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HAZARD MITIGATION PLAN
RIVER EDGE COLORADO
GARFIELD COUNTY, COL ORADO
O W N E R / A P P L I C A N T :
C A R B O N D A L E I N V E S T M E N T S , L L C
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C A R B O N D A L E C O 8 1 6 2 3
970-456-5 3 2 5
C O N S U L T A N T :
8 1 4 0 P A R T N E R S , L L C
P O B O X 0 4 2 6
E A G L E , C O 8 1 6 3 1
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Hazard Mitigation Plan
River Edge Colorado, Garfield County, Colorado
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HAZARD MITIGATION PL AN
RIVER EDGE COLORADO
GARFIELD COUNTY, COL ORADO
TABLE OF CONTENTS
I. INTRODUCTION ........................................................................... 3
A. BASIS .................................................................................................... 3
B. PURPOSE AND SCOPE OF PLAN .............................................................. 3
C. FINDINGS .............................................................................................. 3
II. PROJECT LOCATION AND DESCRIPTION .......................................... 4
A. PROJECT LOCATION ............................................................................... 4
B. PROJECT DESCRIPTION .......................................................................... 4
III. SITE CONDITIONS, POT ENTIAL EFFECTS AND M ITIGATION MEASURES 6
A. SOIL CHARACTERISITIC S ......................................................................... 6
1. EXISTING CONDITIONS .............................................................................. 6
2. MITIGATION MEASURES ............................................................................ 7
B. GEOLOGIC HAZARDS ............................................................................ 10
1. EXISTING CONDITIONS ............................................................................ 10
2. MITIGATION MEASURES .......................................................................... 12
IV. COST ESTIMATE AND ME THOD OF FINANCING .............................. 18
APPENDICES
APPENDIX A: VICINITY MAP AND PROJECT SITE DRAWINGS
APPENDIX B: POSSIBLE UTILITY RELOCATION EXHIBITS
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I. INTRODUCTION
A. BASIS
This Hazard Mitigation Plan ("Plan") has been prepared in support of an application for
PUD Plan Review ("Rezoning") and Subdivision Review ("Preliminary Plan") for the
proposed River Edge Colorado ("Project", "REC", or "REC PUD") in accordance with the
requirements of the Garfield County Unified Land Use Resolution of 2008 ("ULUR"), as
amended and as specifically required pursuant to Section 5-501.G.11.c of the ULUR. This
Plan specifically addresses the requirements of Sections 7-109 and 7-210 of the ULUR.
This Plan is supported by other referenced documents submitted as part of the REC
rezoning and preliminary plan applications, including the River Edge Colorado PUD
(Rezoning) and Subdivision (Preliminary Plan) Drawing Package ("Drawing Package") and
the Geotechnical Engineering Study (Appendix J of the Impact Analysis).
B. PURPOSE AND SCOPE OF PLAN
The primary purpose of this Plan is to provide preliminary mitigation information for the
Project that supports the design and layout of development improvements depicted on
the Preliminary Engineering Plans (Preliminary Engineering Drawings, Series C00-07,
DR01-03, S01, and SW01-07 in the Drawing Package) and the layout of lots and
placement of building envelopes within the REC as depicted on the Preliminary Plan
(Preliminary Plan, Series PRPN01-04 in the Drawing Package). This Plan, in combination
with the Soils and Geological Hazards Plan and Mitigation Details, B01 Series drawings in
the Drawing Package, documents that the Project meets the requirements of the ULUR
in all respects. The Plan further serves as the design framework for final design efforts to
be completed in association with each Final Plat.
C. FINDINGS
Based on the findings of the Geotechnical Engineering Report (Appendix J of the Impact
Analysis) and the preliminary mitigation proposed to deal with identified hazards in this
Plan, William S. Otero P.E. (Colorado Registration #32163) has determined no significant
risks from natural hazards are posed to the Project that cannot be reasonably mitigated
or overcome through design, and the Project will not exacerbate the existing natural
hazards. The Project therefore conforms to Section 7-109 of the ULUR.
In addition, areas subject to alluvial fan and mudflow hazards have been effectively
mitigated by grade breaks provided by the bench created by SH 82 and the RFTA right-
of-way and additional protective measures proposed in this Plan for the single lot
affected. In addition, no development is proposed on slopes of 20% or greater and all
unstable slopes are located outside the development area in open space or common
areas not subject to development. Finally, as described below, where identified
sinkholes cannot be fully avoided, mitigation measures are proposed that will ensure
that roads and utilities are reasonably protected from damage. Based on these
mitigation measures and avoidance of high hazard areas where practicable, the Project
conforms to the requirements of Section 7-210 of the ULUR.
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Steven Pawlek of Hepworth-Pawlek Geotechnical ("HP Geotech") has conducted an
independent review of this Plan and the grading associated with the development plan
and determined that this Plan and the proposed Project grading reasonably conform to
and addresses the known site conditions and recommendations contained in the
Geotechnical Engineering Study date November 15, 2010 (Appendix J of the Impact
Analysis).
