UDC Article 13 - Revised Feb 2013 - Redline Version
Article 13 ? Land Disturbing Activities and Drainage
Schertz Unified Development Code
Article 13Land Disturbing Activities and Drainage
Sec. 21.13.1 Clearing and Grading
A.No removal of trees, land clearing and/or grading activities shall occur without
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and
the required approvals and permits including but not limited to the Grading and
Clearing Permit and Tree Permits. Clearing and grading activity shall be limited
to the limits of grading area identified on an approved grading plan. A Grading
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and ClearingPermit is required for any land disturbing activities greater than 0.1
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the
acres outside of the FEMA designated special flood hazard area (SFHA), and all
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clearing
areas within the SFHA unless specifically exempted in this Section.
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grading
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B.All clearing and grading permits shall be reviewed by the City Engineer and
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by the
Public Works Department and approved by the City Engineer or his/her designee.
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City and can be applied for
Grading and Clearing Permit shall be reviewed in accordance with this Unified
once the final plat has been approved by
Development Code, the Public Works Specification Manual, the City of Schertz
the Planning and Zoning Commission
Code of Ordinances, and any other applicable law, code, or regulation governing
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.
grading and clearing activity.
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Manager
C.The following shall be exempt from the requirement for clearing and grading
permit:
1.grading and clearing in emergency situations involving immediate danger
to life and property or substantial fire hazards;
2.the removal of underbrush, dead trees or diseased or damaged trees which
constitute a hazard to life and property based upon field inspection
verification; and
3.grading and clearing practices associated with agricultural operations,
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normal
excluding timber cutting, grading cuts or fills, and work within SFHA.
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crop
Sec. 21.13.2 Drainage
A.Applicability. The provisions of this section shall apply to any capital
improvement project, application for subdivision plat, master development plan,
site plan, grading and clearing permit, or building permit approval except as
otherwise provided by this chapter. A stormwater management plan shall be
provided as set forth in section 21.13.3.
B.Stormwater Management Program
1.System Criteria
a.All stormwater management facilities, or combination of facilities,
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shall be designed for ultimate development. Facilities with
drainage areas less than one hundred (100) acres none of which
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lies within the SFHA shall be designed for a twenty-five-year
storm. Facilities with drainage areas over one hundred (100) acres
orany part of which is within a SFHA shall be designed for a 100-
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areas
year storm or a twenty-five-year storm plusfreeboard,as defined
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designated floodplain
in the Public Works Specification Manual, if that elevation is
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.¶
higher.
F
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b.Detention facilities and streets are exceptions to the frequency
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(based on Table 21.13.2I)
criteria cited above. Refer to the Public Works Specifications
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Detention facility outflows
Manual for specific drainage design criteria for streets and
will be designed for five (5) year, twenty-
five (25) year and one hundred (100) year
detention facilities.
frequency storms.
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subsection 3(g)
c.Three (3) development conditions shall be analyzed for each
development.
i.Existing Conditions. This refers to current development
conditions in the watershed and on-site. Use as the baseline
analysis for determining the impact of development.
ii.Proposed Conditions. This refers to existing conditions
with the proposed development added. Use to determine if
the increased runoff from the proposed development results
in an adverse impact to other properties.
iii.Ultimate Conditions. This refers to ultimate development
conditions within the watershed used to design the drainage
facilities. This condition may be used in-lieu of subsection
(2) above, to determine if the increased runoff from the
ultimate watershed development results in an adverse
impact to other properties.
d.Responsibility to Accept Stormwater
The owner or developer of property to be developed shall be
responsible for the conveyance of all stormwater flowing through
the property. This responsibility includes the stormwater flowing
onto the property by any other developed property as well as the
drainage naturally flowing through the property by reason of
topography. Future upstream development shall be accounted for
by assuming ultimate development when sizing drainage systems
as specified in this section.
e.Positive Overflow Pathways
Stormwater management facilities for local drainage systems will
be designed to ensure that a positive overflow pathway is provided
to the nearest one hundred (100) year conveyance facility. The
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definition?
overflow pathway must be delineated on a plan that shows all
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existing structures in the vicinity impacted by the overflow
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the
pathway.
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both publicly owned and
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f.Maintenance
144 pt, Hanging: 36 pt
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due to zoning or replatting
i.Maintenance of publicly owned facilities will be the
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<#>Method of Computing
responsibility of the City. Maintenance of private facilities
Runoff¶
<#>Calculation Methods¶
is the responsibility of the property owner or the
<#>For drainage areas less than 640
community association and must be specified in a
acres, the basis for computing runoff shall
be the rational formula or some other
maintenance schedule submitted to the City. A maintenance
method provided it is acceptable to the
schedule for privately owned facilities must be approved by
Director of Public Works. Hydraulic
calculations shall be performed by using
the Director of Public Works prior to the approval of
the U.S. Army Corps of Engineers HEC-
construction drawings.
2 ?Water Surface Profiles? or HEC-RAS
?River Analysis System? computer
models. Normal depth channel
ii.Authorized personnel from the City shall conduct periodic
calculations are permissible for
constructed open channels with a uniform
inspections of these facilities and structures. Any required
geometric cross section where (i) there is
repairs will be consistent with current construction
no potential for the water surface
elevations to be controlled by backwater
standards. Maintenance issues identified by the City or
and (ii) the channel is not in a FEMA
State during inspections shall be the responsibility of the
floodplain.¶
<#>For drainage areas 640 acres or
current owner.
greater, the basis for computing runoff
shall be a unit hydrograph method,
iii. If the current owner fails to address any maintenance issues
preferably the Soil Conservation Service
(SCS) Dimensionless Unitgraph method
identified by the City or State, then the City may perform
as contained in the U.S. Army Corps of
required maintenance at the expense of the current owner.
Engineers Hydrologic Engineering Center
HEC-1 ?Flood Hydrograph Package?,
which document shall be maintained on
g.New Development
file with the Director of Public Works
and is hereby incorporated by this
reference. For the SCS method,
Peak stormwater runoff rates from all new development shall be
antecedent moisture condition II shall be
used in the runoff model. Design rainfall
less than or equal to the peak runoff rates from the site?s
values listed in Table 21.13.2E shall be
predevelopment conditions for the five-year, twenty-five-year and
used for hydrograph calculations.¶
<#>Open channel hydraulic calculations
one-hundred-year (100-yr) design storm events, except as provided
shall be performed by using the U.S.
in subsection B.1, above.
Army Corps of engineers HEC-2 ?Water
Surface Profiles? or HEC-RAS ?River
Analysis System? computer models,
h.Redevelopment
which documents shall be maintained on
file with the Director of Public Works
and is hereby incorporated by this
Peak stormwater runoff rates from an area of redevelopment shall
reference.¶
be less than or equal to the peak runoff rates produced by existing
<#>Certain watersheds have hydrologic
and hydraulic models that are available
development conditions for the five-year, twenty-five-year and one
through and maintained by the City.
hundred (100) year design storm events, except as provided in
Developments proposed within the limits
of these watersheds must have the models
subsection B.1, above.
updated by the consultant to reflect
changes in flow, channel configuration
(including alterations to vegetation) and
C.Drainage Easements/Rights-of-Way
channel structures. The consultants?
models must use the same computer
1.Where a subdivision is traversed by a watercourse, drainageway, natural
program that was used in the existing
model e.g. HEC-RAS models will not be
channel or stream, there shall be provided an easement or right-of-way
accepted where the original model used
conforming substantially to the limit of such watercourse, plus additional HEC-2. The updated models shall be
submitted to the Director of Public Works
width as outlined below.
for incorporation into the master models.
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2.Easements shall be the one hundred (100) year floodplain or the twenty-
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<#>Easement or right-of-way
requirements are specified in the
five-year plus freeboard (Public Works Specifications Manual) whichever
following subsections of this section for
is greater. In floodplain areas where ongoing maintenance is required or
particular stormwater management
facilities:¶
the floodplain will be reserved for use by the public, the drainage
<#>subsection
easements shall be maintained by a public entity and the property will be
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<#>D
dedicated to the City as a multi-use drainage easement. A drivable access
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<#>C.3 Natural Watercourses
way shall be provided in floodplain easements for the length of the
or Floodplains;¶
easement. Diversion of stormwater away from the natural watercourse will
<#>subsection
not be allowed except within the boundaries of the property controlled by
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<#>H
the developer, provided that the diverted water is returned to the
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watercourse within which it would naturally have been flowing prior to
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<#>G.7.g Concrete Lined
leaving the developer?s property. An analysis of the timing of the diverted
Channels;¶
<#>subsection
hydrograph on watersheds greater than twenty (20) acres, as it reenters the
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<#>H
receiving watercourse, must be performed to show that the peak flow rate
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<#>G.8.c and 8.d Vegetated
in the receiving watercourse has not been increased as a result of the
Earth Channels;¶
diversion.
