HomeMy WebLinkAbout1.00 General Application Materials
PERMIT FOR A DEVELOPMENT IN THE 100-
YEAR FLOODPLAIN
for
Three Mile Mobile Home Park Bridge Replacement
Project
January 30, 2025
Prepared for:
Roaring Fork Community Development Corporation
250 County Road 127
Glenwood Springs, CO 81623
Prepared by:
Roaring Fork Engineering
592 CO Highway 133
Carbondale, CO 81623
Table of Contents
Application Forms and Data
Application Form
Proof of Ownership
Statement of Authority
Agreement to Pay Form
Mineral Rights Form
Copy of Pre-Application Summary
Application Form Narrative
List of Owners of property within 200 feet
Mineral Rights
Project Narrative
Overall Project Narrative
Bridge Design Narrative
Water and Sewer System Design
Vicinity Map and Project Drawings
8.5”x11” Vicinity Map
Existing Conditions Map
Design Drawings
Proposed Site Plan
Proposed Bridge Grading Design
Water and Sewer Design
[Included Separately]
[Signature Page Included Separately]
Details
Bridge Design
Floodplain Analysis
Construction Practices
Zero Rise Determination
Scour Analysis
Code Compliance Memo
Community Development Department
108 8th Street, Suite 401
Glenwood Springs, CO 81601
(970)945-8212
www.garfield-county.com
LAND USE CHANGE PERMIT
APPLICATION FORM
TYPE OF APPLICATION
Administrative Review Development in 100-Year Floodplain
Limited Impact Review Development in 100-Year Floodplain Variance
Major Impact Review Code Text Amendment
Amendments to an Approved LUCP
LIR MIR SUP
Rezoning
Zone District PUD PUD Amendment
Minor Temporary Housing Facility Administrative Interpretation
Vacation of a County Road/Public ROW Appeal of Administrative Interpretation
Location and Extent Review Areas and Activities of State Interest
Comprehensive Plan Amendment Accommodation Pursuant to Fair Housing Act
Pipeline Development Variance
Time Extension (also check type of original application)
INVOLVED PARTIES
Owner/Applicant
Name:________________________________________________Phone:(______)_________________
Mailing Address: ______________________________________________________________________
City:_______________________________________ State:_______ Zip Code:____________________
E-mail:_______________________________________________________________________________
Representative (Authorization Required)
Name:________________________________________________Phone:(______)_________________
Mailing Address: ______________________________________________________________________
City:_______________________________________ State:_______ Zip Code:____________________
E-mail:_______________________________________________________________________________
PROJECT NAME AND LOCATION
Project Name:
_____________________________________________________________________________________
Assessor’s Parcel Number: ___ ___ ___ ___ - ___ ___ ___ - ___ ___ - ___ ___ ___
Physical/Street Address: ________________________________________________________________
Legal Description: ______________________________________________________________________
_____________________________________________________________________________________
Zone District:___________________________________ Property Size (acres): __________________
ROJECT NAME AND LOCATION
2 1 8 5 2 7 2 0 0 0 1 0
Roaring Fork Community Development Corporation
PO Box 2026
Carbondale CO 81623
Phillip Supino 970 343-2463
PO Box 9316
Aspen CO 81611
phillip@headwatersplanning.com
Three Mile Vehicle and Pedestrian Bridge Replacement
250 County Road 127, Glenwood Springs, CO 81623
Section 27 Township 6 Range 89 NENESEC.28
Residential Suburban 11
✔
Please see separate signed copy of second page in the project folder.
PROOF OF
OWNERSHIP
CERTIFICATION OF MINERAL OWNER RESEARCH
This form is to be completed and submitted with any application for a Land Use Change Permit.
Mineral interests may be severed from surface right interests in real property. C.R.S. § 24-65.5-101, et seq,
requires notification to mineral owners when a landowner applies for an application for development from a
local government. As such, the landowner must research the current owners of mineral interests for the
property.
The Garfield County Land Use and Development Code of 2013 (“LUDC”) Section 4-101(E)(1)(b)(4) requires
written notice to owners of mineral interests in the subject property in accordance with C.R.S. § 24-65.5-101,
et seq, “as such owners can be identified through the records in the office of the Clerk and Recorder or
Assessor, or through other means.” This form is proof of applicant’s compliance with the Colorado Revised
Statutes and the LUDC.
The undersigned applicant certifies that mineral owners have been researched for the subject property as
required pursuant to C.R.S. § 24-65.5-101, et seq,and Section 4-101 (E)(1)(b)(4) of the Garfield County Land
Use and Development Code, as amended. As a result of that research, the undersigned applicant certifies
the following (Please initial on the blank line next to the statement that accurately reflects the result of
research):
I own the entire mineral estate relative to the subject property; or
Minerals are owned by the parties listed below
The names and addresses of any and all mineral owners identified are provided below (attach additional pages
as necessary):
Name of Mineral Owner Mailing Address of Mineral Owner
I acknowledge I reviewed C.R.S. § 24-65.5-101, et seq, and I am in compliance with said statue and the
LUDC.
_____________________________________________
Applicant’s Signature
__1/31/25_______________________
Date
X
_____________________________________________
Applicant’s Signature
1
Community Development Department
108 8th Street, Suite 401
Glenwood Springs, CO 81601
(970) 945-8212
www.garfield-county.com
PRE-APPLICATION
CONFERENCE SUMMARY
PARCEL NUMBER: 2185-272-00-010 PREPARED: 11/20/24
PROJECT: Three Mile Mobile Home Park Floodplain Development Permit
OWNERS/APPLICANT: Roaring Fork Community Development Corporation
PRACTICAL LOCATION: 250 County Road 127, southeast of the City of Glenwood
Springs on Three Mile Creek
ZONING: Residential Suburban (RS)
TYPE OF APPLICATION: Flood Plain Development Permit
I. GENERAL PROJECT DESCRIPTION
The Applicant has met with Staff to discuss the replacement of an existing bridge over Three
Mile Creek providing access to the mobile home park. The bridge is in need of repair due to
damage from flood waters and high flows on Three Mile Creek. The Applicant has indicated
that they have engineering consultants preparing plans for the replacement of the bridge.
The new structure was described as being located in the same location as the existing bridge
with upgraded abutments and protection from erosion and improved bank stability. Topics
discussed included conveyance of flood waters, location with in the mapping floodplain for
Three Mile Creek and related hydrologic issues reflective of County Land Use and
Development Code provisions and requirements associated with development/structures
within the 100 Year Floodplain.
Code provisions include Section 3-301 including relevant provisions on specific standards,
areas of shallow flooding, placement of fill, and drainage consideration, Section 4-109 with
review criteria, Section 4-203 (O) with details on Floodplain Study requirements.
Staff is further researching related FEMA provisions allowing for repairs to flood damaged
infrastructure including bridges. Additional information on this option will be provided as it
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becomes available from FEMA and may result in changes to the pre-application summary
and permitting requirements.
A copy of the FEMA Floodplain mapping for the site is also attached with this summary.
OTHER COMPLIANCE ISSUES
Given the lack of historic information and permitting for existing mobile homes within the park
that may be located in the 100 Year Floodplain, current documentation of compliance with
standards needs to be provided. A Floodplain Development application may need to include
commitments/updates on meeting standards for mobile homes within the 100 year floodplain.
This includes compliance with Section 3-301(C)(4) standards for Manufactured Homes with
elevation and anchoring requirements.
