FLOOD10337 ‎(2)‎

• County: Arapahoe
• Community: Greenwood Village
• Stream Name: Upper Goldsmith Gulch
• Title: Flood Hazard Area Delineation, Upper Goldsmith Gulch
• Date: 4/1/2005
• Prepared For: Urban Drainage and Flood Control District, Arapahoe County and Greenwood Village
• Prepared By: Moser and Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

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2-1

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vere calculated only for the existing land-use condition due to the area's fully-developed state.
'" with District policy, existing privately-owned detention basins were not included in the
hydrologic models because of the uncertainty associated with their continued existence.
owned and maintained detention basins were included in the models.

lists the one-hour rainfall depths

Additionally,
used for the analysis.

hour

storm duration.

maps.
the 80th percentile (0.5

in/hr )

event was also modeled.

depths
Table 2-1

below

Hydrographs
In accordancp

However,

publiclY-

from the USDCM

2.2.4 Rainfall
The one-hour rainfall

I-Hour Rainfall

depths for the 2-,

5-, 10-, 25-,
The CUHP model

50-,

100-, and 500-year storm
then distributed

the

rainfall

events

over a 2-

were gathered

I

adjusting the
peak flow at
discussed in more detail

in Section 2.4.

Drainage Criteria Manual
Appendix A.

I

. .
ImpervIOUS
Belleview Avenue,

the most

downstream

limit

of this study.

models generate similar
The comparison process

IS

(USDCM,200l).

The distribution of the C soil type can be seen on Figure A-3

In

I

values

The new UDSWM model

was compared to the
and the detention characteristics.

1989 UDSWM model by running several different scenarios
all the

In

cases,

For Type C
These values are

soil

in accordance with District criteria as

the

initial infiltration rate

is 3.0 inches per hour (iph)
referenced

in the Runoff Chapter of the Urban Storm

and the final

infiltration rate

is 0.5 iph.

The routing for the Study Area was generated using the
Water Management Model (UDSWM). Routing of the peak flows UDSWM model requires
information on the physical characteristics of the conveyance system as geometry, roughness, and
The existing conveyance system information was formatted for input into the UDSWM
runoff hydro graphs generated from the runoff was
to obtain peak flow

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system

rate,

stage data,

the COOP analysis,
and time-to-peak flow

information

routed through
at specific

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.

.

Surface retention storage values

Soil

infiltration rates

District's January 1,
using the
such

2001

version of the Storm

2.2.3 Soils
One soil type
Area. Generally,
hydrological soil

Appendix A.

fully-developed state.
levels of existing imperviousness.
to 95 percent.

2.2.2 Watershed Imperviousness
The watershed imperviousness was determined

variants (loamy soils)

with a

from

5

percent

was

only for the existing land-use
The mapping provided by the District along with site reviews were used to identify
Seven different categories of imperviousness were identified and ranged
types and locations A-2 in

The

. .
ImpervIOusness

found in the Goldsmith Gulch Study

are

condition due

shown

in Figure

to

the area's

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.

Time of concentration

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Drainage area
Length of the subwatershed
Subwatershed

District's 2000 version of the Colorado Urban Hydrograph Procedure
in the CUHP analysis include:

2.1
The

for each subwatershed

.

.

.

.

.

Percent imperviousness

slope
Distance to the sub watershed centroid

Information
identified by the Soil Conservation Service (1971)

the predominant soil types are Buick, Fondis, and Renohill

classification of Type C.

5 acres to 82 acres

inspections

contour mapping, provided by the District,
were performed to verify flow paths and boundaries.
with an average drainage area size of

19.5 acres.

to identify
sub watersheds range

The

in size from

METHODOLOGY
storm runoff hydro graphs for the sub watersheds

within

the

Study
(CUHP)

Area were generated using
The physical characteristics

the

The 2-foot

2.2.1 Sub watershed Delineation

A total of 50 subwatersheds were delineated

identification

displayed
characteristics of each sub watershed are described

and

locations

are

was used

on Figure A-I
in Table A-I

in the Goldsmith Gulch Study

In

Appendix
located

subwatershed boundaries.

in Appendix A.

In addition, the physical

sub watershed

A.

Area.

The

Field

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SECTION 2

2.2

CUHP PARAMETERS

HYDROLOGIC ANALYSIS