HomeMy WebLinkAboutFloodplain Analysis 11.01.2020c:oLoRADO
ENGINEERING
November 1, 2020
Mr. Mike Johnson
c/o MWJ LLC
Via Email only: mike@johnconinc.com
RE: Job No. 1213 - Rifle Creek Property - Floodplain Analysis
Dear Mr. Johnson:
Colorado River Engineering, Inc. (CRE) has completed a floodplain analysis for the MWJ
LLC property located along Rifle Creek, upstream of the Rifle Creek Golf Course in Garfield
County. This study has been completed to analyze the development of a home site and
creek crossing. Portions of the property fall within the FEMA mapped floodplain as part
of the National Flood Insurance Program. County, State, and Federal regulations allow
development within mapped floodplains; as long as the development occurs in fringe
areas, can be flood proofed, and does not impact water levels of the stream system
beyond acceptable limits.
Our review shows that the house site is outside of the 100-year flood hazard boundary
identified on the FEMA Flood Insurance Rate Map (FIRM). The analysis also shows that
all work associated with the creek crossing can be completed outside of the calculated
floodplain limits identified in our more detailed hydraulic modeling. In summary, the
house is outside of the floodplain, and although the bridge crossing is within the mapped
area, the work can be completed outside of the actual calculated floodplain extents and
will not have any impact to the 100-year floodplain.
It has been our experience and observation that bridge projects in Garfield County and
other nearby counties do not typically require permitting or adjustments to FEMA
mapping (e.g. LOMA's or LOMR's) to more accurately identify mapping based on updated
engineering. This is likely due to bridges not being subject to any mandatory insurance
requirements such as federally backed mortgages for homes within the National Flood
Insurance Program. A floodplain permit or boundary adjustment does not appear to be
warranted. We recommend this report be submitted with your building permit to Garfield
County to document that the house is outside of the floodplain limits and the bridge does
not impact the floodplain.
Existing Conditions - Proposed Improvements
The property (PIN 212718200027) consists of approximately 60 acres, as shown on the
attached Figure 1. State Highway 325 and Rifle Creek pass through the property. The
existing conditions in the area of the proposed home are illustrated on the attached
Figure 2 (Sheet 2 from CRE Site Plan drawings). The Rifle Creek Canon Ditch (RCCD) and
P.O. Box 1301 • Rifle, CO 81650 • Tel. 970-625-4933 • Fax 970-625-4564
COLORADO
RIVE
E N d 1 N E E R I N 6
INcOHPOIIA ri:r
the West Lateral of the RCCD divert water from Rifle Creek and traverse the property.
The proposed house site is situated along the valley floor at the base of the Grand
Hogback. The home site was chosen based on our recommend siting to mitigate rockfall
hazards. The property and the two irrigation ditch diversion points have historically been
accessed from a dirt drive at the northern portion of the property.
Previous analysis shows that a bridge crossing utilizing the existing drive entry could be
accomplished but would require significant fill in the calculated floodplain limits due to
minimal channel bank depths in the creek and elevated water levels from the diversion
dams. A new access point is proposed from State Highway 325 that will utilize a bridge
crossing the creek at an area with a deep incised channel and greater floodwater carrying
capacity.
Floodplain Mapping - FEMA
According to the current FEMA Flood Insurance Rate Map (FIRM) Panel No. 0802050965B
(attached), the proposed building site falls outside the Flood Hazard Zone A boundary.
Preliminary floodplain mapping updates are also attached. The preliminary mapping is
not a new study but merely provides improved background imagery and consistent
elevation datums for multiple study areas across the county.
Zone A delineates the floodplain boundary for the 100-year frequency event and has been
superimposed by the surveyor on the site plan drawings, as shown on Figure 2. The FEMA
Flood Insurance Rate Map for this area was completed by analyzing a few cross sections
on the property and then extrapolating elevation onto topographic mapping. These can
be considered "approximate" engineering methods due to the distance between cross
sections with respect to the varying topographic transitions. The mapping also shows a
Zone B, representing flooding hazards associated with the 500-year frequency event
where shallow flooding of less than 1-foot in depth could occur. The house site does fall
within Zone B. Zone B does not require the mandatory flood insurance for any federally
backed mortgages unlike those that are associated with Zone A. Private lenders could
require flood insurance. We recommend the home constructed in this zone be
constructed without any basements and with the finished floor elevation constructed 2-
feet above the existing grade.
