[Ord. No. 3801, 11-15-2022]
A.
Scope. This Chapter 430 hereby sets forth the minimum standards for design of storm drainage facilities on public rights-of-way and private property in the City of Bolivar, Missouri.
B.
Authority.
1.
The provisions of this Chapter have been adopted by the Planning
and Zoning Commission and the Board of Aldermen in accordance with
the zoning text amendment procedures set forth in the Bolivar Municipal
Code.
2.
Any development or grading begun after the date of passage of the criteria and standards set forth in this Chapter which does not comply with the requirements of this Chapter shall be deemed to be a municipal ordinance violation; and shall be subject to enforcement measures and penalties set forth in Municipal Code Section 100.220, General Penalty.
C.
Interpretations.
1.
Where any of the provisions of this Chapter may be unclear or
ambiguous as they pertain to a particular site or situation, interpretations
of the policies, criteria and standards set forth herein shall be
made in writing by the Zoning Administrator.
2.
Such written interpretations shall be kept on file for future
reference for use in similar situations and shall be incorporated
in subsequent revisions for the standards, if deemed necessary for
general reference.
D.
Appeals.
1.
Where disagreements may arise over the interpretation of the
requirements set forth herein, appeals may be made to the Zoning Administrator
upon written request.
2.
Information and supporting documentation for the appeal shall
be submitted with the request. The Zoning Administrator shall forward
the information to the Public Works Director, or the City Engineer
within three (3) business days following receipt of the information.
E.
Approvals And Permits Required.
1.
Grading Permit. Storm drainage facilities may not be constructed
or altered without review and approval of the plans by the City and
issuance of a grading permit by the City for subdivisions or for commercial
or other sites.
2.
National Pollutant Discharge Elimination System (NPDES) Stormwater
Permit.
b.
Federal rules for NPDES stormwater discharges are contained
in 40 CFR Parts 122, 123 and 124 of the Code of Federal Regulations.
State NPDES stormwater regulations are contained in 10 CSR 20-6.200
of the Code of State Regulations.
(1)
For certain activities, which involve the discharge
of dredged or fill materials into the waters of the United States,
a Department of the Army permit may be required as set forth in Section
404 of the Clean Water Act. Rules for 404 permits are contained in
33 CFR Parts 320 through 330 of the Code of Federal Regulations.
(2)
Determination of applicability for Section 404
requirements are generally made by the Kansas City District office
of the Corps of Engineers.
(3)
A brochure regarding the Corps of Engineers regulatory
program may be obtained from the Corps offices.
F.
Coordination With Other Jurisdictions.
1.
Where proposed storm drainage facilities are located on property
adjoining to other local government jurisdictions, design of storm
drainage facilities shall include provisions to receive or discharge
storm water in accordance with the requirements of the adjoining jurisdiction,
in addition to meeting City requirements.
2.
In these cases, two (2) additional sets of plans shall be submitted
and will be forwarded to the adjoining jurisdiction for review and
comment.
3.
No grading or construction of storm drainage facilities may
commence without prior notification of the Missouri One Call utility
warning system at 1-800-DIG-RITE, as required by law.
G.
Communications And Correspondence. Communications and correspondence
regarding stormwater plan review, policies, design standards, criteria
or drainage complaints shall be directed to the MS4 Coordinator at
the City of Bolivar, 345 S. Main Ave., Bolivar, Missouri 65613.
H.
Ownership And Maintenance.
1.
Improvements On Public Road Rights-Of-Way. Storm drainage improvements
on public rights-of-way shall, upon acceptance of the constructed
improvements, become the property of, and shall be maintained by,
the City of Bolivar.
[Ord. No. 3801, 11-15-2022]
A.
Stormwater Management Goals. In order to ensure protection of the
general health and welfare of the citizens of the City of Bolivar,
planning and design of stormwater management measures shall meet the
following goals:
1.
Prevent damage to residential dwellings and other building structures
from floodwaters.
2.
Maintain emergency vehicle access to all areas during periods
of high water.
3.
Prevent damage to roads, bridges, utilities, and other valuable
components of the community's infrastructure from damage due
to floodwaters and erosion.
4.
Prevent degradation of surface water and groundwater quality
from storm water runoff; preserve and protect quality of the environment;
and promote conservation of the City's natural resources.
5.
Minimize floodwater and erosion damage to lawns, recreational
facilities, and other outdoors improvements.
6.
Minimize traffic hazards from runoff carried in streets and
roads.
7.
Comply with applicable State and Federal laws and regulations.
8.
Meet the foregoing goals in a manner which is cost effective
and which minimizes the cost of housing and development while encouraging
sound development practices.
9.
Encourage innovative and cost-effective planning and design
of stormwater management facilities.
10.
Encourage multiple purpose design of stormwater management facilities
to provide opportunities for recreational use and other benefits to
the community wherever possible. The standards and criteria set forth
herein provide the minimum standards for planning and design of stormwater
facilities. Where a particular plan or design may be found to be in
conflict with a specific standard, achievement of the goals set forth
above will have precedence.
B.
General Planning And Design Principles.
1.
The City of Bolivar recognizes that stormwater management is
an important component of overall land use planning.
2.
The City of Bolivar further recognizes that proper stormwater
planning significantly reduces the long-term costs to the community
both in terms of infrastructure cost and property losses due to flood
damage. It is more cost effective to prevent flood damage by proper
design and construction than to repair and remediate problems which
have occurred through poor planning and design.
3.
The following general principles must be followed in preparing
the grading and storm drainage plans for all development sites:
a.
Recognize The Existing Drainage System. Proper planning of storm
drainage facilities must begin with the recognition of the existing
system and include necessary provisions for preserving or altering
the existing system to meet the needs of proposed development or construction.
The storm drainage system differs from other utility systems in very
important ways:
4.
Allow For Increase In Runoff Rates Due To Future Urbanization.
a.
As areas urbanize, peak rates of runoff increase significantly.
The City of Bolivar will require storage of increased volumes of urban
runoff in order to minimize increases in flow rates as urbanization
occurs. However, the cumulative effects of on-site detention are difficult
to predict and control and development of comprehensive basin-wide
runoff models to determine these effects do not appear likely in the
foreseeable future.
b.
For this reason, design of storm drainage improvements must
be based upon the assumption of fully urbanized conditions in the
area under consideration. No reduction in peak flow rates due to detention
unless an approved runoff model has been developed for the drainage
basin under consideration. Any detention storage facilities whose
effects are considered must be located within approved drainage easements.
5.
Provide For Acceptance Of Runoff From Upstream Drainage Areas.
a.
Drainage easements or public rights-of-way must extend to a
point where the upstream drainage area is no greater than five (5)
acres.
b.
Drainage easements or public rights-of-way must extend to the
point where existing watercourses enter the site. Where the upstream
drainage area is five (5) acres or greater, but does not discharge
onto the site through a defined watercourse, the drainage easement
shall extend to the point of lowest elevation.
6.
Provide A Means To Convey Runoff Across The Site. Stormwater
shall be conveyed across the site in a system of overland drainage
ways and storm sewers. Overland drainage ways consist of streets,
open channels, swales, and overland flow within drainage easements.
7.
Discharge Of Runoff To Downstream Properties.
a.
Concentrated runoff shall be discharged into existing watercourses,
drainage easements, or public road rights-of-way. Where none of these
exist, a drainage easement which extends to the nearest watercourse,
drainage easement, or public road right-of-way must be obtained from
the downstream property owner and proper provisions made for conveyance
of the peak flow from the one percent (1%) annual probability (one
hundred (100) year) storm within the drainage easement.
b.
