FLOOD09514

• County: Grand
• Community: Ouray Ranch
• Stream Name: Colorado River
• Basin: Colorado Mainstem
• Title: Ouray Ranch Project Phase II Preliminary Plat
• Date: 12/13/1984
• Prepared For: Grand County
• Prepared By: RME
• Floodplain - Doc Type: Educational/Technical/Reference Information

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Mr. Ed Opitz
December 11, 1984
Page Two

6-13

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Using the Log Pearson method, the 100-year peak flow was estimated to
be 8,945 cfs, considerably larger than that estimated for the period prior to lake
operation. Logically, this is not possible, since the outflow from the lake could
not exceed the inflow (as estimated by the pre-reservoir period 100 year flow)
and in fact should be less due to attenuation of the peak through lake storage.
The apparent error in this estimation is due to the unnatural skew in the data
introduced by the large difference in peak flows between most years when releases
are on the order of about 100 cfs and the occasional years when the reservoir
fills and large flows greater than 1,000 cfs occur.

Very truly yours,

The unnatural nature of the flow data is further emphasized by the Gumbel
method analysis. The data plotted for this method could not be fitted by any
straight line or even curved line.

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In summary, an estimate of the 100-year peak discharge was obtainable
for the period prior to the operation of Lake Granby, however, this value should
be utilized only with the nature and limits of probability analysis in mind. An
estimate of the 100-year peak discharge for the period following operation of
Lake Granby was not possible using the available data. As we discussed on the
phone, operation of the dam must be considered in addressing this problem.

In addition to the above analysis, you raised some questions concerning
the use of an instantaneous peak discharge versus an average daily discharge,
and the nature and duration of the snowmelt runoff hydrograph.

First of all, an instantaneous peak discharge is the value most commonly
used in probability analysis for flood predictions. It represents the maximum
velocities and stream stages for the design of structures or flood plain delineation
for a particular event.

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With regard to the snowmelt runoff hydrograph, its shape is quite different
from that produced by a rainfall event in a small or medium sized watershed.
In the Colorado mountains, the snowmelt months of May, June, and July typically
produce about 80 percent of the annual volume of flow. Depending on temperature
and snowpack conditions, the snowmelt can remain at near peak discharge levels
for several days or two or more peaks of similar magnitude can occur separated
by several days of lower flows. In general, you could expect daily maximum
discharges within 10 to 20 percent of the annual peak discharge for a number
of days during the runoff period. In addition, the snowmelt hydrograph exhibits
a consistant daily pattern of flow representing the effects of day and night
temperature changes. The daily peak flow represents the arrival of water derived
from the peak melting rate in late afternoon and the timing of this peak is
dependent on the distance downstream from the primary snowpack areas.

If you have any additional questions, please call.

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HR~ATER CONSULT.ANTS, INC.

lib~ ~

Peter D. Boddie, Hydrologist

~fk;cLut.--

Karen L. Rudeen
Water Resources Engineer

Enclosures