FLOOD07627

• Designation Number: 220
• County: Douglas
• Community: Unincorporated Douglas County
• Stream Name: Marcy Gulch
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Marcy Gulch
• Date: 2/1/1983
• Designation Date: 3/1/1984
• Floodplain - Doc Type: Floodplain Report/Masterplan

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TABLE 1
DESIGN 100-YEAR RAINFALL PATTERN

In this equation L is the basin lenqth (mi), Lca is the distance from the
outlet to the centroid of the basin (mi) and So is the basin slope as
calculated from Figure 3. The measured slope as shown in Figure 3 was
found utilizing the following equation:

-------
Time % of Depth Time % of Depth
(Min. ) 1-Hr. Depth ( In. ) (Min. ) I-Hr. Depth (In. )
5 1 .03 65 4 .10
10 3 .08 70 2 .05
15 4.6 .12 75 2 .05
20 8 .21 80 1.2 .03
25 14 .36 85 1.2 .03
30 25 .64 90 1.2 .03
35 14 .36 95 1.2 .03
40 8 .21 100 1.2 .03
45 6.2 .16 105 1.2 .03
50 5 .13 110 1.2 .03
55 4 .10 115 1.2 .03
60 4 .10 120 1.2 .03
I-Hr. Depth ~2.57 in. 2-Hr. Depth = 2.97 in.

0.24 0.24 0.24 4.17
(L1S1 +L2S2 +....+LnSn )
---~--

S =

L1+L2+L/... .Ln

The val ues of L, Lca and S were determi ned from a topograph ic map for
each of the subareas used in the study. The peaking parameter Cp which is
a 1 so ut il i zed in the CUHP computer program was found us i n9 the fa 11 owi ng
equation:

0.15

Cp = PCtA

Where Ct ~ Coefficient from Fig. 1
P ~ Peaking parameter from Fig. 2
A ~ Basin Area in sq. mi.

The percent imperviousness needed for finding Ct and Cp was estimated
using a composite building density versus percent imperviousness for
Arapahoe County shown in Figure 4 on a semi-log scale. The housing density
was determined from the proposed Development Plan for Highlands Ranch which
includes the Marcy Gulch drainage area and closely approximates the housing
density vs. percent imperviousness for Arapahoe CQunty. For development
such as commercial business, industrial or school sites, or for areas of
housing densities greater than 5 units per acre, percent imperviousness was
estimated by analyzing individual site plans for such areas and/or similar
existing developments. The Planned Community District Development Guide
for the New Town of Highlands Ranch comprehensive plan calls for a minimum
of 10% of industrial and commercial areas to be landscaped, which relates
to an impervious factor of approximately 90%. Adjacent subdivisions for
similar type development have been using an impervious factor of 80%. For
purposes of CUHP runs for Marcy Gulch, an impervious factor of 85% was
used. An impervious factor of 5% was used for open space.

The time to peak parameter utilized in the CUHP computer program was found
USing the following relationship:

Ct Computer = (Ct(Fig. 1)

(L Lca) 0.18
(5) .24
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