<#>subsection
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<#>I
3.An unobstructed access right-of-way connecting the drainage easement
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<#>H.C.3 Storm Sewers.¶
with an alley or roadway parallel to or near the easement shall be provided
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for natural watercourses
at a minimum spacing of one (1) access right-of-way at approximately one
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see Table 21.13.2I of this
thousand-foot intervals. The access right-of-way shall be a minimum of
section
fifteen (15) feet in width and shall be maintained clear of obstructions that
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would limit maintenance vehicular access. If the flow line of the designed
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t when regular maintenance of
channel incorporates grade control structures or vehicular bridges that
the floodplain is required
would prevent maintenance equipment from accessing that portion of the
channel, additional access points may be required. Channel design, earthen
or concrete, shall have ramps in the side slopes near the access points that
would allow maintenance equipment to descend to the floor level of the
channel. The maximum allowable ramp slope for vehicular access is seven
to one (7:1). Access points adjacent to roadways or alleys shall be
provided with a post and cable feature with padlock to prevent
unauthorized use.
4.For single family residential subdivisions drainage easements crossing lots
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D
and property lines are prohibited.Drainage easements should be placed in
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crossing
separate common area lots. Fencing or other structures may be allowed
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across drainage easements only in accordance with the following
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highly discouraged
restrictions:
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Where it is determined that
a.The fence or structure will not interfere with adequate drainage
this is appropriate, the drainage
flow.
conveyance structure shall be constructed
of concrete, and a statement shall be
added to the plat that no fencing or
b.Bottom of fence shall be a minimum of the flow depth, plus
structures that will interfere with
adequate drainage flow will be allowed
freeboard (see Public Works Specifications Manual) above design
on or across such lines
flow line of channel or drain.
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Table 21.13.2I of this section
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c.A hinged gate will be placed across the entire width of the drainage
easement.
d.Fence posts located within the easement must be structurally
designed to resist damage from the stormwater flows and impact
from debris.
e.A floodplain development permit will be required to construct a
fence within an easement within the 100-year floodplain.
5.Interceptor drainage easements and channels shall be provided where the
drainage area to the back of platted lots exceeds the depth of one (1)
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two
platted residential lot or 120 feet whichever is greater. Interceptor drains
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2
shall be constructed prior to the issuing of building permits on any lot that
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average
would be affected by natural drainage being intercepted.
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6.All developments shall provide for adequate drainage outfall at the lower
end of the site into an existing street, alley, drainage, easements or right-
of-way, or to the centerline of an existing natural drain. Where proposed
street, storm sewer, or open channel does not discharge into a natural low
or into an existing adequate drainage easement then facilities and drainage
easements of adequate width to contain the design discharge shall be
constructed and dedicated to the centerline of an existing natural low
within the same watershed. However, where the natural low lies within the
developer?s property, the developer will be required only to plat an
easement to natural low and include the design storage area for the volume
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the centerline of the
of runoff, provided that the easement is adequate to accommodate the
facilities that will be built in conjunction with the future development of
that property to build out.
D.Site Design and Grading
1.All land disturbing or land filling activities or soil storage shall be
undertaken in a manner designed to minimize surface runoff, erosion and
sedimentation, and to safeguard life, limb, property and the public welfare
in accordance with the TPDES General Permit TXR150000, as amended,
and the document entitled ?Complying with the Edwards Aquifer Rules;
Technical Guidance on Best Management Practices,? by Michael E.
Barrett, Ph.D., P.E. Center for Research in Water Resources, Bureau of
Engineering Research, University of Texas at Austin, (RG-348, July
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June
2005), which documents are hereby incorporated by this reference.
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1999
2.Erosion and sedimentation controls in accordance with the specifications
established by the Director of Public Works in compliance with the
National Pollution Discharge Elimination System (NPDES) permitting
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for the Cit
y
requirements and the Public Works Specifications Manual are required.
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Article 13 ? Land Disturbing Activities and Drainage
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3.Projects shall not be considered complete until restoration has been made
in accordance with NPDES requirements.
4.Where possible, multiple uses of drainage facilities and open space shall
be incorporated by the owner or developer of a new subdivision.
Alternative uses such as public recreation, horse/bike/hiking trails,
walking paths, nature preserves, wildlife habitat areas, etc. are encouraged
subject to the approval of the Director of Public Works.
5.A note must be placed on the plat for residential lots, which states that
finished floor elevations must be a minimum of eight (8) inches above
final adjacent grade. A grading plan shall be prepared and submitted to the
City, which indicates typical lot grading for all lots in the subdivision
using typical FHA lot grading types (A, B and C). A more detailed
grading plan is also acceptable.
E.For projects or series of projects with an increased impervious area of greater than
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0.1 acres, stormwater detention shall mitigate peak flow rates to predevelopment
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be required for all new
developments or redevelopment of
or existing development conditions including the initial 0.1 acres increase as
individual parcels of property to
stated in sections 21.13.2.B. of thisArticle.
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C
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6 and 21.13.2.C.7 of thi
1.The maximum allowable outflow rate from the detention facility must be
restricted to the flow rate from the undeveloped or existing development
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section
tract for the five-year, twenty-five-year and one-hundred-year frequency.
Best Management Practices shall be used in the design of detention
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management
facilities in accordance with this section. The timing of the hydrograph
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p
released from the detention facility must be checked against the timing of
the flow rate in the first open watercourse to prevent any increase in the
peak flow rate in the receiving watercourse. For detention basins
constructed in-line on an existing watercourse, the creation of the basin
shall not increase flood elevations in the channel upstream of the new
development boundaries.
2.On-site detention facilities must be privately owned and shall be
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maintained by the community association or property owner. A
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maintenance schedule shall be submitted to the PublicWorks Department
and approved by the Director of Public Works prior to approval of
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d
construction plans. The City will have the right to do periodic inspections
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Where a detention facility
accepts flows from public facilities such
of privately owned and maintained detention facilities to ensure that the
as City rights-of-way, the detention
maintenance schedule is being implemented.
facility will be considered a detention
facility serving a public purpose and will
be dedicated to the City upon completion
3.Multi-use facilities are encouraged, but not required (multi-use facilities
and a drainage easement will be
dedicated to provide for access to the
allows for water quality, satisfy NPDES requirements, enhance ground
facility. When a regional detention
water recharge, provide open space, provide recreation or other amenities,
facility accepts flow from an area
exceeding three hundred (300) acres, the
and/or provide habitat) and may be utilized so long as the facility meets
facility shall be considered serving a
the standards set forth in subsection (B.1.a) of this section and does not
public purpose and shall be dedicated to
the City.
increase the rate or volume of erosion above that which would result from
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a
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Article 13 ? Land Disturbing Activities and Drainage
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the use of a facility without multiple uses. The use of multi-use detention
facilities to alleviate existing flooding problems, enhance and provide
amenities for older neighborhoods, and support the revitalization of
economically depressed areas is encouraged in public and private
redevelopment initiatives.
4.Stormwater retention with permanent wet pool or pumped detention
systems will not be acceptable methods of stormwater mitigation unless
the facility will remain privately owned, operated, and maintained. The
City will approve the use of a pumped facility for private use under the
following conditions:
a.A gravity system is not feasible from an engineering and economic
standpoint.
b.At least two (2) pumps are provided each of which is sized to
pump the design flow rate.
c.The selected design outflow rate must not aggravate downstream
flooding.
d.Controls and pumps shall be designed to prevent unauthorized
operation and vandalism.
e.Adequate assurance is provided that the system will be operated
and maintained on a continuous basis.
