FLOOD10344

• County: Boulder
• Community: Boulder
• Stream Name: South Boulder Creek
• Basin: South Platte
• Title: South Boulder Creek Correspondence
• Floodplain - Doc Type: Correspondence

DRAFT
South Boulder Creek
Outline for Scope of Work
Hydrology - Browning/Jarrett

Component 4c - Flood frequency analysis / regional analysis
The purpose is to develop reasonable flood frequency curves that, at a minimum, meet the requirements of
Water Resource Council Bulletin 17B, FEMA, the Colorado Water Conservation Board, regional, and local
criteria and guidelines.

Regional Flood Frequencv Analysis using at-site and reJ!ional streamflow gaging station data
I) Information to be evaluated
I) Systematic records
2) Historic data
3) Incorporation of Paleoflood data
4) Comparison with similar watersheds
5) Flood estimates from precipitation
Data assumptions
Natural trends
Randomness of events
Watershed changes
Mixed populations
Reliability of flow estimates
Determination of the frequency curve
Annual flood series (vs. partial-duration series)
Log-Pearson Type III (Expected Moments, others?)
Station skew
Modified skew, weighted skew
Outliers
Refining basic curve using relevant info
Confidence limits

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Comparison ofhvdrologic results using published regional peak flow methods
I) USGS "Analysis of the magnitude and frequency of floods in Colorado"
1) Regionalized flood frequency relations for Colorado recently updated (2000)
2) Estimate peak discharges for 2,5,10,25,50,100,200, and 500-yr recurrence intervals
3) Streamflow gaging station records used
4) Ordinary least squares regression was used to determine the best explanatory variables
5) Generalized least squares regression was used to determine the best regression relation
6) Independent variables of drainage area, mean annual precip, and mean basin slope were determined to
be statistically significant
7) Five separate hydrologic regions were identified
8) Can be utilized for verification, calibration, and reasonableness check of rainfall runoff modeling results
IT) CWCB "Guidelines for determining 100-year flood flows for approximate floodplains..."
1) Streamlined hydrologic procedure for quickly estimating peak flows
2) Data collected from existing detailed floodplain studies throughout Colorado
3) Single independent variable (drainage area)
4) Subregions defined for "homogeneous" areas
5) Best-fit regression analysis (power function, Y=a(X)^b)
6) Can be utilized for verification, calibration, and reasonableness check of rainfall runoff modeling results