Los Pinos River between Valkcito Reservoir and the Town of Bayfield were derived from a recently completed detailed study by the U.S. Army Corps of Engineers entitled F!ood Plain Delineation Study, Los Pinos River Below Vallecito Dam 10 the Southern Ute Indian Reservation Boundary, La Plata County, Colorado (1999). Flows in this study are ba1ed upon the results of HEC.l model developed for the 70 sq m.i drainage basin between Vallecito Reservoir and Bayfield. The modeling assumes a base flow rate from the reservoir of 3,100 cfs representing the maximum release with the conservation pool full. Peak flow rates for tributaries to the Los Pinos were determined using regional regression equations developed by the CWCB and documented in Guidelines for Determining lOO.Year Flood Flows for Approximate Floodplains in Colorado (CWCB, January '(99), Regression equations presented 5