FLOOD05988

• Designation Number: 311
• County: Arapahoe
• Community: Aurora
• Stream Name: Granby and Sable Drainageways
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Granby and Sable Drainageways
• Date: 9/1/1990
• Designation Date: 1/1/1991
• Floodplain - Doc Type: Floodplain Report/Masterplan

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V. SUMMARY OF HYDRAULIC ANALYSIS

Hydraulics

Hydraulic analyses of the study area were completed tu

determine the water surface elevations for the 10- and 100-year

storm events. The water surface elevations were compiled using

the U.S. Army Corps of Engineers (COE) HEC-2 Water Surface

Profiles computer program with digitized cross sections. The

digitized sections were taken from the 1" = 100' scale, two-foot

contour topographic mapping, and supplemented with cross sections

taken from the topographic maps, as needed. All cuI vert cross

section data were provided by the UDFCD, and verified by field

measurements.

Channel and overbank roughness coefficients were estimated

by field inspection and photographs along the drainageway.

Mannings 'n' values for channels varied between 0.016 and 0.045

and the overbank roughness varied between 0.02 and 0.08 for HEC-2

modeling purposes. In areas where flood flows move overland, the

roughness coefficient was held constant along the cross section.

In certain areas, higher 'n' values were used to account for

obstructions caused by existing buildings within the floodplain.

The digitized cross sections used for the HEC-2 modeling do not

ref lect the obstructions from buildings, and where appropriate

were modified to reflect building blockage.

Starting water surface elevations for Granby and Sable

Drainageways were estimated by using the data as provided by the

City as a result of Toll Gate Creek's channel improvement

project.

Flows in Granby Drainageway are diverted at Colfax to Toll

Gate Creek. The storm sewer along Colfax Avenue was estimated to

have a capacity of 440 cubic feet per second (cfs). The surface

flow remaining during a larger flood event will split. at the

intersection of Colfax Avenue and Sable Boulevard. Approximately

80 percent of the surface flow is likely to cross Colfax Avenue

and continue In a northwesterly direction unt i 1 it jOlns the

Sable Drainageway near 23rd Avenue and 1-225. The remaining 20

percent of surface flow will flow along the south side of Colfax

Avenue and eventually drain to Toll Gate Creek near 1-225 and

Colfax Avenue.

Major street crossings were assumed to have no obstructions

In the IlEC-2 model, due to limited sources for debris and no

large trees present.

For HEC-2 modeling purposes, three reaches were identified.

For Reaches 1 and 2, the maln channels of Granby and Sable

Drainageways are modeled separately. The overflow section from

the confluence area of Sable and Granby Drainageways to Sand

Creek was modeled as Reach 3.

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Channel and Floodplain
The channel and floodplain for Granby and Sable Drainageways
are described as follows:

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1. Granby Drainageway
The upstream study limit for the Granby Drainageway 1.6 at
Buckley Road. The section of open channel between Buckley Road
and 8th Avenue consists mainly of a sandy channel bottom with
light vegetation. Most of the channel section 1.S shallow and
flat with minimal bank erosion. The floodplain 1n this segment
1.S contained within open space areas.
From 8th Avenue to 13th Avenue, the flow is carried by storm
sewer and streets. Overland flow will likely flood streets and
residential buildings adjacent to the drainageway.
From 13th Avenue to Colfax Avenue, the channel 1S vegetated
with grass and wetland vegetation, sustained by trickle flow from
the upper drainage basin. The channel is generally small 1.n size
with little bank or invert erosion. The floodplain 1.n this
segment is mainly open space.
At Colfax Avenue, the 84-inch storm sewer diverts
approximately 440 cfs of the lOa-year peak flow to Toll Gate
Creek. From Colfax Avenue to Montview Boulevard, the flow i.n
excess of the storm sewer capacity is carried by local streets
and storm sewers. There are no well-defined surface flow paths
for this segment. Overland flow will occur due to limited Rtorm

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