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<br />ENGINEERING MKTnODS <br /> <br />HIdrolol!ic Analvsis <br /> <br />Ta.ble 1 lists all streams and stream reaches for which approximate or detailed f100dplain <br />boundary information is contained in the 1995 FIS. The scope of the 1995 study was not <br />expanded from the original 1981 PIS which covered exactiy the same streams and reaches. <br /> <br />Hydrologic analysis for streams studied by detailed methods in unincorporated La Plata County <br />is based 0:0 one of three approaches: <br /> <br />. Discharge-frequency analysis of stream gage records. <br />. Discharge-frequency analysis of stream gag(' records for similar streams. <br />. Hydrologic modeling using synthetic: unit hydro graph methodologies. <br /> <br />Di~;charge rates for streams studied by detailed metl10cs a.'(' presented in the PIS for the 10-, 50-, <br />100- and ~;OO-year eVI~nts. <br /> <br />Hydrologic analyses and discharge rates for streams studied by approximate methods are not <br />document,~d by FEMA in either the 1981 or 1995 PIS'!;. As previously discussed, Colorado <br />st,nutes r~quire that this information be provided in order for floodplain studies to be <br />"designat(,d" and thus made legally usable for floodplain regulation. Table 1 presents the list of <br />flooding sources in unincorporated La Plata County studied by approximate methods in the 1995 <br />PIS. For the purposes of this HA, 100-year flow rates for streams in La Plata County studied by <br />approximate methods in the 1995 HS have been detenn.ined using one of three infoffilation <br />sources: <br /> <br />. CWCB regional regression equations. <br />. CWCB regional regression equations + dam outlet capacity <br />. 1999lJSACE Floodplain Study for Los Pinos River. <br /> <br />Table 2 presents 100-year discharge rates determined fDr approximate streams in the 1995 PIS <br />and sources of these computed values. Details of these in formation sources and tl1eir application <br />to approximate streams in La Plata County is provided in the following paragraphs. <br /> <br />Di~;charge rates for the Los Pinos River between Valkcito Reservoir and the Town of Bayfield <br />were derived from a recently completed detailed study by the U.S. Army Corps of Engineers <br />entitled F!ood Plain Delineation Study, Los Pinos River Below Vallecito Dam 10 the Southern <br />Ute Indian Reservation Boundary, La Plata County, Colorado (1999). Flows in this study are <br />ba1ed upon the results of HEC-l model developed for the 70 sq m.i drainage basin between <br />Vallecito Reservoir and Bayfield. The modeling assumes a base flow rate from the reservoir of <br />3,100 cfs representing the maximum release with the conservation pool full. Peak flow rates for <br />tributaries to the Los Pinos were determined using regional regression equations developed by <br />the CWeB and documented in Guidelines for Determining lOO-Year Flood Flows for <br />Approximate Floodplains in Colorado (CWCB, January '999). Regression equations presented <br /> <br />5 <br />