FLOOD02900

• County: Adams
• Community: Commerce City
• Basin: South Platte
• Title: Lower First Creek and Direct Flow Area 0055 Major Drainageway Planning
• Date: 5/1/2002
• Prepared For: UDFDC
• Prepared By: Turner Collie & Braden Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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B.

IV. HYDRAULIC ANALYSIS

A.

GENERAl.

The purpose of the hydraulic analysis is to dctcrmine the characteristics and associated capacities of
the various channel reaches and structures. In order to make these determinations, physical
attributes (such as size, length, slope, etc) as well as parameters (such as material, amount of
vegetative cover, boundary conditions, etc) of both channel reaches and structures must be speci fied.

Thc U.S. Army Corps of Engineer's HEC-RAS Water Surface Profile computer program was used
to calculate the water surface elevations for the 10-,50-, 100- and 500-year peak flows. The peak
flows presented in Chapter III were used in the analysis. The geometric attributes for the hydraulic
analysis were obtained from the 2-foot contour mapping provided for the project. A 3-dimensional
digital terrain model of thc existing topography was prepared and cross sections were cut along the
stream alignments using HEC-GeoRAS. HEC-GeoRAS is an ArcVicw GIS extension specifically
designed to process geospatial data for use with HEC-RAS.

HYDRAULIC PARAMETERS

Manning's roughness coefficients en") for the main stem of First Creek wcre obtained from the
master plan prepared by Engineering Consultants, Inc. in 1977 (Reference 8). Those values are
0.045 for the overbanks and 0.040 for the main channel. For the tributaries in Direct Flow Arca
0055 estimates for the roughness coefficients were made after field investigations. The coefficient
used for the main channels is 0.040. For overbank areas a coefficient of 0.045 was used. Expansion
and contraction coefficients were obtained from the HEC-RAS Users Manual.

Streets and railroads crossing the floodplain were modeled by different methods depending on the
capacity of their hydraulic structures. Several of these crossing have relatively small culverts which
are likely to bc pluggcd with debris during major floods and were modeled by simply placing cross-
sections along the crest of the roadway or railroad and thus ignoring the culvert capacity. Crossings
of this type are at Peoria Street, Havana Street and 104th Avenue for First Creek. Other crossings
with larger culverts were modeled using the Special Culvert routine in HEC-RAS. Crossings of this
type are 96th Avenue, State Highway 2, Union Pacific Railroad, 1.76, U. S. 85 and Brighton Road.
There is only one existing bridge in the Lower First Creek watershed, the bridge at BNRR. This
bridge was modeled using the Special Bridge Routine in HEC-RAS. All crossings of the First Creek
tributaries in the study area were modeled using the Special Culvert routine in HEC-RAS.

The starting water surface elevations at the downstream limit for lower First Creek werc bascd on
South Platte River flood elevations from the Adams County Flood Insurance Study (Reference 12).
The I OO.year water surface of South Platte River from this reference (5034 feet) was uscd. For DF A
0055 and other tributaries downstream boundary conditions were based on the corresponding flood
elevations in the receiving stream. DF A 0055 and tributaries were also analyzed for "mixed" (both

D.

E.

sub-critical and super-critical) flow conditions thus requiring the establishment of upstream
boundary conditions. The upstrcam condition used for these streams was critical depth.

c.

FLOW SPLITS

A flow split occurs when a portion of the flood flow separates from the main flow path due to
inadequate flow capacity. This condition occurs at several street and railroad crossings, in thc
Lower First Creek watershed, the cxisting structures and adjacent flow paths are inadequate to
convey total flood flow under future development conditions. In the vicinity of the Union Pacific
Railroad (UPRR), East 104'h Avenue, and Interstate 76, the roadway and railroad profiles and the
topography are such that a portion of the 100.year flow overtops UPRR and US 85 and flows into
the main flood path downstream while another portion of the flow splits from the main flow and
creates a floodplain separate from the First Creek main flow path. This creates a new floodplain to
the north between the Union Pacific Railroad embankment and Havana Street. Other areas of flow
include the area north of East I 08'h Avenue and east of Brighton Road as well as further downstream
on the main stem of First Creek a few hundred feet upstream of Brighton Road on the right bank of
the First Creek.

The flow split at UPRR and East 104'h Avenue was modelcd utilizing the "split flow" option in
HEC-RAS Version 3.0 which allows the computer program to calculate the magnitude of the flow
split internally. For this case, the energy elevations at River Section 8917 on the main stem and
River Section 6068 on the split flow diversion were balanced to calculate a diversion of28% of the
total flow for the 100-year flow. The othcr two flow split areas were accounted for by extending the
main stcm cross-sections across the split flow areas. The hydraulic analysis of UPRR and Rolla
tributaries was conducted as if they were one stream. This was donc because eacb stream has a
minimal existing channel causing their floodplains to merge during major floods.

IMPOUNDMENT AT CROSSINGS

InlPounding at roadways and railroads creates inadvertent detention. This impoundment, which
could potentially reduce flood flows, occurs upstream of the embankmcnts at SH-2, 1-76, the
O'Brian Canal, Union Pacific Railroad, US 85, Old Brighton Road, and Fulton Ditch. Inadvertent
detention was not accounted for in the hydrologic analysis for this study.

RESULTS

The hydraulic discussion presented in this chapter is provided for information purposes only. A
complete presentation of the results, including discussion and delineation of the calculated
floodplain can be found in the FHAD study (Reference 23).

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