ORD NO 28-2023 City of Van Bur
Municipal
1003 Broadway • Van Buren, Arkansas 72956
Ordinance —
Has been MENDE or REPEALED as of
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Please refer to
Ordinance - o
Shawnna M. Reynolds
City Clerk-Treasurer
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G ofBuren,
ity
Municipal Complex
1003 Broadway • Van Buren, Arkansas 72956
Ordinance 40 — Y 6
Has been AMENDED as of
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Please refer to
Ordinance
Shawnna M. Reynolds
City Clerk-Treasurer
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CITY OF VAN BUREN,ARKANSAS
ORDINANCE NO A c?-2023
BE IT ENACTED BY THE CITY COUNCIL,FOR THE CITY OF VAN BUREN,
ARKANSAS,AN ORDINANCE TO BE ENTITLED:
AN ORDINANCE ADOPTING AMENDED ORDINANCE NO. 40-1980 TO
MODIFY THE MINIMUM DRAINAGE STANDARDS FOR THE CITY OF
VAN BUREN,ARKANSAS.
WHEREAS, the City of Van Buren,Arkansas, adopted an Ordinance to implement minimum
drainage standards in 1980 identified as Ordinance No.40-1980; and
WHEREAS, the purpose of the Ordinance is to protect the public as it applies to all storm
drainage facilities,whether an enclosed structure,pipe, open channel, ditch, or
stream; and
WHEREAS, there is now a need to update these standards to update the methods of hydrologic
design, and the design, construction, ownership, and maintenance of detention
basins; and
WHEREAS, notice of a public hearing was duly published and a public hearing was conducted
by the Planning Commission on September 5, 2023,to consider these proposed
modifications; and
WHEREAS, the amended minimum drainage standards were recommended by a majority vote
of the entire Planning Commission.
NOW THEREFORE,BE IT ORDAINED BY THE CITY COUNCIL OF THE CITY
OF VAN BUREN,ARKANSAS,THAT:
SECTION 1: The amended minimum drainage standards set forth in attachment"A"are adopted.
SECTION 2: These standards are the Storm Drainage Standards that are referred to in Ordinance
No. 7-2003 (Subdivision Regulations) as amended.
SECTION 3: The various provisions and parts of this Ordinance are hereby declared severable,
and,if any section or part of a section,or any provision or part of a provision herein,
is declared to be unconstitutional, inappropriate or invalid by a court of competent
jurisdiction, such holding shall not invalidate or affect the remainder of this
Ordinance and to the that extent the provisions hereto are declared to be severable.
SECTION 4: Prior Ordinances or sections of ordinances in conflict with the provisions enacted
herein are repealed to the extent of the conflict.
IN WITNESS WHEREOF,the City of Van Buren,Arkansas,by its City Council, did
pass, approve, and adopt, by a vote of for and against, the foregoing Ordinance at its
regular meeting held on the 25th day of September 2023.
a
Jo e h P Hurst Mayor
ATTESTED: APPROVED AS TO FORM:
Shawnna Re o s
City Clerk/Treasurer City Attorney `�tir `�� -yi�'��
City of Van, Buren, Arkm, sas
MunidRP al CompRex
1003 Broadway o Van Buren, Arkansas 72956
VAN BUREN CITY COUNCIL MEMORANDUM
TO.. MAYOR JOE BURST AND CITY COUNCIL
FROM: WALLY BAILEY,PLANNING DIRECTOR
SUBJECT: AMENDED MINIMUM DRAINAGE STANDARDS
DATE: -SEPTEMBER 18;2023
The City of Van Buren drainage standards need updating and amending to better address
drainage issues for new developments in the City of'Van Buren.
Mr.Neal Morrison,P.E.,has assisted the City staff with preparing the amended minimum
drainage standards. Mr.Morrison has been assisting the.City as.our third-party-reviewerof
plans for subdivisions,which has given him a,practical insight into the problems with our
current standards.
