FLOOD09322

• County: Rio Blanco
• Community: Rangely
• Stream Name: White River
• Basin: Yampa/White
• Title: White River at Rangely
• Date: 10/1/1993
• Prepared For: Rangely
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

TablE! 1
100-Year Peak DiHcharges
White River 'at: Rangely

Location

Drainage Area in
Sauare Miles

Peak Discharge in
Cubic Feet per Second
10::':ie!al: 50,-Year 100-Year: 2.Q.Q'-Year

Downstl:eam from
Coal M:,ne Draw 3,300 7, ~;OO 13,500 20,000 3'5,000
Upstream from
Douglall Creek 2,880 7,000 13,000 18,500 33,,000

for stnady, gradually varied fIo,",. in natural or man-made channels.
'rhe effects of various obstruct. ions such as bridges, cuI verts,
weirs, and structures in .the flood plain may be considered in the
computations. The main data requirement of the HEC-2 program is
that the geometry of the flood plain (in t.he form of crClSS sections
taken 'icross the flood plain) be at. right: angles to the direction
of flOlI. Bridges, culverts, and \,redrs are, defined by special cross
sectio:1S. The program also requirE!s data on the roughness of the
channe l and the overbanlcs in the :Eorm of Manning's Roughness
Coeffi,~ient, often referred to as t:he, "n" value.

T:le cross sections used in th,~ Frs were taken from 1982 aerial
photography. These cross sections did not contain any geometry
below the water surface, so the pe!ak discharges used for the
computation of the water surfac,e profiles were ad:lusted by
subtra~ting out the discharge in the white River at the 1:ime of the
photography (2,300 c.f.s.).

T1.e first step in the! hydraulic analysis for this study was to
restor~ the HEC-2 model from the FIS. Because the input: data from
the FI:, model were not stored elec1:ronically, the data 1,lerE! entered
into t1.e computer from input printouts included in the microfiche
record. Once the input file was cr,eated, the HEC-2 program was
executed to produce the 100-year flood water surface elevation
profile. The output from this file was compared with t:he results
of the original FIS.

A comparison of the results showed t:hat the computed water
surface elevations from tche rest,ored HEC-2 model were t:he same or
within a few hundredths Clf a foot of the results from most of the

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