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<br />I. Colorado extreme precipitation events are significantly smaller in their depth.
<br />duration and aerial coverage than the storm characteristics of HMR 52 that rely
<br />on 29 storms sampled over the eastern two-thirds of the country (See Figure I).
<br />
<br />2. The major axis of the rainfall pattern extends along the direction of the winds in
<br />the storm cloud layer (+/- 10 degrees). The surface wind direction is 40 to 90
<br />degrees to the right of the cloud layer winds (See Figure 2). In effect, this
<br />relationship of meteorology to rainfall pattern must be maintained during the
<br />transposition of storms into a basin to keep the transposition process "physically
<br />possible". The final PMP design storm must also maintain this relationship.
<br />
<br />Figure I shows that Colorado storms are smaller than the HMR 52 storms, The
<br />Colorado storms range from 66% to 91 % of the HMR 52 storms for area sizes ranging
<br />from 5,000 square miles down to 100 square miles. HMR 52 requires the use of its storm
<br />sample together with their rainfall distribution curves without offering any other
<br />alternative. This finding supports the fact that the use ofHMR 52 within/without curves
<br />is inappropriate in the Cherry Creek basin due to major differences between the Colorado
<br />storms and those used in HMR 52,
<br />
<br />Comparison of Average Within / Without Stonn Curves for
<br />Different Study Regions
<br />to-Square Mile Area Size
<br />
<br />10000
<br />
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<br /> u ---.. .u u
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<br />,I 'i I II " , i ,I ' II,i I, 1'1\1.
<br />
<br />--+- Storms Used in HMR 52
<br />
<br />-+-Colorado Front Range
<br />
<br />1000
<br />
<br />-.- Colorado East of Foothills
<br />
<br />--..- Colorado Eastern Plains
<br />
<br />100
<br />
<br />10
<br />~ ~ ~ ~ ~ ~ W ~ ro ~ w ~ ~ ~ m ~
<br />
<br />Percent of 24-hr Rainfall Amounts
<br />
<br />Figure I Comparison of Average Within I Without Stonn Curves for Different Study Regions
<br />
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