The amendments will update our standards with current,engineering practice for
hydrologic design and for design,construction,and maintenance of detention basins.The
proposed standards are consistent with those of other cities in our area.
The attached amended standards reflect the proposed standards as reviewed and amended
by the Planning,Commission.
The Planning Commission voted unanimously to recommend these revised standards to
the City Council for approval.
Please let me know if you require any additional information before the City Council
meeting.
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MEMORANDUM
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To: Wally'Bailey
Director of Planning
City of Van Buren,AR '
From: Neal Morrison
Consulting.Engineer
Date: August 29,,2023
Rom: Drafi Revised Drainage Standards
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The draft revised drainage standards:For the City of Van Buren are intended to update them to be
more .current and to be consistent with other nearby communities via the following general
categories of revisions:
J. Amendments to methods of Zrydrologic and hydraidic calculations. As part of this
amendment,the fist.of design information and calculations required to be submitted in the
basis of design was refined.and.updated. Also, the nomograph's and design charts in the
Appendix are recommended to be deleted since they are rarely.used and readily available
elsewhere. The rainfall frequency curves are also recommended to, be changed to the
AiDOT curves for this part of the State.
2. Amendments to Section 2.5 to include stornm)ater detention. Currently,Section.2.5 is very
vague on limiting drainage system discharges due to development. The recommended
changes clarify how detention basins are to be used and provides design, construction,
ownership and maintenance guidelines for same.
3. Amendment regarding details in the current standard. The details in the current standard
are not needed since they are included in the City's Standard Details for Pubhic Works
Construction.
M. Minor amendments. There are other minor revisions to correct grammar, syntax .and
presentation of formulae.
I look forward to receiving input from the Planning Commission and the public. In the end, I
believe we can create a standard thatprotects the public interestwhile not being overly burdensome
to potential developers. Thank you for the.opportunity to participate.
°-r
MEMORANDUM
To: Wally Bailey
Director of Planning
City of Van Buren,AR
From: Neal Morrison
Consulting.Engineer
E
Date: August 29,2023 {
RE: Draft Revised Drainage Standards
The draft revised drainage standards for the City of Van Buren are intended to update them to be
more current and to be consistent with other nearby communities. via the following general
categories of revisions:
1
1. Amendments to methods of hydrologic and hydrazdic calculations. As part of this
amendment, the list of design information and calculations required to be submitted in the
basis.of design was refined and,updated. Also, the homographs and design charts in the
Appendix are recommended to be deleted since they are rarely used and readily available
elsewhere. The rainfall frequency curves are also. recommended to be changed to the
AiDOT curves for this part of the State.
2: Amendments to Section 2.516 inch ide storinwater detention. Currently,Section 2.5 is very i
vague on limiting drainage system discharges due to development. The recommended
changes clarify how detention basins are to be used and provides design, construction, {
ownership and maintenance guidelines for same. j{
3. Amendment regarding details in the current standard. The details in the current standard j
are not needed since they are included iri the City's Standard Details for Public Works
Construction.
4. Minor ainendinents. There are other minor revisions to correct grammar, syntax and
presentation of formulae. E
I Iook forward to receiving input from the Planning Commission and the public. In the end, I
believe we can_create a standard that protects the public interest while not being,overly burdensome
to potential developers. Thank you for the opportunity to participate.
"ATTACHMENT A"
MINIMUM DRAINAGE STANDARDS
FOR THE
CITY OF VAN BUREN,ARKANSAS
September 2023
Page 1 of 17
1. SCOPE
1.1 These Minimum Storm Drainage Standards shall apply to all storm drainage
facilities,whether an enclosed structure,pipe, open channel, detention basin, ditch
or stream.
1.2 These Minimum Storm Drainage Standards are those referred to in Ordinance No.
7-2003 as amended, (Subdivision Regulations) Section IV,IMPROVEMENTS, c.
Storm Drainage.
