WSP03196

• File Number: 8443.600
• Description: Narrows Unit - Studies
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 6/30/1975
• Author: Dames and Moore
• Title: Flood Study Narrows Dam South Platte River
• Water Supply Pro - Doc Type: Report/Study

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DAMES 8- MOORE

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These actual occurrences emphasize that i.t "is extremely hazardous

to base a prediction of the probable maximu1)l flood peak solely on previous

flood history. So also do the comparisons of peak floods and peak volume

with historical floods, given later as Plates 111-2 and 111-3. The entire

range of conditions that affect flood peaks, including basin topography

and shape, experienced storms, dewpoints, winds, snow accumulations,

temperatures, and ability of the soil to absorb water, must be considered

in the estimation of the probable maximum flood peak. This has been done

in the analyses both the USBR and we have made.

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Thus, while the PMF estimate would be an extremely rare event

(as it should be), it has the potential of occurring. Therefore, if one

should choose to take risks, 'he should not start with trying to reduce

the PMF. He should, instead, recognize the PMF as a ceiling and then

say how far below that ceiling he is willing to venture and take risks.

In the case of Narrows, because of the risk to human life, we recommend

no significant risk, choosing an IDF that is within the confidence band

of PMF.

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Risk of Damage to, Spill~

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Design Philosophtes

The acceptance of risk oJ; damage to the structure involves it

different philosophy than risk of failure of the, dam. Both we and the

USBR are in agreement on the philosophy of failure -- it should not be

accepted as a possibility. We tend to disagree, but in degree only, on

acceptance. of damage to the spillway through floods that do not endanger

the integrity of the dam. 'This difference is a matter of design

philosophies, but involves considerable costs.

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,The USBR has accepted in its design an auxiliary overflow

spillway that will oeprate very rarely (perhaps once in 1000 years), in

conjunction vith a gated service spillway that will operate more

frequently. Our discussions w!ththe USBR indicate that they are 'not

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