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FLOOD11045
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Last modified
5/17/2010 12:51:13 PM
Creation date
11/30/2007 11:08:12 AM
Metadata
Fields
Template:
Floodplain Documents
County
Arapahoe
Douglas
Stream Name
Cherry Creek Reservoir
Basin
South Platte
Title
Probable Maximum Precipitation Study for Cherry Creek Reservoir - Technical Review - Final Report Technical Addendum
Date
8/1/2003
Prepared For
CWCB
Prepared By
Applied Weather Associates, LLC
Floodplain - Doc Type
Educational/Technical/Reference Information
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<br />Ideally, a RF -RO model should be calibrated based on precipitation data collected over the <br />basin and stream gage data at the mouth or point of interest. If such data exists, it will be <br />identified and used for model calibration. However, it is likely that such data are not available <br />for most sub-basins to be modeled in this study. In lieu of having the desired rainfall data, <br />regional regression analyses will be applied to assist in model calibration. Peak discharge for <br />various return periods will be calculated via regional regression. The HEC-l model results (i.e. <br />discharge hydrographs) driven with rainfall for the various return periods will be compared to <br />that determined via regional regression. If there is reasonable agreement, the HEC-l model will <br />be applied per the above discussion, but with adjustment to account for hydrologic characteristics <br />of very large floods. Adjustments would include lag-time, antecedent moisture and infiltration <br />rates (or, curve number). Although not ideal, this approach should be sufficient for the purpose <br />of this task - to assess the reliability of existing rainfall data and to estimate rainfall associated <br />with peak discharge. If there is not reasonable agreement between model results and regional <br />regression, the reasons will be determined and necessary corrections made to the model. <br />Task VIII in the RFP, collection of historic data, will be included here in Task 5. The <br />refinement of the PMP listed in Task V of the RFP will be addressed in Task 6. The approach <br />will used in the site-specific PMP study will be defined and presented in an interim report to the <br />technical review panel. <br /> <br />Task 6 <br />Discussions will be provided comparing the NWSSS with the A W A proposed approach for <br />the site-specific PMP calculations. A table will be developed to compare the two approaches. <br />The calculation ofPMP estimates from Task 5 in the RFP will be combined with the <br />determination of different methodologies from Task VII in the RFP. The A W A approach will be <br />to adhere to the basin definition of PMP and use the historically accepted procedure that has been <br />used in the Hydrometeorological Reports (HMRs). However A W A will incorporate new <br />technologies and techniques in the application of the historic procedure to the Cherry Creek site- <br />specific PMP study. This philosophy is consistent with the NWS approach taken in their newest <br />HMR publications, HMR 57 for the Columbia River Basin and HMR 59 for California, where <br />refined techniques are used. The A W A team will fully document any new techniques used and <br />provide discussion to justify how the new approach improves the PMP analysis. Extreme storms <br />identifies in the previous task will be carefully evaluated using historically accepted procedures. <br />New techniques and procedures will be implemented with any changes thoroughly documented. <br />The particular items listed in Task IX of the RFP will be addressed along with the computation <br />of the new PMP values since they are part of the procedure used in producing PMP estimates. <br />
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