2. GENERAL DESIGN REQUIREMENTS AND DESIGN SUBMITTALS
2.1 All designs, plans and specifications submitted to the Planning Director for
Planning Commission approval shall be prepared by a registered professional
engineer, licensed in the state of Arkansas.
2.1.A PLAN REQUI MENTS
Two(2)hardcopies of the plans shall be submitted on 24"by 36"sheets and
along with a portable document format (PDF) file either by e-mail or on
electronic media. The plans shall include:
o Locations of the project with respect to well-known roads, streets,
subdivision or survey lines on a key map of the entire project.
o Plans and profiles for each storm sewer line shall be provided which
show location, size, flow line elevations, gradients, materials and any
soil boring information. Plan profiles submitted shall have a vertical
scale of not less than one inch equal to five feet.
o All easements, storm sewers (enclosed or open channel), utilities and
facilities,both existing and proposed shall be shown.
o Elevations submitted shall use NAVD 1988. Location and elevation of
all benchmarks shall be indicated on the plans.
o Details of special drainage structures not included in the Van Buren
Standard Drawings for Public Works Construction shall be provided.
2.LB DRAINAGE AREA MAP
An area map showing topography shall be furnished with the drainage basin
and subareas outlined. The drainage area map shall be at a scale of 1"=100'
or larger and shall have a maximum two (2) foot contour interval unless
otherwise approved.
Page 2 of 17
The drainage map shall show a minimum of the following information:
o North arrow and scale
o Project limits, property boundary, street rights of way and and/or
platted lots
o Predevelopment and postdevelopment drainage areas with sizes
indicated
o Locations of study points for pre and post development scenarios
o Locations of drainage inlets, open channels and detention basins
o Limits of regulatory 100-year floodplains and floodways
2.1.0 BASIS OF DESIGN
One (1) hardcopy and a PDF of the Basis of Design shall be submitted to
the Planning Director for all drainage facilities in public rights-of-way and
easements and for detention basins in private developments. The Basis of
Design shall be prepared by a professional engineer licensed in the State of
Arkansas and shall be sealed, signed and dated and shall include:
o A narrative description of the project, its discharge points, proposed
drainage facilities and existing downstream drainage facilities and
deficiencies thereof(if any)
o A statement of hydrologic methods and computer software (if any)
employed in the calculations.
o Drainage area maps as described in 2.1.B.
o Hydraulic and hydrologic calculations for storm drains and culverts
shall be provided for each design storm required to be examined by
applicable sections of these Minimum Design Standards. Calculations
for pipes, pipe ends and junctions may be compiled into tabular forms
and as a minimum shall include:
✓ tributary and cumulative drainage area, runoff coefficients and
time of concentration calculations
✓ invert,hydraulic and energy grade line elevations
✓ junction and entrance loss coefficient and head loss calculations
✓ top of curb,top of road or top of inlet elevations
✓ indication whether inlet is in a sump or grade condition
Page 3 of 17
✓ longitudinal and cross slopes of pavements
✓ flow in pipe velocity
✓ inlet length, gutter flow depth, gutter spread, flow captured and
flow bypassed at curb inlets
✓ grate size, opening area and capture efficiency at grated inlets
o Hydraulic and hydrologic calculations for the capacity evaluation of
existing drainage infrastructure downstream of system discharge(s).
o Hydraulic and hydrologic calculations for detention basins shall be
provided for each design storm required to be examined by applicable
sections of these Minimum Design Standards. Calculations for each
pond shall include:
✓ hydrologic calculations for pre and post development conditions
including runoff parameters, times of concentration, drainage
areas and peak flows
✓ elevation vs. volume curve for the selected pond geometry
✓ discharge flow for each element of the outlet structure
✓ peak water surface elevation for each storm examined
✓ for Modified Rational Method, calculations for determining the
duration factor that yields the maximum detention volume
2.2 DESIGN FLOWS
Storm sewer systems shall be designed to contain the peak flow from a 10-year, storm with
minimum freeboard as required by applicable sections of these regulations. The effects of
the peak 100-year storm shall also be evaluated to ensure that existing and proposed
finished floor elevations of structures of adjacent or affected properties are two(2)or more
feet above the 100-year water surface elevation.