The County anticipates working with the Applicant on the timing for review of a Floodplain
Development Permit Application in conjunction with other Applications for Special Use Permit
Amendments that may be required for the overall mobile home park. Concurrent review
and/or approval with conditions are options that may be available to ensure efficient review
and implementation and scheduling of critical life safety improvements.
Allowances for installation of needed life safety – access improvements while other Special
Use Permit Amendments are in process may be considered and is not inconsistent with past
policies of the Department.
VICNITY MAP
(site shown in red)
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II. REGULATORY PROVISIONS APPLICANT IS REQUIRED TO ADDRESS
Garfield County Land Use and Development Code, as amended (LUDC) including the
following Sections:
• Section 3-301 – Floodplain Overlay Regulations including but not limited to 3-
301(C)(4), and 3-301(E)
• Table 3-301 – Floodplain Overlay Use Restrictions Table
• Section 4-109 – Development in the 100 Year Floodplain, including review criteria
• Table 4-102 – Common Review Procedures and Required Notice
• Table 4-201 – Submittal Requirements
• Section 4-203 – Submittal Requirements as appropriate
• Section 4-203 (O) – Flood Plain Analysis
• Section 4-118 – Waiver from Standards (if needed)
• Section 7-203(B) & (C) – Protection of Water Bodies
III. REVIEW PROCESS
The Floodplain Development permit will be processed pursuant to Section 4 -103,
Administrative Review Process and consistent with Table 4-102 Common Review
procedures for Floodplain Development, and as summarized in the attached flow chart.
If the Application is being heard concurrently with other Special Use Permit Amendment
Applications that will have a public hearing before the Board of County Commissioners
(BOCC), or if the Floodplain Development Permit warrants it, the Director may exercise the
option of referring the Application to the BOCC.
IV. PUBLIC HEARINGS AND NOTICE
Public notice shall be mailed pursuant to Section 4 -101.E.
_X __No Public Hearing (Director’s Decision unless referred to the BOCC – mailed
notice required)
____ Planning Commission
____ Board of County Commissioners
____ Board of Adjustment
Referral agencies may include but are not limited to: Garfield County Consulting
Engineer, Garfield County Attorney, Army Corp of Engineers , FEMA. Garfield County
Road and Bridge, City of Glenwood Springs, Roaring Fork Conservancy, County
Vegetation Management.
V. SUBMITTAL REQUIREMENTS
A. General Application Materials including
o Application Form including signatures
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o Proof of ownership (deed and/or title information)
o Authorization to represent as appropriate
o Statements of authority
o List of owners of property within 200 ft. with addresses (to be reviewed
and accepted by the County Attorney’s Office)
o List of owners of mineral rights on the subject property and/or description
of how this was researched.
o Payment of fees
o Agreement to Pay Form
o Project Description – narrative
o Copy of Pre-Application Summary
B. Vicinity Map – formatted on 8 ½ x 11 size for use with public notice
C. Site Plan – to applicable scale, including all proposed improvements and
significant features.
D. Floodplain Analysis – meeting the requirements of the Land Use and
Development Code including Sections 3-301 and 4-203 (O) as applicable.
The Flood Plain Analysis/memo needs to be completed by a qualified
licensed professional engineer and needs to include the engineer’s stamp.
Some elements may not be applicable based on the type of project. An
explanation – justification in those cases should be provided.
E. The Application will need to include details and plans for the proposed bridge
structure including any materials/structures placed within the floodplain,
floodplain cross sections or elevations and any bank stabilization
improvements.
F. The Analysis will need to address the lack of any impacts from the proposed
improvements (i.e. no impact on flooding potential – Zero Rise Determination)
G. The Analysis will need to include information on construction practices
including staging areas, stock piling areas, protection of water quality from
sediment or erosion potential, and equipment to be used. Revegetation and
restorations plans need to be provided as appropriate.
H. The submittals need to address compliance with the Review Criteria contained
in Section 4-109 as applicable.
I. The submittals need to include any other reports, studies, and permits
supporting the request.
J. The submittals need to generally address compliance with Section 7-203 (B),
Protection of Water Bodies, Structures Permitted in Setbacks that includes
allowance for bridges and other reasonable and necessary structures.
K. If mobile home placements are also being requested within the 100-year
floodplain, information on elevation and anchoring needs to be provided
demonstrating compliance with Section 3-301(C)(4).
Follow-up conversations with Staff is an option if additional questions come up regarding the
submittal requirements. As noted above some elements of the Floodplain Analysis may not
be applicable and the Application submittals should reflect that representation.
5
The Application submittal needs to include 3 hard copies of the entire Application and 1
Digital PDF Copy of the entire Application (on a CD or USB Stick). Both the paper and digital
copies should be split into individual sections and properly labelled. Please refer to this pre-
application summary for submittal requirements that are appropriate for your Application.
VI. APPLICATION REVIEW FEES
This application will be subject to the following fees and deposit requirements:
Planning Review Fees: $ 400 (Additional Staff time charged at hourly rate of $40.50)
Referral Agency Fees: $ tbd (Review fees associated with consulting engineer review)
Total Deposit: $ 400 (additional fees to be determined by hourly rates)
VII. GENERAL APPLICATION PROCESSING
The foregoing summary is advisory in nature only and is not binding on the County. The
summary is based on current zoning, which is subject to change in the future, and upon factual
representations that may or may not be accurate. This summary does not create a legal or
vested right. The summary is valid for a six-month period, after which an update should be
requested. The Applicant is advised that the Application submittal once accepted by the
County becomes public information and will be available (including electronically) for review
by the public. Proprietary information can be redacted from documents prior to submittal.
Pre-application Summary Prepared by:
11/21/24
Glenn Hartmann, CFM Date
Community Development Director
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Note: The notice requirement in
Step 4 has been changed to 15
days pursuant to Section 4-103
of the LUDC.
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FEMA FLOODPLAIN MAPPING
APPLICATION FORMS AND DATA
APPLICATION FORM NARRATIVE
In the application packet, all required forms are completed, signed, and attached in the previous sections.
Two specific items are addressed below in accordance with Garfield County Pre-Application Submittal
requirements.
PROPERTIES WITHIN 200 FEET
All properties within 200 feet include:
1. Debra Rivera and Thomas Morton
a. 1319 117 County Road
b. 1487 117 County Road
2. PCP Hideout LLC
a. 1293 117 County Road
3. Karen Stephan
a. 390 127 County Road
4. Three Mile Property LLC
a. 250 127 County Road
MINERAL RIGHTS
The property has no mineral rights associated with it. Records were pulled from the Mineral and Land
Records System from Bureau of Land Management (BLM). Please see Figure 1 for the project site
(yellow) and only nearby mapped mineral rights (pink diagon al boxes).
FIGURE 1. Mineral rights from mineral and land records system (BLM). Yellow box is the project site
and pink diagonal boxes are the only nearby mineral rights.
PROJECT NARRATIVE
PROJECT DESCRIPTION
The Roaring Fork Community Development Corporation is proposing to replace the vehicle and
pedestrian bridges on the property, the potable water system, and creek crossing of the sewer system.
