FLOOD02921

• County: Eagle
• County: Pitkin
• Community: Basalt
• Stream Name: Roaring Fork River
• Basin: Colorado Mainstem
• Title: Floodplain Information Report - Roaring Fork River
• Date: 11/14/2001
• Prepared For: Basalt
• Prepared By: Matrix Design Group, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

Roaring Fork River
Floodplain Information Report

3J,OXJ

25,OXJ

2O,OXJ

'iil
-
.!:!.
w
Cl
~15,OXJ
:I:
U
U)
i5

10,OXJ

5,OXJ

o
o

,
L"'l..1
'1
1'--1..'--- ,.

a:'-~'~J
w W ,L.._,
~ ~ 51 m ......-..-1
o () > L.lJ ::c::
~ ~ ~ 15 li1 '-
o w a:
9 w w:?: w
o ~ a: c:c ...J
() I l:::l t;
!;i: I- :fr ....
o
I
>-
:J
o
"

5

10

.~'t '""",,",cro,..,.
" "-..- "=='

Town of Basalt, Eaf?le & Pitkin Counties, Colorado

Figure 5

ROARING FORK RIVER

HYDROLOGY SUlVlVlARY

Hy<roIogy data fran the us. PmTf Caps d Er1!jneers assures
I\Jedi Reservoir is ope. ,,1b:J as a flood control reservoir, athc:Ju!tI
the I!u'eau d Redlll"lHion dd not construct the reservoir fa flood
control JUposes and ms rd assigned I""',....eo 4 flood contrd
storage in the reservoir.

0:.----"-
W
>
cr:
~

...
U)
>-
a:
<.:>

---

15

'W
IZ
I :J Notice that the QystaI Rh,.". v.ith a 364 sq. rri. cra;nage adds 6,600 cfs
l$l~
: to the 1QO.year flood fI<m in the Roaring For1< Rver, whereas
'''' the Fryingpan Ai_ v.ith a 2!Kl sq. rri. aainage adds only 1,000 cIS.
'"
I~ I

1G-Year
-- - 5O-Year

"',,1 I
! . ... ... .: ~" .,

'"
'"
to:(
"
"
Ig
1
I

"
W
W
a:
<.:> "
Z W
o lJ!
o <.:> "

"
W
a:
<.:>
, >-
, 0
I 0
o
"

-100-Year
,.,."~Year
I
,
I

,..............

I --1-;
1 L______
"-';-"-"~I"! I
I I -"-"1

"
W
W
a:
o
>:;
::>
o
u:
u.
o

~ += ~
'" <.:>
" a:
o w
>-
Z
::>
I

--....-.....,
,

20 25 3J 35
STAllQl11NG FROM MOUTH (mles)

40

45

EO

Page 14

November 14. 2001

Roaring Fork River
Floodplain Information Report

Town of Basalt, Eaf?le & Pitkin Counties, Colorado

4.2 Hvdraulic Analvsis

The water surface elevations for floods of Ihe selecled recurrence inlervals were computed Ihrough use of
the Corps of Engineers' HEC-RAS backwater computer program, A total of 112 cross sections were
analyzed for the hydraulic analysis of the Roaring Fork River and were secured from topographic
mapping and field surveys, The locations of these cross sections are shown by reference point on the
flooded area maps and the flood profiles in the back of this report, The 10, 50, 100, and SOD-year flood
elevations and discharges are listed in Table 3, Channel roughness factors (Manning's n) for these
computations were assigned on the basis of field inspection of the floodplain areas,

Starting water,surface elevations for Roaring Fork River calculated using critical deplh al the beginning
and end of the study, This is a reasonable assumption due to the steep gradient,

The hydraulic analyses for this study were based on unobstructed flow, The flood elevations shown on
the profiles are, thus, considered valid only if hydraulic structures remain unobstructed, operate properly,
and do not fail.

The flooded area maps, which show the boundaries of the lOa-year and 500-year floods, The low hazard
area is shown on the flooded area maps by cross-hatching,

4.2.1 Lazy Glen Trail Court
Lazy Glen Trailer Court, located upstream of the Highway 82 bridge, is on a historic floodplain, Fill and
small dikes have been placed to control flooding, Detailed analysis shows that the smaller floods such as
the 5 or lO-year flood, would not flood any portion of Lazy Glen but the lOG-year flood would overflow
into the area at two points, Only one ranch building occupies the floodplain in the reach above the Lazy
Glen Trailer Court, The upstream reach of the study area is largely confined within well-defined natural
banks ,

55

4.2.2 South Side Flow Split
The construction of Highway 82 through the town of Basalt effectively divided the Roaring Fork River
and its floodplain between the "North" and "South" Channels, The main channel is included in the North
Channel and carries the regular flows, The resulling South Channel is considered active only during
flood flows and has no distinguished or established natural channel or centerline, Map RF5 shows the
"South Side" Flood Area entire area as well as the various physical and floodplain related infonnation,
The area south of Highway 82, know as "South Side," has developed extensively and can be categorized
into Ihree reaches:
. The upper reach is primarily undeveloped ranch land and continues from Ihe flow split at the
Upper Bypass Bridge to river station 47+75 as shown on Ihe enclosed work maps,
. The middle reach is an urbanized area down stream of the upper reach that creates complex flow
conditions and is the focus of the two-dimensional modeling described below,
. The lower reach, downslream of the urbanized area, is differentialed from the middle reach by the
highway 82 embankment (river station 6+60),

After flood flows are conveyed through the middle reach, they flow over the highway embankment and
down into the lower reach, Floodwater in Ihis area flows through partially developed areas and back into
the main channel downstream of the wastewater treatment plan!. J,F, Sato, FEMA's subcontraclor,
previously completed modeling and floodplain delinealion in order 10 update floodplain maps and reflect
development impacts, including those crealed by the Highway 82 construction, In order to establish the
floodplain on the Soulh Side, J.F. Sato completed a flow split analyses and determined that approximately

'il~~~-"

Page 15

November 14,2001