II. PROJECT LOCATION AND DESCRIPTION
A. PROJECT LOCATION
The Project is located along State Highway 82 ("SH 82") between the City of Glenwood
Springs and Town of Carbondale near the junction of County Road 110/113 ("CR 113")
and SH 82. The property is located almost entirely west of the Roaring Fork Transit
Authority ("RFTA") right-of-way and east of the Roaring Fork River and the Roaring Fork
Conservancy ("RFC") Conservation Easement (i.e., Grant of Conservation Easement
dated February 3, 2000, recorded at Reception Number 559036 and survey map,
recorded December 24th, 2008, recorded at Reception Number 760571 in the real
property records of Garfield County, Colorado). The Project straddles Cattle Creek which
is also located within the RFC Conservation Easement. A vicinity map is provided as
Exhibit 1 in Appendix A. The Project covers approximately 160 acres ("Project Site") as
shown and described on the Project Site drawing [Exhibit 2(a-d), Appendix A]. The
Project is proposed by Carbondale Investments, LLC ("CI").
B. PROJECT DESCRIPTION
The Project is a proposal to create a walkable clustered-form of residential development
with neighborhood amenities including naturalized open space and enhanced wildlife
habitat, community recreation, parks, and neighborhood agriculture that is designed to
serve the residents and preserve and provide reference to the rural character and
agricultural roots of the Roaring Fork Valley. The Project aims to have a strong historic
identity back to the days of ‘old Colorado’ when compact neighborhoods formed with a
strong sense of community based on the land and surrounding landscape. The REC
landscape aesthetic will be simple, informal, and place emphasis in the use of plant and
landscape materials local, adaptable and appropriate to the climate and environment of
the area. The Project will include approximately 366 residential units of various sizes
and types including 55 affordable homes and one exclusive executive lot for a custom
home. Housing types will range from attached homes to small single family attached
and detached garden homes, village homes, and larger estate homes. Smaller garden
homes are anticipated to be designed for younger residents that are looking for their
first home in the County, while village homes and estate homes will provide move up
opportunities for growing families. Densities in the Project are proposed at less than 2½
units per acre. Lot sizes will vary from over 1 acre to approximately 5,000 square feet for
single family homes, and 1,700-5000 square feet of lot area for each garden home. Most
of the units back to either proposed active parks or reclaimed open space to help
enhance the connection to the land. The REC layout and design is depicted in the PUD
Plan, PUD01-03 Series and the Preliminary Plan PRPN01-03 Series of the Drawing
Package.
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The architectural theme will be complementary to the traditional architecture of the
valley. Generally, exterior materials will include wood, stone, brick, stucco and cement
board siding. Varied roof heights and articulation of the front elevations will be used to
break up the massing and provide street-level appeal. Front porches and covered stoops
are included on homes to emphasize the entry and connection to the sidewalk and
street. Roofing will include dimensional shingles, metal, or other materials appropriate
to the building style and that roofs will generally be pitched. Gables, wall plane and
roofline articulation, bays, balconies, porches, canopies and arcades will be used in the
design of various buildings. The selection of materials will minimize the exterior
maintenance of the homes to help maintain a quality appearance for the long term.
The street pattern and pedestrian network are designed to facilitate community
interaction. Streets have detached sidewalks with designated cross walks at major
intersections and landscaped areas that create a comfortable environment for walking.
On‐street parking in most areas will further buffer vehicular and pedestrian uses.
Internal circulation is maximized and dead‐end streets are limited. Alleys are used
where appropriate to enhance the streetscape and achieve a mix of housing styles. A
soft trail system is used to connect open spaces and other common elements with the
sidewalk network. The homes are placed close to the streets to help define the
streetscape space and provide visual interest to pedestrians. Street trees and plantings
are proposed to enhance the aesthetics of the street.
The community is served with a variety of recreational facilities and a neighborhood
center that could include meeting room(s), fitness room, offices, kitchen, restrooms,
recreational facilities, and limited community service use such as a day care facility,
deli/coffee shop, or health club. Parks will provide informal recreational opportunities
within the community and will likely include tot lots, playfields, and trail system. The
west portion of the property is generally set aside as the naturalized area that buffers
the RFC Conservation Easement along the Roaring Fork River. The soft trails around the
property allow residents to enjoy the river and wetland areas without disrupting the
environment in conformance with the terms of the RFC Conservation Easement. More
than the minimum open space requirements will be met by the project. Nearly 50% of
the Project Site is in some form of open space, common area or park. Finally,
opportunities for productive and edible landscapes, including community gardens and
neighborhood orchards are integrated and dispersed in between the residential land
uses as gathering and focal places for residents connecting REC to its agricultural
heritage.