2.3 FLOW TOWARD STREETS.
Any concentration of surface flow in excess of 6 cubic feet per second (cfs) shall be
intercepted before crossing the curb (or curb line) and carried by enclosed storm sewers.
No storm water concentration will be allowed to empty into the street except as stated
above.
2.4 METHODS OF CONVEYING WATER
2.4.A All flows within the right-of-way of non-estate streets not carried in the gutter and
side lot flows to drainage channels at the rear of lots shall be in storm sewers.
2.4.B Storm flows in areas not listed in 2A.A.may be carried in open channels as defined
in Section 5.
Page 4 of 17
2.4.0 Natural drainage channels (not relocated or channelized) may be used in new
developments providing the channel will carry the storm runoff used in the design
storm without erosion problems and sufficient land for a one-foot(1') freeboard is
included in a drainage easement.
2.5 SYSTEM DISCHARGE
2.5.A All storm sewer systems in new developments shall be adequate to contain the
design storm runoff to the discharge point or points at the down-stream property
line. The location(s) of point(s) of storm water discharge from the developed
property shall generally be the same as the pre-development discharge point. All
reasonable effort should be taken to ensure that storm water discharge volume and
velocity will be limited to the predevelopment discharge conditions in the 2, 10, 50
and 100-year storm events for developments with property boundary areas greater
than one(1) acre,including future phases.
If the existing infrastructure downstream of the system discharge(s)lacks hydraulic
capacity to carry the peak 10-year flow, downstream improvements may be
required.
2.5.13 Hydrologic methods used in the design of detention ponds shall conform to Section
3.1. The Modified Rational Method may be used for ponds where the tributary area
is twenty (20) acres or less. For tributary areas greater than 20 acres, one of the
methods listed in Section 3.1 shall be used.
2.5.0 If detention ponds are used to meet the requirements of Section 2.5.A then the
following standards apply to the detention pond:
1. Detention ponds in subdivisions containing two or more building lots shall
be located on a platted lot and its use shall be clearly specified on the plat.
The lot will not be subject to area and frontage requirements of the Zoning
Code, but shall be of a size sufficient to allow operation and maintenance
of the pond within its boundary.
2. Detention ponds shall be designed to meet the objectives of Section 2.5.A
and provide a minimum of(1) foot of freeboard above the peak 100-year
water surface elevation.
3. Outlet structures shall be designed to control the outflow from the pond in
the design storms. Outlet structures may consist of box structures, pipes,
orifices, weirs or any combination thereof. All elements of the structure
shall be constructed from reinforced concrete. Enclosed structures greater
than 4 feet in depth shall include permanent means of access for
maintenance such as steps and manholes. Steps shall be made of steel
reinforced copolymer polypropylene.
Page 5 of 17
4. All embankments and the top six (6) inches of excavated surfaces shall be
compacted to 95% standard Proctor density. Earthen surfaces shall be
covered with a minimum of four(4) inches of topsoil and sodded.
5. Grassed earthen pond side slopes shall not be steeper than 4H:1 V. Steeper
side slopes in combination with slope stability measures will be considered
on a case-by-case basis. Slope stability measures may include,but are not
limited to concrete lining,rip-rap lining,reinforced earth and retaining wall
systems.
6. A minimum eight (8) foot wide clear and flat maintenance access shall be
provided at the tops of berms and around the pond perimeter.
7. Trickle channels shall be provided from each discharge point to the outlet
structure to transmit frequent, small flows and for grade control. Trickle
channels shall be six (6) inch thick Class AAA (4,000 psi) concrete, a
minimum of four (4) feet wide, six (6) inches deep and have a minimum
longitudinal slope of 0.50 percent.