Floodplain Base Flood Elevation Determinations
CRE has completed more detailed Hydraulic Analyses to develop Base Flood Elevations
(BFE's) and floodplain limits for the 1% frequency flow occurrence (100-year flood event).
2
P.O. Box 1301 • Rifle, CO 81650 • Tel. 970-625-4933 • Fax 970-625-4564
COLOfADO
WE
ENGINEERING
iNcoeroien rr.a
Hydrology —100 Year flow amounts (Q1oo)
Review of the FEMA Flood Insurance Study shows that the 100-year flood flow amounts
for Rifle Creek, upstream of its confluence with Government Creek, is 2,070-cfs.
Hydraulic Analyses
Numerous cross sections and elevation points were surveyed along Rifle Creek and were
utilized to develop topographic mapping (Figure 3). The survey included cross sections
near the proposed bridge crossing and home site as well as areas downstream. The cross-
section geometry was input into the US Army Corps of Engineers' HEC-RAS computer
modeling software. This hydraulic model calculates water elevations based on cross
section geometry, Manning's "n" friction coefficients, channel slope, and channel flow.
Final inputs used for the analysis included a Manning's "n" coefficient of 0.045 for the
overbank and channel areas. The Manning's "n" coefficients were chosen based on
coefficients used in the FEMA Flood Insurance Study and our site observations.
The attached printout summarizes the HEC-RAS computer model results. The Base Flood
Elevations (BFE's) and the 100-year floodplain limits in the project area are shown on the
attached Figure 3. The calculated floodplain BFE's fall within the creek channel banks.
The cross section near the proposed bridge (station 1+67) has a corresponding BFE of
5768.49-ft. I recommend that the bottom of the bridge have an minimum elevation of
5771-feet, which provides 2.5-feet of freeboard as a factor of safety.
Summary of Results
The hydraulic analysis shows that the improvements will not impact the Rifle Creek 100-
yr floodplain. We recommend the proposed home's finished floor elevation be
constructed 2-feet or higher than the existing ground elevation. If there are any questions
or comments, please do not hesitate to call (970) 625-4933.
Sincerely,
Christopher Manera, P.E.
3
P.O. Box 1301 • Rifle, CO 81650 • Tel. 970-625-4933 • Fax 970-625-4564
eiPublic,net Garfield County, CO
FIGURE 1
R210668:.,
RlP�E G•4A !SAND291) 34OU01-� 7 'l •.� �P
RA:I -;RIF
COURSE CGOLF
Account Number R210472
Parcel Number 212718200027
Acres 60
Land SgFt 0
Tax Area 021
2019 Mill Levy 63.9580
Date created: 9/12/2020
Last Data Uploaded: 9/12/2020 2:05:43 AM
Developed bytii. Schneider
GEOSPATI A L
Physical Address
Owner Address
0
RIFLE
KMLJ LLC
PO BOX 1926
RIFLE CO 81650
Overview
s'
Legend
ri Parcels
Roads
Parcel/Account
Numbers
Highways
= Limited Access
— Highway
Major Road
Local Road
Minor Road
Other Road
Ramp
Ferry
Pedestrian Way
Owner Name
Lakes & Rivers
— County Boundary
Line
2019 Total Actual Value $298,000 Last 2 Sales
81650 Date Price
1/6/2017 $0
1/1/2017 $0
FIGURE 1
• Proposed
Bridge
I Crossing
PO Box 1301
Rifle, CO 81650
Tel 970-625-4933
Fax 970-625-4564 rite Name: lKt
JOHNSON PROPERTY
FLOODPLAIN ANALYSIS
c
0
a)
W
50
Rifle1 Plan: Plan 06 9/12/2020
Alignment - (8) Alignment - (8)
Legend
100 150 200
Main Channel Distance (ft)
EG PF 1
WS PF 1
Crit PF 1
■
Ground
Rifle1 Plan: Plan 06 9/12/2020
RS = 2.82
5758-
Legend
EGPF1^
WS PF 1
Crit PF 1
Ground
■
Bank Sta
0 20 40 60 80 100 120
Station (ft)
1
Riflel Plan: Plan 06 9/12/2020
RS = 197.69
5780-
Legend
EG PF 1
WS PF 1
Ground
•
Bank Sta
0 20 40 60 80 100 120
Station (ft)
2
0
as
as
W
5778
5760
Rifle1 Plan: Plan 06 9/12/2020
RS = 187.76
045
0
Legend
20 40 60 80 100 120
Station (ft)
-EG PF 1
WS PF 1
Ground
•
Bank Sta
3
5760
0
Rifle1 Plan: Plan 06 9/12/2020
RS = 167.79
o
Legend
EG PF 1
WS PF 1
Crit PF 1
Ground
•
Bank Sta
20 40 60 80 100 120
Station (ft)
4
Riflel Plan: Plan 06 9/12/2020
RS = 148.17
r 045
5776-
5774
5772 ■ ■ -_
5770
5768
co
w
5766-
5764-
5762-
5760
0
Legend
EG PF 1
WS PF 1
—+— -
Crit PF 1
Ground
•
Bank Sta
20 40 60 80 100 120
Station (ft)
5
Rifle1 Plan: Plan 06 9/12/2020
RS = 122.85
— — - 045
776
5768
0
a)
W
Legend
EG PF 1
INS PF 1
Grit PF 1
Ground
•
Bank Sta
20 40 60 80 100 120
Station (ft)
6
w
5764
5756
.z
Riflel Plan: Plan 06 9/12/2020
RS = 85.05
.045
Legend
0 20 40 60 80 100 120
Station (ft)
EG PF 1
WS PF 1
Crit PF 1
Ground
/.