The combination of trails, recreation areas, and open space system with the ability to
engage in ‘interactive community agriculture’ on a small scale will make REC a very
desirable place to live, filling a unique niche not yet met in Garfield County. This unique
combination will help establish a sense of place, foster community, and engage
residents with their immediate environment. It is intended this overall outdoor focus
will set the tone and become a major driver of the identity of REC.
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III. SIT E CONDITIONS, POTENT IAL EFFECTS AND MITI GATION
MEASURES
The following section presents existing site conditions ("natural features") that
influenced the layout and design of the Project. Where hazards could not be fully
avoided, mitigation measures are identified to meet the requirements of Sections 7-109
and 7-210 of the ULUR. The mitigation measures identified herein are considered
reasonable and appropriate preliminary mitigation actions or measures to employ in the
development of the Project Site. Where conditions warrant during final design, location
specific design measures may be identified. In addition, where field investigations
during construction yield more detailed information or expose additional evidence,
mitigation, which may include additional avoidance actions or alternative design
measures not identified herein, may be determined to be necessary. The general
mitigation measures identified herein are based upon the findings of HP Geotech, but
should not be viewed as final in scope or extent. Field observations and investigations
during construction are a key component of identifying and appropriately mitigating any
hazard.
A. SOIL CHARACTERISITIC S
1. EXISTING CONDITIONS
HP Geotech completed an assessment of geologic conditions including the
identification of geologic hazards for the REC PUD in November 2010. The
following discussion concerning the soil conditions is summarized from the
Geotechnical Engineering Study (Appendix J of the Impact Analysis).
The main landforms at the Project Site related to the site's surficial materials
include (1) post-glacial alluvial terraces along the Roaring Fork River and Cattle
Creek, (2) Pinedale glacial outwash terraces along the Roaring Fork River and
related alluvial terraces along Cattle Creek, and (3) coalescing alluvial fans. A
small part of the proposed development area within the REC PUD is located on
the post-glacial alluvial terraces and the remaining development area, except
the Executive Lot at the south end of the Project, sits on the Pinedale glacial
outwash terraces.
The topsoil was stripped from most of the Project Site and stockpiled in 2005 by
Bair Chase in association with the Sanders Ranch PUD approved by Garfield
County. The areas stripped of topsoil and stockpile sites are shown on the
Existing Conditions/Land Suitability Plan, EC01 Series drawings in the Drawing
Package. The previous grading consisted of both cut and fill operations. The fill
areas are mostly composed of coarse-grained terrace alluvium. The terrace
topsoil and upper fine-grained deposits were separated during grading and
were placed in the soil stockpiles. The character of the coarse- and fine-grained
terrace alluvium is described in the Pinedale Terraces as described in the
Geotechnical Engineering Study.
The post-glacial terraces are located as two terraces. The lower terrace stands
about 5 feet above the river and the higher terrace stands about 13 feet above
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the river. The alluvium is described as a clast-supported deposit of silty sand
with occasional bouldery, pebble and cobble gravel interbedded and often
overlain by sandy silt and silty sand. Shallow groundwater is expected to be
present in these areas. A small part of the proposed development area in the
southern part of the Project Site is located on the upper terrace. Elsewhere the
proposed development will be located on the higher Pinedale terraces.
The Pinedale outwash terraces along the Roaring Fork River and the associated
Cattle Creek terraces occur in several levels that formed at different periods.
Grading in 2005 removed all of the mid level terraces. Essentially all of the
proposed development will be on the graded area (originally the fifth and sixth
terraces) and on the third, fourth and seventh terrace levels. The alluvium under
the Pinedale terraces associated with the Roaring Fork River and Cattle Creek
are a clast-supported deposit of rounded gravel, cobbles and boulders in a silty
sand matrix. Pedogenetic soil profiles are well developed in the Pinedale
terraces. This indicates these surfaces have been stable with respect to erosion
and deposition for over about 5,000 years.
2. MITIGATION MEASURES
The following section identifies the potential effects associated with existing
soils conditions. Both general considerations and mitigation measures proposed
to overcome the limitations associated with the identified soil conditions are
provided in accordance with Sections 7-109 and 7-210 of the ULUR.
a) General Considerations
No specific or critical soil-related issues were identified by HP Geotech that
would indicate a problem for development of the site within the
development areas boundaries proposed. The Geotechnical Engineering
Study identifies a series of standard or common considerations with respect
to construction on native soils of this type within the Roaring Fork Valley.
These include general recommendations concerning foundations, floor
slabs, underdrain systems, and surface water management.
Previous site grading results in the most substantial risk to construction.
Risks and issues related to manmade conditions potentially exist due to the
cut and fill actions completed by Bair Chase in association with the Sanders
Ranch PUD. Special considerations must be given to these conditions. These
areas should be specifically identified and appropriately mitigated as
construction progresses. With respect to disturbed fill, HP Geotech noted
that while resistance is high in areas where fill has been placed, they are
uncertain if the fill in all areas has been adequately placed and compacted,
and therefore are unsure if these soils are suitable for building foundations.