Where concentrated flows are discharged to downstream properties
proper provisions must be made to:
8.
Assess Potential Downstream Flooding Problems.
a.
Determine whether there are existing structures, which are subject
to an unacceptable flooding hazard.
b.
If areas having an unacceptable flooding hazard occur downstream
of a development site, either on-site detention for peak flow control
or mutually agreed off-site improvements will be required.
9.
Assess Potential Water Quality Impacts On Receiving Waters.
Sediment, erosion and other water quality controls are required.
C.
Drainage Easements. All areas subject to inundation during the major
storm must be included in drainage easements. Specific standards for
drainage easements to be provided for storm sewers, open channels
and detention facilities.
[Ord. No. 3801, 11-15-2022; Ord. No. 3844, 3-28-2023]
A.
This Section outlines acceptable methods of determining stormwater
runoff.
1.
General Guidelines.
a.
The design storm runoff shall be analyzed using an approved
hydrograph method.
2.
Hydrograph Methods.
a.
Methodologies.
(1)
The Corps of Engineers HEC-1 Flood Hydrograph Package
and NRCS computer models are the preferred runoff models. Other models
may be used with approval from the City Zoning Administrator.
(2)
The runoff model must include the entire drainage
basin upstream of the proposed development. The model shall be prepared
in sufficient detail to ensure that peak runoff rates are reasonably
accurate.
(3)
The runoff model shall be developed for the following
cases:
b.
Rainfall.
(1)
Point precipitation frequency estimates data from
Atlas 14 for City of Bolivar can be found at: https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=mo.
If the above-referenced link is not available, the information can
be obtained from the City's Zoning Administrator.
(2)
Rainfall shall be distributed in time using Huffs
Distribution or the Pilgrim-Cordery Distribution adapted to local
rainfall data (references NOAA Atlas 14 for Bolivar, Missouri, as
shown in the following table). Other distributions may be used upon
approval from the City's Zoning Administrator.
|
Pilgrim-Cordery Method Synthetic Rainfall Mass Curves
| ||||
|---|---|---|---|---|
|
Cumulative Fraction of Depth
|
Cumulative Fraction of Storm Duration
| |||
|
1-Hour
|
2-Hour
|
3-Hour
|
4-Hour
| |
|
0.00
|
0.00
|
0.00
|
0.00
|
0.00
|
|
0.05
|
0.03
|
0.03
|
0.03
|
0.02
|
|
0.10
|
0.07
|
0.05
|
0.05
|
0.03
|
|
0.15
|
0.11
|
0.10
|
0.06
|
0.05
|
|
0.20
|
0.14
|
0.17
|
0.09
|
0.06
|
|
0.25
|
0.17
|
0.22
|
0.11
|
0.08
|
|
0.30
|
0.23
|
0.25
|
0.13
|
0.14
|
|
0.35
|
0.29
|
0.27
|
0.19
|
0.20
|
|
0.40
|
0.35
|
0.29
|
0.31
|
0.27
|
|
0.45
|
0.41
|
0.30
|
0.39
|
0.33
|
|
0.50
|
0.47
|
0.31
|
0.44
|
0.38
|
|
0.55
|
0.56
|
0.41
|
0.47
|
0.47
|
|
0.60
|
0.65
|
0.51
|
0.54
|
0.56
|
|
0.65
|
0.73
|
0.60
|
0.64
|
0.64
|
|
0.70
|
0.82
|
0.69
|
0.70
|
0.74
|
|
0.75
|
0.91
|
0.78
|
0.73
|
0.83
|
|
0.80
|
0.93
|
0.82
|
0.81
|
0.87
|
|
0.85
|
0.95
|
0.87
|
0.89
|
0.90
|
|
0.90
|
0.97
|
0.92
|
0.94
|
0.93
|
|
0.95
|
0.99
|
0.96
|
0.98
|
0.97
|
|
1.00
|
1.00
|
1.00
|
1.00
|
1.00
|
[Ord. No. 3801, 11-15-2022; Ord. No. 3844, 3-28-2023]
A.
Inlets.
1.
Inlet locations. Inlets shall be provided at locations and intervals
and shall have a minimum inflow capacity such that maximum flooding
depths set below are not exceeded for the specified storm; at all
sump locations where ponding of water is not desired and where drainage
cannot be released at the ground surface.
2.
Inlet Interception Capacities.
a.
Inlet capacities shall be determined in accordance with the
Federal Highway Administration HEC-22 Manual (reference 5).
b.
Nomographs and methods presented in the Neenah Inlet Grate Capacities
Report (1987) may also be used where applicable.
c.
The use of commercial software utilizing the methods of HEC-22
is acceptable. It is recommended that software be pre-approved for
use by the City.
3.
Clogging Factors. Clogging factors are not required to be considered
in curb inlet capacities.
4.
Interception And Bypass Flow. It is generally not practical
for inlets on slopes to intercept one hundred percent (100%) of the
flow in gutters. Inlets must intercept sufficient flow to comply with
street flooding depth requirements. Bypass flows shall be considered
at each downstream inlet, until all flow has entered approved storm
sewers or drainage ways.
5.
Allowable Street Depths. Urban streets are a necessary part
of the City drainage system. The design for the collection and conveyance
of stormwater runoff is based on a reasonable frequency and degree
of traffic interference. Depending on the street classification, (i.e.,
local, collector, etc.) portions of the street may be inundated during
storm events. Drainage of streets are controlled by both minor and
major storm events. The minor system is provided to intercept and
convey nuisance flow. Flow depths are limited for the major storm
to provide for access by emergency vehicles during most flood events.
When the depths of flow exceed the criteria presented in this Section
a storm sewer or open channel system is required.
a.
General Design Guidelines.
(1)
Allowable Flow Depths: Flow in the street is permitted
with allowable depths of flow as follows.
(2)
Local Streets: Crown of the street for the runoff
from a 5-year rainfall, top of curb for runoff from a 25-year rainfall.
Runoff from a 100-year rainfall should be contained within the right-of-way.
(3)
Collector Streets: The equivalent of one (1) ten
(10) foot driving lane must remain clear of water during a 5-year
rainfall, top of curb for runoff from a 25-year rainfall. Runoff from
a 100-year rainfall should be contained within the right-of-way.
(4)
Minor Arterials and parkways: Two (2) ten (10)
foot lanes must remain clear of water, one (1) in each direction,
during a 5-year rainfall. Top of curb for runoff from a 25-year rainfall.
Runoff from a 100-year rainfall should be contained within the right-of-way.
Where allowable depths are exceeded a storm sewer system must remove
the excess water.
(5)
Major Arterials and Parkways: Two (2) ten (10)
foot lanes must remain clear of water, one (1) in each direction for
the 25-year storm. For the 100-year storm, a maximum of six (6) inches
at the crown, depth at the gutter shall not exceed eighteen (18) inches.
Where allowable depths are exceeded, a storm sewer system must remove
the excess water.
b.
Cross Flow. Cross flow at intersections is permitted up to the
following depth.
|
Street Classification
|
5-year Storm Allowable Depth
|
25-year Storm Allowable Depth
|
|---|---|---|
|
Local
|
6" in cross pan flow line
|
12" at gutter
|
|
Collector
|
No cross flow permitted
|
6" at gutter
|
|
Arterial or parkway
|
No cross flow permitted
|
No cross flow permitted
|
c.