5.All stormwater detention facilities considered to be dams as defined by
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Texas Commission on Environmental Quality (TCEQ) shall conform to
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shall be located in
topographically depressed areas where
any applicable law, regulation, and ordinance of any regulatory entity and
possible. When necessary, dams may be
the following criteria:
constructed to detain flows. All proposed
dams
a.Alldams shall be approved by the Dam Safety Team of the Texas
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items
Commission on Environmental Quality (TCEQ) for safety. All
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dams
other new dams shall be designed in accordance with acceptable
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over
design criteria as approved by the Director of Public Works, or his
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six (6) feet above existing
authorized representative.
natural around
b.All hydrology and hydraulic properties of a dam will be reviewed
by the Department of Public Works with regard to spillway design,
freeboard hydraulics, backwater curves and downstream effects
due to the dam site.
c.The spillway section of any earthen dam with a height greater than
six (6) feet shall be large enough to pass a PMP (probable
maximum precipitation) flood, as defined by the NRCS, without
overtopping the crest of the dam in accordance with TCEQ
regulations.
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Article 13 ? Land Disturbing Activities and Drainage
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d.A 100-year frequency flood shall be routed through the proposed
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dam and all land subject to flooding shall be dedicated as drainage
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<#>Streets¶
<#>Generally¶
easement or right-of-way. An unobstructed fifteen-foot access
<#>Design of streets shall consider public
easement around the periphery of the flooded area shall be
safety and limit potential conflicts
between stormwater conveyance, traffic,
dedicated as drainage easement for facilities that require regular
parking, pedestrian access, ADA
mowing or other ongoing maintenance, at the discretion of the
requirements, and bicycle traffic.¶
<#>Streets draining a watershed greater
Director of Public Works. An unobstructed fifteen-foot access
than one hundred (100) acres must be
right-of-way shall be established which connects the drainage
designed for the one hundred (100) year
frequency storm.¶
easement adjacent to the dam structure to a road or alley.
<#>Streets may be used for stormwater
drainage only if the calculated stormwater
flow does not exceed the flows outlined
e.Development below existing dams will take into account the
in the Public Works Department
original design conditions of the existing dam. Dam breach
Specifications Manual or the velocity
does not exceed ten feet (10?) per second.¶
analysis checks will be required, dependent upon location of
<#>Where streets are not capable of
development with respect to dam site.
carrying stormwater, as outlined above,
inlets or curb openings discharging to
drainage channels or storm sewers shall
f.All spillway discharges shall be adequately routed to the centerline
be provided. Partial flow past the inlet
of the natural low below the dam site. The adequate routing of
will be allowed when the capacity of all
downstream street systems can
spillway discharges pertains to the hydraulic routing of the one
accommodate the flow.¶
hundred (100) year frequency flood for dedication of drainage
<#>Street width shall not be widened
beyond the width as determined by the
easement limits. Probable maximum precipitation (PMP) defined
street classification for drainage purposes.
¶
PMP on definition section flood routing or breaches will only be
<#>Stormwater conveyance on streets
shall be designed to account for the
considered for safety considerations (that is, the placement of
cumulative impact of peak flows and
building and the setting of minimum floor slab elevations below
runoff volumes on the system as the
stormwater progresses downgrade.¶
the dams). Any proposed concrete dam structure need not have
<#>Curb cuts for driveways on all streets
spillway capable of routing a PMP flood, however, it shall be
shall be designed for compatibility with
the stormwater conveyance function of
shown to be structurally capable of withstanding any range of
streets.¶
flood conditions with regard to possible failure due to sliding,
<#>Potential flooding problems or
conflicts at the connection points where
overturning, and structural integrity, up to and including the PMP
new or modified drainage systems
flood.
(including streets, storm sewers, etc.) and
the existing portions of the downstream
street system and stormwater conveyance
Sec. 21.13.3 Storm Water Management Plan
system shall be identified and resolved
either in the design of the new or
modified drainage system or in
Procedure
modifications to the existing system.¶
<#>Dwelling units located on the
downhill side of a T-intersection with a
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To standardize the review process and minimize the time for approval by
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the City during review of any applications indicated in Section 21.13.2.A
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of this Article, a complete submittal regarding the analysis of existing
First line: 36 pt
drainage conditions and the design of modifications or new drainage
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facilities is necessary. The owner of the property to be developed is
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<#>Number of Copies¶
required by the Director of Public Works to provide, at the owner?s
The applicant shall provide two (2) blue-
expense and as a condition of construction plan approval, a stormwater
line or black-line copies of the plat
together with two (2) copies of
management report for the total development area to be ultimately
construction drawings.¶
constructed. The stormwater management report shall contain all of the
<#>Format¶
Plats shall be drawn in India ink on Mylar
... [3]
necessary support data, methodologies used in calculations, and
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the plat and construction
conclusions as required by the Public Works Specifications Manual. A
drawings
checklist is found in the Public Works Specifications Manual that will be
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for a subdivision
used by the City reviewer as a guide during the evaluation of all
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below
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stormwater management reports submitted to the City. The purpose of the
checklist is to expedite the review process for both the engineer and the
City, and to aid the engineer in the preparation of reports for the City?s
review. The stormwater management report shall be submitted to the
Director of Public Works or his/her designee prior to approval of any
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through the City Manager
construction plans.
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Sec. 21.13.4 Waivers and Amendments
A.General
The City Manager or his/her designee may authorize waivers from the provisions
of this Article when, in his/her opinion, undue hardship will result from requiring
strict compliance. In granting a waiver, the City Manager shall prescribe only
conditions that he/she deems necessary or desirable to the public interest. In
making his/her findings, the City Manager shall take into account the nature of the
proposed use of the land involved and existing uses of land in the vicinity, the
number of persons who will reside or work in the proposed subdivision, and the
probable effect of such waivers upon the public health, safety, convenience, and
welfare in the vicinity. Waivers shall not be granted unless the City Manager
finds:
1.That the granting of the waiver will not be detrimental to the public health,
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safety, or welfare, or injurious to other property in the area, and will not
violate any applicable law or ordinance of any regulatory entity; and
2.Waivers may be granted only when in harmony with the general purpose
and intent of this UDC so that the public health, safety, and welfare may
be secured and justice done. Such findings of the City Manager, together
with the specified facts upon which such findings are based, shall be
incorporated into the official records of any related permit for which such
waiver is granted.
B.The City Manager may establish a time period for execution of each granted
waiver.
C.Any decision of the City Manager or his/her designee regarding waivers to the
provisions of this Article may be appealed to the City Council. When considering
an appeal, the City Council shall consider the same standards as the City Manager
as outlined above.
D. The City Manager or his/her designee is hereby granted authority to amend,
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36 pt, Hanging: 36 pt
supplement, change, modify, or repeal the text of any portion of the Public Works
Specifications Manual in order to establish and maintain appropriate standards for
development and use of property within the City?s corporate limits and
extraterritorial jurisdiction in accordance with Section 21.1.4.
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<#>Report¶
The stormwater management plan shall
End of Article 13
include two (2) copies of a written report
that includes the following information,
as applicable:¶
<#>a vicinity map of the site and affected
reach of the outfall channel;¶
<#>a detailed map of the area and the
outfall channel with all pertinent
physiographic information;¶
<#>a watershed map showing the existing
and proposed drainage area boundary
along with all sub area delineations and
all areas of existing and proposed
development;¶
<#>discharge calculations specifying
methodology and key assumptions used
including a table of discharges at key
locations;¶
<#>hydraulic calculations specifying
methodology used, assumptions and
values of the design parameters;¶
<#>profiles of the affected channels,
including water surface elevations for the
specified design frequencies, all existing
and proposed bridge, culvert and pipeline
crossings, the location of all tributary and
drainage confluences, and the location of
all hydraulic structures;¶
<#>detention basin design calculations,
including those used for design of the
control structure;¶
<#>right-of-way and easement
requirements, and a map showing
locations of all rights-of-way and
easements;¶
<#>a soils report which addresses erosion
and slope stability of new or altered
channels and detention facilities;¶
<#>a computer diskette of all existing
and proposed condition HEC-1 and HEC-
2 models used in analysis; and¶
<#>a checklist for the submittal package
is included as section 21.13.3 B below. A
checklist for the preparation of a HEC-2
model is included as section 21.13.3.C
below.¶
<#>Subdivision Drainage Checklist¶
<#>________U.S.G.S. Quadrangle map
showing overall drainage areas, runoff
coefficients, time of concentration,
intensity and Qs.¶
<#>________Subdivision Master
Drainage Plan with overall interior
drainage area of subdivision showing
drainage area, time of concentration
runoff coefficients, intensities, and Qs for
the street and alley flows and also
channel and underground system design.¶
<#>________Subdivision plat showing
interior drainage areas, time of
concentration, runoff coefficients, and
intensities, Qs for street and alley flows
and also channel and underground system
design.¶
<#>Drainage Calculations Required For:¶
<#>________Open channel design¶
<#>________Underground systems¶
<#>________Box culverts¶
<#>________Pipe culverts¶
... [4]
13-10
50307818.3
Article 13 ? Land Disturbing Activities and Drainage
Schertz Unified Development Code
[This page intentionally left blank.]
