|
<br />Roaring Fork River
<br />Floodplain lnfarmation Report
<br />
<br />Town of Basalt, Eagle & Pitkin Counties, Colorado
<br />
<br />Roaring Fork River
<br />Floodplain Information Report
<br />
<br />Town of Basalt, Eagle & Pitkin Counties. Colorado
<br />
<br />below the mean annual high waler mark, The steep slopes and lack of vegetation reduce the resistance of
<br />the banks to scour,
<br />
<br />Sopris Engineering surveyed 44 cross-seclions in areas of special interest (e,g, bridges, wide valley
<br />bottoms, or where floodplain development had occurred), In addition, bridge measurements were verified
<br />and SpOI elevations taken al critical poinls,
<br />
<br />2.3 Climate
<br />
<br />Precipitation varies widely throughout the Roaring Fork Basin above Basalt. On the continental Divide,
<br />near Independence Pass, the average annual precipitation is 26,3 inches wilh 17,5 inches occurring during
<br />the winter months (November - April), Near Basalt, the average annual precipitation is approximately
<br />17,2 inches with 8,7 inches occurring during the winter months November Ihrough April. Data on
<br />precipitation from the National Oceanic and Atmospheric Administration indicate that in the Aspen area,
<br />the IOO'year 24.hour storm would produce 2,6 inches of precipitation,
<br />
<br />Generally, field surveys agreed well with topographic mapping except in areas of heavy brush where Ihe
<br />topographic contours appeared to be high in certain locations,
<br />
<br />Vertical control points for Ihe cross sections were three-quarter inch rebar pins, which were used as aerial
<br />control for the mapping, These poinls are shown on the mapping and are designed by letters and numbers
<br />similar to CA-35,
<br />
<br />Temperature and precipitation varies greatly from location-Io-Iocation and season,to-season within the
<br />drainage basin and are important variables in flooding conditions, Above normal spring temperatures can
<br />cause early and heavy flows on the Roaring Fork River. Records from the Aspen wealher station indicate
<br />that the month of July has Ihe highest normal total precipilation for the year at 2,06 inches, The month of
<br />March follows closely with 1.98 inches of total precipitation, The first month with a normal spring
<br />temperature above the freezing point is April with a mean monthly lemperature of 38,6 0 Fahrenheit.
<br />
<br />The basis of vertical control for the surveyed Roaring Fork River cross-sections is NA VD 29 sea level
<br />datum originating at USGS benchmark for Township 8 South, Range 87 West, Section ]2 TR62, The
<br />locations of the four benchmarks used in the survey are described below:
<br />
<br />1.
<br />
<br />NAME
<br />TR88 AP4
<br />
<br />ORDER
<br />THIRD
<br />
<br />ELEVATION
<br />6,585, I
<br />
<br />Flood flows on the Roaring Fork River typically result from rapid melting of the mounlain snowpack
<br />during the period from May to early July, Snowmelt runoff may occasionally be augmented by rain, The
<br />snowmelt runoff is characterized by sustained periods of high flows and marked diurnal fluctuation,
<br />Examinalion of meteorological and climatological conditions and precipitation and stream flow records
<br />show that summer cloudbursts are not a great flood threat on these streams,
<br />
<br />DESCRIPTION:
<br />Station mark is small disk attached to a pipe projecting 3 em from a ],6,ft. round concrete post flush with
<br />Ihe ground, It is ]6,l-ft. soulh of, and slightly higher than the road center, 8,2-fl. north of the right-of-
<br />way fence, 13,8-fl. west of the north pOSI to a deer gate at a jog in Ihe fence, 1.3,fl. north of a fiberglass
<br />witness post and 3,6-fl. east of a fiberglass witness post,
<br />
<br />2.4 Channel Instabilitv
<br />