Bank Sta
7
Riflel Plan: Plan 06 9/12/2020
RS = 71.72
.045
5772
5770
5768
c
5764
a�
w
5762
5760
5758
5r 35
Legend
EG PF 1
WS PF 1
Crit PF 1
Ground
■
Bank Sta
20 40 60 80 100 120
Station (ft)
8
Riflel Plan: Plan 06 9/12/2020
RS = 46.09
Legend
EG PF 1
WS PF 1
Crit PF 1
■
Ground
•
Bank Sta
1
5756
0 20 40 60 80 100 120
Station (ft)
9
Plan: Plan 06 Alignment - (8) Alignment - (8) RS: 197.69 Profile: PF 1
E.G. Elev (ft)
5771.10
Element
Left OB
Channel
Right OB
Vel Head (ft)
0.40
Wt. n-Val.
0.045
W.S. Elev (ft)
5770.69
Reach Len. (ft)
9.93
9.93
9.93
Crit W.S. (ft)
Flow Area (sq ft)
405.98
E.G. Slope (ft/ft)
0.002194
Area (sq ft)
405.98
Q Total (cfs)
2070.00
Flow (cfs)
2070.00
Top Width (ft)
59.76
Top Width (ft)
59.76
Vel Total (ft/s)
5.10
Avg. Vel. (ft/s)
5.10
Max Chl Dpth (ft)
9.87
Hydr. Depth (ft)
6.79
Conv. Total (cfs)
44193.7
Conv. (cfs)
44193.7
Length Wtd. (ft)
9.93
Wetted Per. (ft)
67.83
Min Ch El (ft)
5760.82
Shear (Ib/sq ft)
0.82
Alpha
1.00
Stream Power (lb/ft s)
4.18
Frctn Loss (ft)
0.03
Cum Volume (acre-ft)
0.92
C & E Loss (ft)
0.03
Cum SA (acres)
0.23
Plan: Plan 06 Alignment - (8) Alignment - (8) RS: 187.76 Profile: PF 1
E.G. Elev (ft)
5771.04
Element
Left OB
Channel
Right OB
Vel Head (ft)
0.71
Wt. n-Val.
0.045
W.S. Elev (ft)
5770.33
Reach Len. (ft)
19.97
19.97
19.97
Crit W.S. (ft)
Flow Area (sq ft)
307.05
E.G. Slope (ft/ft)
0.004665
Area (sq ft)
307.05
Q Total (cfs)
2070.00
Flow (cfs)
2070.00
Top Width (ft)
51.53
Top Width (ft)
51.53
Vel Total (ft/s)
6.74
Avg. Vel. (ft/s)
6,74
Max Chl Dpth (ft)
9.47
Hydr. Depth (ft)
5.96
Conv. Total (cfs)
30308.7
Conv. (cfs)
30308.7
Length Wtd. (ft)
19.97
Wetted Per. (ft)
59.41
Min Ch El (ft)
5760.86
Shear (lb/sq ft)
1.51
Alpha
1.00
Stream Power (lb/ft s)
10.15
Frctn Loss (ft)
0.17
Cum Volume (acre-ft)
0.84
C & E Loss (ft)
0.15
Cum SA (acres)
0.22
Plan: Plan 06 Alignment (8) Alignment-(8) IRS: 167.79 Profile: PF 1
E.G. Elev (ft)
5770.72
Element
Left OB
Channel
Right OB
Vel Head (ft)
2.18
Wt. n-Val.