Hydraulics. The allowable storm capacity of each street section
with curb and gutter is calculated using the modified Manning's
formula for both the two 2-year and 25-year storm event.
|
Q = 0.56(Z/n)S1/2d8/3
| |||
|
Where:
|
Q
|
=
|
Discharge in cubic feet per second
|
|
Z
|
=
|
Cross slope of the street in feet per foot
| |
|
d
|
=
|
Depth of flow at the gutter in feet
| |
|
S
|
=
|
Longitudinal slope of the street in feet per foot
| |
|
n
|
=
|
Manning's roughness coefficient
| |
6.
Types Of Inlets Allowed.
a.
Public Streets.
(1)
Curb Opening Inlets. Curb inlets shall be required
along public streets with curb and gutter and shall be as required
in the City of Bolivar "Construction Specifications for Public Improvements."[1]
[1]
Editor's Note: "Construction Specifications for Public
Improvements" is on file in the City offices.
(2)
Graded Inlets. The use of grated inlets in streets
will not be permitted. Where conditions are such that curb inlets
cannot intercept the required rate of flow necessary to control street
flooding depth or to provide diversion of flow to detention, sedimentation
or infiltration basins, "trench inlets" with veined grates may be
specified with approval of the City.
(3)
Other types of inlets will not be permitted unless
approved by the City.
b.
Outside Of Public Rights-Of-Way. The type of inlets specified
outside of public rights-of-way is left to the discretion of the designer,
provided the following criteria are met:
(1)
Maximum flooding depths for the major or minor
storm as set forth above are not exceeded.
(2)
General safety requirements set forth below are
met.
(3)
All inlets shall be depressed a minimum of two
(2) inches below the surrounding grade to allow proper drainage to
the inlet and prevent inadvertent ponding in the area around the inlet.
(4)
Inlets in pavements shall be provided with a concrete
apron.
7.
General Safety Requirements. All inlet openings shall:
a.
Provide for the safety of the public from being swept into the
storm drainage system; the maximum allowable opening shall not exceed
six (6) inches in width.
b.
Be sufficiently small to prevent entry of debris which would
clog the storm drainage system.
c.
Be sized and oriented to provide for safety of pedestrians,
bicyclists, etc.
B.
Storm Sewers.
1.
Design Criteria.
a.
Design Storm Frequency. The storm sewer system, beginning at
the upstream end with inlets, is required when allowable street depths
are exceeded. Allowable street depths are specified above.
c.
Vertical Alignment.
(1)
The sewer grade shall be such that a minimum cover
is maintained to withstand AASHTO HS-20 loading on the pipe. The minimum
cover depends upon the pipe size, type and class and soil bedding
condition, but shall not be less than one (1) foot from the top of
pipe to the finished grade at any point along the pipe. If the pipe
encroaches on the street subgrade, approval is required. Manholes
will be required whenever there is a change in size, direction, elevation
grade and slope or where there is a junction of two (2) or more sewers.
The maximum spacing between manholes for storm sewers [cross sectional
area less than twenty-five (25) square feet] shall be four hundred
(400) feet. For large storm sewers [cross sectional area greater than
twenty-five (25) square feet], manholes for maintenance access need
only be placed a minimum of every five hundred (500) feet; access
to the laterals can be obtained from within the larger storm sewer.
(2)
The minimum clearance between storm sewer and water
main (for new construction), either above or below shall be eighteen
(18) inches. For clearances less than eighteen (18) inches, the waterline
shall be constructed in accordance with Section 8.7 of Missouri Department
of Natural Resources, Design Guide for Public Water Systems.
(3)
The minimum clearance between storm sewer and sanitary
sewer (for new construction), either above or below, shall be eighteen
(18) inches. In addition, when an existing sanitary sewer main lies
above a storm sewer or within eighteen (18) inches below, the sanitary
sewer shall have an impervious encasement or be constructed of structural
sewer pipe for a minimum of ten (10) feet on each side of the storm
sewer crossing.
(4)
Siphons or inverted siphons are not allowed in
the storm sewer system.
d.
Horizontal Alignment.
(1)
Storm sewer alignment between manholes shall be
straight except when approved by the City. Approved curvilinear storm
sewers may be constructed by using radius pipe. The radius requirement
for pipe bends is dependent upon the manufacturer's specifications.
(2)
A minimum horizontal clearance of ten (10) feet
is required between the outside diameter of water utilities and the
outside diameter of storm sewer.
(3)
The permitted locations for storm sewer within
a street right-of-way (ROW) are behind the curb. Storm sewer shall
not be placed within pavement except where pipe crosses a roadway.
e.
Pipe Size. For storm sewers less than fifty (50) feet in length,
the minimum allowable diameter is fifteen (15) inches. All other pipe
shall have a minimum diameter of eighteen (18) inches.
f.
Storm Sewer Capacity And Velocity.
(1)
Storm sewers should be designed to meet the required
street spread without surcharging the storm sewer.
(2)
The maximum full flow velocity shall be less than
fifteen (15) fps. Higher velocities may be approved by the City if
the design includes adequate provisions for uplift forces, dynamic
impact forces and abrasion. The minimum velocity in a pipe based on
full flow shall be two and one-half (2.5) feet per second (fps) and
the minimum slope shall be one-half percent (0.50%) to avoid excessive
accumulations of sediment. The energy grade line (EGL) for the design
flow shall be no more than six (6) inches below the final grade at
manholes, inlets or other junctions. To ensure that this objective
is achieved, the hydraulic grade line (HGL) and the energy grade line
(EGL) shall be calculated by accounting for pipe friction losses and
pipe form losses. Total hydraulic losses will include friction, expansion,
contraction, bend, manhole and junction losses. The methods for estimating
these losses are presented in the following Sections.
g.
Storm Sewer Outlets. All storm sewer outlets into open channels
shall be constructed with a headwall and wingwalls or a flared-end
section. Riprap or other approved material shall be provided on all
outlets.
2.
Easements. Easements shall be provided for all storm sewers
constructed in the City of Bolivar that are not located within public
rights-of-way. The minimum easement widths are as follows:
a.
For pipes forty-eight (48) inches or less in diameter or width,
the required easement width is fifteen (15) feet.
b.
For pipes and boxes greater than forty-eight (48) inches in
width, the required easement width is fifteen (15) feet plus one-half
(1/2) the width of the proposed storm sewer.
c.
Storm sewers greater than eight (8) feet in depth to the flow
line require additional easement width at a rate of two (2) feet in
width for every vertical foot greater than eight (8) feet.
d.
All easements required for construction which are not included
on the final plat shall be recorded and filed with the City prior
to approval of the construction drawings.
C.
Design Standards For Culverts.
1.
Structural Design. All culverts shall be designed to withstand
an HS-20 loading in accordance with the design procedures of AASHTO
"Standard Specifications for Highway Bridges." The designer shall
also check the construction loads and utilize the most severe loading
condition. The minimum allowable cover is one (1) foot.
2.
Design Capacity. Culverts shall be designed to pass a 25-year
storm with one (1) foot of freeboard prior to overtopping the road
or driveway.
3.
Headwater. The maximum headwater for the major storm design
flow shall be one and one-half (1.5) times the culvert diameter for
round culverts or one and one-half (1.5) times the culvert rise dimension
for shapes other than round.
4.
Inlet And Outlet Protection. For road and driveway culverts
larger than fifteen (15) inches, culverts are to be designed with
protection at the inlet and outlet areas. Headwalls or end sections
are to be located a sufficient distance from the edge of the shoulder
or the back of walk to allow for a maximum slope of 3H:1V to the back
of the structure. The type of outlet protection required is as follows:
|
V<7FPS
|
7FPS<V<15FPS
|
|---|---|
|
Minimum riprap protection
|
Riprap protection or energy dissipater
|
5.
Velocity Limitations. The maximum allowable discharge velocity
is fifteen (15) feet per second.
