Presented in this chapter of the manual are policies that govern
development of specific standards and criteria for the design, evaluation,
and construction of drainage facilities. These policies are based
on industry standards for stormwater management that have evolved
through experience gained. Policy statements are indicated by bold italic text to distinguish them from background
information, which is presented to assist with decisions regarding
special circumstances that may arise during development.
(Res. 40-08 (§ 401), 3-19-08)
(a) Jurisdictional Cooperation.
Mesa County, the Cities
of Grand Junction and Fruita, the Town of Palisade, and the Grand
Junction Drainage District have contracted through intergovernmental
agreements (IGA) to form the 5-2-1 Drainage Authority (Authority),
under Colorado Revised Statutes, specifically §
29-1-204.2, C.R.S. The stated purpose of the Authority is to pursue unified
stormwater management planning that meets the requirements of the
Colorado Water Quality Control Act (§
25-8-101,
C.R.S. et seq.) and the Federal Water Pollution Control Act, as amended
(
33 U.S.C.
1251 et seq.)
for the discharge of stormwater associated with municipal separate
storm sewer systems.
The Drainage Authority was created to pursue a
jurisdictionally unified effort to promote integrated, comprehensive,
regional stormwater management planning, while recognizing local requirements
may have minor differences in policies, standards and criteria presented
in this manual.
(b) Drainage Planning and Required Space.
All new development and redevelopment shall provide storm drainage
planning that includes allocation of space for drainage facilities,
construction and maintenance, and dedication of rights-of-way and/or
easements.
(c) Multi-Purpose Resource.
Stormwater runoff is an integral
part of Mesa County’s and the City of Grand Junction’s
water resources and may have potential for beneficial uses, such as
groundwater recharge, recreation, and wildlife habitat. These uses,
however, must be compatible with adjacent land uses and applicable
State water laws.
Stormwater runoff shall be considered an integral
part of Mesa County’s water resources.
(d) Water Rights.
A drainage design must be planned and
constructed with recognition given to adjudicated water rights and
applicable water laws.
The existence of adjudicated water rights and
all applicable State laws related thereto shall be recognized in stormwater
management plans.
(e) Drainage Analysis Tributary Area.
Stormwater runoff
will follow pathways established by physical boundaries and not political
or property boundaries. Therefore:
Any drainage analysis must consider all tributary
area and the potential for future land use changes within the tributary
area.
(Res. 40-08 (§ 402), 3-19-08)
(a) Reasonable Use Rule.
The reasonable use rule is defined for drainage planning purposes as permitting the use
of an economic and hydraulically efficient drainage system which is
demonstrated not to adversely impact downstream properties within
reason. This reasonable use of drainage therefore allows development
to occur while preserving the rights of adjacent property owners.
Stormwater discharges from new development and
redevelopment shall be:
(1)
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Discharged to downstream properties within the
pre-developed drainage path and in a manner and quantity and quality
that approximates pre-developed conditions. If developed stormwater
discharges occur in a more concentrated manner, then additional measures
are required to protect downstream properties.
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(2)
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Limited to pre-developed rates, unless downstream
drainage facilities can accommodate increased flow rates from development
and permission/easements are granted.
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(Res. 40-08 (§ 403.1), 3-19-08)
It is the intent of Mesa County and the other
cooperating governmental entities to develop, adopt, and use Watershed
Master Plans to identify stormwater requirements. Once adopted, these
plans will be reviewed and updated regularly. When Watershed Master
Plans are not available, then requirements in the Stormwater Management
Manual will govern.
(Res. 40-08 (§ 403.2), 3-19-08)
Drainage facilities are categorized as part of either the major
drainage system or the outfall system. Recommended public improvements
to major drainage and outfall drainage systems are defined in Watershed
Master Plans, if they exist. If a Master Plan does not exist, other
information will be used to determine the scope of public improvements.
All new development and redevelopment shall participate
in drainage improvements as set forth below:
(a)
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Outfall Drainage System.
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(1)
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Design and construct that portion of the outfall drainage system, as defined by the approved final drainage report (Chapter 28.12 GJMC).
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(2)
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If the outfall system is defined in a Watershed
Master Plan, and traverses the development, the developer shall design
and construct that portion of the outfall system within the development,
in accordance with this manual.
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(3)
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If the outfall system defined in a Watershed Master
Plan does not traverse the development, but is required to convey
stormwater from the development to the major drainageway, the developer
shall design and construct that portion of the outfall system within
the development, in accordance with this manual. The local jurisdiction
may participate in the connection of the outfall to the major drainageway
at their sole discretion.
