FLOOD10493

• County: Adams
• County: Arapahoe
• Community: Aurora
• Stream Name: Upper Box Elder Creek
• Basin: South Platte
• Title: Upper Box Elder Creek Outfall Systems Planning Study - Alternative Evaluation Report
• Date: 12/1/1994
• Prepared For: Aurora, Arapahoe County, Adams County, UDFCD
• Prepared By: CH2M Hill
• Floodplain - Doc Type: Floodplain Report/Masterplan

3.5 Rainfall
3.5.1 Criteria
The design rainfall events modelled for the study area were developed using criteria from
the Urban Storm Drainage Criteria Manual (USDCM) (UDFCD, 1969) and the National
Oceanic and Atmospheric Administration (NOAA) Atlas (NOAA, 1973). Tables 4-1 and
4-2 of the Rainfall Chapter of the USDCM stipulate criteria for general storm durations
and adjustments to rainfall based on watershed area. Table 3-3 provides a summary of
area adjustments, based on Tables 4-1 and 4-2 of the USDCM, used for this study.
3.5.2 Rainfall Depths
Rainfall depths for 1-, 3-, and 6-hour liurations were determined from the NOAA Atlas
(NOAA, 1973). One-hour rainfall depths were used to produce 2-hour duration storms
according to criteria shown in Table 3-1 of the Rainfall chapter of the USDCM (UDFCD,
1969). The I-hour depths were estimated for the 2-year and 100-year storm events using
the equation provided in Table 11 of the NOAA Atlas for Region 1 (South Platte River
Basin) (NOAA, 1973). One-hour depths for the 5-, 10-, and 50-year storm events were
estimated from Figure 6 in the NOAA Atlas (NOAA, 1973).
3-2

3.5.3 Hyetographs
A total of 24 hyetographs were developed for six recurrence intervals (2-, 5-, 10-, 50-,
100-, and 5oo-year), each having four area adjustments as shown in Table 3-3. These
hyetographs are provided in Appendix G. The methods used to create the hyetographs are
shown in Appendix G and briefly described below.
PENlOOlS186.WPS

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I Table 3-4 I
Summary of Unadjusted Rainfall Depths
Unadjusted Rainfall Depths (inches)
Storm Event I-Hour 3-Hour 6-Hour
2- Year 1.0 1.2 1.4
5- Year 1.3 1.7 2.0
10- Year 1.7 2.0 2.2
50- Year 2.3 2.7 3.0
l00-Year 2.6 3.1 3.4
500- Year 3.3 3.9 4.2

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I Table 3-2 I
UDFCD Stream Names and Numbers
UDFCD Stream Appendix F
UDFCD Stream Name Number Stream Number
Coyote Run 8168 100
W ouzles Gulch 8168-01 103
Prairie Dog Draw 8169 106
Rat Run 8170 119
Muskrat Gulch 8171 126
W oodrat Gulch 8172 131
Box Elder Creek 8100 200
Cardboard Draw 8173 204
Corrugated Gulch 8174 211
Empty Draw 8175 210
Younger Gully 8176 216
Statesman Gulch 8177 219
Deacon Draw 8178 223
(no name) 8179 226
Company Gulch 8180 229
Bender Drainage 8181 231
Drop Seed Drainage 8182 233
Forked Gully 8183 243
Beach Coulee 8184 247
Crossing Gulch 8185 249

Six-hour duration rainfall depths were estimated for the 2-, 5-, 10-, 50-, and 100-year
storm events by superimposing the study area onto the rainfall isopluvial maps provided in
the NOAA Atlas (Figures 20-25) (NOAA, 1973). Three-hour duration rainfall depths
were estimated using equation 4 in the NOAA Atlas, which is a function of the I-hour
and 6-hour rainfall depths (NOAA, 1973). The 1-, 3-, and 6-hour rainfall depths for the
500-year storm event were determined by plotting the 2-, 5-, 10-, 50-, and 100-year depths
on log-probability paper and extrapolating. A summary table of the 1-, 3-, and 6-hour
depths is shown in Table 3-4.

50-75

10-20

Watershed
Area (sq.IDi.)
0-10

20-30

6-Hour

3-Hour

Storm
Duration
2-Hour

6-Hour

Table 3-3
Summary of Area Adjustments

Need for Area Locations Where Area
Adjustment Adjustments Were Applied

no - All tributaries
- Upstream portion of
mainstem Coyote Run

- Middle portion of mainstem
Coyote Run (3.1 to 9.8 miles
upstream of north study
limit)

- Downstream 3.1 miles of
mainstem Coyote Run
- Mainstem Box Elder Creek
in study area and upstream
of study area

yes

yes

yes

2

Area Adjustment
Identifier
1

4

3