FLOOD01357

• Designation Number: 107
• County: Jefferson
• Community: Unincorporated Jefferson County
• Stream Name: Green Mountain Area
• Title: FHAD - Green Mountain Area
• Date: 4/1/1978
• Designation Date: 7/11/1978
• Floodplain - Doc Type: Floodplain Report/Masterplan

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

year and 100 year frequency flood peak. However, the storm runoff not
intercepted by the primary sewer system will flow to the secondary system
and exceed the capacity of the sewer. In addition, severe clogging of the
inlets in the secondary system by the sand and silt deposits reduces the
sewer system capacity and causes local flooding problems.
Additional flooding results from the hydraulic characteristics of
the paved channel between Center Drive and Virginia Drive. Because of the
extreme velocities, the 100 year flow will escape the channel at the
alignment deflection and will result in flows across the lawn opposite the
point where this channel meets Virginia Dr. Sheet flow also occurs from
Virginia Drive to Alaska Drive and then into the open space and channel area.
The flood hazards in the reach consist of erosion, debris removal ~nd cleanup,

flood peak, The flood peaks are contained within the channel area with

high velocity flow,
The channel above Zinnia Street varies from a broad parabolic section
near Zinnia Street to a more narrow V-section in the upper reaches, The
channel meanders slightly and has negligible obstructions, The average
slope is about 5.5%, The channel is crossed by Zinnia Street with two
60" RCP culverts. The storm flows have undermined the inlet to the
culverts and the pipes have dropped at the inlet, The channel has adequate

capacity to carry the 100 year flood peak,
The flood hazards in the reach consist of erosion, debris removal
and cleaning, and a moderate life hazard due to the deep channel with

interrupted traffic and services, content and structural damage to the houses,

high velocity flow,

E. Drainageway E

Drainageway E is a tributary to Lakewood Gulch at Simms Street

(also called Jack Rabbit Gulch) draining the northeast portion of Green

probable damage to the paved drainage channel, and a low life hazard due to
the shallow flooding,

2, Station 106+00 to 15+50, Alaska Drive to Union Blvd, Sheets
21, 22 & 23

The channel below Zinnia Street consists of a deeply incised

Mountain,
1. Station 109+00 to 56+00, Upstream Study Limits to 6th Avenue
(Sheets 24 and 25)
Tne channel in this reach varies from a broad detention area at
6th Avenue to a trapezoidal shaped section. The channel, which is part of the
Union Square development, has an average slope of 3.5%. The channel is crossed
by 6th Avenue (42" RCP with inlet structure and trashrack), and by Zang Street
(60" CMP with flared end section). The inlet structure at 6th Avenue serves
to reduce the flood peaks through detention ponding upstream of the frontage

trapezoidal shaped section with nearly vertical side walls and some minor

meandering. The lower portion of the reach consists of the five improved

detention ponds in the Union Square development. The ayerage channel slope

is about 3.5%. The channel has minor bank sloughing and negligible obstruc-

tions. The outlet of the upper pond consists of an inlet structure and

trashrack, a 60" RCP, and an outlet stilling basin. The outlet for two

of the lower ponds consists of an overflow weir constructed from gabion
baskets, The culverts under Van Gordon Street (two 54" RCP's), and
Union Blvd, (two 75"S x 43"H HE-RCP's) serve as outlets for the two ponds,
The detention pond has the adequate capacity for the routed 100 year

road.

14