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Eagle and Colorado Rivers Floodplain Information Report Eagle and Colorado Rivers Floodvlain Information Report Colorado Ea Colorado Ea SURVEY BENCHMARKS AND CONTROL 2.3 Climate Precipitation varies widely throughout the Eagle River Basin. Precipitation data reported by the Natural Resource Conservation Service shows annual precipitation of 12 inches at lower altitudes and up to 40 or more inches at higher altitudes MAP SHEET Colorado Sheet 1 Colorado Sheet 4 Colorado Sheet 4 Eagle Sheet 1 Eagie Sheet 6 Eagle Sheet 7 Eagle Sheet 8 Eagle Sheet 11 Eagle Sheet 13 Eagle Sheet 17 Eagle Sheet 17 Eagle Sheet 20 Eagle Sheet 24 Eagle Sheet 26 Eagle Sheet 27 Eagle Sheet 28 Eagle Sheet 34 ELEVATION (feet) 6152.9 6147.7 .8 .8 6470.5 6414.7 6501 .06 6535.15 6615.6 6800.6 6867.18 6920.03 7113.9 7207.1 7228.5 - 7303.00 7731.29 A total of 46 cross sections on the Colorado River and 425 cross sections on the Eagle River were incorporated into the floodplain models. Eagle County staff surveyed 100 cross-sections in critical areas (e.g. bridges, wide valley bottoms, or where floodplain development had occurred) to improve the accuracy of the hydraulic model. In addition, bridge measurements were verified and spot elevations taken at critical points. sand Survevs The base topograppjc mapping for ti'js study \vas provided by Analytical Surveys, Inc. Aerial imagery and mapping was completed October 1998. This mapping was available at scales of I" = 200' and has a contour interval of 2 feet. the Generally, field surveys agreed well with topographic mapping except in areas of heavy brush where topographic contours appeared to be high in certain locations. Basis of Horizontal Control Colorado State Plane Coordinate System. Central Zone Lambert Conformal Projection NAD 83 U.S. Survey Feet Basis of Vertical Control NAVD 88 Sea Level Datum August 22, 2003 Page 8 titJ ~U.~~~h< aerial and numbers are described August 22, 2003 6161 6161 EASTING {feet} 2547397.27 2559241.80 2561444.37 2561444.37 2589807.64 2598968.23 2602351 .50 2622266.01 2629933.43 2648827.94 2652138.42 2664268.23 2677285.72 2687488.21 2693196.10 2698238.05 2732461.44 Ma 2.4 Vertical control points for the cross sections were three-quarter inch rebar pins, which were used as control for the mapping. These points are shown on the mapping and are designed by letters as shown on the floodplain mapping. The locations of the benchmarks used in the survey below: Page 7 iM1~~~~~ "V" __ NORTHING {feet} 1656717.94 1665024.70 1662348.96 1662348.96 i 666i 79.78 1663920.36 1663317.80 1663120.94 1668548.98 1678324.63 1681420.63 1686197.74 1669614.95 1664869.38 1661795.29 1659882.68 1648196.86 STATION 10 NUMBER GPS 001 GPS 010 DOTSERO DOTSERO T483i 32 T58 6 5 R85W GYPSUM K280 F 280 GPS 006 GPS 014 6864 L 269 GPS 012 GPS 003 CO DOT LP1-2 K2 T 280 Flood flows on the Eagle River typically result from rapid melting of the mountain snowpack during the period from May to early July. Peak events occurring after late July are almost always generated by rainfall events. Snowmelt runoff may occasionally be augmented by rain. The snowmelt runoff is characterized by sustained periods of high flows and marked diurnal fluctuation. Examination of rnptpr\rnll"\n;~~l ":lnrl ,..11"""l')t""1",,....';,.....,.1 ",..,._,.1.;...;............("1 ""...........1 _................1_1+.....+1............ .........,..1 ...+_...............- .!'1.....ny _.....~~_.....1~ _1.......___ ...1.._4- _u______ ..................--.......'-'..1.'-'5..1._......... u.J.J.U "",u..lJ..lULV..lV5.1'-'u..a. ......VUU.1UVlJ~ LULU pl\';\"'JplLaUVU QJJU ~UC;a111 .llUVV l(;\.,VIU~ ~JIUW L11dl ~Ull1111Cl cloudbursts are not a great flood threat on the main stem of the Eagle River. While localized rainfall events can produce peak flows higher than those calculated utilizing snowmelt driven peak flows, these tend to be localized events that do not produce peak flows on the main stem of the Eagle River. Drainage basins from 10 to 20 square miles may produce higher peak flows due to rainfall, but that for larger basins, snowmelt is the primary source of peak flows. Temperature and precipitation varies greatly from location-to-location and season-to-season within the drainage basin and are important variables in flooding conditions. Above normal spring temperatures can cause early and heavy flows on the Eagle River. Peak snowmelt runoff events are generated by runoff from snowmelt from higher altitudes. Visual observations of snowmelt patterns, as well as data from NRCS Sno-tel stations, suggests areas above approximately 10,000 feet in elevation are the primary source of runoff during the peak stream flow season. This is further validated by a statistical comparison of calculated peak runoff vs. drainage basin areas above and below 10,000 feet.