FLOOD04857

• County: Adams
• Community: Adams County
• Basin: South Platte
• Title: Hydrology Addendum Adams County And Incorporated Areas
• Date: 9/1/2000
• Prepared For: Adams County
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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ENGINEERING METHODS

Hydrolol!ic Analysis
The Adams County FrS presents both detailed and approximate floodplain delineations, Detailed
delineations were developed using standard hydrology methods and are mostly well documented.
Hydrologic information used for the FEMA approximate floodplain delineations are not
available or were not documented by FEMA. However, the Colorado Water Conservation Board
(CWCB), using regional regression methodologies for computing 100-year flood-flow values,
has developed acceptable hydrologic information for undeveloped basins, This methodology
was used for undeveloped basins in the eastern portion of the county,

Because of the complex hydrology of flood control improvements associated with urban
development, this methodology was not utilized for basins within the Denver metropolitan area,
For most of the developed basins, drainage reports and basin master plans are available from the
Urban Drainage and Flood Control District. Discharges for the developed basins were obtained
from these reports and are shown in the Appendix.

The CWCB regional regression equation methodology is fully described in a publication by the
CWCB entitled "Guidelines for Determining 100-year Flood Flows for Approximate Floodplains
in Colorado". Due to regional differences in flood flow approximations, three separate equations
were used in the county, The CWCB regression equations are shown in Table 3.

a e . el!lona el!reSSlOn ~quatlOns or n eve ope ams ounty
Sub- Discharge Equation Drainage Basin
Region ID Sub-Re,gion Name (cfs) Limitations (sq,mi.)
SPL-I South Platte, Eastern Plains QIOO = 707,9(A)o6S-l 2 to 1090
SPL-2 South Platte, Southern Foothills QIOO = 1005,5(A)o638 I to 170
SPL-3 South Platte, Northern Foothills QIOO = 800,8(A)om I to 445

T bl 3 R .

IR

E

f U d

dAd

c

For the equations in Table 3, the QIOO is the 100-year peak flow in cubic feet per second, and A is
the upstream (contributing) drainage area in square miles, The regression equations are only valid
for the range of drainage areas as shown, Calculation worksheets are shown in the appendix,
None of the stream gages have a period-of-record long enough to verify the estimated discharges
using USGS Bulletin 17B procedures,

The 100-year peak discharge for each of the undeveloped approximate floodplains is shown in
Table 4, As shown, Todd Creek Tributary 2 and North Tributary 7 have drainage basin areas that
are below the minimum applicable basin size for the equations, Nearly the entire Todd Creek
basin is currently being developed as ranchettes with unknown drainage improvements,
Therefore, the calculated discharges are likely inaccurate for existing and future conditions
within the basin. However, the discharges would have applied at the time that the FrS study was
conducted,

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