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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3-1

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The upstream gage is located at 64th A venue (USGS Station ID 06714215) and has been in operation since 1982. The
gage is upstream of the MWRD CTP outfall and therefore does not reflect the flows discharging from that facility.
Figure 3-1 presents the range of average daily flows recorded at this gage for the last 15 years from 1984 to 1998.

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3.1.2 South Platte River Streamflows
Within the Study Area,

two streamflow gages are operated by the USGS on the South Platte River

mainstem.

Figure 3-3 presents the average daily flows over the entire period of record at both gages (means and maximums
only). The large differences between the peak flows at 64th A venue and Henderson Road indicate that a significant
portion of the runoff from storm events originates from areas located between the two gages. Flood potential along
the South Platte has been significantly reduced, as expected, since the construction of Bear Creek, Chatfield, and
Cherry Creek Dams. However, the frequency of runoff events appears to have increased with the expansion of
upstream urbanization.

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..._.......... 'I' ........................ ..._. ._.......~ ....._....
Averaae Dailv Flow cfs) Calculated
Period of Peak 100-Year
Tributary USGS Gage ID Record Mean Minimum Maximum Flow lcfs)
Sand Creek 394839104570300 1992-1997 59 4 1,100 30,500
Clear Creek 06720000 1914-1990 95 0.4 2,720 23,000
First Creek 06720490 1991-1996 0.79 0.0 18 7,400

is period, daily have
lu The monthly distribution of
average and high flows is similar to the 64th
A venue gage, although flows generally peak in
June, 1 month later than the upstream gage.
Minimum average daily flows are sustained at
this downstream gage at a level ranging from 31 cfs in April

to 231 cfs in June.

II Oct N Dee

Month Figure 3-2
South Platte River at Henderson Road
Range of Average Daily Flows, Period of Record 1984-1998

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Table 3-1

Major tributaries contributing flow to the Study Area include Sand Creek and Clear Creek, with additional
contributions from numerous minor tributaries. Average, minimum, and maximum daily flows recorded on US.
Geological Survey (USGS) gages near the mouths of the two major tributaries and a representative minor tributary
(First Creek) are provided in Table 3-1. Also shown are the estimated peak flows with a 100-year recurrence
interval as reported in the 1984 master plan (UDFCD 1984).

Trib

fl

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PI

During
averaged 593 ci:~

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(263 cfs)

3.1.1 Watershed Inflows

Inflows to the South Platte River watershed include transmountain diversions and tributary drainage areas.
Approximately 154,000 ac-ft per year are imported to the watershed from trans mountain diversions located
upstream of the Denver metropolitan area and utilized for municipal use. A significant portion of water used for
municipal purposes is nonconsumptive and returns to the stream system after its use. Return flows from outdoor
use (primarily irrigation) reenter the South Platte River along its entire length through the metropolitan area.
Return flows from indoor use (sewerage wastewater) primarily return to the river at the MRWD CTP outfall
between 64th Avenue and Sand Creek, currently averaging approximately 170 million gallons per day (mgd)

The second gage is located downstream near
Henderson Road (USGS Station ID 06720500) and
has been in operation since 1927. Flows at this
gage reflect the discharges from MWRD and the
South Adams County Water & Sanitation District
(SACWSD) Williams-Monaco Treatment Plant, as
well as the tributary contributions from Sand
Creek, Clear Creek, and several minor tributaries.
Figure 3-2 presents the range of average daily
flows recorded at this gage for the same period
1984 to 1998 used to evaluate the 64th Avenue
gage. flows

2000

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:; 3000
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graph scale.

5000

6000

7000

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At the downstream boundary of the Study Area (168th Avenue), the South Platte River watershed drains
approximately 4,900 square miles of watershed. The area contributing to stream flows of the South Platte River in
Adams County extends from the Continental Divide in the Rocky Mountains through the foothills and into the
eastern Colorado plains. The hydrologic characteristics of the drainage area differ markedly between the mountain
and plains regions, particularly with respect to high flows. The mountain regions produce heavy snowmelt runoff
during the late spring to early summer period. While snowmelt runoff certainly influences flow in the plains,
summer convective thunderstorms are typically responsible for generating peak flood events in the Study Area.

events. Large flows also occur in other summer
months - the peak average daily flows recorded in
July, August, and October are typically the result
of summer thunderstorm events. Minimum
average daily flows at this gage range from 2 to
14 cfs and are not shown in Figure 3-1 due to the

3.1 Watershed Description

The hydrology of the South Platte River watershed has been extensively documented in prior studies. Information
from these studies was consolidated and presented in the Master Plan. Hydrologic information pertinent to this
FHAD is summarized and updated, as appropriate, in this section.

Over this period, daily flows have averaged 246
cfs. The high flow month is May, when daily flows
average 695 cfs. The highest flows also occur
during May, with a daily flow of 4,110 cfs
recorded in May 1987. This is typical of Front
Range streams where high flows during the
snowmelt season combine with runoff from storm

500

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ii:
~2000
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3000

3500

4500

4000

500

Jun Jul 0
Month Figure 3-1
South Platte River at 64th Avenue
Range of Average Daily Flows, Period of Record 1984-1998

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50p

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is too low at this location

Hydrology

1\

Section 3

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4/8/05