BOARD02224

• Board Meeting Date: 9/27/1999
• Description: Flood Section - Platte River Basin Issues - Report Regarding the Cherry Creek Dam Safety Study
• Board Meetings - Doc Type: Memo

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File: Pl'JPtopics799summary.doc

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July 21, 1999

6. HMR 52 was not developed using storms considered transpositionahle to the a
Cherry Creek drainage basin. Whizt analysis (Le. analysis vs opinion) has been _
performed to demonstrate the HMR '52 is appropriate for use for the non-orographic
component of PMP for the Cherry Creek location? (Tomlinson)

No consensus is reached by the group. The i'-iWS will examine a way to verify ./
this assumption

7. The most recent storm included in HMR 55A analyses is 1978. Was a review of
more recent storms conducted to identify significant rainfall events which could
influence the site-specific PlY/P studj? (Tomlinson)

Storm events are reviewed after they occur to determine if the event would have a
significant impact on the regional stUdy. As of 1995, no storms were identified
that would alter HMR55A.

8. Table 14 in HMR 52 provides a list of the storms used to develop the withinlwithout
curves. Averages of values from these 29 storms were used and smoothed It is stated
that the authors of HMR 52 chose not to consider any regional variation that may exist
in these storm ratios. They additionally state that they consider this conclusion (Le. not
to consider regional variations) to bejustified at this time (1982) but add "..., however,
future study should investigate regional variation in depth-area relations." In the
HMR 55A discussions related to Depth-Area-Duration Relations (Section 11), it states _
that "...DAD relationships in the non-orographic regions west of the HMR No. 51 _
region should decrease with increasing area size at an even faster rate than they do
within the HMR No. 51 region." In light of these identified variations west of the
HMR 51 region and the suggestion that future studies should investigate regional
variations in depth-area relations, why were not regiotwl variations in the
withinlwithout curves considered in the site-specific PMP study for Cherry Creek?
(Tomlinson)

No consensus is reached by the group. The NWS will examine a way to verify .......-
this assumption

9. Among storm K-faatm; were applied to the within storm rainfall values to account
for orographic effects, in agreement with the HMR 52 recommendation that additional
modification to the distributiqra of PMP brought about by terrain effects be considered
(Section 5.1, p43). However, HMR 52 states that these local modifications should
modify or warp the pattern in the direction of I1Uljor storm patterns that have been
observed in the drainage. Does the application of the among storm K-faaors modify
the pattern in the direction of major storm patterns that have been observed in the
drainage? Please provide exmnples. (Tomlinson)

No consensus is reached by the group. The NWS feels that any additional stUdy
results will have a minimal impact on results and there will be no further examination of
this issue at this time.

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