FLOOD10336 ‎(2)‎

• County: Adams
• Community: Brighton, Commerce City and Thornton
• Stream Name: South Platte River
• Title: Flood Hazard Area Delineation, South Platte River, Adams County
• Date: 4/1/2005
• Prepared For: UDFCD; Adams County; Brighton, Commerce City, Thornton, Metro Wastewater
• Prepared By: CDM
• Floodplain - Doc Type: Floodplain Report/Masterplan

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4/8/05

This FHAD includes prediction of 0.2 percent annual probability (500-year recurrence) flows for the South Platte
River that were not included in the Master Plan hydrologic modeling. Methodology used to compute the peak
500-year flows was suggested and approved by the FEMA Region 8 contractor, Michael Baker, Jr. Ine. (Baker) (Jula
2004).

3.3 500- Year Flows

Section 3
Hydrology

The 100-year storm hydrograph developed by Merrick (1983) was utilized in the CUHP model runs to generate the
hydrographs for the Master Plan, using a time step of 15-minutes (see Figure 3-5). The modeled hydrographs were
then input to the UDSWM model to route the flows in the South Platte River through the Study Area.

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Baker suggested using the 500-year flow rate from the 1995 Adams County Flood Insurance Study (FIS) at the
confluence of the South Platte River with Todd Creek to calculate peak 500-year flows at other locations along the
South Platte River. The following formula was thus used to calculate 500-year flows:

QIOO,
--

Q'OO,i-1

3.2.3 Results

Simulated hydro graphs at selected locations for the 100-year storm event are shown in Figure 3-6. This figure
demonstrates both the progression and attenuation of the flood event as the flood wave traverses the Study Area.
At Franklin Street, the peak flow is approximately 21,500 cfs and is a result of the flood originating from upstream
of the Study Area. Below the confluences of Sand Creek and Clear Creek, the peak flow rises to nearly 40,800 cfs.
The peak flows begin to attenuate in the downstream direction, producing at l04th A venue a flow of 36,400 cfs and
at Henderson Road a flow of 32,200 cfs. The peak flow does not decrease any further downstream, remaining at
cfs at least until the downstream 45.000

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QSOO,i-1

In the above formula, QSOO,i is the 500-year flow rate at cross-section i; Q500, i-I is the 500-year flow rate at the
downstream cross-section; Ql00,i is the 100-year flow rate at cross-section i, and Ql00, i-I is the 100-year flow rate at
the downstream cross-section. 500-year flows were developed for each model cross-section throughout the entire
Study Area, using Baker's methodology with the 1977/1985 FHAD 100-year flows that were used in the Master
Plan. Table 3-5 provides a summary comparison of 100- and 500-year flows.

Qsoo,

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Cross- Master Plan FIS FHAO
Section 100-Year Q 500- Year Q 500- Year Q
10 Station (cfs) (cfs) (cts) Notes
32,500 57,500 57,500 US Hwy 85
FISA 0+42 58,000
10 128+66 33,600 59,446 Todd Creek inflow
29 426+14 35,400 62,631 120th Ave
FIS Z 451+00
30 465+68 36,000 63,692 Bull Seep outfall
FISAE 600+20
36 602+68 35,600 62,985 104th Ave
FISAI 710+30
43 710+81 36,800 65,108
FISAK 779+30
45 779+41 37,200 65,815 88th Ave
FISAM 842+00 37,200 66,000 66,000
50 854+73 37,200 66,000 UPRR Crossinq
53 887+50 37,600 66,710 Clear Creek
61.085 954+32 38,000 67,419 Sand Creek
22,300 35,000 39,565 15th Street

Table 3-5100/500-Year Flows

eonnuence wi CI_ ere.A:

00 0 0 0 0 0 0 0 0 0 0 6 b b b 6 0 b 6 6 6 0 0 0 0 0 0 0
~ ~ ~ g ~ ~ ~ ~ ~ g ~ ~ ; ; ~ ~ ~ ~ ~ ~ g E ~ ~ ~ ~ ~ ~

Time from Start of Storm Event (hours)

40,000
35,000
30,000
~
.2 25,000
t
"
1: 20.000
6
15,000
Sase/Ill
10,000
.000

Figure 3-7 presents the 1977/1985 FHAD
and modeled flow rates throughout the
study reach. The modeled flow rates
computed in this study are within plus or
minus 8 percent of the published
1977/1985 FHAD values. For the HEC-2
modeling of existing conditions conducted
for the Master Plan, the 1977/1985 FHAD
peak flows were used, as these represent
the regulatory flows and their use
provides a direct comparison of modeled
flooding extent to that projected in the
1977/1985 FHAD studies.

32,200
boundary of the Study Area.

197711985 FHAD 100-yr
. - . COM Model100-yr Tl
f ~ ~ ____1984 Phase A 2-yr ~ c ~ 2
;;; . . .
c U U ~"'<~"= Interpolated 5-yr U . (; .
. c
C ." . -4 ~ 1984 Phase A 10...yr i! ." ." ~
C C ."
E . . - .- 1984 Phase A SO-yr u: . 0
~ l/) <; :to I- 1Il
r-_
--
-
--
- 4- _ _ _ _ _ _ _ . _ _ _ _ _ _ _ _ _ _. _._ _ _ _ _ _ _ _ _ _ _ _ __
--
.....~
T-~.------------+-~._-------
- . ...
i . . 1 . ----f

Figure 3-6
South Platte River
100- Year Hydrographs at Selected Locations

Simulated

60000

<0000

50000

Also shown in Figure 3-7 are flow rates for
the 50-, 10-, 5-, and 2-year flood events.
The 50-, 10-, and 2-year events are as
reported in the 1984 Master Plan Phase A
Report (UDFCD 1984), The 5-year event
was estimated by interpolation between the
10-year and 2-year events. These flows
were define the

water surface
with the HEC-

used to
profiles and flooding extent
2 model

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Four locations that bracket the Study Area were identified along the South Platte with 500-year flow rates from the
Adams County, Weld County, and City and County of Denver FrS reports: 15th Street in Denver, Clear Creek,
Todd Creek, and U.S. Highway 85 near Evans. U.s. Highway 85, Clear Creek, and Todd Creek flows were used to
calculate 500-year flows at the model cross-sections. The 500-year flow at 15th Street was compared to the derived
results, and shows an approximately 4,600 cfs (13 percent) difference. As 15th Street is a considerable distance
upstream of the Study Area (approximately 3 miles), this difference was considered acceptable.

1000+00 800+00 600+00 400+00 200+00 0+00
Stationing from County Line (ft,)
Figure 3-7
South Platte River
1 DO-Year Design Discharges

30000

2lJOOO

10000

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1200+00

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