FLOOD11045

• County: Arapahoe
• County: Douglas
• Stream Name: Cherry Creek Reservoir
• Basin: South Platte
• Title: Probable Maximum Precipitation Study for Cherry Creek Reservoir - Technical Review - Final Report Technical Addendum
• Date: 8/1/2003
• Prepared For: CWCB
• Prepared By: Applied Weather Associates, LLC
• Floodplain - Doc Type: Educational/Technical/Reference Information

5

values on Plates Ib through IVb were reproducible. In all
instances, the original and reproduced values departed by less
than one percent.

Using the barrier elevation map (Figure 3.4, HMR 55A and
Appendix B of this study) for the region in and around the Cherry
Creek Drainage, the raster fields of non-orographic values were
next adjusted to sea level assuming a saturated, pseudo-adiabatic
atmosphere. Basin average values of 10-mi2 non-orographic PMP
for each duration adjusted to sea level were then obtained using
GRASS software. These average values are shown in Table 1. To
reiterate what has just been done at this stage: 13,568 values
(depths) of 10-mi2, among-storm PMP were made to be centered at
each of the 13,568 grid locations in the Cherry Creek Drainage,
at sea level for each of four durations. A smoothed depth-
duration curve (Figure 1) was drawn for these four values and
depths at 12 and 48 hours were obtained. Five of these 10-mi2
values were then used in combination with the depth-area curves
of Figure 11.4 of HMR 55A (including depth-area curves, not
shown, interpolated for 12 and 48 hours) to obtain the values
shown in Table 2. Appendix B of this study contains Figure 11.4.
The Table 2 values were then read into a software program from
the BOSS Corporation referenced hereafter as PRHMR52 which
emulates the procedures of HMR 52.

PRHMR52 calculated labels for isohyets of within-storm
non-orographic PMP at sea level. For durations of 1-, 6-, 24-,
and 72-hours, raster fields were made starting from the PRHMR52
derived isohyets using GRASS software. In contrast to what was
done earlier, the GRASS software at this stage has produced
13,568 point values of within-storm PMP at sea level in the
Cherry Creek Drainage. We assumed that there was no variation in
the point value of precipitation in the .03 mi2 (.19 times .15)
area represented by a grid point. This assumption is more likely
to be correct for the distribution of rain rates from a PMS in a
.03 mi2 area than for everyday rain rates across the same sized
area. The 13,568 point values at sea level were then "returned"
to the barrier elevation using the inverse of the procedure which
"brought" the among-storm non-orographic values to sea level.
The within-storm, non-orographic point values of PMP for each of
the four durations were then multiplied by corresponding
K-factors to create a raster field of complete or total PMP.

Isohyetal depictions of these values are presented at a
scale of approximately 1:250,000 in Figures 2-5. The complete or
total PMP raster field was smoothed to prepare Figures 2-5. The
next step was to obtain average values of within-storm, total PMP
for the Cherry Creek Drainage at the durations of 1, 6, 24, and
72 hours from the unsmoothed raster field of total PMP. These
values are shown in Table 3 and are plotted in Figure 6 as filled
circles and a smooth curve was drawn through them. Values were
extracted from this curve for each 6-hour duration and 6-hourly