FLOOD07967

• Designation Number: 181
• County: Larimer
• Community: Loveland
• Stream Name: Big Thompson River
• Basin: South Platte
• Title: Floodplain Information Report - Big Thompson River, Loveland
• Date: 10/1/1981
• Designation Date: 12/1/1981
• Floodplain - Doc Type: Floodplain Report/Masterplan

Hydraulic Anulysis

Analysis of the hydraulic characteristics of the st~dy reach of the
Big rhoapson River was carried out to provide estimates of the water-
surfac~ elevations of flood3 at the selected recurrence intervals aloog
each 5trea~. The water-surface elevations were computed through U5e
of the Corps of I>nginecra HEC-2 step-backwater computer program (Ref. 3).
Cross sections for the backwater analysis of the Big Thompson River were
field-su.veyed at close intervals above and below bridges and culverts in
order to compute the significant backwater effects of these structures.
Additional digitized cross sections were secured when the topographic maps
were compiled (Ref. 4). The locations of the field-surveyed CrosS sections
and the digitized crosa sections from the topographic mapa are shown by
reference points on the flooded arca maps (Plates 2 through 10) and the
flood profiles (Plates 11 through 13). The reference points, 10-, 50-,
100-, and SaG-year flood elevations, are alao listed in Table 5.

Channel roughn<;ss factors (Manning's "n") for these computations "'ere
assigned on the basis of field inspection of the flood-plain areas (Ref.
8). These roughness factors ran!\edfromO.030 to 0.040 for channel an d
from 0.040 to 0.080 for overbank areas.

AveraRe velocities in the floodways ran~e from 1 to 14 feer per
second. As shown on the profiles in PIa res 11 through 13 there are scour
holes under many of the bridges. This is an actual occurrence .~nd in SOllIe
locations scour holes are 4 feet deep. Cross section data from the topo-
graphic mappin?, were adjusted for actual streambed elevations in most areas
where field survey data wcrc available.

Scour holes undcr and near bridge ntructures in most cases continue
under the entire bridge and incrcase the hydraulic capacity of thc struc-
ture slightly; therefore theae depressions were considered in the hydraulic
aaalyais. These scour holes are the result of the major flood events in
1976and1980butdonotdPpeartoboeflllingwiths"di.."ntduringlow-flo'"
pe,iods. In the area above Railroad Avenue the IO-year flo~d pr~file i5
above the 100-year flood profile. This result is a combination of several
factors. Thc first factor is a flood overflo..' channel near Railroad Avenue
which during the lo-year flood divid<'.s the flow while the 50-, 100-, and

RE:\OURCE: CON\UlT~NT\ INC

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500-year flolo's are not divided. The second fsctor is a result of gravel
mining in the area which hss created levees that contain the frequent flows
and the effect of the narrow channel during these frequent flows is a large
backwaterduringtheIO-yearflood. Duringthe50-,100-,and500-yoear
floods, flows overtop the levees upstream of the area and flow into the
flood overflow channel, and the overbank flow is not restricted by the
levees creating less backwater effect from the channel.

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A detailed description of the hydraulic analysis i5 includcd in the
Technical Addendum.

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RE:\OURCE: CON\UlT~NT\ INC