As a result, additional geotechnical work will need to be done prior to
development as part of pre-development reclamation (Phase 0) as well as
during grading of the Project Site. Disturbed soils and existing fill will need
to be further tested during pre-development reclamation (Phase 0) and
development to determine if they should be removed, replaced and
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compacted where existing conditions would not support planned
development features.
Soils conditions in some Open Space Tracts along the western and southern
edges of the Project are on steep slopes and subject to erosion. In low lying
areas adjacent to Cattle Creek, soils are subject to wetting. In both cases,
these areas are avoided by the development except for a bridge crossing of
Cattle Creek which is discussed further below. The edges along the western
and southern boundaries of the Project are proposed for reclamation to
stabilize these areas and avoid further deformation and degradation in the
Reclamation Plan (Phase 0) (Appendix U of the Impact Report).
b) Detailed Mitigation Measures
The following represent the detailed mitigation measures that will be
implemented as appropriate to support the development of the REC PUD.
All of the measures proposed are consistent with information presented by
HP Geotech in the Geotechnical Study. However, additional mitigation
measures or options are also proposed to best achieve the goals of the
Project and meet the requirements of Sections 7-109 and 7-210 of the
ULUR.
(1) Foundations
With respect to native soils in place, HP Geotech noted that while
bearing conditions will vary depending on the specific location, in
general, shallow foundations placed on the upper natural soils
should typically be suitable for structure support. HP Geotech
expects allowable bearing pressures in the range of 1,500 pounds
per square foot (psf) to 2,500 psf for footings bearing on the natural
fine-grained soils in the Pinedale terraces. Some potential for
settlement or heave due may be encountered if fine grained soils
are present. Due to the depositional environment that created the
alluvial terraces on which development is proposed, expansive clays
could be encountered in small lenses.
Some specific mitigation measures that may be required as a result
of the above conditions include:
Relatively rigid foundations such as reinforced
structural slabs or other appropriate foundation
design shall be used to reduce the risk of
differential settlement and building distress, where
determined necessary as a result of site specific soil
and differential settlement conditions associated
with geologic hazards discussed later in this Plan.
If encountered, the expansive clays shall be
excavated and replaced with select fill or the
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footings designed to impose a minimum dead load
pressure to limit potential heave.
Footings for heavily loaded structures (i.e. bridge
abutments) bearing entirely on the natural gravel
alluvium shall be sized for allowable bearing
pressures in the range of 3,000 psf to 5,000 psf.
Foundation walls shall be designed to span local
anomalies and to resist lateral earth loadings when
acting as retaining structures.
Below grade areas, such as basements, and
retaining walls should be protected from wetting
and hydrostatic loading by use of an underdrain
system where soils conditions indicate a threat of
wetting.
Footings shall have a minimum depth of 36 inches
for frost protection or as otherwise required by the
Garfield County Building Code
Footings placed on fill areas shall be designed for
allowable loads recommended by the onsite
geotech once the fill is placed and tested.
Loose colluvial and alluvial fan soils with collapse
potential may need treatment such as enlarging
footings or placing compacted structural fill to
protect against settlement.
(2) Floor Slabs
Slab-on-grade construction should be feasible for bearing on the
natural soils or compacted structural fill. There could be some
potential for post-construction slab movement at sites with
collapsible soils or expansive clays. Some specific mitigation
measures that may be required as a result of the above conditions
include:
Removal of the moisture sensitive soils and
replacement with select fill shall be used to reduce
the movement risk where collapsible or expansive
clays are identified.
(3) Surface Water Management
It should be noted that the Geotechnical Engineering Study notes
that piping caused by excess surface water infiltration has proven to
be a potential source of soils instability, predominately along the
upper edges of the steep escarpments located along the RFC
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Conservation Easement. Some specific mitigation measures that
may be required as a result of the above conditions include:
Community space irrigation will be managed to
lessen infiltration by controlling the amount of
water placed (i.e. the amount placed will be
consistent with evapotranspiration parameters
specific to the region).
Limited areas will be subject to irrigation and high
water demand vegetation in accordance with the
landscape provisions of the PUD Plan.
Automatic irrigation control systems will be
required in accordance with the landscape
provisions of the PUD Plan to limit and zone
irrigation to appropriate levels consistent with
water demand.
B. GEO LOGIC HAZARDS
1. EXISTING CONDITIONS
HP Geotech completed an assessment of geologic conditions including the
identification of geologic hazards for the REC PUD in November 2010. The
Geotechnical Engineering Study is included in Appendix J of the Impact Report.