The proposed scope of work addresses decades of deferred maintenance at 3 Mile MHP and brings
essential infrastructure up to modern standards. It was developed in close coordination with relevant
regulatory agencies, including Glenwood Fire, CDPHE, and Garfield County. The condition of the bridges
and water and sewer systems present life safety hazards to residents and park visitors. The lack of on-
site fire suppression capacity is a threat to structures, residents, and adjacent properties. Since taking
ownership of the park in 2023, the RFCDC has invested significant time and resources to addressing this
deferred maintenance. The goal is to ensure basic life safety for park residents and improve the
resilience of the park and its infrastructure to natural and human hazards. Failure to address these
deficiencies will exacerbate existing life safety risks and threaten the health and well being of residents of
the park. This floodplain permit application is an essential step in bringing the park up to code and
improving life safety conditions for residents.
The proposed bridges will meet all required clearances 2 feet above base flood elevations, and provide a
resilient solution for critical infrastructure on the property. Scour protection of the abutments is proposed
to minimize damage to the bridges in the event of a flood event. Where the channel is disturbed from
construction work, rip-rap will be placed to prevent erosion and transport of material through the creek.
The new bridges will require grading around the approach and descent. An arched bridge is being
proposed to help mitigate changing grade and provide adequate clearance at both bridges.
Both bridges will be constructed of steel material, with a steel deck, and concrete abutments. Rip-rap will
be placed around the abutments, and in the disturbed creek area to facilitate erosion control, and any
disturbed banks will be stabilized using native vegetation and rip-rap.
In addition to the replacement of both bridges, water system buried pipeline and sewer crossing are being
replaced. The entire water system distribution system is being replaced with CDPHE compliant watertight
materials of C900. The current crossing of the waterline above the creek does not meet 2 feet freeboard
requirements and as such the crossing will be relocated to the downstream edge of the vehicle bridge.
The waterline will be HDPE across the bridge, and will be insulated to ensure proper operatio n of the line
and freeze prevention. A water tank is proposed on-site to provide the community with emergency
storage, increased treatment ability, and a more robust pumping system to meet the community’s
demands. The storage tank is located directly adjacent to the 100-year floodplain, and due to its
proximity is watertight, designed with an anti-buoyancy anchoring system to prevent flotation during a
flood event, and all access hatches, and vents are proposed to be a minimum of 2 feet above the base
flood elevation. The chlorination building is relocated from a subgrade vault to a small shed outside and
setback from the floodplain.
The existing sewer crossing is not located outside of the floodplain or above the base flood elevations
and as such is being proposed to be buried under the creek for maximum protection. The buried pipeline
is designed to meet anti-buoyancy forces present during a flood, and will provide the sewer collection
system with a robust design to prevent water infiltration or impact of debris in the creek. A manhole is
being added at the crossing to eliminate vertical bends in the system and provide cleaning and
maintenance access. The manhole is located outside of the 100-year floodplain.
BRIDGE DESIGN DETAILS
The existing vehicle bridge in place does not currently meet HL-93 standards and was derated to a limit
such that emergency vehicles (e.g. fire trucks, ambulances) can not cross the bridge. Upon inspection of
the bridge, it is evident there are several structural deficiencies, which necessitate the replacement of the
vehicle bridge and pedestrian bridges.
The proposed bridges are provided by Contech bridge manufacturer and AASHTO LRFD Bridge Design
Specifications, 9th edition, 2020. All bridges are designed to meet structural standards, seismic loadings,
wind loads, and dead loads for anticipated use (HL-93 Design Vehicle). The vehicle bridge can support
the crossing of a fire truck including width and loading ratings. The materials of both the vehicle and
pedestrian bridge design include:
1. Steel superstructure
2. Concrete abutments
3. Steel and concrete deck materials
WATER AND SEWER SYSTEM DETAILS
The existing water system does not meet current CDPHE regulation, and is at risk of failure if not
replaced soon. The system currently consistent of galvanized steel of varying sizes, a creek crossing
below flood elevation, and no adequate contact time for 4-log Virus removal. The proposed system will
address these deficiencies and provide a robust drinking water system for this affordable housing
community. The scope of the water system replacement project includes:
1. Redrill a new well with well casing 2 feet above base flood elevation per County and CDPHE
requirements.
a. The new well is proposed to be directly adjacent to the 100-year floodplain and since it is
critical infrastructure will meet all floodplain requirements.
2. Install a 10,000 gallon watertight, anchored tank for emergency storage, additional contact time,
and fire storage. The tank will partially overlap the 100-year floodplain and as such will meet all
installation requirements.
a. All above ground appurtenances (vent, manhole, fire hose connection) are minimum 2
feet above base flood elevation.
b. The tank will be anchored with an anti-buoyancy system to prevent flotation, or
movement during a 100-year flood event. See calculations attached to the end of the
report.
3. Demolish existing subgrade vault with chlorination equipment, and move to building out of the
floodplain with safe ingress and egress available during a flood event. The equipment housed in
building will include chlorination tank and pump for disinfection of groundwater.
4. Replace all distribution piping with 4-inch C900 piping, install isolation valves for proper
maintenance and troubleshooting. Some
5. Install an insulated bridge crossing for the water line on new vehicle bridge.
The existing sewer crossing is located within the floodplain and is below base flood elevations and as
such is being proposed to be buried under the creek for maximum protection during a flood event. The
buried pipeline is designed to counter anti-buoyancy forces present during a flood, and will provide the
sewer collection system with a robust design to prevent water infiltration or impact of debris in the creek.
A manhole is being added at the crossing to eliminate vertical bends in the system and provide cleaning
and maintenance access. The manhole is located outside of the floodplain elevation.
Scope of work for sewer system improvement project includes:
1. Installation of access manhole outside of floodplain
2. Replacement of sewer crossing with buried ductile iron piping
3. Installation of new manhole downstream of existing property manhole (outside of floodplain)
4. Installation of new clean-outs (outside of floodplain)
The sewer collection is privately owned and operated and discharges to the City of Glenwood sewer
system. The improvements will ensure continued discharge of sewage, protection from infiltration, and
provide protection against spilling of sewage into the creek during a flood event.
VICINITY MAP AND PROJECT DRAWINGS
COVER SHEET ANDDRAWING INDEX
3 MILE MOBILE HOME PARKINFRASTRUCTURE IMPROVEMENTS
250 COUNTY RD. 127 | GLENWOOD SPRINGS, CO
ENGINEERING
PLAN SET
PROJECT DESCRIPTION:
THE PROJECT INCLUDES THE REMOVAL AND REPLACEMENT
OF ALL SITE BRIDGES, WATER SYSTEM NETWORK, AND
SEWER PIPE CROSSING
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PROPOSED SITE PLAN
C4
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PROPOSED VEHICLEBRIDGE GRADINGPLAN, PROFILE, DETAIL
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PROPOSED PEDESTRIANBRIDGE GRADINGPLAN, PROFILE, DETAIL
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#
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PROPOSED SEWERCROSSING UPGRADESPLAN AND PROFILE
THR
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PROPOSED WATER SYSTEMPLAN AND PROFILE (1)
C8
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PROPOSED WATER SYSTEMPLAN AND PROFILE (3)
C10
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#
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12"12"
12
"
MI
N
ZONE
BEDDING
6" MIN
C11
SEWER DETAILS
·
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C12
WATER TANK DETAIL
CHECKED BY:
#
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C13
WATER DETAILS
CHECKED BY:
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R
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M
ANTI-BUOYANCY CALCULATIONS
WWS
Client: RFE
Project: 3 Mile MHP
Detail: 10k Gallon Fiberglass Tank
Job No.: 2025-RFE-3M
Date Chkd:
Chkd By:
Comp. By: TAW
Date: 01.28.2025
Page No.: 1
10,000 Gallon Buried Fiberglass Water Storage Tank - Uplift Check
This spreadsheet will check a buried fiberglass tank in a high groundwater condition for uplift.
indicates user input field
Overall Structure Weight:
Unit Dimensions (ft)Total Wt
Item Weight Units Length Width Height (kips)
10k gallon tank (empty)*5175.00 LBS 5.18
Base slab 150.00 PCF 22.00 13.17 0.73 31.69
TOTAL STRUCTURE DL 36.87 kips
*Nominal tank weight - 4500 lbs (Xerxes) includes 15% increase for tank appurtenances.
