FLOOD04416

• Designation Number: 304
• County: Adams
• Community: Unincorporated Adams County
• Stream Name: Big Dry Creek, (ADCO), North area tributaries
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Big Dry Creek, ADCO, North Area Tributaries
• Date: 7/1/1989
• Designation Date: 1/1/1990
• Floodplain - Doc Type: Floodplain Report/Masterplan

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SECTION IV
HYDRAULIC ANALYSIS

A.

HEC-2 MODEL
The purpose of the hydraulic analysis in the FHAD is to define the 100-
year floodplain and 10-, 50- and 100-year water surface profiles. The
methodology used and the results of the analysis are presented in this
section. The floodplains and profiles were defined utilizing the dis-
charges associated with the projected future development condition and
the existing drainage facilities. The hydraulic calculations for this
study are available at the offices of the Urban Drainage and Flood Con-
trol District (UDFCD).

The floodplains for the tributaries were determined using a computer-
ized backwater program titled "HEC-2 Water Surface Profiles". The re~
quired input to the program is the channel cross section (taken from
project mapping), the hydraulic properties of each sub-section within
the cross section, and the discharge value. The longitudinal spacing
of the cross sections varies with channel conditions, but typically the
spacing was approximately 500 feet.

Estimates of channel and overbank roughness factors ("n") were made
after field investigations of the study area. The typical Manning's
"n" value used for the channel was 0.045, with values ranging from
0.045 to 0.060. For the overbank areas, Manning's "n" values ranged
from 0.045 to 0.080.

The starting water surface elevations for each of the tributaries were
critical depth for the 10- and 50-year storms and Big Dry Creek's 10-
year water surface for the lOG-year event.

IV-I

The capacities of the existing culvel1s under roadways and railroads
were determined in accordance with guidelines set by the Colorado Water
Conservation Board (CWCB) and UDFCD. Culverts smaller than 36 inches
in diameter were considered to be completely blocked during the 100-
year event. For culverts greater than 36 inches in diameter, the
effective cross-sectional area of the culvert was reduced by an amount
equivalent to a 36~inch diameter pipe.

Most existing culverts in the study area are 36 inches or less in dia-
meter and consequently were considered fully blocked for the floodplain
determination. Hand calculations based on weir~f1ow approximations
were made to determine the depth and width of overtopping at roadways
and railways embankments.

Discharges and starting water surface elevations for the projected
future development condition for the 10-, 50- and 100-year storms were
input into the HEC-2 model and water surface profiles calculated. This
data was used to plot on the 1" = 100', 2-foOI contour interval mapping
the I OO~year floodplain and profile. The appendix to this report con-
tains the FHAD drawings.

B.

INADVERTENT DETENTION AREAS
The Union Pacific Railroad (UPRR) and the
ments create areas of inadverdent detention
ing from 20 to 150 acre-feet. In all
created by these embankments was not
tributary peak flows.

Interstate 25 (1-25) embank~
storage with volumes rang-
cases, the detention storage
considered in determining

However, at two locations, the 1-25 crossing of Shay Ditch and South
Fork Preble Creek, the inadvertent detention created by the roadway
embankment was considered In the delineation of the floodplain and
floodway upstream. At these crossings, significant portions of runoff
would be hydraulically disconnected flow (i.e., split flow) rather than

IV-2