Five primary hazards affecting the Project Site were identified in the
Geotechnical Engineering Study. These hazards include evaporite sinkholes,
steep terrace escarpments, active stream bank erosion, debris flows and floods,
and earthquakes. The following key information and considerations are drawn
from the Geotechnical Engineering Study.
a) Evaporite Sinkholes
HP Geotech notes that the near surface formation rock in the area is the
Eagle Valley Evaporite. The evaporite between Carbondale and about 3
miles south of Glenwood Springs is part of the Roaring Fork diapir which
coincides with the Grand Hogback monocline that marks the western limit
of the Carbondale evaporite collapse center. The Carbondale evaporite
collapse center is the western of two regional evaporite collapse centers
present in the western Colorado evaporite region. As much as 4,000 feet of
regional ground subsidence is believed to have occurred during the past 10
million years as a result of dissolution and flowage of evaporite from
beneath the region. HP Geotech notes that it is uncertain if the regional
subsidence and evaporite deformation along the Roaring Fork diapir are still
an active geomorphic process or if evaporite deformations have stopped. If
still active, present deformations are likely occurring at rates similar to past
long-term rates of between 0.5 and 1.6 inches per 100 years. HP Geotech
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concludes that these slow deformation rates should not present a potential
risk to buildings and other facilities being considered at the Project Site.
HP Geotech identified nine general sinkhole areas in the field and on aerial
photographs in and close to the Project Site. These locations are identified
on the Existing Conditions/Land Suitability Plan, EC01 Series of the Drawing
Package. Evaporite sinkholes in western Colorado are typically 10- to 50-
foot diameter, circular depressions at the ground surface that result from
upward caving of a soil rubble pipe to the ground surface. The soil rubble
pipe is formed by subsurface erosion (piping) of near surface soils into
subsurface voids. Sinkhole development or reactivation in the specific
locations (Hazard Zone 1) is still an active geomorphic process.
b) Steep Terrace Escarpments
Steep terrace escarpments that commonly have slopes of about 60 percent
and vary from 40 to 80 feet high are present along the Roaring Fork River
and the lower reaches of Cattle Creek. These escarpments are potentially
unstable and in some cases have been further destabilized due to piping
associated with irrigation water from the previous agricultural activities on
the Project Site. The escarpments are located along the western most
property line and encroach into the RFC Conservation Easement. These
areas can contribute to sediment production during rain and flood events or
in association with excessive irrigation.
c) Active Stream B ank Erosion
Active stream bank erosion during high flood flow is occurring along the
Roaring Fork River and Cattle Creek in several areas where these streams
flow along the base of the steep terrace escarpments further destabilizing
the steep terrace escarpments. These areas contribute to sediment
production during rain and flood events. These areas all lie outside the
boundaries of the Project Site within the RFC Conservation Easement.
d) Debris Flow and Floods
HP Geotech noted that coalescing alluvial fans developed at the mouth of
the numerous, small drainage basins on the east side of the Roaring Fork
Valley where the ephemeral streams discharge on terrace surfaces. Before
construction of SH 82 and development to the east of the highway, the
alluvial fan formed a continuous apron at the terrace-valley transition. Most
of the upper parts of the fans have been removed by grading for SH 82 and
the rail corridor. With the exception of the Executive Lot at the southern
end of the Project, development is not being proposed on the alluvial fans.
Swell-consolidation tests show that the deposits do not have a high collapse
potential (settlement after wetting under a constant load) and are
moderately compressible under increased loading after wetting. This
indicates that the fans are geologically young landforms and are still
potential sites of debris flow and flood deposition.
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e) Earthquakes
HP details the faults present in the vicinity of the Project. They conclude
that geologically young faults related to evaporite tectonics are present in
the Carbondale evaporite collapse center in the vicinity of the Project Site
but considering the nature of evaporite tectonics these faults are not
considered capable generating large earthquakes.
The closest geologically young faults considered capable of generating large
earthquakes are located in the Rio Grande rift to the between 62 and 67
miles east of the Project Site. For firm rock sites with shear wave velocities
of 2,500 fps in the upper 100 feet the U. S. Geological Survey 2002 National
Seismic Hazard Map indicates that a peak ground acceleration of 0.06g has a
10% exceedance probability for a 50 year exposure time and a peak ground
acceleration of 0.22g has a 2% exceedance probability for a 50 year
exposure time at the Project Site. This corresponds to a statistical
recurrence time of about 500 years and 2,500 years, respectively. HP again
concludes that at these distances, large earthquakes on the two closest
geologically young fault zones should not produce strong ground shaking at
the Project Site. HP concludes that earthquake risk is consistent with other
areas of the Roaring Fork Valley.
2. MITIGATION MEASURES
The following section identifies the potential effects associated with existing
geologic conditions. Both general considerations and mitigation measures
proposed to overcome the limitations associated with the identified geologic
conditions are provided in accordance with Sections 7-109 and 7-210 of the
ULUR.
a) General Considerations
The Geotechnical Engineering Study provides several general considerations
which are incorporated into this Plan and PUD Guide. These include general
recommendations concerning foundations, floor slabs, underdrain systems,
and surface water management.