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(b)
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Major Drainage System.
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(1)
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If new development (i.e., the placement of fill
or structures) encroaches into a 100-year floodplain (whether mapped
or not), the developer will be required to construct improvements
as described in the Watershed Master Plan. If a Watershed Master Plan
is not available, the developer shall have prepared a channel stabilization
analysis, under the guidance of the local jurisdiction, to identify
required improvements and shall implement the mitigation plan.
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(2)
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Additional improvements to protect health, safety,
and welfare may be required by the local jurisdiction if new development
is within the vicinity of a 100-year floodplain, whether mapped or
not. The developer may be required to participate in a channel stabilization
analysis, under the guidance of the local jurisdiction, and may be
required to participate in the implementation of the mitigation plan.
For the purpose of this policy, “vicinity” shall mean
any portion of the property that lies within a setback area defined
by a slope of 10 feet horizontal to one foot vertical (10:1) from
the channel invert to the point where the slope daylights.
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(Res. 40-08 (§ 403.3), 3-19-08)
Fees may be collected by the local jurisdiction and used by
the local jurisdiction solely for the planning, design and construction
of drainage improvements to the outfall drainage system and major
drainage systems.
When drainage fees are assessed for new development and redevelopment,
the drainage fee will be determined based on the following equation:
Drainage Fee = B(CD – CH)A0.7
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(28.16-1)
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where:
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B
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=
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Fee constant (U.S. dollars, see local jurisdiction for value)
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CD
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=
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100-year runoff coefficient (expressed as a decimal) based on
developed land use conditions
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CH
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=
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100-year runoff coefficient (expressed as a decimal) based on
pre-developed land use conditions
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A
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=
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Area of development (acres)
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(Res. 40-08 (§ 403.4), 3-19-08)
All investigations, reports, and construction
plans prepared for drainage improvements shall be submitted for review
and acceptance before construction of said improvements, and shall
be consistent with this Stormwater Management Manual and Watershed
Master Plans.
(Res. 40-08 (§ 403.5), 3-19-08)
Floodplains within Mesa County, Grand Junction,
Fruita and Palisade shall be regulated in accordance with the provisions
of the applicable land development/use code.
(Res. 40-08 (§ 403.6), 3-19-08)
Since urban development can increase the rate, volume, duration,
and frequency of stormwater runoff, measures must be implemented to
avoid harm to downstream properties. Detention is considered a viable
method to reduce development impacts and drainage costs. Temporarily
detaining storm runoff can significantly reduce downstream flood hazards
as well as reduce pipe and channel sizes in developed areas. Temporary
storage also provides for sediment and debris collection which helps
to maintain water quality in downstream channels and streams. However,
detention may not be necessary where downstream drainage facilities
have adequate capacity to convey runoff from fully developed upstream
areas without negatively impacting downstream properties.
(a) Detention Requirements.
(1)
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On-site detention storage is required for all new development or redevelopment to limit 10-year peak stormwater discharges to the 10-year storm runoff based on pre-developed conditions. In addition to the 10-year detention, detention of the 100-year storm event and release at pre-developed rates will be required, if the capacity of the downstream drainage system will be exceeded. With local jurisdiction approval, detention may be considered for off-site flows draining to a site. See Chapter 28.56 GJMC for details.
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(2)
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A fee in lieu of detention may be required in lieu of on-site detention that is not required (see GJMC § 28.56.050 through 28.56.070). Granting fee in lieu of detention does not exempt the development from providing post-construction BMPs.
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(b) Detention Exemption.
At the sole discretion
of the local jurisdiction, exemptions to on-site detention may be
granted for the following conditions. Note that in all cases, post-construction
BMPs are required:
(1) A development which discharges to a regional drainage facility is
exempt provided the regional facility is completed in accordance with
a Watershed Master Plan, the regional facility was designed to include
runoff from the proposed development, and there is adequate conveyance
capacity for the drainage system from the development to the regional
facility.
(2) A development lying within the limits of a Watershed Master Plan
which explicitly exempts on-site detention for development.
(3) A development which discharges to a local detention facility is exempt
provided the local detention is completed in accordance with the final
drainage plan for the development and was designed to include runoff
from the proposed development.
(4) A development which discharges to an outfall drainage system that
has adequate conveyance capacity for the 100-year flood from a fully
developed watershed.
