AN ORDINANCE AMENDING CHAPTER 9, "SUBDIVISIONS"
OF THE BASTROP CITY CODE, APPENDIX "A," SECTION 1, ENTITLED
"GENERAL", RELATED TO DRAINAGE DESIGN CRITERIA, AND ESTABLISHING
DRAINAGE DESIGN REQUIREMENTS; PROVIDING A SAVINGS CLAUSE; AND PROVIDING
AN EFFECTIVE DATE.
WHEREAS, the rules and regulations of the City established
by ordinance, governing plats and subdivision of the land, in the
corporate limits of the City of Bastrop apply to all of the area under
the extraterritorial jurisdiction of said City, as provided for in
the laws enacted by the State of Texas and, including Local Government
Code Chapter 212, et seq., and
WHEREAS,
the City Council has determined that, in order to ensure safe and
orderly development within the City's jurisdiction, an amendment to
Chapter 9 of the City's Subdivision Code, Appendix "A,"
Section 1, entitled "General," should include drainage criteria,
regulations and standards applicable to all development within the
City's jurisdiction.
NOW, THEREFORE,
BE IT ORDAINED BY THE CITY COUNCIL OF THE CITY OF BASTROP, TEXAS:
PART 1: That Chapter
9 of the Code of Ordinances of the City of Bastrop, Subdivision Ordinance,
Exhibit "A," Section 1 - General - shall be, and it is hereby,
amended so that it shall hereafter read as follows:
SECTION 1 GENERAL
1.10
1.20
1.30
1.40 DRAINAGE CRITERIA AND DESIGN STANDARDS
1.40.1 General Provisions
A. The owner or
developer of property to be developed shall be responsible
for the conveyance of all storm drainage flowing through or
abutting subject property. This responsibility includes the
drainage directed to that property by prior development as
well as the drainage naturally flowing through the property.
B. Where the improvement
or construction of a storm drainage facility is required along
a property line common to two (2) or more owners, the owner
hereafter proposing development or use of his property, shall
be responsible for all the required improvements on either
side of the common property line, regardless of ownership,
at the time of development, including the dedication by the
legal owner(s) of all necessary rights-of-way or easements,
to accommodate the construction and maintenance of improvements.
C. Where a property
owner proposes development or use of only a portion of his
property, provision for storm drainage shall only be required
in that portion of the property proposed for immediate development
or use, except as construction or improvements of a drainage
facility outside that designated portion. However, future
development runoff should be considered in the design of the
proposed development.
D. The owner or
owners shall dedicate to the City the required drainage easements
and/or rights-of-way. Determination of minimum easements and/or
rights-of-way required shall be made by the City Engineer.
E. The responsibility
of the owner or developer shall extend to provision of adequate
drainage improvements to accommodate the full effects of the
development of his property. Such drainage improvements shall
prevent a diversion, impounding or increase of the natural
flow of surface waters caused by the development of the property
from damaging the property of another. The term "natural
flow" as used herein is meant to describe the conditions
existing down stream of the property prior to and after the
proposed development. Such improvements may be on-site or
off-site, or a combination of both, and shall be made at the
expense of the owner or developer. Such drainage improvements
shall be a condition of plat approval.
F. All storm sewers,
inlets, head walls and manholes in the drainage system shall
be designed and built in accordance with the City of Bastrop
Construction Standards, as periodically amended.
G All drainage
design must comply with the City of Bastrop Comprehensive
Master Plan and the effective Flood Insurance Study (FIS)
and
effective Flood Insurance Rate Maps (FIRM) prepared by the
Federal Emergency Management Agency.
H. All buildings
and structures shall have a minimum finished floor elevation
as stipulated in the City of Bastrop's Flood Damage Prevention
Regulations.
I. Storm water
detention facilities shall be required where deemed appropriate
by the City. Storm water detention shall be used to reduce
the net increase in storm water run-off due to development
of the property at the 5, 10. 25, 50 and 100 year events.
Maintenance of storm water detention facilities shall be the
responsibility of the property owners and/or homeowners association.