Soil Volume:
Unit Total V
Item Weight Units Diam Length Width Height
(ft3)
Soil cube 130.00 PCF 22.00 13.17 15.50 4490.97
Tank 10.46 18.93 1625.92
TOTAL SOIL VOLUME 2865.05 ft
3
TOTAL SOIL WEIGHT 372.46 kips
WSE =100.00 ft water surface elevation
Liquid Density = 62.40 lb/cf
Top of Base Slab EL = 86.54 ft
Height to WSE = 13.46 ft
Volume of displaced water = 4111.17 ft
3 (ht to WSE + slab t) * slab L * slab W
Buoyancy / uplift force, U = 256.54 kips volume * liquid density
Uplift Resistance w/o soil = 36.87 kips Structure w/o soil
F.S. Uplift = 0.14 Not OK resistance/uplift
*It will be important for the contractor to dewater the excavation during construction until backfill is complete.
Uplift Resistance w/ soil = 409.32 kips Structure w/ soil
F.S. Uplift = 1.60 OK
F.S. Uplift Req'd = 1.50
Dimensions (ft)
WWS Calculation
Sheet Page 1 of 2 Printed On: 1/28/2025
WWS
Client: RFE
Project: 3 Mile MHP
Detail: 10k Gallon Fiberglass Tank
Job No.: 2025-RFE-3M
Date Chkd:
Chkd By:
Comp. By: TAW
Date: 01.28.2025
Page No.: 1
Bearing Capacity Check: In Service Loading - Tank Full:
Wt of Structure = 409.32 kips
Xerxes nominal tank volume =10144.00 gallons
weight of water volume = 84.62 kips
total weight of structure = 493.94 kips
Base Slab Footprint Area = 289.74 ft
2
Bearing Pressure = 1704.77 psf
Allowable Bearing Capacity = 4000.00 psf
Check =OK
WWS Calculation
Sheet Page 2 of 2 Printed On: 1/28/2025
Project Name: 3Mile MHP
Sewer Pipe Crossing Burial
Date: 1/29/2025
Calcs By: RNM
Soil Cover Depth
(ft) [MIN]
Saturated Unit Weight
(pcf)
Dry Density
(pcf) [MIN]
Total Density
(pcf) [MIN]
Pipe Outer Diameter 8.4 in 0.5 115 180 198
0.70 ft 1 89 127 140
Pipe Area 0.38 ft2 1.5 80 110 121
2 76 101 111
Unit Weight of Water 62.4 lbs/ft3 2.5 73 96 105
3 71 92 102
Weight of Pipe 5.63 lb/ft 3.5 70 90 99
4 69 88 97
Bouyant Force per Unit Length 24.0 lb/ft 4.5 68 87 95
**assumed fully submerged pipe 5 68 85 94
5.5 67 84 93
Target Soil Porosity 0.4 6 67 84 92
Target Water Content 10%6.5 66 83 91
Factor of Safety 2 7 66 82 91
7.5 66 82 90
8 66 81 90
8.5 65 81 89
9 65 81 89
9.5 65 80 88
10 65 80 88
10.5 65 80 88
11 65 80 88
11.5 65 79 87
Assumptions:
1. Sewer pipe to be 6-inch encased in an 8-inch pipe and
covered with 24-inch of rirap and 6-inch of bedding layer.
2. Riprap rock weights ~150 lbs/cf.
3. Calculations estiamted to needed backfill compaction
density to counteract the bouyant forces at the specified
depth.
Page 1 of 1
FLOODPLAIN ANALYSIS
PREPARED MEMORANDUMS
Three different memorandums were prepared for the floodplain analysis and are subsequently included in
the following order:
1. No Rise Certification
2. Bridge Scour Analysis
3. Specific Permit Response
CONSTRUCTION PRACTICES
During construction the project will adhere to all best management practices to prevent erosion control
on-site during and after construction. The following mitigation practices will be implemented:
1. All staging and stockpiling areas will be located outside of the 35-foot setback from the floodplain.
Vehicle staging will include drip pans, and stockpile areas will include erosion control mitigation
measures including logs around the bottom of all stockpiles and covers for materials to prevent
runoff. See erosion control detail drawing for more information
2. By placing all stockpiles and staging areas outside of the 35-foot setback, erosion potential which
impacts water quality of the creek will be mitigated. Furthermore, soil logs and tarp covers will
prevent material migration from stockpiles.
3. The creek does not have existing vegetation but will require restoration of rip-rap and rock in the
channel after construction. The banks are steep and will be armored with appropriately sized rip -
rap (e.g. native material on site) to ensure no bank erosion or degradation.
Page 1
TO: Garfield County Engineering Department
FROM: Ryan Martin, P.E.
Design Engineer, Roaring Fork Engineering
CC: Maggie McHugh, P.E.
Project Manager, Roaring Fork Engineering
DATE: January 23, 2025
RE: Garfield County Floodplain Development Permit Requirements
THREE MILE MOBILE HOME PARK
NO-RISE CERTIFICATION MEMORANDUM
PROJECT BACKGROUND
Three Mile Mobile Home Park (MHP) is located along Threemile Creek approximately 0.6 miles upstream
from its confluence with the Roaring Fork River in Glenwood Spring, Colorado. The project is located within
a previously mapped floodplain within Garfield County and as such will require a floodplain development
permit for all proposed work. Three mile Creek flows through the MHP property limits and was last officially
mapped/published by FEMA on January 3, 1986 (Panel No. 0802051434B). The mapping includes the 100-
yr and 500-yr floodplain limits but excludes the floodway limits. The panel indicates that the MHP is in a
special flood hazard area with known base flood elevations (BFE). A LOMR, conducted by SGM Inc. in
2016, was issued to the City of Glenwood Springs but was not submitted to FEMA for map updating. Since
the LOMR was not approved by FEMA, it is not considered the current effective model and as such will be
excluded from this study. However, the scope of work for the 2016 LOMR did not seek to alter the BFE at
the MHP, but only to alter the mapping information downstream of the project limits.
Both the FEMA map and the 2016 LOMR did not have a sufficient level of detail of the bridge hydraulics
and other key hydraulic features within the property to identify the true limits of flood/debris flow concern
and true base flood elevations. Roaring Fork Engineering (RFE) was hired to replac e the bridges within the
property and to evaluate the extent of the flooding/debris flow risk. It should be noted that the intent of this
no-rise memorandum is not to officially alter the base flood elevation of the project area or update the FEMA
panel mapping, but only to provide justification of design choices for the project.
RFE is requesting a no-rise certification for the proposed work as part of the floodplain development permit
for Garfield County. The no-rise certificate demonstrates “No impact on the 100-year flood elevations,
floodway elevations, or floodway widths at the new cross-sections and at all existing cross-sections
anywhere in the current effective model.” The no-rise certification is supported by technical data based
upon two separate analyses:
1. A step-backwater analysis
2. A conveyance compensation analysis
Page 2
Engineering design drawings will be submitted separately with the Garfield County Floodplain Development
permit. These drawings include surveyed information regarding the project limits, proposed bridge
replacement, and other ancillary floodplain work.