13-11
50307818.3
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Method of Computing Runoff
Calculation Methods
For drainage areas less than 640 acres, the basis for computing
runoff shall be the rational formula or some other method
provided it is acceptable to the Director of Public Works.
Hydraulic calculations shall be performed by using the U.S.
Army Corps of Engineers HEC-2 ?Water Surface Profiles?
or HEC-RAS ?River Analysis System? computer models.
Normal depth channel calculations are permissible for
constructed open channels with a uniform geometric cross
section where (i) there is no potential for the water surface
elevations to be controlled by backwater and (ii) the
channel is not in a FEMA floodplain.
For drainage areas 640 acres or greater, the basis for computing
runoff shall be a unit hydrograph method, preferably the
Soil Conservation Service (SCS) Dimensionless Unitgraph
method as contained in the U.S. Army Corps of Engineers
Hydrologic Engineering Center HEC-1 ?Flood Hydrograph
Package?, which document shall be maintained on file with
the Director of Public Works and is hereby incorporated by
this reference. For the SCS method, antecedent moisture
condition II shall be used in the runoff model. Design
rainfall values listed in Table 21.13.2E shall be used for
hydrograph calculations.
Open channel hydraulic calculations shall be performed by using
the U.S. Army Corps of engineers HEC-2 ?Water Surface
Profiles? or HEC-RAS ?River Analysis System? computer
models, which documents shall be maintained on file with
the Director of Public Works and is hereby incorporated by
this reference.
Certain watersheds have hydrologic and hydraulic models that are
available through and maintained by the City.
Developments proposed within the limits of these
watersheds must have the models updated by the consultant
to reflect changes in flow, channel configuration (including
alterations to vegetation) and channel structures. The
consultants? models must use the same computer program
that was used in the existing model e.g. HEC-RAS models
will not be accepted where the original model used HEC-2.
The updated models shall be submitted to the Director of
Public Works for incorporation into the master models. The
City will periodically update the master models to reflect
current watershed development conditions. The updated
models will be made available for use and distribution as
the latest existing condition models for the watershed.
Time of Concentration
Overland (sheet) flow, shallow concentrated flow and channel
flows are components that need to be considered in the
calculation of time of concentration. The following
methods are recommended for time of concentration
calculation:
Overland flow - flow over plane surfaces: Maximum allowable
time is twenty (20) minutes. Minimum is five (5) minutes.
The overland flow time chart from ?Design? by Elwyn E.
Seelye may be used to calculate overland flow times. Note
that the minimum time has been reduced to five (5)
minutes.
Shallow concentrated flow - overland flow usually becomes
shallow concentrated flow after a maximum of 300 feet:
Use Manning?s equation to estimate travel time for defined
swales, bar ditches and street sections, etc. Figure 3-1 from
TR-55 ?Urban Hydrology for Small Watersheds?, SCS
1986, may be used where a geometric section has not been
defined.
Channel flow: Use existing computer models where available or
Manning?s equation if data is not available. Non-floodplain
channel velocities for ultimate watershed development
should not be less than six (6) fps when estimating time of
concentration.
Runoff Coefficients
Runoff coefficients (C value) for use in the rational formula shall
not be less than the values shown in Tables 21.13.2A or 21.13.2B,
as appropriate.
Table 21.13.2A
Runoff Coefficients (C) ? Percentage
Slope
Character of Area
Up to Over 1% Over 3% Flow
1%up to 3% up to 5% over 5%
Business or commercial areas (90% or more
95 96 97 97
impervious), Existing Pavement / Buildings
Table 21.13.2A
Runoff Coefficients (C) ? Percentage
Slope
Character of Area
Up to Over 1% Over 3% Flow
1%up to 3% up to 5% over 5%
Densely developed areas (80% to 90%
85 88 91 95
impervious)
Closely built residential areas and school
75 77 80 84
sites
Undeveloped areas* - Present land is
undeveloped and ultimate land use is
68 70 72 75
unknown. C values for use in ultimate
development calculations
Large lot residential area 55 57 62 64
Undeveloped areas * - Existing conditions
See Table 21.13.2B
Average residentials area 65 67 69 72
__________________
* Areas included within parks, green belts, or regulatory floodplains shall be considered to remain
undeveloped per Table 21.13.2B.
Table 21.13.2B
Runoff Coefficients (C) ? Percentage
Slope
Character of Area
Up to Over 1% Over 3% Flow
1%up to 3% up to 5% over 5%
Cultivated or Range (Grass Cover <50% of
44 47 53 55
Area)
Range (Grass Cover 50?75% of Area) 37 41 49 53
Forest or Range (Grass Cover >75% of Area) 35 39 47 52
Rainfall Intensity
Use Table 21.13.2C to determine rainfall intensity.
Table 21.13.2C
Rainfall Intensities (inches/hour)
DurationFrequency
Minutes
2-Year 5-Year 10-Year 25-Year 50-Year 100-Year 500-Year
1 6.94 8.00 8.84 9.99 11.09 11.92 13.55
2 6.69 7.72 8.53 9.67 10.69 11.53 13.24
3 6.45 7.46 8.24 9.36 10.31 11.15 12.93
4 6.22 7.21 7.95 9.05 9.95 10.79 12.62
5 6.00 6.96 7.68 8.76 9.60 10.44 12.30
6 5.79 6.73 7.42 8.48 9.27 10.10 11.98
7 5.59 6.50 7.17 8.20 8.95 9.78 11.66
Table 21.13.2C
Rainfall Intensities (inches/hour)
DurationFrequency
Minutes
2-Year 5-Year 10-Year 25-Year 50-Year 100-Year 500-Year
8 5.40 6.28 6.93 7.94 8.65 9.47 11.34
9 5.21 6.08 6.70 7.69 8.37 9.17 11.01
10 5.04 5.88 6.48 7.44 8.10 8.88 10.68
11 4.88 5.69 6.27 7.21 7.85 8.61 10.35
12 4.72 5.52 6.08 6.98 7.61 8.35 10.02
13 4.58 5.35 5.89 6.76 7.39 8.10 9.68
14 4.45 5.19 5.72 6.56 7.19 7.86 9.34
15 4.32 5.04 5.56 6.36 7.00 7.64 9.00
16 4.22 4.94 5.46 6.26 6.89 7.53 8.89
17 4.12 4.84 5.36 6.16 6.79 7.42 8.78
18 4.03 4.75 5.27 6.06 6.68 7.31 8.68
19 3.94 4.66 5.17 5.96 6.58 7.20 8.57
20 3.85 4.56 5.08 5.86 6.48 7.09 8.47
21 3.76 4.48 4.99 5.77 6.38 6.99 8.36
22 3.67 4.39 4.90 5.68 6.28 6.88 8.26
23 3.59 4.30 4.82 5.59 6.18 6.78 8.16
24 3.51 4.22 4.73 5.50 6.09 6.68 8.06
25 3.43 4.14 4.65 5.41 6.00 6.58 7.96
26 3.35 4.06 4.57 5.33 5.91 6.49 7.86
27 3.27 3.98 4.49 5.24 5.82 6.39 7.76
28 3.20 3.91 4.41 5.16 5.73 6.30 7.67
29 3.13 3.83 4.33 5.08 5.64 6.21 7.57
30 3.06 3.76 4.26 5.00 5.56 6.12 7.48
31 2.99 3.69 4.19 4.92 5.48 6.03 7.39
32 2.93 3.62 4.12 4.85 5.40 5.95 7.30
33 2.87 3.56 4.05 4.77 5.32 5.86 7.21
34 2.81 3.49 3.98 4.70 5.24 5.78 7.12
35 2.75 3.43 3.92 4.63 5.17 5.70 7.03
36 2.69 3.37 3.86 4.56 5.09 5.62 6.94
37 2.64 3.31 3.80 4.50 5.02 5.54 6.86
38 2.59 3.26 3.74 4.43 4.95 5.47 6.77
39 2.54 3.21 3.68 4.37 4.88 5.40 6.69
40 2.49 3.15 3.62 4.31 4.82 5.32 6.61
41 2.45 3.10 3.57 4.25 4.75 5.25 6.53
42 2.40 3.06 3.52 4.19 4.69 5.19 6.45