0.045
W.S. Elev (ft)
5768.54
Reach Len. (ft)
19.62
19.62
19.62
Crit W.S. (ft)
5768.38
Flow Area (sq ft)
174.97
E.G. Slope (ft/ft)
0.020662
Area (sq ft)
174.97
Q Total (cfs)
2070.00
Flow (cfs)
2070.00
Top. Width (ft)
37.29
Top Width (ft)
37.29
Vel Total (ft/s)
11.83
Avg. Vel. (ft/s)
11.83
Max Chl Dpth (ft)
7.99
Hydr. Depth (ft)
4.69
Conv. Total (cfs)
14400.7
Conv. (cfs)
14400.7
Length Wtd. (ft)
19.62
Wetted Per. (ft)
44.47
Min Ch El (ft)
5760.55
Shear (lb/sq ft)
5.08
Alpha
1.00
Stream Power (lb/ft s)
60.05
Frctn Loss (ft)
0.35
Cum Volume (acre-ft)
0.73
C & E Loss (ft)
112
Cum SA (acres)
0.20
Plan: Plan 06 Alignment - (8) Alignment - (8) RS: 148.17 Profile: PF 1
E.G. Elev (ft)
5770.25 Element
Left OB
Channel
Right OB
Vel Head (ft)
1.76 Wt. n-Val.
0.045
W.S. Elev (ft)
5768.49
Reach Len. (ft)
25.32
25.32
25,32
Crit W.S. (ft)
5767.87
Flow Area (sq ft)
194.43
E.G. Slope (ft/ft)
0.015257
Area (sq ft)
194.43
Q Total (cfs)
2070.00
Flow (cfs)
2070.00
Top Width (ft)
40.42
Top Width (ft)
40,42
Vel Total (ft/s)
10.65
Avg. Vel. (ft/s)
10.65
Max Chl Dpth (ft)
8.44
Hydr. Depth (ft)
4.81
Conv. Total (cfs)
16758.6
Conv. (cfs)
16758.6
Length Wtd. (ft)
25.32
Wetted Per. _(ft)
46.10
Min Ch El (ft)
5760.05
Shear (Ib/sq ft)
4.02
Alpha
1.00
Stream Power (lb/ft s)
42.77
Frctn Loss (ft)
0.45
Cum Volume (acre-ft)
0.64
C & E Loss (ft)
0.06
Cum SA (acres)
0.18
Plan: Plan 06 Alignment -(8) Alignment -.(8) RS: 122.85 Profile: PF 1
E.G. Elev (ft)
5769.73
Element
Left OB
Channel
Right OB
Vel Head (ft)
2.35
Wt. n-Val.
0.045
W.S. Elev (ft)
5767.38
Reach Len. (ft)
37.80
37.80
37.80
Crit W.S. (ft)
5767.38
Flow Area (sq ft)
168.27
E.G. Slope (ft/ft)
0.021475
Area (sq ft)
168.27
Q Total (cfs)
2070,00
Flow (cfs)
2070.00
Top Width (ft)
35,83
Top Width (ft)
35.83
Vel Total (ft/s)
12.30
Avg. Vel. (ft/s)
12.30
Max Chl Dpth (ft)
8.74
Hydr. Depth (ft)
4.70
Conv. Total (cfs)
14125.6
Conv. (cfs)
14125.6
Length Wtd. (ft)
37.80
Wetted Per. (ft)
41.51
Min Ch El (ft)
5758.64
Shear (Ib/sq ft)
5.43
Alpha
1.00
Stream Power (lb/ft s)
66.85
Frctn Loss (ft)
0.82
Cum Volume (acre-ft)
0.54
C & E Loss (ft)
0.17
Cum SA (acres)
0.16
Plan: Plan 06 Alignment - (8) Alignment - (8) RS: 85.05 Profile: PF 1
E.G. Elev (ft)
5766.55
Element
Left OB
Channel
Right OB
Vel Head (ft)
1.78
Wt. n-Val.