RFE obtained both the FEMA panel mapping and the 2016 LOMR report issued by SGM. The FEMA panel
mapping is assumed to be the current effective model at the time of this study. The panel mapping does
not include the two bridges, two pipe crossings, and only four cross sections throughout the entire project
area. It was decided by RFE that the current effective model does not contain the required accuracy to both
effectively model the true floodplain limits and establish a no-rise condition with the addition of all pertinent
hydraulic structures. In-lieu of the current effective model, the entire project area was surveyed by RFE in
late 2024 and cross sections were generated from the terrain surface using the RASmapper tool within
HEC-RAS. Land cover roughness coefficients were determined from following site visits by RFE staff and
are described in the following section.
Site Characteristics
Threemile Creek flows through the MHP property for approximately 680 feet and can be considered a
mountain stream with no vegetation within the channel with steep banks with trees and brush that are
submerged at high stages (Figure 1). A final channel mann ing’s n-value of 0.042 was determined to produce
the least amount of cell error and uncertainty during model calibration. The channel overbanks were
assumed to have a manning’s n-value of 0.05 which is consistent of floodplains with light brush, trees and
heavy weeds (Figure 2). The remainder of the site’s land cover was determined from field surveying
conducted by RFE in 2024 and is listed below:
1. Asphalt; n = 0.016
2. Building; n > 1 (friction values set extremely high to prevent flow through building cells)
3. Concrete n = 0.012
4. Gravel; n = 0.023
5. No Data; n = 0.035
Page 3
Figure 1 – Threemile Creek through MHP property
Figure 2 – Threemile Creek Bank Vegetation
Page 4
Step-Backwater Analysis
Typically to complete the steps-backwater analysis, the current effective model of the stream reach is
obtained from the FEMA engineering library. However, the current effective model was established in 1986
well before modern step-backwater analyses software or access to computational methods was widely
available. The original 1986 Flood Insurance Rate Map (FIRM) neglected to include the two bridges and
two pipe crossings within the project area and determined cross sections from hand surveying techniques.
It was decided by RFE that a new model of the project reach be created to better understand the project
hydraulics and base flood elevations.
A sitewide survey was collected by RFE in September 2024 with a following site visit in October 2024 to
determine land cover types (see following section for more detail). A digital elevation model was generated
from the survey and imported into HEC-RAS (v6.4.1) to generate cross sections and insert hydraulic
structures. A total of 22 cross sections were generated from the new model. The five original cross sections
that span the project reach were added for additional clarity between the new model and the current
effective mapping area. The existing conditions model was generated to establish a true baseline for
comparison with the proposed conditions model. Then the proposed development was added to a copy of
the existing conditions model to evaluate any changes in the base flood elevation at the project site and
official FEMA cross sections. Copies of the new models, existing and proposed, are included in the attached
appendices.
The proposed work includes removing and replacing the existing bridges and pipe crossings in the project
area. This includes one vehicle bridge, and one pedestrian bridge. Both bridges will be of arch span type
to increase the allowable flow area beneath the structure.
Both the existing and proposed conditions were modeled using the predetermined FEMA 100-yr return
period flood magnitude of 710-cfs. Results from this model will be considered the true base flood elevation
for the project area.
Proposed conditions modeling indicated that there would not be a rise greater than 0.5 feet in the base
flood elevation at any official cross sections. Table 1 shows the predicted base flood elevation at key cross
sections for both existing and proposed condition.
Table 1 – Flood Modeling Results
Official FEMA
Cross Section
Current
Effective FIRM
BFE
(ft-amsl)
BFE
Existing
Conditions
(ft-amsl)
BFE
Proposed
Conditions
(ft-amsl)
Change in
BFE
(feet)
V 5958.7 5960.6 5960.6 0
U 5947.3 5947.1 5945.9 -1.2
T 5943.3 5942.5 5942.4 -0.1
S 5937.4 5935.6 5935.6 0
R 5923.4 5924.8 5924.8 0
The proposed conditions modeling indicates that removal and replacement of the two bridges will not cause
a rise in the base flood elevation by more than 0.5-ft per the FEMA floodplain definition. No changes to
official FEMA cross sections or other key hydraulic structures downstream were predicted by proposed
conditions model. Appendix A includes model outputs and copies of key cross sections.
Page 5
Conveyance Compensation Analysis
A conveyance compensation analysis of the study reach was investigated since it is FEMA’s position that
any blockage to flow in the floodway will result in a rise to the one-hundred-year flood profile, regardless of
the tabular results comparing BFE's in pre- and post-development conditions in the backwater analysis.
Computed conveyance compensation calculations using the cross section and 100-year encroached
hydraulic data in the modified existing conditions model comprised of the following:
1. Calculation of the reduction in conveyance (K) caused by the proposed obstruction, assuming no
change in floodway water-surface elevation, and using the "n" value appropriate for the site of the
proposed obstruction.
2. Calculation of the increase in conveyance (K) obtained by the proposed offsetting measure, using the
"n" value appropriate for the site of this measure.
3. Comparison showing that the conveyance increase computed in (2) equals or exceeds the loss
computed in (1) and evidence that the increase in effective conveyance provided for in (2) will be
maintained perpetually. This should be in the form of a self-maintaining measure or certified
maintenance plans for the measure provided.
Conveyance is derived from the Manning’s equation and is defined as follows:
𝐾=1.486
𝑛𝐴𝑅𝐻
2/3 where;
𝐾= conveyance-flow-carrying capacity of cross section directly proportional to discharge (cfs)
𝑛= manning’s n-value of cross section
𝐴= flow area of cross section (sq.ft)
𝑃= wetted perimeter of cross section (ft)
𝑅𝐻= 𝐴
𝑃= hydraulic radius of cross section (ft)
Existing and proposed conditions models predicted conveyance values are listed in Table 2 below.
Page 6
Table 2 – conveyance [K] at key cross sections
Key Cross
Section No.
Description Existing
Conditions
K
Proposed
Conditions
K
Change in K
(%)
654
Official FEMA cross section V
Approx. 210 feet upstream of the
vehicle bridge
2,229 2,258 +1.3
420 US Vehicle bridge upstream cross
section 1,666 5,162 +309.8
420 DS
Official FEMA cross section U
Immediately downstream of the
vehicle bridge
2,679 2,653 -1.0
384 US Waterline crossing upstream cross
section 4,978 3,576 -28.2
384 DS
Official FEMA cross section T
Immediately downstream of the
waterline crossing
883 991 +12.2
323
Official FEMA cross section S
Approx. 130 feet downstream of
the waterline crossing
3,589 3,870 +7.8
163 US Pedestrian bridge upstream cross
section 2,629 2,888 +9.9
43
Official FEMA cross section R
Approx. 130 feet downstream of
the pedestrian bridge
3,058 3,047 -0.4
3 US Approx. 30 feet downstream of
official cross section R 2,711 3,368 +24.2
Existing conditions modeling predicts a discontinued conveyance within the main channel with constrictions
at both bridges and the sewer line crossing. Both channel constrictions were alleviated, and greatly
increased, at the hydraulic structures during the proposed conditions modeling due to the inclusion of arch
type bridge spans and the removal of low hanging pipe crossings.