43 2.36 3.01 3.47 4.13 4.63 5.12 6.37
44 2.32 2.97 3.42 4.08 4.57 5.05 6.29
45 2.29 2.92 3.37 4.02 4.51 4.99 6.21
46 2.25 2.88 3.33 3.97 4.45 4.93 6.14
47 2.22 2.85 3.29 3.92 4.40 4.87 6.06
48 2.19 2.81 3.25 3.87 4.34 4.81 5.99
Table 21.13.2C
Rainfall Intensities (inches/hour)
DurationFrequency
Minutes
2-Year 5-Year 10-Year 25-Year 50-Year 100-Year 500-Year
49 2.16 2.78 3.21 3.83 4.29 4.76 5.92
50 2.14 2.74 3.17 3.78 4.24 4.70 5.85
51 2.11 2.71 3.13 3.74 4.19 4.65 5.78
52 2.09 2.69 3.10 3.70 4.15 4.60 5.71
53 2.07 2.66 3.07 3.66 4.10 4.55 5.64
54 2.06 2.63 3.04 3.62 4.06 4.50 5.58
55 2.04 2.61 3.01 3.59 4.02 4.45 5.51
56 2.03 2.59 2.99 3.55 3.98 4.41 5.45
57 2.02 2.57 2.96 3.52 3.94 4.37 5.38
58 2.01 2.56 2.94 3.49 3.91 4.33 5.32
59 2.00 2.54 2.92 3.46 3.87 4.29 5.26
60 2.00 2.53 2.90 3.43 3.84 4.25 5.20
120 1.10 1.54 1.83 2.21 2.50 2.78 3.48
180 0.86 1.19 1.41 1.68 1.88 2.08 2.53
240 0.70 0.97 1.13 1.33 1.50 1.65 1.99
360 0.51 0.71 0.83 0.98 1.09 1.19 1.41
720 0.28 0.39 0.46 0.55 0.61 0.67 0.81
1440 0.165 0.227 0.273 0.324 0.366 0.413 0.513
SCS Curve Numbers
The SCS curve numbers adopted for use by the City are shown in
Table 21.13.2D. The hydrologic soil groups are listed in the latest
version of the United States Natural Resources Conservation
Service [formerly the Soil Conservation Service], ?Urban
Hydrology for Small Watersheds?, Technical Release No. 55 (TR
55), which document is hereby incorporated by this reference. Soil
types that relate to the hydrologic soil group may be found in the
latest version of the United States Natural Resources Conservation
Service Soil Surveys for Bexar, Guadalupe and Comal Counties,
Texas which documents are hereby incorporated by this reference.
Soil types may also be based on a Geotechnical Engineering
Report.
Table 21.13.2D
SCS Curve Number by Soil Type
Hydrologic SCS Curve
Description
Soil Group Number
Soils having a low runoff potential due to high infiltration rates.
These soils consist primarily of deep, well drained sand and
A25
gravels.
Table 21.13.2D
SCS Curve Number by Soil Type
Soils having a moderately low runoff potential due to moderate
infiltration rates. These soils consist primarily of moderately
B55
deep to deep, moderately well to well drained soils with
moderately fine to moderately coarse textures.
Soils having moderately high runoff potential due to slow
infiltration rates. These soils consist primarily of soils in which a
C70
layer exists near the surface that impedes the downward
movement of water or soils with moderately fine to fine texture.
Soils having a high runoff potential due to very slow infiltration
rates. These soils consist primarily of clays with high swelling
Dpotential, soils with permanently high water tables, soils with a 77
clay pan or clay layer at or near the surface, and shallow soils
over nearly impervious parent material.
Percent Impervious Cover
The percent impervious cover for typical land use types in the City
are presented in Table 21.13.2E.
Table 21.13.2E
Percent Impervious Cover by Land Use
Average Percent Impervious
Land Use Category
Cover
1/8 acre Residential Lots, or
garden or townhouse 65 ? 85%
apartments,
1/4 acre Residential Lots 38%
Residential
1/3 acre Residential Lots 30%
1/2 acre Residential Lots 25%
1 acre Residential Lots or 20%
Industrial 72 ? 85%
Business or Commercial 85 ? 95%
Densely developed (apartments) 65 ? 85%
Streets, Roads, and Parking Areas 98%
Design Rainfall
A twenty-four-hour (24 hr) rainfall distribution shall be applied for
runoff calculations. Rainfall intensities as adopted for the City are
given in Table 21.13.2F and should be used for HEC-1 input. The
lag value for a subarea shall be calculated as 0.6 times the time of
concentration.
Table 21.13.2F
Design Rainfall Values (inches)
Duration Frequency
Minutes/
5-Year 10-Year 25-Year 50-Year 100-Year 500-Year
Hours
5 minute 0.58 0.64 0.73 0.8 0.87 1.03
15 minute 1.26 1.39 1.59 1.75 1.91 2.25
60 minute 2.53 2.9 3.43 3.84 4.25 5.2
2 hour 3.08 3.66 4.42 4.99 5.57 6.95
3 hour 3.57 4.23 5.04 5.64 6.23 7.6
6 hour 4.26 4.99 5.89 6.52 7.13 8.47
12 hour 4.68 5.55 6.58 7.32 8.05 9.68
24 hour 5.45 6.55 7.78 8.78 9.91 12.75
Routing of Runoff
Routing of the runoff hydrograph through the channel from one
subarea calculation point to the next in the HEC-1 shall be
computed using one of the following methods:
Overbank/channel storage not significant: Use normal depth
channel routing.
Overbank/channel storage is significant: use the Muskingum
method where a hydraulic model is not available. Use
Modified Puls storage method where a hydraulic model is
available to develop storage/out flow relationship.
Kinematic wave method for channel reaches where inflow from
overbank runoff or multiple point sources (Example: storm
sewer outfalls) is significant and where hydrograph
attenuation is insignificant. Channel routing methodologies
currently being applied in the existing HEC-1 model of the
watershed shall not be replaced with a different
methodology without approval or direction from the
Director of Public Works.
Manning?s Roughness Coefficient
Manning?s roughness coefficients (?N? values) for use in routing
methods or in hydraulic calculations shall be consistent with the
values listed in Table 21.13.2G.
Table 21.13.2G
Manning?s Roughness Coefficient
Channel Description Manning?s ?N? Value
Concrete Lined Channel 0.015
Table 21.13.2G
Manning?s Roughness Coefficient
Grass Lined Channel with regular maintenance 0.035
Grass Lined Channel without recent maintenance 0.050
Vegetated Channel with trees, little or no underbrush 0.055
Natural Channel with trees, moderate underbrush 0.075
Natural Channel with trees, dense underbrush 0.090
Natural Channel with dense trees and dense underbrush 0.100
Overbank Description Manning?s ?N? Value
Pasture 0.035 ? 0.055
Trees, little or no underbrush, scattered structures 0.060 ? 0.075
Dense vegetation, multiple fences and structures 0.075 ? 0.090
The ?N? value to be used in Manning?s Formula shall conform to
the following for design purposes:
earth channels--0.035
concrete lined channels--0.015
reinforced concrete pipe--0.013
concrete box culverts--0.013
corrugated metal pipe:
unpaved ½? corrugated--0.024
unpaved one (1) inch corrugated--0.027
any other ?N? value shall be based on generally accepted
engineering principles.
Page 13-8: [2] Deleted larry.busch 4/16/2012 9:32:00 AM
Streets
Generally
Design of streets shall consider public safety and limit potential
conflicts between stormwater conveyance, traffic, parking,
pedestrian access, ADA requirements, and bicycle traffic.
Streets draining a watershed greater than one hundred (100) acres
must be designed for the one hundred (100) year frequency
storm.
Streets may be used for stormwater drainage only if the calculated
stormwater flow does not exceed the flows outlined in the
Public Works Department Specifications Manual or the
velocity does not exceed ten feet (10?) per second.