8. The pond bottom shall have a minimum one (1)percent slope from the toe
of slope toward the trickle channel(s).
9. Underground detention systems, retention ponds and parking lot detention
will be considered on a case-by-case basis.
10. Access from a public street to a detention pond shall be provided via a
minimum of twenty (20) feet of frontage on a public street or by a twenty
(20) foot dedicated access road easement between the pond lot and public
street. Other means and/or widths of access may be considered by the City
on a case-by-case basis. The access road shall be all-weather and a
minimum of twelve (12) feet wide. A concrete driveway approach
conforming to the requirements of Ordinance No. 23-1996 shall also be
provided at the connection to the public street.
11. Fencing is not required but will be allowed. Fencing and gates shall
conform to the Standard Drawings for Public Works Construction with the
exception that the fabric be four (4) foot polymer coated woven wire and
installed with a 10-inch gap between the ground and the bottom of the
fabric. Gates shall be provided as necessary for maintenance access.
12. After completion of construction, an "as-built" survey of the detention
facility shall be conducted to verify that it conforms to the approved design.
A record of the survey in the form of a scale drawing sealed and signed by
a professional engineer or land surveyor licensed to practice in the State of
Arkansas shall be submitted to the City. As a minimum, the as-built
drawing shall include the following:
Page 6 of 17
a. Date of survey, scale and north arrow;
b. Boundary of the detention basin parcel, if applicable;
C. Elevation contours of the surface at an interval not exceeding one
foot;
d. The elevations of berms,pipe inverts,weirs,boxes,trickle channels
and other elements of the pond;
e. A table showing the elevation vs. volume relationship from the
lowest elevation of the pond to the top of the berm.
f. Material testing reports for concrete structures and compacted
earthen embankments showing that the materials meet the minimum
requirements of the approved plans and specifications shall be
provided. Testing shall be done by a certified materials testing
laboratory.
If the as-built features of the detention pond are determined by the City to
be significantly different from the approved design, corrective action may
be required,or an analysis by a professional engineer demonstrating that the
pond, as constructed, meets the objectives of the Minimum Drainage
Standards.
2.5.1) Ownership of detention ponds for single tract developments shall be vested
in the property owner. In residential subdivisions containing two or more
lots,the City of Van Buren will assume ownership of the detention pond(s)
after the following requirements are met:
1. At least eighty(80)percent of the lots in all phases of the subdivision
shall have been developed for their intended use.
2. The detention pond maintenance items listed in Section 2.5.E are
current.
2.5.E Routine maintenance items shall include:
o Sediment removal
o Repair of eroded areas and areas that do not support grass cover
o Outlet structure cleaning and maintenance
o Mowing and weed control
o Litter and debris removal
o Access road and fence maintenance
If the property owner fails to maintain a detention pond as specified above,
the City of Van Buren Code Enforcement Officer may determine the
property owner to be in violation of Section 504 of the Van Buren Municipal
Code.
Page 7 of 17
2.6 EASEMENTS
All storm sewers shall be located in street rights-of-way or in an easement dedicated
to public use. Easement widths shall conform to the following:
2.6.A Easements for enclosed drainage systems shall have a minimum width of
fifteen(15) feet. For larger or deeper systems,the easement width shall be
width of the drainage conduit plus ten(10)feet or fifteen(15) feet plus the
amount of depth in excess of five(5)feet,whichever is greater.
2.6.13 Open channel easements shall be required to contain the entire channel
design width including freeboard and maintenance accessway with the
minimum width being fifteen(15) feet.
3. HYDROLOGY
3.1 HYDROLOGIC DESIGN METHOD
The Rational Method may be used to determine storm water runoff characteristics
for storm sewer design when tributary drainage area is 200 acres or less. Other
methods such as the SCS Method, the US EPA Storm Water Management Model
(SWMM) and adaptations thereof and US Army Corps of Engineers HEC-HMS
may be used for tributary areas of any size.