<br />To reach from the junction of State Highway 82 and the Basalt turnoff (at stoplight, about 0.4 miles south
<br />of Basalt), go west on Highway 82 for 1.35 miles to a paved road right just before reaching the crossroad,
<br />Turn right, north, for 0,05 miles to aT-intersection, Turn right, east, on paved road for 0,] miles to the
<br />station on the right at jog in the fence line,
<br />
<br />Cobble-bed streams such as the Roaring Fork River exhibit inslability problems when the cobble particles
<br />are mobilized, Those particles begin to move when the waler exceeds a shear stress on the bed particles
<br />beyond Ihe threshold value for incipient motion, When flow rates and velocities are high enough to
<br />mobilize the cobble, the channel becomes unstable, Calculations can estimate the flow conditions under
<br />which cobble will be mobilized, Bedload calculations and sediment rating curves have been developed
<br />specifically to estimate the flow frequency (i,e, 5-year flow condilions, IO-year flow conditions, 25-year
<br />flow conditions) under which particle mobilization will occur at particular locations of stream instability,
<br />
<br />2,
<br />
<br />NAME
<br />DI58
<br />
<br />ORDER
<br />SECOND
<br />
<br />ELEVATION
<br />6,898,60
<br />
<br />2.5 MaDS and Survevs
<br />
<br />DESCRIPTION:
<br />A standard disk, stamped 158 1934 and set in the top of a concrete post 5,8 miles northwest from Woody.
<br />Creek, It is located 5,8 miles northwest along the Denver & Rio Grande Western Railroad from
<br />benchmark A 158 at Woody Creek, Pitkin Counly, 0,2 miles east of the station at Rose, 330 feet east of
<br />milepost 387, 56 feet west of a corrugated pipe culvert, 40 feet northwest of the center of a road crossing,
<br />30 feet southeast of pole 2547, 20 feet north of the centerline of the track, and 4 feet higher than the top of
<br />the rail.
<br />
<br />The topographic mapping for this study from the GarfieldlEagle County line through the Wingo Bridge
<br />was provided by the Greenhorne & O'Mara, This mapping was available at scales of 1" = 200', The
<br />contour interval of the mapping was 2 feel. The upper Pilkin County mapping from the Wingo Bridge to
<br />the confluence with Snowmass Creek was obtained from Ihe Roaring Fork Railroad Holding Authority
<br />and was produced with a contour interval of 5 feel.
<br />
<br />3,
<br />
<br />NAME
<br />El58
<br />
<br />ORDER
<br />SECOND
<br />
<br />ELEVATION
<br />6,749,18, THIS SURVEY - 6,749.43 PUBLISHED
<br />
<br />On November 8, 2000, Aero.Metric, Inc, of Fort Collins, Colorado flew over the Town of Basalt to
<br />produce high resolution imagery and topographic mapping for the town, and specifically for the area
<br />known as "South Side," Horizontal and vertical Ground control was set by Sopris Engineering, LLC of
<br />Carbondale, Colorado, This new topographic information with 2-foot contour interval was incorporated
<br />into the base topographic mapping for Ihis study,
<br />
<br />DESCRIPTION:
<br />A standard disk, slamped E 158 1934, The slalion is located about 2,9 miles east-soulheast of Emma, 1.8
<br />miles southeast of Basalt and 1.3 miles south of he Eagle-Pitkin County Line, in the northeast '4 of
<br />Section 20, T 8 S, R 86 W, of the 61h PM at Stale Highway 82 milepost 24,8, Land ownership is Ihe old
<br />railroad righl-of,way, To reach the station from the bridge, go over the Fryingpan River just upstream of
<br />Ihe confluence wilh the Roaring Fork River in Basall, go soulheast on the old State Highway 82 for 1. 75
<br />miles to the crossing of the railroad tracks and the highway and the station on the right. The station is a
<br />
<br />,..!;- """"""'Clu1>"
<br />.. ....,_..,... -,-
<br />, ._~
<br />.. --
<br />
<br />Page 4
<br />
<br />November 14.2001
<br />
<br />"I!~~~,;;
<br />
<br />Page 5
<br />
<br />November 14,2001
<br />
|