(5) When it can be demonstrated that the peak flows from the development
will not increase the peak flows from the watershed for storm events
up to the 100-year flood. The burden of proof is on the developer
to demonstrate this condition.
A fee in lieu of detention may be required in
lieu of on-site detention that is not required.
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(Res. 40-08 (§ 403.7), 3-19-08)
Storm runoff retention or over-detention may be used when downstream
drainage facilities lack adequate conveyance capacity or are essentially
nonexistent and construction of an outfall drainage system is impractical.
Retention or over-detention may be used in those
instances where there are severe limitations on the downstream conveyance
capacity or where there is essentially no outfall or drainage system
to convey storm runoff from the development. The acceptability of
retention will be determined on a case-by-case basis.
(Res. 40-08 (§ 403.8), 3-19-08)
Studies by the Environmental Protection Agency (EPA) and others
have shown that land disturbances due to construction activities and
the resulting development decrease the quality of storm runoff. The
CDPS stormwater discharge permit requires that construction activities
and new development be controlled to minimize the discharge of pollutants
to the maximum extent practicable. As such, it is recognized that
construction sediment and erosion control, stream stabilization, and
permanent best management practices (BMPs) are necessary to protect
the quality of the waters of the State.
All significant development and redevelopment
disturbing more than one acre within the urban areas of Mesa County
shown on Figure 28.16.110 shall implement:
(a)
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Sediment and erosion control measures during construction
activities;
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(b)
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Stream stabilization measures for the major drainageways;
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(c)
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Post-construction best management practices to
control the discharge of pollutants to the municipal separate stormwater
system (MS4).
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(Res. 40-08 (§ 403.9), 3-19-08)
An important part of all storm drainage facilities is continued
maintenance of the facilities to ensure they will function as designed.
Maintenance of drainage facilities includes a number of routine tasks,
such as removal of debris and sediment, and nonroutine tasks, such
as restoring damaged structures.
All drainage facilities will be maintained to
preserve their function, and shall:
(a)
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Be designed to minimize and facilitate maintenance.
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(b)
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Include access to the entire drainage facility
by dedication of rights-of-way, easements and tracts of land specifically
for drainage purposes. Tracts or easement dedications shall prohibit
uses and the construction of permanent improvements that restrict
or block access. Specifically, detention and retention basins in subdivisions
shall be located in tracts owned by the property owners’ association
with an easement granted to the local jurisdiction. Basins for individual
sites require neither a tract nor an easement but do require a maintenance
agreement.
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(c)
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Be incorporated in the lot grading for residential
development in conformance with FHA lot grading Type A (all drainage
to street) or Type B (drainage to front and rear lot line). Typical
FHA three dimension Type A and Type B grading plans are shown below:
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(d)
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Be maintained by the property owner, the developer
and/or a homeowners’ association. Should the property owner
fail to adequately maintain drainage facilities, the right is reserved
to enter the property, upon proper notice, for the purpose of performing
drainage maintenance. All maintenance costs shall be assessed against
the owner(s) of the property.
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(e)
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Include v-pans (two-foot minimum width) or storm
sewer systems with an inlet in each yard when the grading plan creates
backyard swales (regardless of swale slope). These must be installed
prior to curb and gutter installation. Check with local municipality
to confirm this requirement.
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(Res. 40-08 (§ 403.10), 3-19-08)
Drainage law recognizes the inequity of transferring the burden
of managing storm drainage from one location or property to another.
Liability questions also arise when the historic drainage continuum
is altered. The diversion of storm runoff from one basin to another
shall be avoided unless specific and prudent reasons justify and dictate
such a transfer.
Stormwater diversion may be investigated during development
of Watershed Master Plans and may be shown, in some instances, to
be a viable option to address drainage and flooding problems. Tests
for reasonableness of the potential transfer include:
(a) Determine where the runoff would go if the diversion system were
to fail. If the flow stays in the historic basin, then the diversion
may be reasonable.
(b) Determine if the downstream channel to which the drainage flows are
being transferred is adequate to handle increased flows. If the channel
has capacity, the diversion may be reasonable.
(c) Check condition where diversion fails during the major flood. If
the downstream natural channel would have adequate capacity without
adverse effects, the diversion may be reasonable.
(d) Determine if there would be any adverse water quality impact in either
drainageway. If there is little or no impact, the diversion may be
reasonable.
The development process can and will significantly alter the
historic or natural drainage paths. When these alterations result
in development outfall system that discharges back into the natural
drainageway at or near the historic location, then the alterations
(i.e., intra-basin transfer) are generally acceptable. However, when
the subdivision outfall system does not return to the historic drainageway,
an inter-basin transfer may result.
