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

HYDROLOGIC AND HYDRAULIC DETERMINATIONS

Recent flood5 on the Big Thompson River which have caused major damage
~ere in 1951, 1976, and 1980.

Flood Characteristics

On August 2 and 3, 1951, intense raini Over much of the Big Thompson
River basin caused a dam to break on the Buckhorn Creek on August J. This
cauied Severe flooding fro~ the mouth of Buckhorn Creek to the mouth of the
Big Thomp50n River, especially through the Loveland area, Approximately 1
mile of V.S. High~ay 34 was destroyed just west of Loveland, irrigation
works were destroyed, crop loss was heavy, and much sediment and erosion
damage occurred. The lives of four people were lost and many were left
homeless. Total damages from the flood were estimated at $602,000. The
estimated discharge from thia flood was 22,000 cfs at Loveland,

Flood flows on the Big Thompson River re5ult mainly from heavy rains
in the late spring and the summer. Rainfall runoff may occasionally be
augmented by snoWlllelL The rainatotll> runoff is charscterized by shorter
duration peaks on the order of several hours to a few days rather than the
lower duration seasonal peak runoff caused by snowmelt.

Hydrolo~ic Analysis

Amore recent flood occurred on July 31, 1976,as torrential rains in
the Big Thompson Canyon caused a severe flood, greater than the 10o-year
flood. A total of l39 people lost thcir lives. At Loveland, the flood and
damage wcre much le56 severe; however, damages totaled as much a5 $500,000
for some businesses. The peak discharge at a gaging station lcoated at the
mouth ot the canyon was estimated at 31,000 cfs. At Loveland, the peak
discharge measured approximately 20,000 cfa, approximately the same as the
100-year discharge (Ref, 8).

A hydrologic anilysls was not conducted as part of this study.
Diacharges for the study reach of the Big Thompaon River (References I, 2,
6, and 7) were reviewed to verify the consistency of discharges uaed in
other flood-plain studies conducted for this reach and the adjacent reaches
of the Big Thompson River.

Table 4 summarizes the peak discharges and drainage areaS used in this
study based on a review of the previous studies.

The most recent flood occurred on April 30 and May 1, 1980. Rainfall
of 3 to 4 inches fell over the entire Front Range area. The peak
discharges wcre estimated at 8,000 cfs on the Big Thompson River at the
confluence with Euckhorn Creek. The peak discharge at LaSalle Wa5 6,000
cfs on the afternoon of May 1. DWlIage from the flood was estimated at $2.2
mill1on, with erosion da=ge as "evere as the 1976 flood becouse of the
longer peak flow.

Table 4
Sununaryof Dhcharges

Location

O....iu..,,"
area
(sq-mi)

lO-year

Peak discharge",
50-year 100-year
----(cfs)--

50o-year

Eig Thompson River at
lower study limits
(County Rd. 9E)

5lS

4,700

12,300

19,000

44,000

Big Thompson River at
upper study limits

472

4,700

12,300

19,000

44,000

n

RE:\OURCE: CON\ULT~NT) INC

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