Where streets are not capable of carrying stormwater, as outlined
above, inlets or curb openings discharging to drainage
channels or storm sewers shall be provided. Partial flow
past the inlet will be allowed when the capacity of all
downstream street systems can accommodate the flow.
Street width shall not be widened beyond the width as determined
by the street classification for drainage purposes.
Stormwater conveyance on streets shall be designed to account for
the cumulative impact of peak flows and runoff volumes on
the system as the stormwater progresses downgrade.
Curb cuts for driveways on all streets shall be designed for
compatibility with the stormwater conveyance function of
streets.
Potential flooding problems or conflicts at the connection points
where new or modified drainage systems (including streets,
storm sewers, etc.) and the existing portions of the
downstream street system and stormwater conveyance
system shall be identified and resolved either in the design
of the new or modified drainage system or in modifications
to the existing system.
Dwelling units located on the downhill side of a T-intersection
with a street or drainage channel discharging onto the
intersection shall be sited so as to avoid obstruction of the
drainage patterns.
An arterial street is a street so designated on the current Master
Thoroughfare Plan. One (1) lane in each direction on arterial
streets shall remain passable with a flow depth not to exceed 0.30
feet during a twenty-five-year storm event. The maximum depth of
water in the street section must not exceed seven (7) inches (the
height of a standard City curb).
A maximum flow depth to the top of curb on a collector street section will
be allowed during a twenty-five-year storm event. A collector
street is a street with a width of forty-two (42) feet or more and not
shown as an arterial street on the current Master Thoroughfare
Plan.
Local Streets. Local streets shall be designed on a basis of a five-year
frequency. A twenty-five-year frequency storm must be contained
within the street right-of-way.
Alleys shall be designed for five-year frequency within the limits of the
alley pavement/curbs and twenty-five-year frequency within the
right-of-way/easement to carry stormwater.
All-Weather Crossings
Where streets cross existing or proposed watercourses, all weather
crossings shall be required. Culverts or bridges shall be
adequate to allow passage of the design storm identified in
section 21.13.2.B.1.
All crossings, culverts and bridges shall be designed for an H-20-
44 or HS-20 loading.
Dangerous conditions for existing crossings are defined by the
Public Works Department Specifications Manual
(Dangerous Conditions on Crossing during Floods).
This section addresses proposed improvements or modifications to drainage
channels and watercourses required to convey stormwater runoff from or
through the proposed development. Refer to section 21.13.2.B.1 for storm
frequency design criteria.
Except as authorized by a development plan approved by the Director of
Public Works or his/her designee, no person shall place or cause to
be placed any obstruction of any kind in any watercourse within
the City and its ETJ. The owner of any property within the City,
through which any watercourse may pass, shall keep the
watercourse free from any obstruction not authorized by a
development plan.
Channel Modifications
Modifications to existing watercourses or newly created open
channels may be designed as earth channels, sod channels
or as concrete lined channels. Liners other than sod or
concrete which enhance the aesthetics or habitat value of
the watercourse and which reduce future maintenance
requirements are encouraged. Preliminary planning for the
applicability of other channel liners shall be reviewed with
the Director of Public Works or his/her representative prior
to the submittal of construction plans for approval.
Runoff that results from upstream development and is discharged
to an unimproved waterway can cause flood damage to
properties adjacent to the waterway. Natural undeveloped
waterways do not receive regular maintenance. Design of
natural waterways shall take into consideration fluvial
geomorphologic principals and practices. Consulting
engineers and development review officials shall work to
resolve potential downstream impact issues.
Design of new channels or alterations to existing channels shall consider
future maintenance requirements. A maintenance schedule for any
private channel shall be submitted to and approved by the Director
of Public Works prior to approval of construction plans.
Maintenance requirements of concrete channels consist of de-
silting activities, prevention of vegetation establishment in
construction joints, and repair of concrete as necessary.
Maintenance of earthen channels includes regular observation and
repair, as necessary, of erosion, scouring, and removal of silt
deposits, as necessary to maintain design parameters. Developers
shall be responsible for maintaining newly planted channels until
coverage is established throughout eighty-five percent (85%) of the
area. This area shall include slopes, floor, and any attendant
maintenance easement. New earthen channels shall be planted with
drought resistant, low growth, native species grasses, which will
allow unobstructed passage of floodwaters. Johnson grass, giant
tagweed and other invasive species shall not be allowed to
promulgate in channels. Suggested species shall include, but not be
limited to, common Bermuda, coastal Bermuda, buffalo grass,
sideoats grama, seep Muhly, little bluestem, and Indian grass.
Mowing frequencies vary with the vegetation growth rates, but is
required when the grass exceeds the design roughness coefficient
of the channel.
Planned multiple-use of a watercourse is allowed (e.g. bike paths or
greenbelt). If multiple use of the watercourse is to be incorporated,
the applicant shall form a property owners? association that shall
assume maintenance responsibility for private amenities. The
appropriate government agency will be responsible for
maintenance of public amenities. The applicant shall provide
overlay easements for public or private use.
Table 21.13.2H shall be used to determine maximum permissible channel
velocity.
Table 21.13.2H
Velocity Control
Maximum
Type of Facility Hydraulic Correction
Velocity (fps) Permissible
Required Radius (ft.) Factor
Velocity (fps)
0--10.85
1 to 6 1--30.95.5
(Maximum Vegetated 3--51.056.3
Average Earthen Channel 5--81.156.9
Velocity = 6 fps) 8--101.2257.35
Over 10 1.257.5
6 to 8 Concrete Retards NA NA NA
Concrete Lining
> 8 or Drop NA NA NA
Structures
Where velocities are in the supercritical range, allowance shall be
made in the design for the proper handling of the water.
Ensure that the channel will contain the hydraulic jump (sequent
depth) throughout the extent of the supercritical profile. An
exception to this criterion is where concrete lined lateral
channels discharge down the side slopes of channels. These
channels may be designed for normal depth plus freeboard
provided velocity controls are established at the main
channel flow line.
Ensure that the energy grade of the channel will not result in
upstream flooding at existing or proposed lateral facility
connections.
Retard spacing shall be computed as follows when using the City standard
retard section Table in the Public Works Department
Specifications Manual and the following equations: L = 1.0? ÷ (S1
- S2)
Where: L = Distance required between retards in feet.
S1 = Actual slope of channel in ft./ft.
S2 = Slope of proposed channel for maximum permissible velocity
established from Table 21.13.2D, i.e.: and
S2 = [(NV) ÷ (1.486R 2/3 )] 2
Where: V = maximum permissible velocity established from Table
21.13.2H
N = .035R = area/wetted perimeter
Concrete Lined Channels
The design of concrete lined channels shall comply with the
following general requirements:
Freeboard consistent with Table 21.13.2I will be applied to the
twenty-five-year design.
From the top of the concrete lining to the top of the ditch, a side
slope not steeper than three (3) horizontal to one (1)
vertical shall be required; nor shall the slope be less than
twelve to one (12:1).
For normal conditions, the concrete lining shall be a minimum of
five inches (5?) thick and reinforced with No. 3 round bars
at twelve inches (12?) on center each way. Where
surcharge, nature of ground, height and steepness of slope,
etc., becomes critical, design shall be in accordance with
latest structural standards. All concrete lining shall develop
a minimum compressive strength of not less than three
thousand (3,000) pounds per square inch in twenty-eight
(28) days. The depth of all toe downs shall be thirty-six
inches (36?) upstream, twenty-four inches (24?)
downstream, and eighteen inches (18?) for side slopes. The
City?s construction inspector may permit an eighteen inch
(18?) toe down in rock subgrade in-lieu of the above toe
down requirements. The horizontal dimensions of toe
downs shall not be less than six inches (6?).
Maximum concrete riprap side slopes shall be one and one-half (1
1/2) horizontal to one (1) vertical, unless soil tests made by
a geotechnical engineer show that a greater slope, or a
special design, will be stable. Where vehicular traffic may
travel within a horizontal distance equal to one-half (1/2)
the vertical rise of the slope, a two foot (2?) surcharge load
shall be included in the design.
Fencing will be required adjacent to the channel where channel
vertical wall heights exceed two feet (2?). Fencing will also
be required adjacent to the channel where channel side
slopes exceed two to one (2:1) and the channel depth is
greater than two feet (2?). The fencing must not cause sight
distance problems for motorists.