6.
Culvert Hydraulics. It is recommended that the procedures outlined
in the publication "FHA Hydraulic Design of Highway Culverts," Hydraulic
Design Series No. 5, 1985, be used for the hydraulic design of culverts.
Backwater calculations demonstrating the backwater effects of the
culvert may be required.
D.
Design Standards For Bridges.
1.
Structural Design. All bridges shall be designed to withstand
an HS-20 loading in accordance with the design procedures of AASHTO
"Standard Specifications for Highway Bridges." The designer shall
also check the construction loads and utilize the most severe loading
condition.
2.
Design Capacity. Bridges shall be designed to pass the one hundred
100-year storm with one (1) foot of freeboard between the water surface
and the bridge low chord.
3.
Backwater. "Backwater" is defined as the rise in the water surface
due to the constriction created by the bridge approach road fills.
The maximum backwater for the 100-year storm design flow shall be
one (1) foot.
4.
Velocity Limitations. Discharge velocities through bridge openings
shall be limited to fifteen (15) feet per second. Abutment and channel
scour protection shall be provided at all bridges.
5.
Bridge Hydraulics. All bridge hydraulics shall be evaluated
using the procedures presented the publication "FHA Hydraulics of
Bridge Waterways." Backwater calculations demonstrating the effects
of the bridge and approach fills compared to the existing flood stages
shall be submitted for all bridges.
E.
Design Standards For Open Channels.
1.
General Design Guidelines.
a.
Natural Channels. The hydraulic properties of natural channels
vary along the channel reach and can be either controlled to the extent
desired or altered to meet the given requirements. Natural channels
used as part of the drainage system must be evaluated for the effects
of increased peak flow, flow duration and volume of runoff due to
urbanization.
b.
Grass-Lined Channels. Grass-lined channels are the most desirable
of the artificial channels. The channel storage, lower velocities,
and the greenbelt multiple use benefits obtained create significant
advantages over other artificial channels. Unless existing development
restricts the availability of right-of-way, channels lined with grass
should be given preference over other artificial types. The minimum
slope in a grass-lined channel shall be one percent (1.0%) unless
a concrete low-flow channel is installed.
c.
Concrete-Lined Channels. Concrete-lined channels arc sometimes
required where right-of-way restrictions within existing development
prohibit grass-lined channels. The lining must be designed to withstand
the various forces and actions, which tend to overtop the bank, deteriorate
the lining, erode the soil beneath the lining, and erode unlined areas.
The minimum slope in a concrete-lined channel shall be one-half percent
(0.50%).
d.
Rock-Lined Channels. Rock-lined channels are constructed from
ordinary riprap or wire-enclosed riprap (gabions, etc.). The rock
lining permits higher design velocity than for grass-lined channels.
Rock linings will normally be used only for erosion control at culvert/storm
sewer outlets, at sharp channel bends, at channel confluences, and
at locally steepened channel sections.
e.
Other Lining Types. The use of fabrics and other synthetic materials
for channel linings has increased over the past several years. Proposed
improvements of this type will be reviewed on an individual basis
as for applicability and performance.
2.
Hydraulics. An open channel is a conduit in which water flows
with a free surface. The calculations for uniform and gradually varied
flow are relatively straightforward and are based upon similar assumptions
(e.g., parallel streamlines). The basic equations and computational
procedures are presented in this Section.
a.
Uniform Flow. Open channel flow is said to be uniform if the
depth of flow is the same at every section of the channel. For a given
channel geometry, roughness, discharge and slope, there is only one
(1) possible depth, the normal depth. For a channel of uniform cross
section, the water surface will be parallel to the channel bottom
for uniform flow.
b.
The computation of normal depth for uniform flow shall be based
upon Manning's formula as follows:
|
Q = (1.49/n) AR2/3S1/2
| |||
|
Where:
|
Q
|
=
|
Discharge in cubic feet per second (cfs)
|
|
n
|
=
|
Roughness coefficient (Table I)
| |
|
A
|
=
|
Cross sectional flow area in square feet
| |
|
R
|
=
|
Hydraulic radius, A/P, in feet
| |
|
P
|
=
|
Wetted perimeter in feet
| |
|
S
|
=
|
Slope of the energy grade line (EGL) in feet/foot
| |
|
For channels with a uniform cross section the EGL slope and
the bottom slope are assumed to be the same.
|
c.
Critical Flow. The design of earth or rock channels in the critical
flow regime (Froude numbers from 0.9 to 1.2) is not permitted. The
Froude number is defined as follows:
|
F = V/(gD)0.5
| |||
|
Where:
|
F
|
=
|
Froude number
|
|
V
|
=
|
Velocity in feet per second (fps)
| |
|
g
|
=
|
Acceleration of gravity, 32.2 ft/sec2
| |
|
D
|
=
|
Hydraulic depth in feet = A/T
| |
|
A
|
=
|
Cross sectional flow area in square feet
| |
|
T
|
=
|
Top width of flow area in feet
| |
|
The Froude number shall be calculated for the design of all
open channels.
|
d.
Gradually Varied Flow.
(1)
The most common occurrence of gradually varied
flow in storm drainage is the backwater created by culverts, storm
sewer inlets or channel constrictions. For these conditions the flow
depth will be greater than normal depth in the channel, and the water
surface profile must be computed using backwater techniques.
(2)
Backwater computations can be made using the methods
presented in "Open Channel Hydraulics" (V.T. Chow, 1959). Many computer
programs are available for computation of backwater curves. The most
widely used program is HecRas, Water Surface Profiles, developed by
the U.S. Army Corps of Engineers (Hydraulic Reference Manual, Version
4.1, 2010) and is the program recommended for backwater profile computations.
Another program by the Federal Highway Administration is WSPRO and
is acceptable for use in backwater computations.
3.
Design Standards.
a.
Flow Velocity. Maximum flow velocities shall not exceed the
following:
|
Channel Type
|
Max. Velocity
(fps)
|
|---|---|
|
Grass-lined*
|
5
|
|
Concrete
|
15
|
|
Rock-lined
|
10
|
|
*
|
Refer to Subsection (E)(3)(f) below.
|
b.
Maximum Depth. The maximum allowable channel depth of flow is
three (3) feet for the design flow.
c.
Freeboard Requirements.
(1)
"Freeboard" is defined as the vertical distance
between the computed water surface elevation for the design flow and
the minimum top of bank elevation for a given cross section.
(2)
For all channels one (1) foot minimum of freeboard
is required.
(3)
Freeboard shall be in addition to super elevation.
d.
Curvature. The minimum channel centerline radius shall be three
(3) times the top width of the design flow.
e.
Super Elevation. Super elevation shall be calculated for all
curves. An approximation of the super elevation h may be calculated
from the following formula:
|
H = V2T/(gr)
| |||
|
Where:
|
h
|
=
|
Super elevation in feet
|
|
V
|
=
|
Velocity in fps
| |
|
T
|
=
|
Top width of flow area in feet
| |
|
G
|
=
|
Acceleration of gravity, 32.2 ft/sec2
| |
|
r
|
=
|
Radius of curvature in feet
| |
|
Freeboard shall be measured above the super elevated water surface.
|
f.
Grass Channels.
(1)
Side slopes shall be three (3) (horizontal) to
one (1) (vertical) or flatter. Steeper slopes may be used subject
to additional erosion protection and approval from the City.
(2)
For design discharges greater than fifty (50) cubic
feet per second (cfs), grade checks shall be provided at a maximum
of two hundred (200) feet horizontal spacing.
(3)
Channel drops shall be provided as necessary to
control the design velocities within acceptable limits.