(1) Foundations
Foundations, as they relate to soil conditions, are discussed in the
Soils section (Section III.A) of this Plan. As noted in the Soils section,
bearing strengths will vary across the Project Site, but within the
development areas can be expected to support residential
structures on a shallow or spread footer foundation. However, the
ability of the soils to support a structure may be compromised by
the risk of ground subsidence associated with the underlying Eagle
Valley Evaporite and the potential for sinkhole development
discussed below and the differential settlement that might occur
under these conditions. Some specific mitigation measures that may
be required as a result of the above conditions include:
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Building sites should be specifically evaluated as
development progresses and the use of relatively
rigid foundations such as reinforced structural slabs
or similar foundations may be appropriate, in some
situations, to reduce the associated risk of
differential settlement.
Deepened foundations or other ridged foundations
may be considered for the Executive Lot located on
the southern end of the Project Site depending on
the findings from the additional investigations and
the proposed structure location.
(2) Floor Slabs
HP Geotech has determined that slab-on-grade construction should
be feasible for bearing on the natural soils or compacted structural
fill. To reduce the effects of potential differential movement, some
specific mitigation measures that may be required include:
Non-structural floor slabs shall be separated from
all bearing walls and columns with expansion joints
and floor slab control joints shall be used to reduce
damage due to shrinkage cracking where identified
as a potential issue affecting specific building site. In
addition, a minimum 4-inch thick layer of free-
draining gravel should underlie basement level floor
slabs to facilitate drainage.
(3) Underdrain Systems
Although HP Geotech notes that groundwater was not encountered,
perched groundwater may occur in the subsurface materials during
heavy rain events. Where evidence of seasonal groundwater or soil
wetting is encountered, some specific mitigation measures that may
be required include:
An underdrain system is recommended to protect
below-grade construction, such as retaining walls,
deep crawlspace and basement areas from wetting
and hydrostatic pressure. The drain should be
placed at each level of excavation and at least 1
foot below lowest adjacent finish grade and sloped
at a minimum of 1 percent to a suitable gravity
outlet.
(4) Surface Water Management
With respect to surface water management, it is important to
maintain positive drainage and limit infiltration of irrigation and
stormwater on the Project Site to reduce impacts to structure, soil
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piping, and water available to place the underlying evaporite into
solution. Some specific mitigation measures that may be required as
a result of the above conditions include:
Pond/detention basins and major drainage ditch
liners have been introduced to reduce water loss to
the subsurface in areas where water is or can
become concentrated.
Community space irrigation will be managed to
lessen infiltration by controlling the amount of
water placed (i.e. the amount placed will be
consistent with evapotranspiration parameters
specific to the region).
Limited areas will be subject to irrigation and high
water demand vegetation in accordance with the
landscape provisions of the PUD Plan.
Automatic irrigation control systems will be
required in accordance with the landscape
provisions of the PUD Plan to limit and zone
irrigation to appropriate levels consistent with
water demand.
These additional surface water management efforts help to
minimize excess water from entering the subsurface and
exacerbating any existing hazards in conformance with Section 7-
109 of the ULUR.
b) Detailed Considerations
The following represent the detailed mitigation measures that will be
implemented as appropriate to support the development of the REC PUD.
All of the measures proposed are consistent with information presented by
HP Geotech in the Geotechnical Study. However, additional mitigation
measures or options are also proposed to best achieve the goals of the
Project and meet the requirements of Sections 7-109 and 7-210 of the
ULUR.
(1) Sinkholes
During pre-development reclamation (Phase 0) site grading
activities, field observations will be performed, by an onsite
geotechnical engineer, as existing soils are disturbed to identify
areas of possible concern. Should questionable soils be observed,
the immediate area of the observed soils will be further investigated
to determine the level of sinkhole hazard present and identify any
specific necessary mitigation. It should be noted that no structures
are sited within Hazard Zone 1; therefore, no further mitigation is
Hazards Mitigation Plan
River Edge Colorado, Garfield County, Colorado
15
presented within these areas. Housing sites located within Hazard
Zone 2 will require further investigations (i.e. borings) to determine
if subsurface voids are present prior to initiating their development
or if its designation should be changed to Hazard Zone 1. The
following presents preliminary measures to be followed should a
development feature be located within an existing or newly
identified sinkhole hazard or within 80 feet of an identified sinkhole:
Soft Trails: Short segments of soft trails cross
Hazard Zone 1 areas (see Soils and Geotechnical
Hazards Plan, B01 Series of the Drawing Package for
locations). In these areas, existing soils will be over-
excavated to a depth deemed necessary by the
geotechnical engineer and select fill placed prior to
placement of the trail materials. A preliminary
depth of 3 feet below the trail design grade is
presented in engineering drawings. Fill will be
compacted to 90 percent Standard Proctor or
better. No further mitigation is anticipated for this
development feature.
