FLOOD03772

• County: Larimer
• Community: Loveland
• Stream Name: Big Thompson River
• Basin: South Platte
• Title: Floodplain Information Report
• Date: 10/1/1981
• Prepared For: Loveland, Larimer County
• Prepared By: Resource Consultants
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

Hydraulic Anulysis

Analysis of the hydraulic characteristics of the study reach of the
Big rhoapson River was carried out to provide estimates of the water-
surfac~ elevations of flood3 at the selected reCurrence intervals along
each strea~. .he water-surface elevations were computed through use
of the Corps of Bngineers HEC-2 step-backwater computer program (Ref. 3).
Cross sections for the backwater analysis of the Big Thompson River were
field-surveyed 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 cross sections from the topographic maps are shown by
reference points on the flooded area maps (Plates 2 through 10) and the
flood profiles (Plates 11 through 13). The reference points, 10-, 50-,
100-, and SOG-year flood elevations, are also listed in Table 5.

Channel roughn<;ss factors (Manning's "n") for these computations were
assigned on the basis of field inspection of the flood-plain areas (Ref.
8). These roughness factors ran!\edfromO.030 taO.040 for channel an d
from 0.040 to 0.080 far overbank areas.

AveraRe velocities in the fload~ays ran~c from I to 14 feet per
second. As shown on the profiles in Plates 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. Crosa section data from the topo-
graphic mappinp, were adjusted for actual streambed elevations in most areas
wnere field survey data were available.

Scour holes under and nCar bridge ntructures in most cases continue
under the entirc bridge nnd increase the hydraulic capacity of the struc-
eure slightly; therefore these depressions were considered in the hydraulic
analysis. These scour holes are the result of the major flood evenes in
1976and1980butdonotdppeartab"fill1ngwiths"di.."ntduringlow-no",
pc,iads. In the area above Railroad Avenue the la-year fla~d pr~file is
above the lOO-year flood profile. This result is a combination of several
factors. The 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 flows are not divided. The second factor is a result of gravel
mining in the area which has created levees that contain the frequent flows
and the effect of the narrow channel during these frequent flows is a large
bael<.~ater during the 10-year flood. During the 50-, 100-, and SOO-year
floods, flows overtop the levees upstream of the area and flow into the
flood overflot.' 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 is includcd in the
Technical Addendum.

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