(4)
Vertical drops may be used up to three (3) feet
in height. Drops greater than three (3) feet shall be baffled chutes
or similar structures.
(5)
The variation of "Manning's n" coefficient with
the retardance and the product of mean velocity and hydraulic radius
as shown in Figure 7.23 in "Open Channel Hydraulics" by Richard French
shall be used in the capacity calculations. Retardance curve C shall
be used to determine the channel capacity, and retardance curve D
shall be used to determine the velocity
4.
Easements.
a.
Easements shall be provided for all open channels constructed
in the City of Bolivar that are not located within public rights-of-way.
The minimum easement width for open channels is the flow width inundated
by a 100-year event plus fifteen (15) feet.
b.
All easements required for construction, which are not included
on the final plat shall be recorded and filed with the City prior
to approval of the construction drawings.
[Ord. No. 3844, 3-28-2023]
A.
Purpose.
1.
The primary goal of the City of Bolivar stormwater management
program is to mitigate flood damage to residential, commercial, and
public property.
B.
Methods Of Analysis. Detailed analysis will be required for the design
of detention facilities, including, but not limited to:
1.
In areas where residences or other structures located downstream
of a development can be shown to have an imminent flooding hazard.
2.
Residences or other structures will be defined as having an
imminent flooding hazard when the lowest point at which surface runoff
may gain entry is located at or below the estimated flooding level
which would result from a storm with an annual probability of one
percent (1%) or greater under conditions existing in the basin prior
to development of the applicant's property (i.e., affected by the
"100-year" storm).
3.
Consideration of downstream flooding problems will be limited
to the area which may reasonably be expected to be significantly affected
by runoff from the applicant's property.
C.
Alternatives To Detention.
1.
Detention Facilities: Provision to provide detention may be
waived in part or in whole, provided one (1) or more of the following
are met:
a.
Development is to discharge within a Federal Insurance Study
defined one percent (1%) AP floodplain.
b.
Developer provides downstream improvements to meet the tributary
area peak discharge requirements to the satisfaction of the City Engineer.
c.
Development cannot be fully and/or practically served by surface
or underground detention facilities.
d.
It is shown that construction of detention facilities will result
in an increase of peak flow in the drainageway.
2.
Fee In Lieu Of Detention. A fee in lieu of detention is not
automatic and will be considered on a case-by-case basis. If detention
requirements are waived for one (1) of the above reasons, a fee in
lieu of detention will be considered.
a.
If detention requirement is waived for one (1) on the above
reasons, a fee in accordance with the following schedule will be applied.
|
Volume of Detention
(c.f.)
|
Payment Rate
(per c.f.)
|
|---|---|
|
0 to 24,000
|
$3.50
|
|
Greater than 24,000
|
$2.50
|
b.
The fee may be offset by the construction of downstream improvements.
The downstream improvements must be approved by the City. Cost of
the downstream improvements will be determined by the City Engineer.
3.
For the following development, detention is not required:
a.
Additions to, improvements, and repair of existing single-family
and duplex dwellings.
b.
Construction of any buildings, structures, and/or appurtenant
service roads, drives, and walks on a site having previously provided
stormwater control as part of a larger unit of development.
c.
Additions, remodeling, repair, replacement, and improvements
to any existing structure or facility and appurtenances that do not
cause an increased area of impervious surface on the site in excess
of one thousand (1,000) square feet of that previously existing.
d.
Construction of any one (1) new single-family or duplex dwelling
unit, irrespective of the site area on which the same may be situated.
D.
Innovation In Design.
1.
It is the desire of the City that detention facilities be designed
and constructed in a manner to enhance aesthetic and environmental
quality of the City as much as possible.
2.
The City of Bolivar therefore encourages designs which utilize
and enhance natural settings and minimize disturbance and destruction
of wooded areas, natural channels, and wetlands.
F.
Design Criteria.
1.
General.
a.
Detention facilities shall discharge into a drainage easement
or public right-of-way.
b.
One (1) foot of freeboard shall be provided between the maximum
water surface elevation from a one percent (1%) annual probability
event and the minimum top of berm or wall elevation.
c.
Earthen embankment slopes steeper than three (3) horizontals
to one (1) vertical (3H:1V) are not permitted.
d.
In certain instances, such as when the existing development
conditions runoff from a watershed would exceed the capacity of the
existing downstream facilities, detention basins (i.e., with a release
rate at the capacity of the downstream facilities) for the stormwater
runoff may be required by the City.
e.
Dry detention basins shall maintain a minimum bottom slope of
two (2) feet per hundred (100) feet [two percent (2%)] to drain across
grass. A minimum of one-half percent (0.5%) slope may be used if a
trickle channel is installed.
g.
The maximum allowable depth of ponding for parking lot detention
is twelve (12) inches, and its water surface elevation shall not exceed
the elevation of the minimum top of berm or wall.
h.
Parking lot detention may not inundate more than ten percent
(10%) of the total parking area.
i.
All parking lot detention areas shall have a minimum of two
(2) signs posted identifying the detention pond area. The signs shall
have a minimum area of one and one-half (1.5) square feet and contain
the following message:
|
WARNING:
|
|
This area is a stormwater detention pond and is subject
to periodic flooding to a depth of twelve (12) inches.
|
j.
The sign shall be reflective and have a minimum height of forty-eight
(48) inches from the bottom of the sign to the parking space finished
grade. Any suitable materials and geometry of the sign are permissible,
subject to approval by the City.
2.
Detailed Analysis.
a.
Detailed analysis shall be performed using hydrograph methodologies
and reservoir routing techniques.
b.
The most common techniques are those developed by the Corps
of Engineers and the Natural Resources Conservation Service [NRCS,
formerly the Soil Conservation Service (SCS)]. These methods are preferred;
however, other proven techniques will be accepted.
c.
Detention basins designed by detailed methods shall be designed
on the basis of multiple storm recurrence frequencies to ensure that
they function properly for both frequent storms and large infrequent
storms.
d.
A minimum of three (3) recurrence frequencies, the fifty percent
(50%), ten percent (10%), twenty-five percent (25%), and one percent
(1%) annual probability storms (the "2-year, 10-year, 25-year and
100-year" storms) must be considered.
e.
The runoff model must include the entire drainage basin upstream
of the proposed detention pond. The model shall be prepared in sufficient
detail to ensure that peak runoff rates are reasonably accurate.
f.
The runoff model shall be developed for the following cases:
(1)
Case 1: Existing conditions in the drainage basin
prior to development of the applicant's property.
(2)
Case 2: Existing conditions in the drainage basin
with developed conditions on the applicant's property.
(3)
Cases 1 and 2 are utilized to determine the required
detention volume and the type of outlet structure to be provided and
shall be analyzed for the three (3) storm recurrence frequencies required
above.
(4)
The detention facility shall be designed such that
peak outflow rates from the facility for Case 2 are no greater than
the rates determined in Case 1 for each of the storm recurrence frequencies
required above.
(5)
The storage volume provided shall not be less than
the difference in total runoff volume between Case 1 and Case 2.
(6)
The overflow spillway shall be sized for the one
percent (1%) annual probability (100-year) event, assuming the primary
spillway is clogged.
(7)
The overflow spillway will, in most cases, be combined
with the outlet structure.
3.
Submittals. The following information must be submitted for
detention ponds designed by detailed methods:
a.
Information regarding analytical methods and software to be
used, including:
b.
Map(s) showing sub-basin delineation, topography, presumed flow
routes and pertinent points of interest; soil types; existing basin
development conditions used in the model; fully developed conditions
used in the model.
c.
Routing diagram for the runoff model.
d.