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1.40.2 Inundation
By A 100-Year Frequency Storm
A. Any water course
with a contributing drainage area greater than ten (10) acres,
whether natural or man made, shall have provision to accommodate
the rainfall runoff generated by a one hundred (100) year
frequency storm such that there is no loss of, or be detrimental
to, property or to create an undue inconvenience to the public.
B. Delineation
of the limits of areas subject to inundation by a one hundred
(100) year frequency storm shall be shown on a drainage plan
and shall be based on detailed hydrologic and hydraulic computations
prepared by a Registered Professional Engineer of the State
of Texas. Effective FEMA floodplain information shall be shown
when available.
C. Easements shall
be provided to contain areas inundated by a one hundred (100)
year frequency storm along natural and man-made drainage ways
and any additional width necessary to provide sufficient ingress
and egress for maintenance purposes.
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1.40.3
Runoff Computations Sizing of inlets, storm sewers, outfalls,
culverts and drainage ditches shall be based on the following:
A.
Design Storm: The design storm shall be a twenty-five (25)
year frequency storm. The one hundred (100) year frequency
storm shall be used to determine flooding impacts in extreme
events.
B.
Hydrologic Methods
(1) Rational Method:
For drainage areas less than 200 acres, the Rational Method
is acceptable. To determine the runoff rates for the various
areas, the standard rational method shall be used, utilizing
the formula Q=CIA, where:
Q = rate of runoff (cfs)
C = runoff coefficient
I = rainfall intensity for the particular duration (in/hr)
A = the drainage area (acres)
Rainfall intensity
for use of the Rational Method shall be computed using
the following equation and coefficients.
I = b/(tc + d )e Where:
I = intensity
(in/hr)
tc = time of concentration (min)
Coefficient 2-year 5-year 10-year 25-year 50-year 100-year
e (in) 0.802 0.8 0.765 0.762 0.748 0.747
b 62 72.0 77 88 93 103
d (mins) 8.2 8.4 8.4 8.4 8.4 8.2
Without substantiation by the subdivision designing engineer,
the maximum allowed overland flow time of concentration
to any storm sewer inlet will be fifteen (15) minutes.
Where computed time of concentration (Tc) is less than
10 minutes, the intensity corresponding to a 10 minute
Tc may be used
(2)
Unit Hydrograph Methods
Soil Conservation
Service (Now called National Resources Conservation
Service) unit hydrograph methods are acceptable for
any size drainage area, and are required for drainage
areas larger than 200 acres. The engineer can propose
to use other hydrologic methods but must have their
acceptability approved by the City Engineer.
Detention
ponds shall be designed using SCS unit hydrograph methods.
The engineer can propose to use other hydrologic methods
but must have their acceptability approved by the City
Engineer.
When unit
hydrograph methods for computing runoff are proposed,
a 24-hour SCS Type III storm shall be modeled as the
rainfall event, with the following rainfall depths.
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2-year |
5-year
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10-year |
125-year
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50-year |
100-year
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500-year |
24-hour
depth (in) |
3.6
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5.1 |
6.2 |
7.7
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9.0 |
10.2 |
13.5 |
1.40.4. Storm
Sewers
A. Storm sewers
shall be sized to carry the discharge (Q) derived above. Capacity
of storm sewers shall be determined by the use of Manning's
formula on the basis of hydraulic gradients rather than physical
slope of the pipe. The minimum size of the storm sewer shall
be eighteen (18) inches. Storm sewers will be required where
subsurface conditions indicate a potential for seepage or
underground flow as determined by the City Engineer.
B. Closed conduits
or storm sewers shall be designed to a twenty-five (25) year
frequency storm and provisions made to accommodate a one hundred
(100) year frequency storm runoff. The twenty-five (25) year
hydraulic grade line shall be at or below the gutter line
and shall in no case surcharge back through an inlet or inlets.
All storm sewers, inlets, manholes or junctions shall be designed
in accordance to Texas Department of Transportation hydraulic
criteria.
C. Inlets: shall
be spaced so that the maximum allowable ponding width shall
not exceed fifteen (15) feet of the pavement width.