The proposed conditions model also included the proposed offsetting measure of creek bank maintenance
(clearing overgrown trees, removing debris, etc.) and slope armouring in locations of noted bank erosion.
To model this, all cross sections between the two bridges had their channel n -values reduced from 0.042
to 0.035. All other cross sections, where only vegetation maintenance occurred, had their channel n-values
reduced from 0.042 to 0.040.
Proposed conditions modeling predicted that total theoretical conveyance increases in Threemile Creek
throughout the study reach. This is indicative of the inclusion of arch type bridge spans, removing of riverine
obstructions, and minor vegetation maintenance. No conveyance reduction is expected in the project area.
Page 7
It should be noted that the reduction in channel conveyance on the downstream end of the vehicle bridge
(420 DS) was caused by altering the geometry of the existing conditions structure from outlet control to
yield inlet control. Therefore, a channel conveyance from existing to proposed conditions can not be directly
compared.
Property Owners of the Three Mile MHP should maintain vegetation overgrowth and remove debris piles
post storm events to maintain proper channel hydraulics.
Engineering “No-Rise” Certification
This is to certify that I am a duly qualified engineer licensed to practice in the State of Colorado.
It is to further certify that the attached technical data supports the fact that the replacement of the vehicle
bridge and pedestrian bridge will not significantly impact the base flood elevations, floodway elevations and
floodway widths on at published sections of Threemile Creek in the FIS for the Town of Glenwood Springs
located in Garfield County effective August 2, 2006 and will not impact the true 100-year flood elevations,
floodway elevations, and floodway widths at unpublished cross -sections in the vicinity of the proposed
development.
Page 1
3-MILE MOBILE HOME PARK
BRIDGE SCOUR DESIGN MEMORANDUM
PROJECT BACKGROUND
3-Mile Mobile Home Park (MHP) is located along Threemile Creek approximately 0.6 miles upstream from
its confluence with the Roaring Fork River in Glenwood Spring, Colorado. The project includes removal and
replacement of one vehicle bridge and one pedestrian bridge that each span Threemile Creek. Both
replacement bridges are designed as to not impact the 100-yr base flood elevation of Threemile Creek and
to increase the channel conveyance through the project reach. Both replacement bridges will be truss type
spans with no central supporting piers. The abutments will be concrete wingwalls with spread footings to
support the full bridge span. A scour analysis was performed for both bridges to determine the necessary
elevations for the top of footings.
BRIDGE SCOUR ANALYSIS
The bridge scour analysis was performed using the Hydraulic Design Calculator included in HEC -RAS
(v.6.4.1). The proposed bridge geometries were imported into the stream reach in the same locations as
the existing structures. For the analyses, it was assumed that both abutments will be constructed to the
same specifications and that test pits are representative of both abutments soil conditions. It was also
assumed that scour results for the left abutment at the vehicle bridge are the most indicative of truth due to
the geometry of the study reach with Threemile Creek bending directly at the left abutment while the right
abutment would expect to see eddy’s form; 1D models exclude superelevation and overestimate the lagging
side of the cross section. Test pits were dug near the left abutment for both proposed bridges. Gradation
results are included in the published geotechnical report for the project and noted the D50 and D95 for each
test pit, see Table 1. The D50 and D95 are key input parameters for the scour analysis since they represent
the bearing materials ability to become mobilized during high flow events.
Table 1 – Test Pit Gradation Results
Test Pit
Location
D50
(mm)
D50
Sieve No.
D95
mm
D95
Sieve No.
Vehicle Bridge 1 No.16 35 2”
Pedestrian Bridge 0.075 No.200 63.5 2.5”
Geotechnical gradation results indicate a large percentage of sand and finer grain material. Accompanied
images provided in the geotechnical report show a large percentage of larger material, boulders, and
cobbles, were excluded from the representative sample in which the test results were conducted from. In -
situ material would include this larger percentage of cobbles and boulders raising both the D50 and D95
for the bedding material. Initial models with the inputs provided from the geotechnical report see med to
greatly overestimate the scour depths at the vehicle bridge. To better understand the theoretical scour
depths at the vehicle bridge, both the published FEMA and StreamStats 100-yr return period peak flood
discharges were compared with several iterations of the D50 and D95 to determine the sensitivity of the
Page 2
total scour depths (Table 2 below). Both the published laboratory data and selected bed material grain sizes
used to monitor the sensitivity of the total maximum scour depth.
Table 2 – Vehicle Bridge Scour Depth Sensitivity Analysis
100-yr Peak Flow
(cfs)
D50
(mm)
D95
(mm) [in]
Maximum Scour Depth Elevation
(ft-amsl)
309 (StreamStats)
1
35 [1.4]
76.2 [3.0]
152.4 [6.0]
5945.00
3
35 [1.4]
76.2 [3.0]
152.4 [6.0]
5947.75
8.75 35 [1.4] 5946.50
8.75 76.2 [3.0]
152.4 [6.0] 5947.75
19 76.2 [3.0] 5946.50
19 152.4 [6.0] 5947.75
38 152.4 [6.0] 5946.25
Dataset Mean and
Standard Deviation
5946.73 ± 1.14
(5945.59 – 5947.87)
710 (FEMA)
1
35 [1.4]
76.2 [3.0]
152.4 [6.0]
5937.50
3
35 [1.4]
76.2 [3.0]
152.4 [6.0]
5945.00
8.75 35 [1.4] 5942.00
8.75 76.2 [3.0]
152.4 [6.0] 5945.00
19 76.2 [3.0] 5942.00
19 152.4 [6.0] 5945.00
38 152.4 [6.0] 5942.00
Dataset Mean and
Standard Deviation
5942.37 ± 3.07
(5939.31 – 5945.44)
The sensitivity analysis shows that the ratio of D50/D95 produces a large range of total scour depth s at the
vehicle bridge. All scenarios with grain sizes of 1 mm produce the maximum scour depth regardless of the
D95 value entered. Increasing the D95 only produces increased scour depth when the ratio of D50/D95 is
Page 3
close to 25%. Increasing the channel discharge value also increases the variance of the dataset due to
increasing velocities and potential transportation of bed material.
Typical design elevation of the top of footings (ToF) just below frost depth which is approximately four feet
below finished grade. This would place the ToF at an elevation 5949 which is higher than all predicted scour
depths. It is recommended that the ToF elevation fall within the intersection of both dataset ranges due to
the large discrepancies between the predetermined FEMA 100-yr flow and the StreamStats 100-yr flow.
For purposes of this design, it is recommended that the ToF elevation for the vehicle bridge be at
or below EL. 5945.50 for both abutments (Figure 1).
Using the provided geotechnical data from Table 1 for the pedestrian bridge, initial modeling results for the
total scour depth seemed to estimate reasonable and realistic results, even at the published FEMA 100-yr
flow (Figure 2). Both abutments had a predicted scour depth at or below the typical footing design depth of
four feet to prevent frost depth penetration and heaving of the underline material. It is recommended that
the left abutment at the pedestrian bridge have a ToF elevation at or below EL. 5929.75 and the right
abutment have a ToF elevation at or below EL. 5930.50.