The Rational Method uses the basic formula Q = CiA for estimating runoff from
rainfall,where:
Q= Rate of runoff in cubic feet per second
i= Average rainfall intensity in inches per hour for the design storm having
duration equal to the time of concentration for the critical upstream drainage
area
C= Runoff coefficient, which is the fraction of the rainfall which becomes
runoff
A= Tributary drainage area in acres
3.2 RAINFALL AND INTENSITY
3.2.A. If the Rational Method is used for design, the duration—intensity
relationships from the Arkansas Department of Transportation (ArDOT)
Drainage manual for Area IV(see Figure 1) shall apply.
Page 8 of 17
AREA S 3a-is-ao
RAINFALL. INTENSITY DURATION FR'E4UENCY
20.0 RELATIONSHIP
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DURATION
FIGURE 1: ArDOT IDF Curves for Area IV
3.2.B. The time of concentration equals the overland flow time plus(if applicable)
shallow concentrated flow time and the time for the water to flow down the
pipe or channel to the point in question.
The overland flow time,Tc,may be calculated using the following equation:
L= 0.93 f(n)(L)/(S)0.5j0.6
10.4
Page 9 of 17
Where:
n= Manning's n of flow path(See Table 1)
S= Slope of flow path,feet per foot
I= Rainfall intensity for a storm with a return period equal to T�,in./hr.
TABLE 1: Manning's n for Overland Flow(AASHTO)
0.011 Smooth surfaces (concrete, asphalt,gravel,bare soil)
0.05 Fallow(no residue)
Cultivated soils:
0.06 Residue cover<20 percent
0.17 Residue cover>20 percent
Grasses:
0.15 Short grass prairie
0.24 Dense grasses
0.41 Bermuda grass
0.13 Range(natural)Grassed waterway
Woods:
0.40 Light Underbrush
0.80 Dense Underbrush
The shallow concentrated flow time, Tt, may be calculated using the
following equation:
Tt= L
(K)(So.$)
Where:
Tt= overland flow time,seconds
L= shallow concentrated flow path length,feet
K= conveyance factor which depends on ground cover(See Table 2)
S= ground slope,percent
Page 10 of 17
TABLE 2: Conveyance Factors for Shallow Concentrated Flow(K)
0.25 Forest with heavy ground litter and meadow
0.47 Fallow or minimum tillage cultivations
0.71 Short grass pasture and lawns
1.00 Nearly bare ground
1.52 Grassed waterway
2.00 Paved area(sheet flow) and shallow gutter flow
Open channel flow time shall be estimated using the Manning equation as
presented in Section 4.1 of these design standards.
Other methods for estimate time of concentration may be used with the prior
approval of the City.