Vertical walls will not be permissible for depths greater than two
feet (2?) unless properly fenced or enclosed. Walls will
have a minimum thickness of six inches (6?).
Easements or rights-of-way for concrete lined channels shall
extend a minimum of two feet (2?) on both sides of the
extreme limits of the channel. ?Extreme limits? of the
channel shall mean the side slope intercept with the natural
ground or proposed finished ground elevation.
A minimum ?n? value of roughness coefficient of 0.015 shall be
used for a wood float type surface finish. This ?n? value is
as used in Manning?s formula.
Table 21.13.2I
Drainage Freeboard for Concrete Lined and
Earth Channels for Twenty-Five Year Storm
Design Depth of Flow Required Freeboard
0 to feet 5 feet 0.5 foot
5 to 10 feet 10% of design depth
10 feet and over 1.0 foot
Vegetated Earth Channels
Freeboard consistent with Table 21.13.2I will be applied to the
twenty five (25) year design.
The side slope shall not be steeper than three (3) horizontal to one
(1) vertical.
Easements or rights-of-way for improved earth channels shall
conform to the requirements stated in subsection (d) of this
section and shall extend a minimum of two feet (2?) on one
(1) side and fifteen feet (15?) for an access road on the
opposite side of the extreme limits of the channels when
such channels do not parallel and adjoin an alley or
roadway. When such channels do parallel and adjoin an
alley or roadway, the easement or right-of-way shall extend
a minimum of two feet (2?) on both sides of the extreme
limits of the channel. Where utilities are installed in the
access road of the drainage right-of-way, the right-of-way
shall extend two feet (2?) on one (1) side and seventeen feet
(17?) on the opposite side of the design limits of the
channel. These seventeen feet (17?) are to provide an access
way along the channel with a maximum cross slope of one
inch (1?) per foot toward the channel. Where designed
channel bottoms exceed one hundred feet (100?) in width,
the fifteen foot (15?) extra width shall be provided on both
sides of the channel.
Interceptor drainage easements shall extend a minimum of two (2)
feet on both sides of the extreme limits of the channel.
Refer to Table 21.13.2E. Improved earthen channels will be
vegetated by seeding or sodding. Eighty-five percent (85%)
of the channel surface area must have established
vegetation before the City will accept the channel for
maintenance.
Freeboard
Allowance for extra freeboard shall be made when the centerline
radius of the channel is less than three (3) times the bottom width.
Where sharp bends or high velocities are involved, the applicant
shall use the following formula for computing the extra freeboard:
d2 - d1 = V 2 (T + B) ÷ 2gR
Where: d1 = depth of flow at the inside of the bend in feet.
d2 = depth of flow at the outside of the bend in feet.
B = bottom width of the channel in feet.
V = the average approach velocity in the channel in feet per
second.
T = width of flow at the water surface in feet.
g = 32.2 feet/second squared.
R = the center line radius of the turn or bend in feet.
The quantity d2 ? d1 divided by two (2) shall be added to the
normal depth of flow before adding the required freeboard
in calculating required right-of-way widths.
Where sharp turns are used without curved sections, the depth
required shall be large enough to provide for all head
losses. Allowance shall be made for any backwater head
that may result.
For normal design conditions no extra freeboard is required. An
accepted rule of thumb to follow is this: Centerline radius
of channel should be at least three (3) times the bottom
width.
Storm Sewers
For all ordinary conditions, storm sewers shall be designed on the
assumption that they will flow full under the design discharge;
however, whenever the system is placed under a pressure head, or
there are constrictions, turns, submerged or inadequate outfall, etc.,
the hydraulic and energy grade lines shall be computed and plotted
in profile. In all cases adequate outfalls shall be provided and the
system adequately designed.
No storm sewers shall be less than twenty-four inches (24?) in diameter.
Minimum easement widths for storm sewers will be the greater of fifteen
feet (15?) or six feet (6?) on both sides of the extreme limits of the
storm sewer width (e.g. the easement width for a three (3) barrel
ten-foot wide box culvert with six inch (6?) walls would be (3 x
10?)+(4 x 0.5?)+(2 x 6?) = 44?).
Inlets and Openings
Drop Curb Openings - Sidewalk Does Not Abut Opening
Where drop curb openings are used to take stormwater off the
streets and into drains, the length of the curb opening can be
calculated from the weir formula using the coefficient of 3.087 in
the following formula:
L = Q ÷ Ch 3/2
Where: L = the length of drop curb opening required in feet.
Q = amount of flow in CFS based on twenty-five-year design
frequency.
C = 3.087.
h = head of weir in feet.
Gutter line depressions will be permitted where such depressions
will not hamper the flow of traffic. For amount of curb exposure,
conform to City inlet standards.
Curb or Drop Inlets
Where drop inlets are used, the City standard inlets with adequate
reinforcing steel may be used. All other types or designs shall be
subject to the approval of the director of developments services in
consultation with the Director of Public Works. The following
formulas for inlet capacity are based on drop inlets in sag points.
Inlet capacities on grades will be considered less, the amount of
which depends on street grades, deflections, cross slopes,
depressions, etc.
Grate Inlets
The flow of water through grate openings may be treated as the
flow of water through a rectangular orifice. The following formula
may be used for determining grate capacity:
Q = CA (2gh) 1/2
Where: Q = discharge in cubic feet per second.
C = orifice coefficient of discharge (taken as 0.70).
g = acceleration due to gravity (32.2 ft./sec.2).
h = head on the grate in feet.
A = net area of the openings in the grate in square feet.
This formula gives the theoretical capacity of the grate inlet. Since
grate inlets are subject to considerable clogging, capacity of the
grate inlet will be taken as one-half (1/2) on the value given by this
formula.
Curb Opening Inlets
The capacity of curb opening inlets will depend on whether or not
the opening is running partially full or submerged. If the depth of
flow at the curb opening inlet is such as to cause a partially full
opening, a weir effect will develop and the following formula will
apply:
Q = CwL(h) 3/2
Where: Q = the discharge of capacity in cubic feet per second.
Cw = the weir coefficient of discharge (3.087).
L = the length of curb opening in feet.
h = the head or depth of water at the opening in feet.
If the depth of flow at the curb opening is such as to fully
submerge the opening, the orifice effect will develop and the
formula used shall be identical to that given under grate inlets with
the exception that the head (h) on the curb opening orifice shall be
taken as the depth from the top of the water surface to the center of
orifice or opening; one hundred percent (100%) efficiency will be
allowed for curb opening inlets.
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Number of Copies
The applicant shall provide two (2) blue-line or black-line copies
of the plat together with two (2) copies of construction drawings.
Format
Plats shall be drawn in India ink on Mylar on sheets eighteen
inches (18?) wide and twenty-four inches (24?) long, with a
margin of two and one-half inches (2 ½?) on the left side of the
sheet, and appropriate margins on the other three (3) sides. Plats
shall be drawn at a scale of one hundred feet (100?) to one inch
(1?) unless a smaller scale is approved by the City Manager or
his/her designee. Plats which include one-half (1/2) acre or less in
area shall be drawn at a scale of fifty feet (50?) to one inch (1?).
Where more than one (1) sheet is necessary to accommodate the
entire area to be subdivided, an index sheet showing the entire
subdivision at an appropriate scale shall be attached to the plat.
Contents
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Report
The stormwater management plan shall include two (2) copies of a
written report that includes the following information, as
applicable:
a vicinity map of the site and affected reach of the outfall channel;
a detailed map of the area and the outfall channel with all pertinent
physiographic information;
a watershed map showing the existing and proposed drainage area
boundary along with all sub area delineations and all areas
of existing and proposed development;
discharge calculations specifying methodology and key
assumptions used including a table of discharges at key
locations;
hydraulic calculations specifying methodology used, assumptions
and values of the design parameters;
profiles of the affected channels, including water surface
elevations for the specified design frequencies, all existing
and proposed bridge, culvert and pipeline crossings, the
location of all tributary and drainage confluences, and the
location of all hydraulic structures;
detention basin design calculations, including those used for design
of the control structure;
right-of-way and easement requirements, and a map showing
locations of all rights-of-way and easements;
a soils report which addresses erosion and slope stability of new or
altered channels and detention facilities;
a computer diskette of all existing and proposed condition HEC-1
and HEC-2 models used in analysis; and
a checklist for the submittal package is included as section 21.13.3
B below. A checklist for the preparation of a HEC-2 model
is included as section 21.13.3.C below.