A summary of sub-basin characteristics used for program input.
e.
Stage-area or stage-storage characteristics for the basin in
tabular or graphic form.
f.
Stage-discharge characteristics for the outlet structure and
overflow spillway in tabular or graphic form; hydraulic data for weirs,
orifices, and other components of the control structure.
g.
A printout of the input data file.
h.
A summary printout of program output, including plots of hydrographs.
(These are intended to be the printer plots generated by the software.)
4.
Control Structures.
a.
Detention facilities designed by the simplified analysis shall
be provided with obvious and effective outlet control structures.
These outlet structures may include v-notch weirs or rectangular weirs,
as well as pipe. Plan view and sections of the structure with adequate
detail shall be included in plans.
b.
Sizing of a low-flow pipe shall be supported with calculations
shown for the pipe rating curve and tailwater considerations.
c.
Low-flow pipes shall not be smaller than four (4) inches in
diameter to minimize maintenance and operating problems, except in
parking lot and roof detention where minimum size and configuration
of opening shall be designed specifically for each condition.
d.
Overflow spillways will be required on all detention facilities.
[Ord. No. 3801, 11-15-2022]
A.
Goals And Objectives. The goal of the regulation is to effectively
minimize erosion and discharge of sediment by application of relatively
simple and cost-effective best management practices. Detention basin(s)
should be constructed at the beginning of the project to maintain
the goals. The following goals shall be met:
1.
Minimize the area disturbed by construction at any given time.
2.
Stabilize disturbed areas as soon as possible by re-establishing
sod, other forms of landscaping and completing proposed structures,
pavements and storm drainage systems.
3.
Provide for containment of sediment until areas are stabilized.
4.
Provide permanent erosion controls.
B.
General Design Guidelines. The following items must be considered
in preparing a sediment and erosion control plan:
1.
Temporary Versus Permanent Controls.
a.
The greatest potential for soil erosion occurs during construction.
Temporary controls are those that are provided for the purpose of
controlling erosion and containing sediment until construction is
complete.
b.
Temporary controls include straw or hay bale dikes, silt fences,
erosion control blankets, etc., which are not needed after the area
is stabilized.
c.
Permanent controls consist of riprap, concrete trickle channels,
detention basins, etc., which will remain in place through the life
of the development.
d.
It is possible for the same facility to serve both a temporary
and permanent purpose. The difference between temporary and permanent
erosion control should be clearly recognized in preparing a sediment
and erosion control plan.
2.
Sheet Flow Versus Concentrated Flow.
a.
In areas where runoff occurs primarily as sheet flow, containment
of sediment is relatively simple. In these areas, straw or hay bales,
silt fences and vegetative filter areas can be very effective.
b.
Where concentrations of flow occur containment of sediment becomes
more difficult as the rate and volume of flow increase. In these areas
more sophisticated controls such as sedimentation basins must be provided.
3.
Slope. Control of erosion becomes progressively more difficult
as the slope of the ground increases. Areas with steeply sloping topography
and cut and fill slopes must be given special consideration.
4.
Soils And Geologic Setting. Area soils and the geologic setting
must be considered in preparing the plan and any special considerations
deemed necessary for a particular site provided.
5.
Environmentally Sensitive Areas. Where construction occurs within
the vicinity of permanent streams, springs, sinkholes, lakes or wetlands,
special attention must be given to preventing discharge of sediment.
C.
Grading Permits.
1.
Permit Requirements. Grading permits are required for all construction
sites with the following exceptions:
a.
Grading for single-family or duplex residences constructed in
subdivisions where approved sediment and erosion controls have been
constructed.
b.
Emergency construction required repairing or replacing roads,
utilities or other items affecting the general safety and well-being
of the public.
c.
For emergency construction sites which would otherwise be required
to obtain a permit and for which remedial construction will take more
than fourteen (14) calendar days, application for the permit must
be made within three (3) calendar days from the start of construction.
d.
The following activities, provided that they are not located
within twenty-five (25) feet of a spring, sinkhole, wetland or watercourse:
(1)
Gardening or landscaping normally associated with
single-family residences that cover less than one-half (1/2) acre.
(2)
Grading and repair of existing roads or driveways.
(3)
Cleaning and routine maintenance of roadside ditches
or utilities.
(4)
Utility construction where the actual trench width
is two (2) feet or less.
2.
Permit Procedure. The following items must be received prior
to issuance of a grading permit:
a.
An approved grading, sediment and erosion control plan. The
submittal and approval procedure is as follows for subdivisions, commercial
and other sites.
3.
Plan Requirements. Plans must be prepared by and bear the seal
of an engineer registered to practice in the State of Missouri. Plans
will not be required in the following cases:
a.
Grading associated solely with a single-family residence.
b.
Grading or filling of less than one (1) acre (of overall plan
development) if located outside of the allowable building areas and
not located within twenty-five (25) feet of spring, sinkhole, wetland
or watercourse. In these instances, a grading permit can be issued,
providing an inspection of the site by a representative of the City
does not reveal conditions that would warrant preparation of a detailed
plan.
D.
Other Permits.
1.
NPDES Storm Water Permit. Effective October 1, 1992, construction
sites where the area to be disturbed is five (5) acres or more must
apply for a stormwater discharge permit from the Missouri Department
of Natural Resources. Permit requirements are set forth in 10 CSR
20-6.200 of the Missouri Clean Water Laws.
2.
"404" Permit. Grading activities in streams or wetlands may
require a Department of the Army permit under Section 404 of the Clean
Water Act.
E.
Design Standards And Criteria.
1.
Grading.
a.
Maximum Grades. Cut or fill slopes (excluding detention basins)
shall not exceed four (4) to one (1).
b.
Maximum Height. Cut or fill slopes shall not exceed fifteen
(15) feet in vertical height unless a horizontal bench area at least
five (5) feet in width is provided for each fifteen (15) feet in vertical
height.
c.
Minimum Slope. Slope in grassed areas (excluding detention basins)
shall not be less than one percent (1%).
d.
Construction Specifications. Construction for streets must comply
with specifications set forth by the City of Bolivar. For all other
areas, construction specifications stating requirements for stripping,
materials, subgrade compaction, placement of fills, moisture and density
control, preparation and maintenance of subgrade must be included
or referenced on the plans or accompanying specifications submitted.
e.
Spoil Areas.
(1)
Broken concrete, asphalt and other spoil materials
may not be buried in fills within proposed building or pavement areas.
(2)
Outside of proposed building and pavement areas,
broken concrete or stone may be buried in fills, provided it is covered
by a minimum of two (2) feet of earth.
(3)
Burying of other materials in fills is prohibited.
f.
Stockpile Areas. Location of proposed stockpile areas shall
be outlined on the plans and specifications for proper drainage included.
g.
Borrow Areas. The proposed limits of temporary borrow areas
shall be outlined in the plans and a proposed operating plan described
on the grading plan. Temporary slopes in borrow areas may exceed the
maximums set forth above. At the time that borrow operations are completed,
the area shall be graded in accordance with the criteria set forth
above and reseeded.
2.
Sediment Containment.
a.
Existing Vegetative Filter Area. Existing vegetative filter
areas may be used where:
(1)
Unconcentrated sheet flow occurs.
(2)
An area of existing vegetation a minimum of twenty-five
(25) feet in width can be maintained between the area to be graded
and a property line, watercourse, sinkhole, spring, wetland, or classified
lake.
(3)
Existing ground slope is no greater than five (5)
to one (1) [twenty percent (20%)].
(4)
The existing vegetative growth is of sufficient
density and in sufficiently good condition to provide for filtration
of sediment.
