FLOOD09640

• County: Weld
• Basin: South Platte
• Title: Supplementing Flood Frequency Analysis in Eastern Colorado using Rainfall Modeling
• Date: 8/2/1999
• Prepared For: CWCB
• Floodplain - Doc Type: Flood Documentation Report

outcrops and badland formatIOns. Length averaged channel slope for all sites are between 0.020
and 0.025 with slopes varying between 0.015 and 0.035 within the basins.

All sites are ephemeral streams having no flow most of the year (Cochran, et aI, 1983). The
selected sites were selected with preference for drainage areas less than five square miles. This
restriction aids in the assumption that a given rainfall event will be uniformly distributed over the
entire basin.

2.2 Data

All storm data used in this study comes from the 5-minute rainfall-runoff data compiled by
Cochran, et al (1983). The actual devices are separate stage and rainfall recorders activated by a
single timer that record both stage and incremental precipitation at 5-minute intervals once
rainfall begins (Livingston and Minges, 1987). The stage discharge relationship for each site
was originally developed using a step-backwater analysis. Further refinements were made using
slope-area methods for significant floods.

Indirect slope-area measurements were made at Geary Creek tributary and Owl Creek tributary.
Problems were encountered with the reported data at Owl Creek tributary that required
adjustment prior to analysis. The problems stem from a discrepancy between the paleoflood
evidence and the published flow values. The details of the problems, adjustments and
justification are in Section 4. I of this report.

3. Methodology

3.1 Precipitation frequency analysis

Hydraulic structures are designed by assuming a storm of a given return period. A storm of
return period n has a probability of Un of occurring in any given year. The lOa-year rainfall
event is a standard design storm although a more or less frequent storm may be used depending
on the nature of the structure and the risk the designer is willing to take. Storm depths ranging
between the 2- and lOa-year event are used in *is analysis. .

Flooding in the plains basins is largely caused by short duration thunderstorms. Most urban
drainage design is based on the I-hour storm of a given return period (UDFCD, 1984). Miller
(1973) compiled data throughout the western United States and summarized the precipitation
frequency results in the National Oceanic and Atmospheric Administration (NOAA) Atlas 2.
The isopluvial maps for Colorado pertinent to this study are in Appendix A. The I aD-year I-hour
rainfall event in the study region is derived from the I aD-year maps for 6- and 24-hour duration
according to (Miller, 1973),

YIOO = 1.897 + 0.439[X3(XyX.j)] - 0.0082

where YlOo is the I DO-year I-hour depth, X3 is the I DO-year 6-hour depth, X4 is the I aD-year
24-hour depth, and Z is the station elevation in hundreds of feet. All depths are in inches.
Similarly, the 2-year I-hour rainfall event, h is found by,

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