0.045
W.S. Elev (ft)
5764.77
Reach Len. (ft)
13.33
13.33
13.33
Crit W.S. (ft)
5764.77
Flow Area (sq ft)
193.37
E.G. Slope (ft/ft)
0.021874
Area (sq ft)
193.37
Q Total (cfs)
2070.00
Flow (cfs)
2070.00
Top Width (ft)
54.97
Top Width (ft)
54.97
Vel Total (ft/s)
10.70
Avg. Vel. (ft/s)
10.70
Max Chl Dpth (ft)
7.17
Hydr. Depth (ft)
3.52
Conv. Total (cfs)
13996.1
Conv. (cfs)
13996.1
Length Wtd. (ft)
13.33
Wetted Per. (ft)
59.58
Min Ch El (ft)
5757.60
Shear (Ib/sq ft)
4.43
Alpha
1.00
Stream Power (lb/ft s)
47.44
Frctn Loss (ft)
0.29
Cum Volume (acre-ft)
0.38
C & E Loss (ft)
0,01
Cum SA (acres)
0.12
Plan: Plan 06 Alignment - (8) Alignment - (8) RS: 71.72 Profile: PF 1
E.G. Elev (ft)
5764.83
Element
Left OB
Channel
Right OB
Vel Head (ft)
1.74
Wt. n-Val.
0.045
W.S. Elev (ft)
5763.09
Reach Len. (ft)
25.63
25.63
25.63
Crit W.S. (ft)
5763.09
Flow Area (sq ft)
195.70
E.G. Slope (ft/ft)
0.021426
Area (sq ft)
195.70
Q Total (cfs)
2070.00
Flow (cfs)
2070.00
Top Width (ft)
56.35
Top Width (ft)
56.35
Vel Total (ft/s)
10.58
Avg. Vel. (ft/s)
10.58
Max Chl Dpth (ft)
5.89
Hydr. Depth (ft)
3.47
Conv. Total (cfs)
14141.6
Conv. (cfs)
14141.6
Length Wtd. (ft)
25.63
Wetted Per. (ft)
60.45
Min Ch El (ft)
5757.20
Shear (lb/sq ft)
4.33
Alpha
1.00
Stream Power (Ib/ft s)
45.80
Frctn Loss (ft)
0.56
Cum Volume (acre-ft)
0.32
C & E Loss (ft)
0.04
Cum SA (acres)
0.10
Plan: Plan 06 Alignment - (8) Alignment - (8) RS: 46.09 Profile: PF 1
E.G. Elev (ft)
5763.76
Element Left OB
Channel
Right OB
Vel Head (ft)
1.61
Wt. n-Val.
0.045
W.S. Elev (ft)
5762.15
Reach Len. (ft)
43.27
43.27
43.27
Crit W.S. (ft)
5762.15
Flow Area (sq ft)
_
203.21
E.G. Slope (ft/ft)
0.022057
Area (sq ft)
203.21
Q Total (cfs)
2070.00
Flow (cfs)
2070.00
Top Width (ft)
64.65
Top Width (ft)
64.65
Vel Total (ft/s)
10.19
Avg. Vel. (ft/s)
10.19
Max Chl Dpth (ft)
5.62
Hydr. Depth (ft)
3.14
Conv. Total (cfs)
13937.9
Conv. (cfs)
13937.9
Length Wtd. (ft)
43.27
Wetted Per. (ft)
67.88
Min Ch El (ft)
5756.53
Shear (lb/sq ft)
4.12
Alpha
1.00
Stream Power (lb/ft s)
41.99
Frctn Loss (ft)
0.91
Cum Volume (acre-ft)
0.20
C & E Loss (ft)
0.03
Cum SA (acres)
0.06
Plan: Plan 06 Alignment - (8) Alignment - (8) RS: 2.82 Profile: PF 1
E.G. Elev (ft)
5761.91
Element
Left OB
Channel
Right OB
Vel Head (ft)
1.53
Wt. n-Val.
0.045
W.S. Elev (ft)
5760.38
Reach Len. (ft)
Crit W.S. (ft)
5760.31
Flow Area (sq ft)
208.93
E.G. Slope (ft/ft)
0.020003
Area (sq ft)
208.93
Q Total (cfs)
2070.00
Flow (cfs)
2070.00
Top Width (ft)
65.43
Top Width (ft)
65.43
Vel Total (ft/s)
9.91
Avg. Vel. (ft/s)
9.91
Max Chl Dpth (ft)
5.08
Hydr. Depth (ft)
3.19
Conv. Total (cfs)
_14636.2
Conv. (cfs)
14636.2
Length Wtd. (ft)
Wetted Per. (ft)
67.61
Min Ch El (ft)
5755.30
Shear (Ib/sq ft)
3.86
Alpha
1.00
Stream Power (lb/ft s)
38.23
Frctn Loss (ft)
Cum Volume (acre-ft)
C & E Loss (ft)
Cum SA (acres)