FLOOD06859

• County: Grand
• Community: Near Winter Park
• Stream Name: Jim Creek
• Basin: Colorado Mainstem
• Title: Flood Deposits in Jim Creek
• Date: 9/6/1989
• Prepared For: CWCB
• Prepared By: USGS
• Floodplain - Doc Type: Floodplain Report/Masterplan

in Albertson, M.L, Ed., Proceedings of the International
Symposium on Design of Hydraulic Structures, American
Society of Civil Engineers, Fort Collins, Colorado,
June 26-29, 1989.

HYDROLOGY AND PAI.:::OHY:J'~OLOGY USED TO IMPROVE TilE
UNDERSTANDING OF FLOOD nYDqO~ETEOROLOGY IN COLOnADO

Robert D. Jarrett
u~s_ Geological Surve~l, HS ~12, Box 25046,
Denver, Colorado 80225

ABSTRACT
A multidisciplinary study of streamflow, precipitation. and
paleohydrology was conducted to improve the understanding of flood
hydrometeorology in Colorado. Conventional flood-frequency analyses
do not adequately characterize the flood hydrology in the foothills
and mountains of Colorado. Annual peak flows are caused by snowmelt
at higher elevations In the mountains and by rainfall at lower
elevations. Above 2,300 meters (this elevation is lower in some
river basins), snowmelt rather than rainfall contributes to the
flood potential. Below 2,300 m. large rainfall-generated floods are
common. Regional flood-frequency methods. supported by paleoflood
information. were developed that indicate the 1976 Big Thompson
River flood has a recurrence interval of approximately 10,000 years.
The approach and results may be useful in decreasing the uncertainty
in the design .of hydraulic structures as discribed for the spillway
of Olympus Dam in the Big Thompson River basin or for other
hydrologic studies.

1 INTRODUCTION

The design of dams and other flood-control structures. land-use
management, and the siting of critical installations such as nuclear
powerplants and waste-storage facilities require evaluating risk
from floods. Because risk assessment of large floods and their im.
pact are uncertain, there is a need for a better understanding of
flood processes. Because the length of precipitation and streamflow
records at most sites in the United States generally is much less
than 100 years, it is difficult to accurately estim$te the magnitude
and frequency of large infrequent floods using conventional flood-
frequency analysis. When one or more large floods occur within a
short record. there is considerable uncertainty in estimating the
recurrence intervals. Consequently, conventional flood. frequency
analysis may not provide the most accurate representation of flood
risk.

The 1976 Big Thompson River flash flood
as much as 305 millimeters of rainfall in
discharge cf 883 cubic meters per second.

in Colorado resulted from
a few hours and had a peak
The flood killed 139

.....

"

people and caused property damage estimated at $35 million. Th.
flood occurred a short distance downstream from Estes Park. Colorado
(elevation 2.300 m). where the U.S. Bureau of Recl~ation's Olympus
Dam forms Lake Estes on the Big Thompson River. The inflow to Lake
Estes during the 1976 flood had a recurrence interval of about 2
years. The existing spillway is designed for a probable maximum
flood (PMF) of 637 m'/s. As a result of the nearby loca~ion of tha
1976 flood, a reevaluation of the capacity of the Olympus Dam
spillway was initiated by the U.S. Bureau of Recl~tion. The
revised PMF (u.S. Buresu of Reclamation, written commun., 1984),
based on new probable maximum precipitation estimates. is 2,380
mS/s. This revised design discharge would increase dramatically
the size of the spillway.

Two basic questions were posed as a result of the 1976 flood: (1)
What was the frequency of the flood, and (2) could a flood of thia
magnitude occur anywhere in Colorado? Conventional flood-frequency
analysis indicated that the recurrence interval of the 1976 flood
was between 100 and 300 years. It commonly was believed that a
flood like the 1976 flood could occur anywhere in Colorado.

A multidisciplinary study of streamflow and precipitation records
and paleoflood hydrology was conducted to improve the understanding
of flood hydrometeorology in the foothills and mountains of
Colorado. The purposes of this paper are (1) to provide an overview
of the study And (2) to provide an example to show how the informaA
tion obtained may be useful in the design of a hydraulic structure
specifically the spillway of Olympus Dam on the Big Thompson River.

2 OVERVIEW OF PALEOHYDROLOGY

To extend climatic and hydrologic records, hydrologists have used
paleohydrologic techniques (Costs, 1987; Patton, 1987; Stedinger and
Baker, 1987; Baker et al., 1988). Paleohydrology is the study of
the movement of water and sediment before the time of continuous
hydrologic records (Costa, 1987). Evidence of historic or prehis.
toric floods often are preserved in channels as distinctive sedimen.
tologic deposits or landforms as well as in botanical evidence.
These records are relatively easy to recognize and are long lasting.
generally for as long as 10,000 years (Jarrett. 1987). Tree-ring
data have been used to reconstruct past precipitation. temperature,
and average discharge for several hundred to thousands of years
(Fritts, 1976). Paleohydrology can provide important supplemencal
information about the spatial occurrence, magnitude, and frequency
of floods, droughts, and hydrologic variability. Paleohydrologic
information complements short. term streamflow records and can
provide information at ungaged locations. Paleoflood studies can
provide information about reasonable upper limits of the maxisum
size of floods that have occurred in a river basin. The results of
paleoflood investigations enhance conventional hydrologic studies
and help reduce the uncertainty in the flood hydrology.