BOARD00961

• Board Meeting Date: 9/22/2003
• Description: Flood Section - Probable Maximum Precipitation Site-Specific Study for the Cherry Creek Reservoir-Study Findings and Recommendations
• Board Meetings - Doc Type: Memo

l. Colorado extreme precipitation events are significantly smaller in their dePth.
duration and aerial coverage than the stonn characteristics of HMR 52 that rely on
29 stonns sampled over the eastern I\vo-thirds of the country (See Figure I).
2. The major axis of the rainfall pattern extends along the direction of the winds in
the stonn cloud layer (+/- 10 degrees). The surface wind direction is 40 to 90
degrees to the right of the cloud layer winds (See Figure 2). In effect, Ihis
relationship of meteorology 10 rainfall pattern must be maintained during the
transposition of stonns into a basin to keep the transposition process "physically
possible". The final PMP design stoml must also maintain this relationship.

Figure I shows thai Colorado stonns are smaller than the H~vlR 52 stonns. The
Colorado stonns range from 66% to 91 % of the HMR 52 slonns for area sizes ranging
from 5.000 square miles down to 100 square miles. H:-"1R 52 requires the use of its storm
sample together with their rainfall distribution curves without offering any other
alternative. This finding supports the fact that the use of H\lR 52 within/without curves
is inappropriate in the Cherry Creek basin due to major differences between the Colorado
stomlS and those used in HMR 52.

Comparison of Average \'rilhin / WilhoUl SlOrm Cun-es for
Different Srudy Regions
IO-SqtJ;ue~fileAreaSiu

,-

'00

_ColoudoFron,R~nll<

,
,;

,

1: 1000
i
,7-

_CoI.-.r~doE~.'ofF",,,"ill.

:.-
'J-.
,
~

_Co~)udoEa"<mPlain.

.
~

_S'orm.L'K1IinHMR51

,.
~ ~ ~ ~ ~ " ~ 65 ro ~ 80 ~ ~ ~ 1001~

P..-rc..-nt <If !4-h. lUinb.lI Amount.

Figure I Comparison of An:rage Within I Without Storm Curves for Different STUdy Regions

IX