D. Manholes: Manholes
(inlets and junction boxes) shall be provided at all changes
in grade or alignment sewer intersections, and at a maximum
of 500 (500) feet on straight lines. Design of manholes shall
conform to the City of Bastrop Construction Standards, as
periodically amended.
E. Pipe: Pipe for storm drains shall be reinforced concrete
pipe (RCP) in sizes as shown on the approved plans. All RCP
shall be (18) inches or larger in diameter shall be reinforced
concrete pipe minimum Class III. Where, in the opinion of
the City Engineer, added strength of pipe is needed for traffic
loads over minimum cover or for excessive height of backfill,
concrete pipe shall be ASTM C14 Extra Strength or ASTM C76,
Class IV or Class V. Pipe shall have a minimum cover of not
less than one (1) foot over the top of the pipe.
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1.40.5 Culverts
A. All culverts
shall be designed to convey the twenty-five (25) year frequency
storm, and the headwater surface elevation shall not exceed
the minimum road surface elevation. The headwater depth for
a one hundred (100) year frequency storm shall not exceed
one foot (1') over the minimum roadway surface elevation.
B. Culvert hydraulics
shall be analyzed using Federal Highway Administration (FHWA)
Hydraulic Design Series Number 5 (HDS-5) HYDRAULIC DESIGN
OF HIGHWAY CULVERTS methods.
C. Box culverts shall conform to Texas Department of Transportation
(TxDOT) design standards and details.
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1.40.6 Headwalls
And Wingwalls
A. All headwall
and wingwalls shall conform to TxDOT design standards and
details.
B. No headwall, wingwall or other structural member shall
protrude above the surface of the traveled roadway.
C. All headwall and wingwalls within the "clear zone"
as defined by TxDOT of any roadway shall conform to TxDOT
design standards and details for safety end treatment, or
shall be protected by a traffic barrier.
D. Outfalls: Whenever
possible, outfalls from storm sewers and ditches into natural
drainage ways shall enter at the grade of the natural drainage
channel. The engineer will design drop-type outfall structures,
or otherwise provide adequate protection against erosion.
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1.40.7 Drainage
Channels
A. Drainage ditches,
where approved by the City Council, may be used for outfalls
to natural or major drainage channels. Ditches shall be designed
to have a minimum of one (1) foot of freeboard at design flow
and side slopes shall not be steeper than 3:1.
B. Channels with
slopes less than 1.0% shall be constructed with a reinforced
concrete trickle channel.
C. Water surface
profiles for all channels shall be computed using a standard
step backwater model, such as US Army Corps of Engineers (USACE)
HEC-RAS. The engineer can propose to use other hydraulic methods
but must have their acceptability approved by the City Engineer.
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1.40.8 Roadside
Ditches & Driveway Culverts
A. Roadside drainage
ditches shall conform to the following. -
Minimum grade - 0.5%
Maximum grade in sandy soils - 5%
Maximum grade in clay soils - 8%
B. Rock or riprap retards shall be used to control the erosive
characteristics of drainage in roadside ditches on steep slopes.
Retards shall be designed to reduce drainage water velocity
to an acceptable level and to prevent drainage water from
encroaching on the driving surface. Retards shall not project
onto shoulder surfaces and shall blend into ditch lines so
that normal roadside ditch maintenance is possible.
C. Roadside ditches shall be designed to carry the 25-year
event. Roadside bar ditches shall have a maximum front slope
of 6:1 (horizontal: vertical). The maximum back-slope shall
be 4:1 (horizontal: vertical). Exceptions to the slopes may
be made by the City's Engineer for unusual circumstances,
provided slopes are adequate for maintenance, soil stability
and traffic safety.
D. The engineer shall calculate the culvert sizes for every
lot within the subdivision and provide a table identifying
each lot, culvert size and elevations. Corrugated metal pipe
(CMP) minimum 15-inch diameter is an acceptable driveway culvert
material. The engineer shall determine if soils are suitable
for CMP, and specify coatings or other materials if corrosive
soils are encountered.
