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TABLE 7, Summary of Hydroloqy Location Triochera Creek Basin Trinehera Creek at Mouth Trincherd Creek. below Smith ReserVOir Trincher~ Creek ~bove Mountaln Borne Reservoir Trineher~ Creek ne~r Fort Carland Sangfe de cristo Creek above Smith Reservoir Sangre de Cristo Creek near Fort Garland Ute Creek near Fort Garland Culebra Creek Basin Culebra Creek at Mouth Culebra Creek below San Luis Culebra Creek at San Luis Ventero Cfeek below Sanchez ReserVOir Rito Seco at San Luis East Basin Rito Seeo at San LUiS North Ba~in Rito Seco at San LUiS Costilla Creek Basin Costilla Creek at Mouth Costilla ~reek at Garcia (St;ateline) Costilla Creek above Costilla Dam. New Mexico Drainage Area (SQ mil 100-year Peak Flow (cfs) Historic Peak Flows (cfs) 420a 2,420 396<1 2.420 61 630 45 no 231 1. 780 190 1.5eO " 5" 1.320 m 689 1.520 630 333b 1.860 255b 1.860 220b 1.740 W 630 42.5 '" '" 640 4.5 190 '" 65< 290c 200e 25 1.640 1.640 500 LOOO 3.870 '- 61 square miles tribut~ry ~o to calculate peak flows. '"'~,,~~~,_ '-'__~ D~"~...~'" ..~.~ '_~'..~,' ..~~......... ..~..'. ...~.. .~.. w~... u...~" " b. 60 square ~iles tributary to Sanchez Reservoir were incluae~ to calculate peak flows. c. 25 square miles tributary to Costilla Reservoir were included to calculate peak flows. _29_ 4.0 HYDRAULIC ANALYSIS tn view of the size of Costilla County and the limited amount of time and money available for preparing floodplain delineations for the county. approximate methods of hydraUlic analysis were selected. 4.1 Ease Maps 7 1/2 minute U.S.G.S. guadrangle maps were used as base topographic maps to show the 100-year floodplain. 21 quadrangle maps were used to provide base mapping for the study area. Of these 21 quadrangles, 6 had a contour interval of 40 feet. 6 had a contour interval of 20 teet (with 1 of those having selected intermediate 10 foot contours and 1 havin'l selected intermediate 5 foot contours). 8 had a contour interval ot 10 teet (with 2 of those having selected intermediate 5 foot contours), and I had a contour interval of 5 feet. ! tn the San ~uis study area field surveyed cross-sections were superimposed on a blown-up version Of the San Luis quadrangle. Likewise, the CSU Maps of I'loodprone Areas wete prepared by superimposin'l field-surveyed cross-sections on quadran'lle maps. In the case of the CSU maps, however. the cross_sections were far less detailed and much more widely spaced than those used for the San Luis study. 4.2 Flood Depths r I The basic procedure for floodplain mapping was to determine a representative depth of floodin'l for the 100-year peak flow based on an elevation-discharge relationship. That depth was assumed to be constant for a 'liven stream reach. Flood contours were drawn on the maps, and the boundary was determined by connectin'l the end-points of the flood contours. The key step in this process was to determine appropriate flood depths. The first sources of information for this selection were the gage analyses performed by the various governmental agencies. These analyses generally inclUded flood stages foe the calculated 100-year flow. The second set of data sources was a set of typical cross_sections that were derived from the base maps (usually 10' contour intervals were used). Hydraulic calculations were performed for these cross-sections. In the area around the Town of San Luis. the elevation-discharge relationships determined in the December. 1988 report. Master DealnaQe St~qy. Ipwn of san Lui~, ~QJ~ado. were used. l"rom these analY6ee draft maps wen' preparec!. Final depths of tloodinq could not be selected until a field review of these draft maps had been conducted. . , r I I . . . -30-