Roads: Roads are planned to generally avoid Hazard
Zone 1 areas. Short road segments cross Hazard
Zone 1 areas (see Soils and Geotechnical Hazards
Plan, B01 Series of the Drawing Package for
locations). In these areas, further investigations will
be performed and a site-specific mitigation action
will be developed as part of final design and field
construction activities. Based on currently available
information, at a minimum, the areas will be over-
excavated to a minimum depth of 5 feet below
design grades, and select fill placed and compacted
to 95 percent Standard Proctor or better. Should
further mitigation be required, ether compaction
grouting or structure bridging may be considered.
Whichever mitigation is selected, the primary
objective is to lessen the impacts of differential
settlement on the road. The placement of
geosynthetics beneath the pavement may also be
considered to further lessen the load on the
subsurface soils is some instances.
Utilities: Utilities have been currently planned to
generally avoid Hazard Zone 1 areas. At this
preliminary level of design, short utility segments
do cross Hazard Zone 1 areas (see Soils and
Geotechnical Hazards Plan, B01 Series of the
Drawing Package for possible locations). Prior to
Hazard Mitigation Plan
River Edge Colorado, Garfield County, Colorado
16
final design, further investigations will be
performed and a location-specific mitigation
program may be developed in coordination within
the geotechnical engineer. Based on currently
available information, at a minimum, the areas will
be over-excavated to a minimum depth of 5 feet
below the design invert, select fill placed
(compaction at 95 percent Standard Proctor) and a
sleeved utility installed (i.e. allowing bridging of the
identified void). Should investigations deem further
mitigation is required to achieve the necessary level
of protection; all efforts will be based on a
geotechnical engineer's recommendation, with the
focus being on minimizing damage to the installed
utility. In the event that mitigation to the proper
level of protection cannot be achieved, the
preliminarily planned utility in that area will be
realigned to appropriate location. See Appendix B
Exhibits 1 and 2 for potential relocation alignments.
Stormwater: Stormwater conveyance channels and
storage facilities have been currently planned to
generally avoid Hazard Zone 1 areas. In areas where
short segments of conveyance channels cross
Hazard Zone 1 areas (see Soils and Geotechnical
Hazards Plan, B01 Series of the Drawing Package for
locations), further investigations will be performed
and a location-specific mitigation program will be
developed. In addition, in areas where storage
facilities are located in the vicinity of Hazard Zone 1
areas (none are currently sited in this zone), further
investigations will be performed and a location-
specific mitigation program will be developed.
Based on currently available information, it has
been determined that a Best Management Practice
(BMPs) for these conditions, at a minimum, is to
line all major water conveyance and storage
facilities in an effort to minimize water infiltration.
In the event that mitigation to the proper level of
protection cannot be achieved, the preliminarily
planned channel or storage facility in that area will
be realigned or relocated to an appropriate
location.
(2) Steep Terrace Escarpments
Planned development within the Project Site does not directly
encroach into any existing steep escarpments. However, in an effort
to reduce further degradation and increase the safety of the
Hazards Mitigation Plan
River Edge Colorado, Garfield County, Colorado
17
transitional areas (i.e. areas accessible to home owners at the top of
the escarpments) the pre-development reclamation action, as
described in the Reclamation Plan (Phase 0)(Appendix U of the
Impact Report), proposes activities to stabilize many of these areas
within the Project Site. Additional mitigation efforts outside the
Project Site, on the RFC Conservation Easement, have been
discussed with the RFC. Based on available information, many of
these transitional areas have become unstable or are sinking
primarily due to what is known as "piping failures" (i.e. the removal
of fine particles). The level of mitigation for these areas will depend
on the site conditions (determined through further investigations)
and level of development activity within the vicinity of the steep
escarpment. For example, in areas where "limited activity" open
space is planned, one form of mitigation for these areas may include
the excavation of the affected areas, placement of filter fabric sized
to reduce the release of finer particles, compaction of select fill and
armoring the outer edges with rock. In other areas where
development infrastructure is planned in the vicinity of a steep
escarpment (no homes sites are sited near steep escarpments),
more robust forms mitigation may be required to achieve the
necessary level of protection. All mitigation efforts will be based on
a geotechnical engineer's recommendation, with the focus being on
minimizing further piping and stabilizing the already impacted areas
to prevent continued degradation. In the event that mitigation to
the proper level of protection cannot be achieved, the preliminarily
planned infrastructure in that area will be relocated to an
appropriate location.
As presented in Section III.A.2, irrigation within these areas will be
managed to reduce further degradation of these areas (i.e. control
irrigation application rates to evapotranspiration parameters
specific to the region). The primary factor that has impacted these
areas to date is flood irrigation returns to the Roaring Fork River in
the form of excess irrigation water. With flood irrigation and
farming generally removed, except as these steep escarpments are
affected by undercutting by active stream bank erosion areas along
the Roaring Fork River these areas should be relatively easy to
stabilize through the actions described.