(5)
Vegetative filter areas are a temporary and permanent
practice.
b.
Hay/Straw Bale Dike Or Silt Fence. Containment areas constructed
of hay or straw bales or silt fence may be provided in areas where:
(1)
Unconcentrated sheet flow occurs;
(2)
An area of existing vegetation a minimum of twenty-five
(25) feet in width cannot be maintained between the area to be graded
and a property line, watercourse, sinkhole, spring, wetland or classified
lake;
(3)
Existing ground slope is no greater than five (5)
to one (1) [twenty percent (20%)];
(4)
Concentrated flow from an area no greater than
one (1) acre occurs and a minimum volume of one thousand (1,000) cubic
feet per acre is contained behind the dike. Either cereal grain straw
or hay may be used for bale dikes. Straw/hay bale dikes shall be constructed
with a minimum of six (6) inches of backfill on the upstream side
and a minimum of four (4) inches of cut (embedment) below existing
grade on the downstream side. Straw/hay bale dikes and silt fences
are temporary practices.
c.
Temporary Containment Berms.
(1)
Temporary containment berms may be provided for
areas where concentrated flow from areas greater than one (1) acre
and less than five (5) acres occurs. Temporary containment berms must
contain a volume of one thousand (1,000) cubic feet per acre of drainage
area.
(2)
Temporary containment berms shall have a riprap
outlet with a sediment filter. Details shall be included in the Stormwater
Pollution and Prevention Plan (SWPPP).
(3)
Temporary containment berms and accumulated sediment
may be completely removed after the tributary area is stabilized and
must be removed prior to final acceptance and release of escrow.
d.
Sediment Basin.
(1)
Sediment basins shall be provided for all areas
where concentrated flow occurs from an area of five (5) or more acres.
Sediment basins shall be designed to detain the runoff from one (1)
inch of rainfall for a period of at least twenty-four (24) hours.
Runoff shall be calculated using the methods contained in Chapter
2 of TR-55 (Reference 11), using the recommended curve number for
newly graded areas from Table 2-2a.
Note: For construction sites in Bolivar, an average value of
runoff volume from one (1) inch of rainfall is approximately one thousand
two hundred (1,200) cubic feet per acre, using a Curve Number of 90,
as indicative of a mixture of Type B and C soils. This value may be
used in sizing sediment basins or the runoff volume determined using
the values from Figure 2-1 of TR-55.
(2)
Sediment basins shall be provided with an outflow
structure consisting of:
(a)
A flow restriction device which provides for the
required detention time;
(b)
An outfall pipe sized to carry the maximum estimated
outflow rate;
(c)
Protective structures at the pipe outlet to prevent
crushing or damage of the end of the pipe;
(d)
Protective structures to prevent blockage of the
pipe with debris;
(e)
Erosion protection at the pipe outlet.
(3)
An overflow spillway capable of discharging the
peak flow rate for the four percent (4%) annual probability (25-year)
storm while maintaining a minimum freeboard of one (1) foot.
(4)
Overflow spillways may be sodded where the depth
of flow at the crest is limited to no greater than six (6) inches
and outlet channel velocities do not exceed five (5) feet per second
for the minor (5-year) storm.
(5)
Overflow spillways not meeting these restrictions
must be constructed of riprap, concrete or other approved, non-erodible
material.
3.
Erosion Protection.
a.
Seeding And Mulching.
(1)
Permanent Seeding. Permanent seeding, fertilizer
and mulch shall be applied at the rates as follows or according to
other specifications, which are approved with the grading permit.
|
Common Name
|
Rate of Pure Live Seed
(pounds per acre)
|
|---|---|
|
Kentucky 31 fescue
|
100
|
|
Rye grass
|
25
|
|
Kentucky blue grass
|
15
|
|
Creeping red fescue
|
10
|
|
150 pounds
|
|
Commercial fertilizer containing not less than 1.0 lb of actual
nitrogen per one thousand (1,000) square feet of area seeded. Provide
nitrogen in form that will be available to the lawn during initial
period of growth.
|
(2)
Permanent seeding seasons are from March 1 to June
15 and September 1 to October 30.
(3)
Mulching. Where slopes are less than four (4) to
one (1), cereal grain mulch is required at the rate of one hundred
(100) pounds per one thousand (1,000) square feet [four thousand five
hundred (4,500) pounds per acre]. Cereal grain mulch shall meet the
requirements of Section 802 of the MHTD State Specifications for Type
1 mulch.
(4)
Where slopes are four (4) to one (1) or greater
Type 3 mulch ("hydro mulch") meeting the requirements of Section 802
of the MHTD State Specifications shall be used.
(5)
Temporary Seeding. Whenever grading operations
are suspended for more than thirty (30) calendar days between permanent
grass or seeding periods, all disturbed areas must be seeded. Temporary
seeding season runs from May 15 to November 15.
(6)
Overseeding. During the winter season (November
15 to March 1) temporary seed and mulch shall be placed in on all
completed areas or areas where grading is suspended for more than
thirty (30) calendar days. During this period seed, mulch and soil
amendments shall be applied at the following rates:
Fertilizer: seventy-five percent (75%) of specified quantity.
Seed: fifty percent (50%) of specified quantity.
Mulch: one hundred percent (100%) of specified quantity.
Areas seeded during this period shall be reseeded and mulched
during the next permanent seeding season according to seeding requirements.
(7)
Maintenance. Seeded areas must be maintained for
one (1) year following permanent seeding.
b.
Cut And Fill Slopes. Cut and fill slopes shall be protected
from erosion by construction of straw bale dikes, silt fences, diversion
berms, or swales along the top of the slope.
(1)
Where drainage must be carried down the slopes,
pipe drains, concrete flumes, riprap chutes or other impervious areas
must be provided. Suitable erosion control measures such as riprap
stilling basins must be provided at the bottom of the slope.
(2)
Diversions shall be maintained until permanent
growth is firmly established on the slopes.
c.
Channels And Swales. Permanent channels and swales shall be
provided with a stabilized invert consisting of one (1) of the following
materials:
(1)
Sod. Where the average velocity of flow is five
(5) feet per second or less and there is no base flow, the channel
shall be lined with sod.
(a)
For channels with a bottom width less than fifteen
(15) feet, sod shall extend up the side slope to a minimum height
of six (6) inches above the toe.
(b)
Channels with a bottom width of fifteen (15) feet
or greater, the center fifteen (15) feet shall be lined with sod and
the remainder of the outer area seeded and mulched.
(2)
Erosion Control Blanket. Commercial erosion control
blankets may be used in lieu of sod, provided that samples are submitted
and approved by the City. The guaranteed maintenance period shall
be one (1) year.
(3)
Non-Erosive Lining. In grass channels where base
flow occurs, a nonerosive low-flow channel of riprap or concrete must
be provided. Low-flow channels shall have a minimum capacity of five
(5) cubic feet per second. Other suitable non-erosive materials may
be specified with approval of the City.
(4)
For channels which have an average velocity of
five (5) feet per second or greater a non-erosive lining of riprap
concrete or other approved material must be provided.
d.
Storm Sewer And Culvert Outlets. Erosion protection shall be
provided at storm sewer and culvert outlets. Minimum erosion protection
shall consist of a concrete toe wall and non-erosive lining, meeting
the City's specifications for public improvements.
(1)
The required length of non-erosive lining will
not be decreased where flared end sections or headwalls are provided
unless calculations and data to support the decrease in length are
submitted and approved.
(2)
Non-erosive lining shall consist of riprap unless
otherwise specified and approved. Field stone, gabions or riprap shall
extend to the point at which average channel velocity for the peak
flow rate from the minor (5-year) storm has decreased to five (5)
feet per second maximum.
e.