0 20 40 60 80 100 120 140 160
5940
5945
5950
5955
5960
5965
5970
Bridge Scour RS = 420
Station (ft)
El
e
v
a
t
i
o
n
(
f
t
)
Legend
WS 100yr FEMA
Ground
Bank Sta
Abutment Toe
Contr Scour
Total Scour
1
FIGURE 1
VEHICLE BRIDGE
TOTAL SCOUR DEPTH
ASSUMED TO BE MORE RESONABLE
GIVEN THE GEOMETRY OF THE
APPROACHING RIVER REACH
0 50 100 150 200 250 300
5920
5930
5940
5950
5960
5970
Bridge Scour RS = 163
Station (ft)
El
e
v
a
t
i
o
n
(
f
t
)
Legend
WS 100yr FEMA
Ground
Bank Sta
Abutment Toe
Contr Scour
Total Scour
1
FIGURE 2
PEDESTRIAN BRIDGE
TOTAL SCOUR DEPTH
Page 1
TO: Garfield County Planning Department
FROM: Ryan Martin, P.E.
Design Engineer, Roaring Fork Engineering
CC: Maggie McHugh, P.E.
Project Manager, Roaring Fork Engineering
DATE: January 31, 2025
RE: Garfield County Floodplain Development Permit Requirements
3 MILE MOBILE HOME PARK
LAND USE AND DEVELOPMENT CODE RESPONSES
The following sections were requested for response by the Garfield County Floodplain Manager during the
Pre-Application meeting. Several additional sections were noted to provide required information per the
Land Use Code for drinking water and sewer facility improvements.
• Section 3-301 – Floodplain Overlay Regulations
▪ Section 3-301 (C)(2) – Flood proofing of non-residential structures
▪ Section 3-301 (C)(4) – Manufactured Homes
▪ Section 3-301 (E) – Standards for Areas of Shallow Flooding
▪ Section 3-302 Drinking Water System Overlay Regulations
• Section 4-109 – Review Criteria
• Section 4-203 (O) – Floodplain Study Requirements
• Section 7-203 B – Protection of Waterbodies
Section 3-301 – Floodplain Overlay Regulations
A. Use Restrictions in the Floodplain Overlay
The effective FIRM only indicates the limits of the 100-year and 500-year and excludes the limits of the
floodway. It was assumed that the upper bank limits of Three mile Creek through the project area fully
contain the typical seasonal flows and thus establish the floodway limits. Occupation of permanent or
temporary structures is permitted between the floodway and 100-year, per Table 3-301. All occupied
structures are located within the floodway and 100-year limits within the project area. The project scope of
work does not include any modifications or expansions of residential structures and accessory
improvements between the floodway and the 100-year limits within the project area.
The replacement of both bridges within the project area will not cause any increase in the base flood
elevation (BFE). The replacement of buried infrastructure including water tank, well, waterline, and sewer
lines will not increase the BFE. Refer to the separate no-rise certification for more details.
Page 2
Lawns, open areas, gardens, driveways, and play areas that are accessory to residential uses are permitted
both within the floodway and the 100-year limits. No change to these structures is proposed as part of this
work.
Underground utilities are permitted both within the floodway and the 100-year limits.
Bank restoration and stabilization are permitted both within the floodway and the 100-year limits.
All other line items listed in Table 3-301 are not applicable to the project.
B. General Standards
1. The proposed bridge replacements will include concrete abutments with footings installed below the
predicted scour depth to prevent flotation, collapse, and lateral movement of the structure from
hydrodynamic and hydrostatic loads, including buoyancy effects. Additional bank stability measures (rip-
rap) will be installed in the general vicinity of both bridge replacements to reduce erosion potential near the
abutments. See engineering drawings for details.
The proposed water tank and well replacement will be installed with access hatches, vents, and top of well
casing 2 feet above the base flood elevation. A majority of the tank is located outside of the 100 -year
floodplain, but is designed with anti-buoyancy anchoring system to resistant anticipated hydrodynamic and
hydrostatic loads.
2. The proposed bridge replacement will be constructed under specifications and industry best practices to
minimize flood damage. The waterline and sewer lines will be constructed to minimize flood damage
ensuring proper pressure testing and installation to prevent flood damage.
3. The proposed bridge replacement will be constructed with steel spans bolted to concrete abutments that
will minimize flood damage.
The water tank is watertight, and all buried lines within the floodplain are watertight to prevent flood damage.
All buried infrastructure within the floodplain is designed to combat hydrostatic forces of the floodwaters.
4. No electrical, heating, ventilation, plumbing, and air conditioning equipment are included in the proposed
scope of work or are located within the floodplain.
5. No substantial modification or replacement of manufactured homes is a part of the proposed scope of
work.
6. The replacement of the water system utility is designed to minimize infiltration from floodwaters. The
following technical details demonstrate this compliance:
• Table 3-301 states that water utilities (underground utilities) may be located within the floodplain
as long as all requirements are met. The proposed design meets all County requirements and
CDPHE requirements.
• Tank is watertight and all access hatches and vents are designed to be a minimum of 2 feet above
the base flood elevation.
• All water line materials (C900 DR-18) are manufactured and designed to be watertight and meet
current CDPHE regulations.
• The well will be redrilled within 20 feet of the existing location and the final casing height will be 2
feet above the base flood elevation.
Page 3
• The water line crossing is being located to the bridge to meet 2 feet above base flood elevation
protection height, and will be encased and insulated to protect the utility from outdoor elements.
6. The improvement of the sanitary sewer system includes the installation of an access manhole outside of
the floodplain and improvement of the sewer crossing across the creek. The partial realignment of the
sewer line will bring more of the utility out of the floodplain.
The sewer crossing across the creek is designed to eliminate infiltration of floodwaters into the system and
is resistant to all buoyant forces during a flood event as demonstrated by the calculations provided in the
application.
8. No OWTS are on-site and therefore are not part of the project scope.
C. Specific Standards
1. No residential construction or modification to existing residential structures and improvements is included
in the scope of work.
2. The potable water system is being completely replaced and improved to meet current CDPHE design
standards and provide a more resilient system for the community. As part of the improvements, a storage
tank is proposed to provide community members with emergency storage, more contact time and treatment,
and ability to take the system offline during well or source maintenance. The storage tank was designed
by a licensed structural engineer to withstand flooding conditions modeled on -site. The tank is constructed
to be impassable and impermeable to any water (e.g. certified watertight from manufacturer), and can resist
all anticipated hydrostatic forces on site. Furthermore, the access hatch terminates 2 feet above the base
flood elevation, and the vent terminates 4 feet above the base flood elevation (to meet snow accumulation
requirements by CDPHE). The fire department connection terminates 2 feet above base flood elevation.
The sewer crossing is designed to include a ductile iron pipe under Three Mile Creek. The pipe is joined
using mega-lug connection joints, which are industry standard watertight connections. The weight of the
pipe and trench material prevents buoyant action on the pipe, and calculations are included in a subsequent
section of this report. The pipe will be pressure tested prior to being put in service, which will confirm
manufacturer’s certification of a watertight product. Furthermore, the pipe is buried at a depth to protect
against rock and sediment movement within the creek and insulating to prevent against freezing conditions
under the creek bed. All of these proposed design elements will protect the pipe from damage during a
flood event.
A record of the certification for all facilities can be provided in any format desired by the County.
3. No enclosures are included in the scope of work.
4. The manufactured homes within the property limits are existing structures. Some of the homes are
located within the 100-year floodplain limits and are classified in a Special Flood Hazard Area (SFHA) with
a known base flood elevation (BFE). No modification or changes to the existing homes is part of this scope
of work.
