FLOOD01680

• Designation Number: 126
• County: Boulder
• Community: Unincorporated Boulder County
• Stream Name: St. Vrain Canyon upstream of Lyons
• Title: Floodplain Information Report - South St. Vrain Canyon upstream of Lyons, Boulder County, CO
• Date: 2/1/1978
• Designation Date: 1/1/1979
• Floodplain - Doc Type: Floodplain Report/Masterplan

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the 100-year flood boundary will be designated as floodplain area and

may be regulated accordingly by the local officials.

Flood Elevations

Flood crest elevations for the 10-, 50-, 100- and SOD-year floods may

be found in four separate locations in this report. The Flood Fre-

quency-Elevation and Discharge Data table, Table 3, lists these eleva-

tions at selected reference points. The flooded area plates 5 through

39 give the plan view of the flooded area on a contour base map and the

high water elevations for the 100-year flood can be interpolated from

this. The flood profile plates, Plates 40 through 105 show the stream-

bed elevation and the high water elevations for all four frequency

floods. Also, the cross section plates, Plates 106 and 107 show a

graphical representation of the high water elevations at typical can-

yon and valley cross sections throughout the study reach.

The flood profiles may be used in areas where controversy arises over

the 100-year flood boundary on the flooded area plates. Since the

flood profile plates give the elevations and distance, or stationing,

from a known point, the high water elevations can be surveyed on the

ground to alleviate any discrepancies on the base map.

Control Markers

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In order to locate points on the ground relating to information given

in this report, 12 benchmarks set by federal agencies have been identi-

fied in the study area. These benchmarks are tied to United States

Coast and Geodetic survey lines 112 and 113. In addition, nine control

markers have been set in the field at strategic locations by the

mapping contractor at the request of Boulder County. The additional

Boulder County control markers are brass caps set in concrete. The

location of all benchmarks and control markers are shown on the topo-

graphic maps prepared for this study. The coordinates, elevations and

general location of the benchmarks and control markers are listed in

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Table 7. Additional survey notes and monument descriptions are avail-

able for review at the Colorado Water Conservation Board.