FLOOD05894

• Designation Number: 342
• County: Jefferson
• Community: Arvada
• Stream Name: Moon Gulch Watershed
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Moon Gulch Watershed
• Date: 8/1/1992
• Designation Date: 9/1/1992
• Floodplain - Doc Type: Floodplain Report/Masterplan

r
I I
I I
I
I
I
I I
I j
I
I
I
I
I
I
I
I
I I
I I
, I
! I
,

C/)
~
Ow
....J(!J
LLcr:
~~
<(~
Wo
CLC/)
I~
Ocr:
....J~
:::>z
C!)Q
t-
Z~
OC/)
o
~

<D
T""
ni w
1i' '"
~ :'S
d <C
'" W
'"
J: "
" F
~
" --L .q
J:
"
a: T""
"
J:
" .
., a:
'"
<C
W >-
'"
<
" 0
0 C\I
-L 0
T"" La

V. HYDRAULIC ANALYSIS

Hydraulic computations were made along the channel of Moon Gulch to determine the

extent of flooding which would occur during the predicted 100-year flood. The principal tool

used in the hydraulic analysis was the computer program developed by the U.S. Army Corps

of Engineers entitled "HEC-2, Water Surface Profiles." Data relating to channel slope,

roughness, cross section shape, and existing drainage facilities were developed from the

-'
~
(j
w
z
::;
J:
'"
:;:

topographic maps, facilities plans, and field reconnaissance. These data were then utilized

J

0 , in the computer program to calculate flow depths, widths, and velocities at selected cross
T"" .
a:
.......... >- sections along the channel. Typical channel cross sections and water surfaces are shown on
en
-' Z "'0
~ 0
(j 0 C 0 each of the FHAD sheets. The depths of flooding in the 100-year flood were determined
1:1 I- m T""
Ii! co en
Q. ~ ::::l at each cross section, and the plotted profile and topographic maps were used to interpolate
,..:
.. 0
~
< (fJ ..c
a t:::.. flooded outlines between cross sections.
;:;
-L .
r <D a:
>-
0 Values of Manning's "n" were selected in order to model the roughness of the stream bed
La
" and overbank areas. Each cross section or group of cross sections was evaluated and
"
z .q
0
~ assigned an "n" value for the left overbank, the channel and the right overbank. At a typical
"l
~
j" channel section, consisting of a natural channel with natural overbank areas, the generally
w .
" a:
z
w
~ C\I >- selected "n" values were between 0.030 and 0.035 in the main channel, and between 0.045
z
8 0
T"" -
I and 0.050 in the overbanks. Based on site visits and examination of the FHAD sheets, an

:l
w
U
z
o
;::
z
w
l:ii
~-L
~
c
z
~

f;j
w
a:
....
..
w
8
1i'
c
-'
w

I
I
I
I
,
J
;
;
;
;
;
;
/
,
;
I
,

0
0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0
La 0 La 0 La 0 LO 0 LO v
.q .q M M C\I C\I T"" T"" W
a:
:J
S,:j8 '38l:J'tH8SIO ~
u::
-14-

obstructed flow path between cross sections with significant development are sometimes

liriJited or non-existent. For hydraulic purposes, cross sections were modeled to reflect the

loss of conveyance due to channel and overbank development including buildings.

-15-