3.3 RUNOFF COEFFICIENT
If the Rational Method is used in storm sewer design, a runoff coefficient for the
ultimate development of an offite drainage area must be used. For offsite areas
(areas outside the project property boundary),the runoff coefficient shall reflect the
current condition of the area. The values listed in Table 3 shall be used as a guide
in selecting the runoff coefficient:
Page 11 of 17
TABLE 3: Rational Method Runoff Coefficients
Description of Area Runoff Coefficient(C)
Single Family Residential 0.30 to 0.50
Multi-units, detached 0.40 to 0.60
Multi-units, attached 0.60 to 0.75
Residential, 1 acre or more 0.25 to 0.40
Apartments 0.50 to 0.70
Industrial -light 0.50 to 0.80
Industrial-heavy 0.60 to 0.90
Parks and cemeteries 0.10 to 0.25
Playgrounds 0.20 to 0.35
Railroad yard 0.20 to 0.35
Unimproved 0.10 to 0.30
In some cases, it may be necessary to develop a composite runoff coefficient, in
which case the following value should be used:
TABLE 4: Runoff Coefficients for Developing Composite C
Character of Surface Runoff Coefficient(C)
Pavement, asphalt and concrete 0.70 to 0.95
Pavement,brick 0.70 to 0.85
Roofs 0.75 to 0.95
Lawns, sandy soil
Flat,<2 percent 0.05 to 0.10
Average,2 to 7 percent 0.10 to.0.15
Steep,>7 percent 0.15 to 0.20
Lawns, clayey soil
Flat,<2 percent 0.13 to 0.17
Average,2 to 7 percent 0.18 to 0.22
Steep,>7 percent 0.25 to 0.35
Page 12 of 17
4. PIPES AND CULVERTS—GENERAL REQUIREMENTS AND DESIGN CRITERIA
4.1 CULVERT HYDRAULICS
Culvert flow may be limited by conditions existing at either the inlet or the outlet of
the pipe. When inlet control governs the cross-sectional area of the barrel,the shape
of the inlet and the amount of ponding (headwater) at the inlet are primary design
considerations. Outlet control is dependent upon the depth of water in the outlet
channel (tailwater), the slope of the barrel, type of culvert material and length of
the barrel. The basis for all hydraulic design calculations will be Manning's
Formula and the Continuity Equation:
V= 1.486 (0.67)(S1.5)
n L
Q=AV
Where:
V= Mean velocity of flow in feet per second,ft./sec.
n= Manning's coefficient of roughness
R= hydraulic radius, feet
S = Slope,ft./ft.
Q= Flow,cfs
A= Flow area, sq. ft.
4.1A Coefficients of Roughness
The coefficients of roughness to be used for evaluating capacity of culverts
of the various kinds of conduit shall be selected according to Table 5:
TABLE 5: Coefficients of Roughness for Pipe(n)
Type of Pipe Roughness Coefficient(n)
Reinforced concrete 0.013
Corrugated metal pipe 0.024
Smooth wall corrugated metal pipe 0.013
Smooth wall plastic or HDPE pipe 0.012
Page 13 of 17
4.LA Inlet and Outlet Control
Culverts and drainage inlets shall be designed so that the greater of inlet or
outlet control headwater is provided plus freeboard. The minimum
freeboard shall be one(1)foot below the top of road for culverts, or the top
of curb for curb inlets or the top of grate for grated inlets in the 10-year
storm.
4.2 CULVERT TYPES AND SIZES
The permissible types of culverts are reinforced concrete box,round pipe,elliptical
pipe and pipe arch. The minimum size of pipe for all culverts shall be fifteen(15)
inches or the equivalent thereof. Box culverts may be constructed in sizes equal
to or larger than 4 feet by 2 feet(span vs. height).
4.4 VELOCITY
All storm drainage pipes and culverts shall be designed to maintain a minimum
velocity of 2.0 feet per second and a maximum velocity of 15 feet per second in the
peak 10-year storm flow.
4.6 STRUCTURAL CONSIDERATIONS
The minimum allowable fill or cover for structures (RCP, CMP, concrete culverts)
under roadways shall be one foot or a minimum clearance of six inches from top of
structures to the bottom of pavement base,except for a special box culvert designed
to carry traffic on the top slab. Structural protection,such as special bedding,shall
be provided where adequate cover cannot be attained. Maximum fill shall be
determined on the basis of structural strength of pipe and design loads. Outside of
street rights-of-way, a minimum cover of twelve (12) inches is required for
vegetation and protection against unusual loading.
5 OPEN CHANNELS—GENERAL REQUIREMENTS AND DESIGN CRITERIA
5.4 OPEN CHANNELS
Open channels may only be used when the 10-year peak flow is greater than fifty
(50) cfs unless otherwise approved by the City. Open channels shall be designed
using the Manning formula and"n"values shown in Table 6.
