FLOOD03875

• County: Arapahoe
• Community: Aurora
• Stream Name: Granby/Sable Ditch
• Basin: South Platte
• Title: Flood Hazard Area Delineation
• Date: 4/1/1977
• Prepared For: Arapahoe County
• Prepared By: UDFCD
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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HYDROLOGIC AND HYDRAULIC DETERMINATIONS

-4-

One previous approximate hydrology study of existing conditions
was also made for Sable Ditch by the City of Aurora. _AjIai,I12.
the Rational Method was used utilizing a runoff coefficient
of 0.4 throughout the basin. The previous studies were
preliminary and not totally representative of future basin
conditions, therefore, the basin hydrology was reevaluated
for this study using the land use data given in Table II.

Hydrologic Analysis

A hydrologic analysis was completed to establish the peak
discharge frequency relationships for floods of 10-, 50-
and 100-year recurrence intervals.

S-2

Granby Ditch

5% low density (34% impervious)
40% medium density (45% impervious)
40% high density (80% impervious)
15% commercial (95% impervious)

65% low density (35% impervious)
5% medium density (45% impervious)
15% high density (80% impervious)
15% commercial (95% impervious)

50% semi-developed (20% impervious)
25% undeveloped (10% impervious)
15% industrial (95% impervious)
10% low density (35% impervious)

Sable Ditch

5% high density (80% impervious)
15% commercial (95% imperviouus)
20% industrial (95% impervious)
20% medium density (45% impervious)
40% low density (35% impervious)

5% commercial (95% impervious)
15% medium density (45% impervious)
20% undeveloped (10% impervious)
60% low density (35% impervious)

The Colorado Urban Hydrograph Procedure, as outlined in the
Urban Storm Drainage Criteria Manual was used to determine the
peak discharges for the 10- and 100-year storm frequency
(Ref.l). The 50-year peak discharge was interpolated on
log probability paper from the 10- and 100-year peak discharges
at the various design points. The calculations used rainfall
data characteristic of the study area as provided in the ,
Rainfall/Runoff Information Report (Ref.2). A summary
of the rainfall data and the drainage basin characteristics used
in this study is provided in Tables III and IV respectively.
The subbasin boundaries are shown on Sheet 1 of 12, Basin
Boundary, in the Drawings Section of this report.

Table II
Future Land Use Data
(Based on Current Planning Data)

Basin

G-l

G-2

G-3

Table III
Rainfall Data

Basin

Maximum Total Rainfall Depth
(Inches)

10-year 100-year

S-l

Granby Ditch

G-l*
G-2*
G-3*

Sable Ditch

S-l*
S-2**

1. 73 2.46
1. 93 2.64
1. 93 2.64
1. 73 2.46
1. 93 2.64

*Storm duration, 2 hrs.; T3S, R66W
**Storm duration, 2 hrs.; T4S, R66W