FLOOD10157

• Designation Number: 74
• County: Arapahoe
• Community: Aurora
• Stream Name: Granby and Sable Ditches
• Basin: South Platte
• Title: FHAD - Granby and Sable Ditches, Aurora, Colorado
• Date: 4/1/1977
• Designation Date: 7/27/1977
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Sheets 2 through 12. The computed 100-year flood elevations
at each reference point are tabulated in Tables VI and VII.
A description of the extent of flooding is discussed in the
following paragraphs.

Sable Ditch - The flood plain limits and flood profiles for
Sable Ditch are shown on the Flood Hazard Area Delineation
maps, Sheets 2 through 5.

Sable and Granby Ditches - Below Confluence

In the reach from the confluence with Granby Ditch to Sable
Boulevard, Sable Ditch is a deep straight channel which
conveys most of the 100-year flood. An exception to this is
the block from 23rd Avenue to 24th Avenue west of Atchison
Street. This area is subject to inundation caused by overflows
beginning near Station 15+00 and extending to 1-225. Much of
the area north of 24th Avenue not within the 100-year flood
plain is subject to shallow flooding. Because a major portion
of the combined Sable-Granby discharge flows to the north along
1-225 rather than directly into Toll Gate Creek, a water surface
profile was computed for this overflow and is shown on Sheet 4.

The flood plain limits and flood profiles for this reach, which
extends from Toll Gate Creek through 1-225, up to the confluence
of the Granby and Sable Ditch flood plain~ are shown on the
Flood Hazard Area Delineation Map, Sheet 2.

Downstream from 1-225, the entire area is inundated from the
Toll Gate 100-year flood plain and Granby and Sable Ditch
have virtually no effect on this area.

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Upstream from 1-225, two-thirds of the flow, 1700 cfs, spills
out of the Granby and Sable Ditch channel and flows north
down the east side of 1-225 to Sand Creek, inundating several
houses along Zion Street and Zion Street extended. North of
Cottonwood Park, none of the houses are affected by the overflow
from Granby and Sable Ditch. The first few rows of houses east
of Zion Street and south of Cottonwood Park, not in the main
flood prone area along 1-225, are subject to shallow flooding
from spillover from Granby and Sable Ditch. Shallow flooding
is defined as those areas subject to inundation with depths
less than one foot. The reason for this large overflow
from Granby and Sable Ditch is in part due to the low
capacity of the culvert under 1-225 and in part due to the
general slope of the land just north of Sable Ditch and west
of Sable Boulevard. The north bank of the Sable Ditch channel
is too low to contain the 100-year discharge in the ditch.

Above Billings Street, the block south of 24th Avenue is also
subject to shallow flooding. The water flows down 24th Avenue
and between the two rows of houses south of 24th Avenue. This
shallow flooding is caused by the inadequate culverts under
Sable Boulevard and the low point in Sable Boulevard located
approximately 250 feet east of the 23rd Avenue intersection.
Although most of this overflow is eventually conveyed back
into the Sable Ditch channel, approximately 100 cfs is lost
causing the shallow flooding along 24th Avenue above Billings
Street.

Drainage structures in the reach from Sable Boulevard to
Eagle Street include two 60-inch concrete culverts under both
Sable Boulevard and Eagle Street. These carry less than half
of the 100-year flood, resulting in inundation of both streets.
Some property damage will occur just upstream from both streets

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