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

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20,000

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Source:

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Area boundary to the downstream
Study Area boundary. The model was
developed and calibrated to simulate
the 100-year flood event, although it
could also be used to simulate flows
of other magnitudes and frequencies
of occurrence if hydrographs and
rainfall amounts are available.

Whenever published flow data were
available for a drainage watershed
tributary to the South Platte River
within the Study Area, that
information was incorporated into the
UDSWM model cases where
did

-Henderson. MaximU11 Daily Flow
~ - . Henderson - Average Daily Flow

14,000

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4.000

2,000

8.000

8.000

12.000

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Section 3
Hydrology

Over the last 25 years, average and peak flows do
not appear to be increasing as the Denver
metropolitan area urbanizes. To further
demonstrate this, Figure 3-4 presents the 10-year
moving averages of the flows at the two gages
over the period of record. Since 1988, both average
and peak flows appear to be trending Imver at
both gage locations. The net impact of
urbanization in the Study Area may be that local
flood control measures have effectively reduced
the magnitude of flooding, while urbanization has
increased flooding frequency.

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5,000

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Figure 3-5
South Platte River 100- Year
Design Storm and Hydrograph at Franklin Street

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Precipitation Event
Hydrograph at Franklin Road

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00000
00000
~ ~ ~ ~ ~
Time from Start of Storm Event (hours)

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have any
hydrographs
the CUHP

River
for

In
not
data,
with

South
been

watersheds
published flow
were developed
program.

1900

Figure 3-3
South Platte River at 64th Ave, and Henderson Road
Mean Annual Average Daily Flows

1985

1980

1975

,

1965 1970
Month

1960

1955

MaKlIT1urn Daily Flow
Average Da~y Flow

1950

-Hend~
~ ~ - Henderson

1945

1940

o
1935

5.000

4,000

6.000

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3.2 Flood Hydrology

The extent of flooding during peak flow events
was evaluated using the us. Army Corps of
Engineers (USACE) HEC-RAS version 3.1.1
computer model. The peak flows modeled with
HEC-RAS were developed from a combination
of data from prior studies and updated analysis
and hydrologic modeling, as described in the
following sections. Flood events simulated in
this study include the 10-, 100-, and 500-year
peak flows.

t

UDSWM hydrology was developed using the Merrick (1983) lOO-year hydrograph at Franklin Street as a starting
point, as shown in Figure 3-5. The downstream watersheds were added to the model in the following fashion:

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If a watershed tributary to the South Platte River had been modeled in another study, the hydrograph at its
confluence with the South Platte River was directly input as a hydrograph element.

1.

I

was established for that particular
(UDFCD 1990), with the

If a tributary watershed had no published hydrograph, a CUHP model
watershed using the parameters set forth in Drainage Watershed Description Maps
following exceptions:

2.

1965 1990 1995 2000
Figure 3-4
South Platte River at 64th Ave. and Henderson Road
10- Year Moving Mean of Annual Average Daily Flows

3.2.1 Prior Studies
South Platte River peak flows from Chatfield
Dam to the Adams/Weld County line have been
analyzed extensively in prior studies. USACE
studied the South Platte and its tributaries,
completing their analyses in 1968. The original
FHAD study was completed based on the
USACE's hydrologiC data (Gingery Associates,
Ine. 1977). Flood hydrology from Chatfield Dam to Sand Creek was updated by Merrick and Company in 1983,
and the 1977 FHAD floodplain was revised to reflect the 1983 hydrology (UDFCD 1985b)

,000

1980

1965 1970
Month

1950

1955

1950

1945

1940

o
1935

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For some direct flow watersheds, the parameters were not included in Drainage Watershed Description
Maps and were therefore determined specifically for this study.

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Sand Creek hydrology was established with additional UDSWM models for Tollgate Creek and Sand
Creek, in the same fashion as the South Platte River model. The published hydrograph on Sand Creek
(Kiowa 1990) below the Coal Creek confluence was the upstream hydrograph starting element for these
models.

The hydrology from the prior studies and the 1977/1985 FHADs 100-year peak flows were used in conducting the
hydraulic modeling component of the Master Plan. In that study, the 2-, 10-, 50-, and 100-year flood events were
considered. For the Master Plan, these peak flows and the 5-year event were considered in evaluating the extent of
flooding utilizing the HEC-2 model (BOSS RMS for AutoCAD HEC-2 Analysis version 2000). The 100-year event
represents the regulatory flood flows for the South Platte River in Adams County.

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Because the first estimate of the modeled hydrograph at Sand Creek's confluence with the South Platte River
exceeded the published values, a proportional reduction was made to the results of the hydrograph to match
the predicted peak to the published lOO-year peak flow for Sand Creek (UDFCD 1977), and the modified
hydrograph was entered into the South Platte River model as a hydrograph element. Furthermore, this

3.

Modeling

To substantiate the applicability of these flows to current conditions, a hydrologic model of the South Platte was
developed as part of the Master Plan effort. The model was developed using the Urban Drainage Stormwater
Model (UDSWM) (Boyle 1985a) and Colorado Urban Hydrograph Procedure (CUHP) (Guo 1997a) computer
programs. The UDSWM model routes the flow hydrograph in the South Platte River from the upstream Study

3.2.2 Hydrologic

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3-2