(3) Active Stream Bank Erosion
Areas of observed active stream bank erosion are located entirely
offsite within the RFC Conservation Easement. While these areas do
impact the steep terrace escarpments, they do not generally place
the Project at risk. No mitigation is required. However, as with the
steep terraces, CI has proposed to the RFC that some mitigation be
done to these areas to preserve the conservation values within the
RFC Conservation Easement. Approval of the pre-development
reclamation plan actions, as described in the Reclamation Plan
Hazard Mitigation Plan
River Edge Colorado, Garfield County, Colorado
18
(Phase 0)(Appendix U of the Impact Report), proposed within the
RFC Conservation Easement will be sought prior to undertaking such
actions. As proposed, these reclamation activities include armoring
these areas to prevent further erosion. These actions could have
benefit to the environment and the Project Site by minimizing the
release of sediment and potential destabilization of the steep
escarpments. If approved by the RFC, these areas will be further
investigated and a detailed mitigation program developed as part of
the Reclamation Plan (Phase 0).
(4) Debris Flows and Floods
Based on current observations, HP Geotech notes that conventional
surface drainage design should be adequate to account for sheet
flow on the terrace surface down slope of the fans. Grading for
Highway 82 and the development to the east of the highway has
substantially modified flow patterns on the fans. This grading should
cause debris deposition at the grade change between the road cuts
and road platform and reduce the extent of future deposition on
the fan limiting the risk to structures located thereon. Mitigation
measures, such as flow diversion or deepened foundations, on the
Executive Lot should be incorporated into the final designs based on
further field investigations and the construction activities planned
as provided for by the PUD Guide and PUD Plan.
IV. COST ESTIMATE AND ME THOD OF FINANCING
A cost analysis and estimate will be provided for hazard mitigation under separate cover
in conformance with Section 6-301C.8.r.(3) of the ULUR at the time of and in association
with each Subdivision Application for Final Plat. The cost estimate shall include
estimates for detailed mitigation programs as necessary to support the development of
the lots being proposed for creation within the boundaries of the Final Plat being
submitted for review.
Based on the preliminary design submitted for review in association with the current
rezoning and preliminary plan application supported by this Plan, preliminary costs have
been developed and reviewed by 8140 Partners, LLC. These costs have been determined
to be reasonable and support the feasibility of implementing the above proposed
mitigation as part of the Project. Preliminary cost estimates are viewed by Carbondale
Investments, LLC ("CI") as proprietary information and of limited or no value to the
rezoning and preliminary plan review and approval process since no construction is
specifically allowed nor is any construction security required by any such approval
granted by Garfield County.
Hazard Mitigation Plan
River Edge Colorado, Garfield County, Colorado
APPENDIX A: VICINITY MAP AND PROJECT SITE DRAWINGS
App. A-1
12/01/10VICINITY MAPExhibit:Date:8140 Partners, LLCTitle:Prepared by:Owner/Developer: Carbondale Investments, LLC7999 HWY 82Carbondale, CO 81623Phone No:970.456.5325App. A-2
Exhibit:Date:8140 Partners, LLCTitle:Prepared by:Owner/Developer: Carbondale Investments, LLC7999 HWY 82Carbondale, CO 81623Phone No:970.456.532512/01/10App. A-3
Exhibit:Date:8140 Partners, LLCTitle:Prepared by:Owner/Developer: Carbondale Investments, LLC7999 HWY 82Carbondale, CO 81623Phone No:970.456.532512/01/10App. A-4
Exhibit:Date:8140 Partners, LLCTitle:Prepared by:Owner/Developer: Carbondale Investments, LLC7999 HWY 82Carbondale, CO 81623Phone No:970.456.532512/01/10App. A-5
Exhibit:Date:8140 Partners, LLCTitle:Prepared by:Owner/Developer: Carbondale Investments, LLC7999 HWY 82Carbondale, CO 81623Phone No:970.456.532512/01/10App. A-6
Hazard Mitigation Plan
River Edge Colorado, Garfield County, Colorado
APPENDIX B: POSSIBLE UTILITY RELOCATION EXHIBITS
App. B-1
Utility Relocation Example 01/14/2011
1
Exhibit:
Date:
8140 Partners, LLC
Title:
Prepared by:
Owner/Developer:
Carbondale Investments, LLC
7999 HWY 82
Carbondale, CO 81623
Phone No:
970.456.5325 App. B-2
Utility Relocation Example 01/14/2011
2
Exhibit:
Date:
8140 Partners, LLC
Title:
Prepared by:
Owner/Developer:
Carbondale Investments, LLC
7999 HWY 82
Carbondale, CO 81623
Phone No:
970.456.5325 App. B-3