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Inter-basin transfer as a drainage solution for new development is prohibited, unless allowed as a deviation/variance to this manual (see Chapter 28.08 GJMC) or as part of the Watershed Master Plan. During development of Watershed Master Plans where retrofit facilities are evaluated, inter-basin transfer may be a viable alternative in certain instances and will be reviewed on a case-by-case basis.
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(Res. 40-08 (§ 403.11), 3-19-08)
All storm drainage facilities shall be planned
and designed in accordance with this manual. Criteria in this manual
may be revised or amended as new technology is developed and/or experience
gained in the use of these criteria. Exemptions to criteria and standards
may be approved on a case-by-case basis.
Construction of any drainage facility not initiated
within a two-year period from the time of construction plan approval
must be reevaluated for approval and must receive re-approval.
(Res. 40-08 (§ 404.1), 3-19-08)
All new development and redevelopment shall plan
for, design and construct drainage facilities to convey storm runoff
from the minor storm and major storm events. Emergency overflow paths
for drainage may be required for storms greater than the 100-year
flood event as evaluated on a case-by-case basis.
Analysis and design of all drainage improvements
for new development and significant redevelopment shall be based on
projected future land use conditions, in accordance with local jurisdictional
requirements and plans, except as may be modified by adopted Watershed
Master Plans.
Culverts for local categorized streets may be
designed for a flood more frequent than the 100-year, subject to the
following conditions:
(a)
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The overtopping of the roadway during the 100-year
event is limited to depths no greater than one foot and the downstream
embankment is protected from erosion during overtopping.
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(b)
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The culvert must pass the 10-year flood with a
minimum of one foot of freeboard from the water surface to the lowest
point in the roadway.
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(c)
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The water surface elevation during the 100-year
flood shall not cause additional flooding of adjacent properties and
shall be compliant with local floodplain regulations.
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All drainage systems shall be sized without accounting
for peak flow reductions from on-site or local detention unless noted
otherwise in the tables below. When required to size best management
practices, the mean annual storm event shall be used.
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A summary of requirements and design storms for
the different types of stormwater facilities is presented in the following
table.
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STREETS
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Minor Storm
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Design Requirements
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2-year
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Flow depth ≤ 0.5 feet at gutter flow line
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Flow depth ≤ top of curb
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Flow velocity ≤ 8.0 feet per second
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Major Storm
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Design Requirements
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100-year
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Flow depth ≤ 1.0 foot at gutter flow line
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Flow velocity ≤ 8.0 feet per second
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Freeboard ≥ 1.0 foot at all finished floor elevations
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STORM DRAIN SYSTEMS
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Minor Storm
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Design Requirements
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2-year*
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Flow velocity between 2.5 and 15 feet per second
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EGL must remain below manhole rims and inlet throats
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Major Storm
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Design Requirements
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100-year*
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Flow velocity between 2.5 and 15 feet per second
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EGL must remain below manhole rims and inlet throats
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*These flow rates will be whatever the street
inlets cumulatively accept during the two-year and 100-year storm
events.
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CULVERTS
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Minor Storm
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Design Requirements for Roadways with right-of-way ≤ 80 feet
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10-year †
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Freeboard ≥ 1.0 foot at lowest point of drive lane(s)
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Approval of local jurisdiction to use minor storm
design
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Major Storm
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Design Requirements for Roadways with right-of-way ≤ 80 feet
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100-year †
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Overtopping ≤ 1.0 foot at crown of roadway
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No additional flooding of adjacent properties
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Comply with all floodplain regulations
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Freeboard ≥ 1.0 foot at all finished floor elevations
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Design Requirements for Roadways with right-of-way ≥ 80 feet
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Flow depth ≤ 0.0 feet at crown of roadway
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Freeboard ≥ 1.0 foot at all finished floor elevations
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†Design of culverts may consider the effects
of detention.
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DETENTION/WATER QUALITY
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Water Quality
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Design Requirements
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WQCV
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Design is per Volume 3 of the USDCM (see Chapter
28.8 GJMC)
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Detention
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Minor Storm
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Design Requirements
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10-year
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Design volume is the detention volume plus the
WQCV
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Allowable release rates apply
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Major Storm
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Design Requirements
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100-year
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Design volume is the detention volume (do not
add WQCV)
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Allowable release rates apply
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(Res. 40-08 (§ 404.2), 3-19-08)
Storm runoff peaks and volumes for the design
and evaluation of storm drainage facilities shall be determined using
statistical analysis of recorded stream gage data, the rational method,
or synthetic rainfall runoff models.