Page 14 of 17
TABLE 6: Manning's Roughness Coefficients for
Open Channel Flow n)
Channel Lining n
Grass 0.03 to 0.05
Concrete 0.013 to 0.015
Rip-Rap 0.017 to 0.03
All open channels shall have a minimum of one (1) foot of freeboard or ample
freeboard to contain the 100-year storm,whichever is less.
All open channels shall have a maintenance accessway along one side of the channel
conforming to the following:
o The accessway shall have a minimum width of eight(8)feet for channel less than
or equal to three(3)feet deep including freeboard. For channels segments greater
than three (3) feet deep, the accessway width shall be three (3) times the depth
with a maximum width of twenty(20)feet.
o The accessway shall be constructed with a 2%cross slope toward the toward the
channel.
o The accessway shall be contained within the drainage easement and shall provide
access points to public streets at intervals not greater than six hundred(600)feet.
o The accessway shall be free of obstructions, including utility poles and
pedestals, fences, landscaping or other items that interfere with maintenance
operations.
5.2. OPEN CHANNELS (EARTHEN)
Earthen(grass lined) channels may be used where the velocities during the peak 10-
year storm runoff are less than or equal to six (6) feet per second. The sideslopes
shall not be steeper than 3H:1V. All earthen channels shall have a minimum
longitudinal slope of 0.50 percent and shall be sodded immediately after their
construction and adequate measures taken to prevent erosion.
5.3. OPEN CHANNELS (LINED)
Where velocities are greater than 6 feet per second, the channel section shall be
lined with concrete, rip-rap or other approved nonerosive materials. The lined
channel may have either vertical sides or sloped sidewalls with a slope not steeper
than 1.5H:1 V.
Page 15 of 17
Channels with vertical sides greater than thirty (30) inches in height shall have
barrier rails conforming to the Van Buren Standard Drawings for Public Works
Construction installed at the top of the walls unless the vertical drop is adjacent to
a sidewalk or pedestrian way. If the vertical drop is adjacent to a sidewalk or other
type of pedestrian way, guardrails conforming to the Arkansas Fire Prevention
Code shall be installed at the top of the walls.
The minimum flat bottom width for lined channels shall be three (3) feet and the
minimum longitudinal slope shall be 0.30 percent.
5.4 OPEN CHANNEL EROSION PROTECTION
Special protection such as headwalls,riprap, energy dissipators, or concrete lining
will be required in places such as bends,junctions, and inlets and outlets for storm
sewers where erosion is likely.
5.5 SUPERELEVATION OF HORIZONTAL CURVES IN OPEN CHANNELS
The amount of superelevation of raised water surfaces on walls of horizontal curves
of open channels shall be calculated and the height of the outside channel side shall
be increased accordingly to protect against overflow. The inside channel wall may
not be reduced due to superelevation. The freeboard criterion of Section 5.1 shall
apply to superelevated sections.
6 CURB INLETS—GENERAL REQUIREMENTS AND DESIGN CRITERIA
6.1. CURB INLETS
Curb inlets and extensions thereof shall be constructed in accordance with the Van
Buren Standard Drawings for Public Works Construction. Curb inlets will be
required at low points of streets, angle changes in storm drain alignment, and at all
other locations where water is removed from street gutters.
The maximum length of a curb inlet opening shall be sixteen (16) feet unless
otherwise approved by the City.
6.2. INLETS NEAR PEDESTRIAN AREAS
Inlets shall be required near intersections to keep crosswalks free of storm water.
6.3. ARTERIAL STREETS
Curb inlets shall be located on arterial streets so that the center two lanes are free
from water during runoff from a 50-year storm.
Page 16 of 17
6.4. MINOR AND COLLECTOR STREETS
Curb inlets shall be located on minor and collector streets so that depth of runoff
from a 10-year storm does not exceed the top of standard 6-inch curbs.
6.5. INTERSECTION SWALES
Intersection swales shall be constructed in accordance with the Van Buren Standard
Drawings for Public Works Construction. Swales shall not be permitted across
through streets.
Page 17 of 17
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