Subdivision Drainage Checklist
________U.S.G.S. Quadrangle map showing overall drainage areas,
runoff coefficients, time of concentration, intensity and Qs.
________Subdivision Master Drainage Plan with overall interior drainage
area of subdivision showing drainage area, time of concentration
runoff coefficients, intensities, and Qs for the street and alley flows
and also channel and underground system design.
________Subdivision plat showing interior drainage areas, time of
concentration, runoff coefficients, and intensities, Qs for street and
alley flows and also channel and underground system design.
Drainage Calculations Required For:
________Open channel design
________Underground systems
________Box culverts
________Pipe culverts
________Hydraulic jump
________Super elevation in channel bends
________Retard spacing
________Backwater curves with cross sections
________Draw down curves with cross sections
________Energy dissipaters
________Hydraulic grade lines of pipes
________(1) Inlets on grades ________(2) Inlets in sump
________Drop curb openings
________Sidewalk culverts
________AR2/3 calculations with cross sections
________Weir formulas structures
________Orifice formulas
________Grade to drain channels
________Upstream pickup and flared section
________Downstream backwater control and flare to match
downstream condition
________Show required free board
________Improper ?N? value
________Improper velocity used
________Improper easement width
________Show access road on each sodded channel
________Improper runoff coefficient used
________Improper time of concentration used
________Improper Qs used
________Steel calculations for box culvert
________Street Qs for 5 yr.(30? street) and 25 yr. (greater than 44?
street) frequency showing street capacities are correct
based on Figure IX in Subdivision Regulations
________Subdivision Plat showing all interior drainage easements, outfall
drainage easements, U.S.G.S. contour map and all other necessary
drainage information
Show outfall drainage easements to the centerline of existing
natural low
Show finished fill contours
Show interceptor drainage easements
________Typical Details Required on Plans for:
________Box culvert with headwalls or wing walls
________Pipe culverts with headwalls or wing walls
________Culvert headwalls shown with proper safety measures
________Drop curb openings
________(1) Inlets on grade ________(2) Inlets on sump
________Drop structures
________Retards
________Sidewalks over drains
________Guard post installations
________Guard rail on structures
________Header curb
________ Energy dissipaters
________Junction boxes
________Concrete lined channels with free board
________Earth sodded channels with free board
________Other concrete structures:
________ Grade to drain sections
________Transition sections
________Fencing for vertical wall channels greater than 2? deep
________Other: _________
________Side slope
________Note: Adjacent lots shall be graded to provide access and
drainage to adjacent street and drainage systems.
________Complete Street Plans and Profiles
________Complete Drainage Plan and Profile Including the Following
Requirements:
________Proposed flowline slopes with grades and elevations
shown every 50? in profile
________Proposed top of channel profile
________Existing ground right and left profile at property line
________Finished fill profiles
________Locations and size of culverts
________Drop structures
________Retards
________Grade to drain profiles
________Flowline elevations at every 50? station and at each
structure and change in grade
________Junction boxes
________Channel plan views
________Channel sections
________Pipes with hydraulic grade lines on profile
________Cross sections of existing natural channels or lows which
are not to be improved, but left in natural state and
dedicated to high water calculated
________Angles, bearings, distances, etc., for structures, channels,
etc.
________Lot grading layout drains
________Culvert structural details
________Unit and Storm Hydrographs For Major Streams (Over 2,000
acres)
________Drainage Easements to the Centerline of Natural Low
________Cost Estimate
________ Engineer?s Seal
________Other
HEC-2 Submittal Checklist
Floodplain submittal checklist supersedes this attachment for projects in
the FEMA Floodplain.
Project __________ Engineer __________ Stream
__________ Date __________
The purpose of this checklist is to aid the engineer in the preparation of
HEC-2 studies and reports and to expedite the City of Schertz review
procedure.
Submission Package
_________ Signed, sealed, and dated by a engineer certified to
practice in the State of Texas
_________ Signed checklist
_________ 3 ½? diskette with all input files
_________ Copy of condensed printouts
Narrative
_________ Table of Contents
_________ Abstract or executive summary
_________ Introduction
_________ Project description and history
_________ Location
_________ Scope and objective of analysis
_________ Previous and related studies that may affect this
analysis
_________ Methodology
_________ Sources of discharges
_________ Bridge routines
_________ Base or effective models (mention source)
_________ Revised-base model
_________ Proposed model
_________ Summary, conclusions, and recommendations
_________ Water surface elevation impacts
Tables
_________ Water surface comparison table at each cross section
_________ Floodway table
_________ Cross section numbering table (if stationing changes)
_________ Exhibits
_________ Vicinity map
_________ Plan view of project reach
_________ Water surface profiles for design storm
_________ Channel cross sections showing limits of drainage
easements and property lines
_________ Bridge cross sections
_________ Plan view of bridge
_________ Photographs (if available)
Appendices
_________ Pertinent correspondence (meeting notes, etc.)
_________ Survey and/or Certified ?As-Built? information for all
revisions to base model
_________ Sample calculations
Name of submitter_________ Date_________
Certification. The stormwater management report must include a
letter signed and sealed by a professional engineer with text
descriptions, exhibits, calculations and models.
The stormwater management plan shall include a performance
bond executed as follows:
?State of Texas
County of _______
KNOW ALL PERSONS BY THESE PRESENTS:
That we, _________, the undersigned developer as principal, and
_________, as surety, do hereby acknowledge ourselves to be held
and firmly bound unto the City of Schertz, a municipal corporation
of the State of Texas, in the full and just sum of $_________, for
the payment of which will and truly to be made, we hereby bind
ourselves and our respective heirs, administrators, executors and
assigns jointly and severally, firmly by these presents.
Whereas, the principal had petitioned the Floodplain Administrator
of the City of Schertz for permission to _________ within the
jurisdiction of the City of Schertz which is shown on plans entitled
_________, and which is more particularly described as follows, to
wit:
WHEREAS, plans and a floodplain development permit for such
development were approved by the Floodplain Administrator; and
WHEREAS, the Floodplain Ordinance of the City of Schertz
requires that the site improvements set out below be completed by
the principal in conformance with the standards established by that
ordinance within three (3) years of the date on which the
Floodplain Development Permit was approved and
WHEREAS, the aforesaid ordinance requires that a guarantee of
performance that such site improvements will have been completed
and will have been accepted by the City within three (3) years of
the date on which the Floodplain Development Permit was
approved; and
WHEREAS, the undersigned developer has elected to provide to
the City of Schertz such a guarantee of performance;
NOW THEREFORE, the condition of this obligation is such that if
the principal shall, on or before the ________ day of _________,
20________, construct or cause to be constructed the above
mentioned improvements in accordance with the requirement of
the City of Schertz Floodplain Ordinance, then this obligation shall
be void; otherwise the obligations under this bond shall remain in
full force and effect.
IN TESTIMONY WHEREOF, WITNESS OUR HAND AND
SEAL this ________ day of _________, 20________.
DEVELOPER AND PRINCIPAL
BY: _________
TITLE: _________
BY:
ATTORNEY-IN-FACT
APPROVED AND ACCEPTED THIS ________ day of
_________, 20_______.
CITY OF SCHERTZ
BY: _________
TITLE: CITY MANAGER
APPROVED AS TO FORM: _________ CITY ATTORNEY?
P.E. Registration Number
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Table 21.14.1
Street Improvement Standards
Sidewalk Hike/Bike
Classification ROW Pavement Drainage Width
Width Trail
Curb or Curb 8 feet other
and Gutter side (unless
48 feet with 12 5 feet
Principal Arterial 120 feet State Hwy
foot median one side
then both
sides)
Curb or Curb 5 feet 8 feet other
Secondary Arterial 86 feet 48 feet
and Gutter one side side
Curb or Curb 5 feet
Collector 60 feet 42 feet -
and Gutter both sides
Local Street ? Curb or Curb 5 feet
50 feet 30 feet -
Residential and Gutter both sides
Local Street ? Curb and Gutter
5 feet
Commercial/Indust60 feet 42 feet -
both sides
rial
Curb or Curb
Paved Alley 20 feet 20 feet None -
and Gutter