(Res. 40-08 (§ 404.3), 3-19-08)
Streets and street rights-of-way may be used to
convey storm runoff from the minor and major events, subject to limitations
set forth in this manual.
Storm runoff is permitted to cross street crowns
at intersections, subject to limitations set forth in this manual.
Storm runoff is permitted to cross street crowns
at culverts and bridges, subject to limitations set forth in this
manual.
(Res. 40-08 (§ 404.4), 3-19-08)
Flood proofing can be defined as those measures which reduce
the potential for flood damages to properties within a floodplain.
Measures can range from elevating structures to intentional flooding
of noncritical building spaces (i.e., basement) to minimize structural
damages.
Flood proofing is permitted in accordance with
the provisions of the Mesa County Land Development Code. For other
jurisdictions, contact the specific jurisdiction for specific guidance.
(Res. 40-08 (§ 404.5), 3-19-08)
Alluvial fans, consisting of sand and fine sediment, are subject
to radical changes in shape, direction, depth, and flow carrying capacity
during storm events. These changes increase the potential flood hazards
of developing on alluvial fan areas.
Additional analysis and specialized design is
required for any development located within an active alluvial fan.
Requirements for analysis and design will be determined on a case-by-case
basis.
(Res. 40-08 (§ 404.6), 3-19-08)
Irrigation conveyance facilities and reservoirs in the Mesa
County area have historically intercepted the storm runoff from the
rural and agricultural watersheds, generally without major problems.
With urbanization of the watersheds, however, the storm runoff has
increased in rate, quantity and frequency, as well as changing water
quality. In developed areas, the irrigation facilities can no longer
be utilized indiscriminately to convey storm runoff. New developments
and redevelopment are prohibited from using irrigation facilities
to convey storm runoff, except as described below.
(Res. 40-08 (§ 405), 3-19-08)
Drainage analysis and design shall be based on
the assumption that an irrigation conveyance facility does not intercept
storm runoff from the upper watershed and that the upper watershed
is tributary to the watershed downstream of the irrigation conveyance
facility, unless such irrigation conveyance facility has been designed
to accommodate storm runoff and is part of an adopted Watershed Master
Plan.
(Res. 40-08 (§ 405.1), 3-19-08)
Storm runoff shall be directed into pre-developed
drainageways downstream of the irrigation conveyance facility. Storm
runoff shall not be discharged into an irrigation conveyance facility,
except as required by water rights law or the discharge is part of
an adopted Watershed Master Plan or negotiated with an irrigation
company.
Where irrigation ditches will serve as the outfall
for a detention facility, the ditch water surface elevation shall
be determined for the maximum irrigation flow of the ditch, and the
stormwater surface elevation shall be determined for the combination
of the maximum irrigation flow and the 100-year stormwater discharge
of the detention facility.
(Res. 40-08 (§ 405.2), 3-19-08)
New development is restricted to areas outside
of:
(a)
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The reservoir’s high water line created
by the design flood for the emergency spillway.
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(b)
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The high water line created by the breach of a
dam (except high hazard classified dams which have passed inspection
by the State Engineer’s Office in accordance with § 37-87-105, C.R.S. et seq.). For dams without the breach high waterline identified,
the developer shall analyze and define the high waterline resulting
from such a breach.
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(c)
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Existing or potential future emergency spillway
paths, beginning at the dam and proceeding to the point where the
flood water returns to the natural drainage course.
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(Res. 40-08 (§ 405.3), 3-19-08)
Natural drainageways are considered an important environmental
feature that contributes to the image and livability of a community.
Their value includes the ability to convey floodwater, to provide
opportunities for trails and open space corridors, and to maintain
natural vegetation and wildlife habitat to the greatest degree possible.
Preservation of natural drainageways, based on
developed land use hydrology, is encouraged. Development of property
shall not adversely affect any natural drainage facility or natural
watercourse, and shall be subject to the following provisions:
(a)
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Drainageways shall remain in as near a natural
state as is practicable. All proposed modification to the natural
drainageway shall be subject to approval.
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(b)
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When the flow rates, velocities, side slopes or
other characteristics indicate a potential negative impact to the
natural drainageway, the impact shall be mitigated in accordance with
criteria set forth in this manual.
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(Res. 40-08 (§ 406), 3-19-08)