FLOOD04806

• County: Larimer
• Stream Name: Big Thompson/Cache la Poudre
• Basin: South Platte
• Title: Storm and Flood of July 31-August 1, 1976 in the Big Thompson River and Cache la Poudre River Basins
• Date: 1/1/1979
• Prepared For: Colroado Geological Survey
• Prepared By: USGS/NOAA
• Floodplain - Doc Type: Flood Documentation Report

CONTENTS

p,,,,
Abstract................................................ 1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Acknowledgments. . . , . . . . . . . . . . . . . . . . . , . . . , . . . . . . . . . 3
The setting.,.,.,.......,... ,_..............,......,.... 3
Location............................................ 3
Terrain features related to flooding. . . . . . . . . . . . . . . . . . . . 3
Seasonal distribution of floods ,..................,... 5
Precipitation........................................ 5
Meteorology of the storm................................ 9
Conditions prior to storm development. . . . . . . . . . . . . . . , 10
Conditions during the storm.......................... 28
Analysis of conditions in region of flash flooding. , . . 32
Physical models of thunderstorms. . . . . . . . . . . . . . . . . 40
Rainfall analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Hydrologic analysis of the flood. .................. 56
Flood data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
Station descriptions and streamflow data. . 56
Peak stages and discharges. . . . . . . . . . . . . . . 57
Velocities and depths ..................... 57
Flood marks and profiles. . . . . . , . , . , . . . . . , . . , . . . . . 62
Detailed description of flood areas....,.,.,.,.... ,..... 62
Big Thompson River basin: upstream from Olympus
Dam........................................ 62

Hydrologic analysis of the flood -Continued Page
Detailed description of flood areas-Continued
Big Thompson River basin: Olympus Dam to Love-
land Heights. , . , . . . . . . . . . . . . . . . . . . , . . . . . . . . . . 62
Big Thompson River basin: Loveland Heights to
Drake ....................................... 63
North Fork Big Thompson River basin: Glen Haven
vicinity to Drake. . . . . . . . . . , . , . . . , . . . . . . . . . . . . 64
Big Thompson River basin: Drake to mouth of can-
yon......................................... 65
Big Thompson River basin: mouth of canyon to
South Platte River........................... 66
Cache la Poudre River basin....................... 67
Flood frequency...................................,., 69
The aftermath. . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . , . . . . . , 70
The human element. . . . . . . . . . . . , . . . . . . . . . . , . , . , . . . , . . 70
The damage. . . . , . . . . . . . . . . . . , . , . . . . . . . . . . , . , . , . . . , . , 70
Comparison with previous rainstorms and floods, . , . , . . . , . , 71
Sources of data. . . . , . . . . . . . . . . . . . , . . . . . . . . . . . . . . , . , . . . , . , 72
Gaging-station and miscellaneous-site data. . . . . , . , . , . . . . . , 72
Selected references. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
Index............................. ...................... 83

ILLUSTRA nONS

PlIge
FIGURE 1. Map showing location of flood area in relation to west-central United States. . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . 2
2. Map showing detailed location of flood area.,........................................ ... ....................... 4
3. Photographs showing typical view of terrain in the Big Thompson River basin near the storm center. . . . . . . . . . . . . . . , 7
4. Map showing altitude range in area where flood originated. . . , . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . 9
5. Graph showing relation of area to altitude for approximate storm area upstream from Drake..............,...,.,.. 10
6. Map showing mean values of precipitation occurring during July in Colorado........... ,....... ........ .....,.,., 11
7. Map showing location of geographic features in Colorado and surrounding States referred to in the meteorological
discussion...................,.... ,....................................................,................. 12
8-11. Maps of the western conterminous United States, southwestern Canada, and northern Mexico showing:
8. Surface analysis, 0600 MDT, July 31, 1976... , ......................... .... , ............... ........... 13
9. 700-millibaranalysis, 0600 MDT, July 31. 1976.,.........................,............................ 14
10. 500-millibar analysis, 0600 MDT, July 31, 1976 .........,............... .... . ..........,., .,........... 15
11. Stability analysis, 0600 MDT, July 31, 1976........................................................... 16
12. Plot of rawinsonde data obtained at Denver, Colo., 0600 MDT, July 31, 1976..,.,.,...,....................,...... 17
13. Plot of raw ins on de data obtained at Sterling, Colo., 0740 MDT, July 31, 1976..................................... 18
14. Map of Colorado and surrounding States showing regional surface analysis, 1200 MDT, July 31, 1976............... 19
15. Map of Colorado and surrounding States showing radar summary for 1135 MDT, with locations of fronts and squall
line for 1200 MDT, July 31, 1976''''''''' ..... ,. ",. "...""""""", """,.,.""""",.,... 20
16. Geostationary Operational Environmental Satellite photograph, 1200 MDT, July 31, 1976......................... 21
17. Plot of raw ins on de data obtained at Sterling, Colo.,1320MDT,July31,1976..................................... 22
18. Map of Colorado and surrounding States showing regional surface analysis. 1400 MDT, July 31, 1976............... 23
19. Map of Colorado and surrounding States showing radar summary for 1335 MDT, with locations of fronts and squall
line for 1400 MDT, July 31, 1976 ",.".""""" ".,..",...,.,.""""""", .,.,. """'" .,...". 24
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