5. No recreational vehicle sites are included in the scope of work.
D. Floodway
Page 4
1. The proposed bridge replacements are to be located along the same alignment as the existing bridge
structures and have been designed to pass the 100-yr flood magnitude flow without raising the base flood
elevation. See the no-rise certification memorandum for additional information. Additional fill and rip-rap
material will be included for the approach ramps for the vehicle bridge and will be protected from floodwaters
by extended wingwall abutments which are above the base flood elevation.
The sewer line is located below the creek and within the floodway. The design prevents movement or uplift
from buoyant forces and prevents infiltration from floodwaters.
2. All proposed work at Three Mile MHP will comply with all applicable flood hazard reduction provisions
stated in the Garfield County Land Use and Development Code.
3. A no-rise certification was determined for the proposed development. Any changes to the FEMA FIRM
for the project area are not needed for the proposed development.
E. Standards for Areas of Shallow Flooding
Three Mile MHP property limits lie within zone A1 where a SFHA is identified, BFE are known, but no flood
hazard factors are determined.
1. No residential construction is included in the scope of work.
2. See response in Section 3-301 (C)(2).
3. Finished grade of the project area will include surface drainage features to guide flood waters around
and away from and new or existing structures back towards Three mile Creek.
F. Properties Removed from the Floodplain by Fill
No properties or structures are requesting removal from the FIRM by use of fill material.
G. Alteration of a Watercourse
No alternation of Three mile Creek is included in the scope of work. Replacement of the two bridges will be
located along the existing alignments. Channel bank stability improvements near the bridge will not
decrease the existing capacity of Three mile Creek or alter the path of floodwaters.
H. Standards for Critical Facilities
1. Three Mile MHP surrounds Three mile Creek with access to the south side of the property by vehicle in
one location only (i.e. the vehicle bridge). The existing vehicle bridge is no longer rated to allow fire trucks
or large emergency vehicles to pass over the bridge. Removal and replacement of this bridge is deemed a
critical structure for the health and safety of the residence. The proposed vehicle bridge replacement will
allow fire truck and other emergency vehicles to access the south side of the prop erty. The bridge is located
within a SFHA so a minimum of 2 feet of freeboard above the BFE will be included in the proposed bridge
design.
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The water tank is designed to remain in operation during a flood event providing essential water service to
the community. The tank hatch is designed to be 2 feet above base flood elevation, and vent is designed
to be 4 feet above base flood elevation. A new well will be drilled with the final casing located 2 feet above
base flood elevation. All buried utilities will be installed with industry standard C900 piping, which is
watertight. The crossing across the creek is being relocated to the downstream side of the bridge, which
is located 2 feet above base flood elevation for maximum protection of the critical utility.
The sewer system improvements include the addition of two manholes (located outside of the 100-year
floodplain) and burying of the sewer main to prevent impact from flood waters or debris events. The pipe
will be installed using watertight materials, compacted and protected with insulation and will remain in
operation during a flood event.
2. Ingress and egress to both sides of the property will have continuous non -inundated access during a
100-yr flood event.
3. The bridges, water, and sewer systems are all critical facilities within the community as they provide a
critical service to community members. As such all infrastructure was designed to meet 2 feet minimum
clearance above the base flood elevation. In instances where these critical facilities are located directly
adjacent to the floodway all floodway design criteria are met (e.g. water storage tank and well head).
Section 3-302 – Drinking Water Constraint Overlay Regulations
The proposed water system improvements are all designed to meet current CDPHE design criteria and
regulations as the system is currently operated under CDPHE permit. The design is under review by the
CDPHE for approval. No construction work of the system can be completed without CDPHE approval.
Section 4-109 – Development in the 100-year Floodplain
C. Review Criteria
1. The proposed development will decrease the danger to life and property due flooding or erosion damage
with the inclusion of the upgraded bridge structures and flood control features.
Sewer and water utilities are being improved to be better protected from flood events and provide continual
service during a flood event.
2. The proposed development will decrease the susceptibility of the proposed facilities and its contents to
flood damage to the owner. The bridge structures will have their lowest structural members a minimum of
2 feet above the BFE and their abutment footings below the predicted scour depths.
The Sewer crossing is buried under the creek to protect the crossing from all flood events. The pipe is
ductile iron pipe and buoyancy calculations were conducted to ensure that under flooding water elevations
the pipe will remain buried. The water utility crossing will be moved to the downstream end of the bridge
to ensure that no flood event will impact the water crossing. Currently the water crossing is within the 100 -
year flood base flow elevation and could experience a break or failure in a flood sc enario.
3. The updated bridge structures and relocation of the pipe crossings over Three mile Creek will reduce the
risk of materials becoming dislodged and swept downstream onto other lands.
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4. The updated bridges are to be located along the same alignment as the existing bridges to maximize site
compatibility between existing and proposed conditions.
5. The proposed vehicle bridge will allow access for emergency vehicles and residence on the south side
of the property to escape to the Three mile Road during flood events.
6. The Three Mile MHP bridges are all privately owned and proposed design should prevent costly work
after a flood event. In the event that damage is done, funding would be pursued from emergency state
funds for affordable housing communities (or mobile home park communities) to ensure the quick fix of any
critical infrastructure.
7. The expected heights, velocities, duration, rate of rise, and sediment transport of the flood waters are
included in the Appendix of the no-rise certification memorandum.
8. Three Mile MHP is an existing residential development along Three mile Creek.
9. The purpose of the project is to replace the vehicle and pedestrian bridges to increase the resiliency and
durability for the MHP residence within the property limits. The MHP can not be relocated further back from
Three mile Creek. Utilities cannot be relocated out of the floodplain as crossing the floodplain is required
to provide services to all community members.
The water tank and building location must be located where shown given the site constraints, property
boundary constraints, and necessity to provide water from the existing decreed well location. The water
tank has been designed to withstand flood events and meet CDPHE requirements. The tank was moved
out of the floodplain as much as possible given site constraints.
10. See Section 4-109 (C)(9) for response.
Section 4-203 (O) – Floodplain Analysis
A floodplain specific site plan and an engineering no-rise certificate were developed for the floodplain
analysis and is included with the Floodplain Permit Submission forms. The developed analysis and
drawings include the following required information:
1. Base Flood Boundary and Water surface elevations
2. Floodway boundary
3. Channel of water course
4. Existing and proposed topographic contours
5. No residential structures are being modified therefore, not structure elevations are
included in the report or analysis
6. Existing and proposed locations of water supply and sanitation facilities
Calculations were performed for the two utility components that are within the 100-year floodplain. First
the potable water tank proposed for the community will provide much needed storage and fire protection
for the residences. The tank is constructed t o be watertight, and the hatch and vent are all designed
to have a terminating elevation 1 foot above the 100 -year base flood elevation. This design meets
Colorado Department of Public Health and Environment (CDPHE) requirements. Details on anchoring
the tank and buoyancy calculations are included as an appendix to this report.
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Certification of critical facility elevations (well, tank hatch, vent) will be completed by a licensed surveyor
after construction is complete. The proposed shed with treatment equipment is located outside of the
floodplain but the floor of the structure will be designed to 2 feet above the base flood elevation for
added protection.
Section 7-203(B)
The proposed project includes the replacement of two bridges within the floodplain setbacks. As 7 -203.B.
states bridges requiring disturbance within the 35 foot setback may be permitted.
Utilities (e.g water and sewer infrastructure) are allowed within the setback when necessary, as stated in
B.