E. The length of culvert pipe, where used, shall be sufficient
to allow for driveway base width (including radius as applicable)
plus three times the pipe diameter plus three feet (3'), but
in any case, no less than twenty feet (20'). All driveway
culvert ends shall be constructed with concrete safety end
treatments.
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1.40.9 Preliminary
Drainaqe Plan Submittal Requirements
A preliminary drainage plan shall be submitted with the preliminary
plat and shall include as a minimum the following:
A. The drainage
plan shall be accompanied by a copy of a topographical drawing
at the same scale as the preliminary plan, showing the location,
type and size of all required drainage structures, and shall
indicate the route of proposed drainage. Where a lot is located
adjacent to a major drainage course or overflow channel the
drainage plan shall show proposed building sites and minimum
finished floor elevations as stipulated in the City of Bastrop's
Flood Damage Prevention Regulations, as periodically amended.
B. Drainage plans shall be drawn to a scale no smaller than
the preliminary plan scale. The scale of supplementary plans,
profiles and cross-sections shall be sufficient to clearly
show details, if required to demonstrate the adequacy of existing
or proposed facilities.
C. Plans shall
show storm (flood) water routing and all drainage structures
with sizes of culverts, retarding and retaining structures,
drainage easements with course and distance of centerline
and boundaries, lot lines, street layout, proposed inlets,
culverts, roadside ditches, channel sections and slopes, bridges,
channel improvements, levees, or berms, fills necessary to
elevate land above flood levels, and remove same from the
flood area.
D. The drainage
plan shall be prepared by a Licensed Professional Engineer
of the State of Texas, whose seal and signature shall appear
on the plan.
E. All drainage calculations are required to be present on
the plans or in an engineering report signed and sealed by
an engineer licensed in the State of Texas. Computations shall
be complete and orderly and shall clearly state all assumptions
and design basis.
F. Profiles, cross-sections,
or substantiating data may be required at the City's request
as necessary to support flood levels and backwater analysis.
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1.40.10 Final
Drainaqe Plan Submittal Requirements
A. Final drainage
plans shall be submitted with the final plat and shall include
as a minimum the following:
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(1) Existing
and proposed topographic information, with minimum two foot
contour intervals.
(2) Location
map.
(3) Offsite and
onsite drainage area maps.
(4) Centerline of watercourses.
(5) Regulatory
flood elevations and boundaries of flood prone areas, including
Floodways where designated.
(6) Drainage
easements.
(7) All street
widths and grades.
(8) Calculations
showing the anticipated storm water flow, including watershed
area, runoff coefficient, and time of concentration. When
a
drainage structure or storm sewer is proposed, calculations
shall be sub-mitted showing basis for design.
(9) Storm sewer
plans and profiles showing size, grade, and pipe or culvert
material. Runoff, inlet, conduit hydraulic grade line calculations
are required.
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B. All drainage
calculations are required to be present on the plans or in
an engineering report signed and sealed by an engineer licensed
in the State of Texas. Computations shall be complete and
orderly and shall clearly state all assumptions and design
basis.
C. A grading plan
shall be prepared for each subdivision and show in sufficient
detail grading of all roads, streets, drainage structures,
channels, swales, or other drainage related features and provide
minimum finished floor elevations, based on an acceptable
elevation datum, for proposed structures to assure no inundation
of such structures by the rainfall run-off by a one hundred
(100) year frequency storm.
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PART 2: That if
any part of this Ordinance is held to be unenforceable, then the validity
of the remaining parts shall not be affected by such holding.
PART 3: That any
provisions of the Bastrop City Code and any ordinances in conflict
with this ordinance are, to the extent of any such conflict, hereby
repealed.
PART 4: That, a
public hearing having been held, this Ordinance shall take full force
and effect upon passage by the City Council and upon publication in
accordance with State and local laws.
READ and Acknowledged on
First Reading on the 12th day of June, 2007.
READ and Adopted on Second Reading on the 26th day of June, 2007.
APPROVED:
By: Tom Scott
Mayor
ATTEST:
Teresa Valdez
City Secretary
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