Curb Openings. Where drainage has been approved by the City
to flow from paved areas to grass areas through curb openings erosion
protection shall be provided with a minimum three-foot concrete apron
that is recessed three (3) inches below pavement height.
f.
Ditch Checks And Drop Structures. In grass channels grades and
velocities may be controlled by use of ditch checks and drop structures.
Riprap ditch checks may be required in natural channels where average
velocity for the peak flow rate from the 5-year storm exceeds five
(5) feet per second for post-development conditions.
g.
Spillways. Erosion protection must be provided at spillways
and outlet structures for detention ponds. Erosion protection shall
extend to the point where flow has stabilized and average velocity
in the outlet channel is five (5) feet per second or less.
4.
Temporary Construction Entrance.
a.
A minimum of one (1) temporary construction entrance is required
at each site. Additional temporary entrances may be provided if approved.
The location of each construction entrance shall be shown on the plan.
b.
Only construction entrances designated on the sediment and erosion
control plan may be used. Barricades shall be maintained if necessary
to prevent access at other points until construction is complete.
c.
Construction entrances shall be constructed of crushed limestone
meeting the following specifications:
(1)
Construction entrances shall be a minimum of twenty-five
(25) feet wide and fifty (50) feet long.
(2)
Minimum thickness of crushed limestone surface
shall be six (6) inches. Additional two (2) inch lifts of crushed
limestone shall be added at the discretion of the County if the surface
of the initial drive deteriorates or becomes too muddy to be effective.
(3)
In locations where an existing drive or street
extends at least fifty (50) feet into the site, the existing drive
may be designated as the construction entrance and construction of
a new gravel entrance is not required, unless job conditions warrant
as set forth in the preceding paragraph.
5.
Cleaning Streets. Streets both interior and adjacent to the
site shall be completely cleaned of sediment at the end of construction
and prior to release of security.
6.
Dust Control. The contractor will be required to use water trucks
to water all roads and construction areas to minimize dust leaving
the site when conditions warrant.
7.
Sequencing And Scheduling. Costs of sediment and erosion control
can be minimized if proper consideration is given to sequencing and
scheduling construction. Any special sequencing and scheduling considerations
should be noted in the grading plan. A detailed schedule must be received
from the contractor at the pre-construction conference.
A.
Introduction. This Section covers the design of best management practices
(BMPs) to minimize the adverse effects of urban stormwater runoff
on the quality of receiving waters.
1.
It is recognized that specific water quality standards, other
than those contained in the Missouri Clean Water Laws, have not been
developed or adopted for these receiving waters. The objective of
this policy is not to meet specific reductions of targeted pollutants,
but rather to provide a generally effective level of pollutant removal
by using reasonable, cost effective measures. The goal is to minimize,
to the maximum extent practical, adverse impacts on the quality of
the receiving waters.
2.
It is important to recognize that the structural best management
practices (BMPs) for which design guidance is given in this Section
represent only one (1) aspect of stormwater quality management. The
most effective means of managing stormwater quality lie in overall
watershed planning and zoning controls, and other nonstructural practices
which are generally beyond the control of an individual development.
3.
Data from communities across the country has shown that, as
the total impervious area in a watershed exceeds ten percent (10%)
to fifteen percent (15%), water quality declines unless mitigative
measures are taken. The most important management tool is to limit
the impervious area in these watersheds to these values. While these
limits may be attainable for the watershed as a whole, they may not
be possible for individual development or sub-basins. Structural BMP's
will be required for these developments.
B.
General Design Guidelines.
1.
Minimize The Amount Of Runoff. The total quantity of pollutants
transported to receiving waters can be minimized most effectively
by minimizing the amount of runoff. Both the quantity of runoff and
the amount of pollutant wash-off can be minimized by reducing the
amount of directly connected impervious area (DCIA). Impervious areas
are considered connected when runoff travels directly from roofs,
drives, pavement, and other impervious areas to street gutters, closed
storm drains or concrete, or other impervious lined channels. Impervious
areas are considered disconnected when runoff passes as sheet flow
over grass areas, or through properly designed BMPs, prior to discharge
from the site.
2.
Maximize Contact With Grass And Soil. The opportunity for pollutants
to settle out is maximized by providing maximum contact with grass
and soil. Directing runoff over vegetative filter strips and grass
swales enhances settling of pollutants as the velocity of flow is
reduced. Infiltration of runoff into the soil is also increased.
3.
Maximize Holding And Settling Time. According to ASCE (Reference
(F)(1) of this Section), the most effective runoff quality controls
reduce the runoff peak and volume. The next most effective controls
reduce peak runoff rates only. For small storms the runoff rate should
not exceed the pre-project peak flow rate from the fifty percent (50%)
AP (2-year) storm. Most obnoxious pollutants (exceptions include water
soluble nutrients and metals) can be settled out. By reducing the
rate of outflow and increasing the time of detention storage, settling
of pollutants and infiltration of runoff is maximized.
4.
Design For Small, Frequent Storms. Drainage systems for flood
control are designed for large, infrequent storm events. In contrast,
stormwater quality controls must be designed for small, frequent storm
events. Eighty percent (80%) to ninety percent (90%) of all twenty-four
(24) hour rainfalls are one (1) inch or less. Most pollutants are
washed off in the "first flush," generally considered the first one-half
(1/2) inch of runoff.
5.
Utilize BMPs In Series Where Possible. Performance monitoring
of BMPs in Florida, Maryland, and Delaware has shown that the combined
effect of providing several BMPs in a series can be much more effective
in reducing the level of pollutants than providing a single BMP at
the point of discharge. To the greatest extent practical, runoff should
be directed first to vegetative filter strips, then to grass swales
or channels, and then to extended detention basins, sand filters,
etc.
6.
Incorporate both flood control and water quality objectives
in designs, where practical. Incorporating both flood control and
water quality criteria into a single stormwater management facility
is not only possible but is encouraged. Whenever practical, combining
several objectives, such as water quality enhancement and flood control,
maximizes the cost-effectiveness of stormwater management facilities.
C.
Requirements. The following requirements will apply to all development.
1.
Stormwater runoff from any new development for which the total
impervious area exceeds ten percent (10%) of the total land area of
the development must be directed through an extended wet or dry detention
basin or other properly designed BMP prior to discharge from the site.
2.
Runoff from fueling areas and other areas having a high concentration
of pollutants will be required to be directed to a sand filter or
other properly designed BMP which provides filtration as well as settling.
3.
The required volume for capture and treatment shall be designed as the water quality capture volume (WQCV) and shall be determined as set forth in Subsection (D)(1) of this Section.
D.
Design Criteria.
1.
Water Quality Capture Volume:
a.
Water quality BMPs shall be designed to capture the runoff from
the one (1) inch 24-hour rainfall as well as to capture the first
flush of pollutants from directly connected impervious areas within
the proposed development.
b.
The required water quality capture volume (WQCV) to be used
in design of extended wet and dry detention basins and other BMPs
whose design is based upon capture and treatment of storm water, shall
be the greater of the following:
2.
Directly Connected Impervious Area:
a.
Impervious areas are considered connected when runoff travels
directly from roofs, drives, pavement, and other impervious areas
to street gutters, closed storm drains or concrete, or other impervious
lined channels.
b.
In order for an impervious area to be considered disconnected,
runoff from the area must pass through a vegetative filter strip or
other BMP meeting the requirements set forth in this Section.
c.
For determining the amount of impervious area, the following
assumptions shall apply in the absence of more detailed data:
(1)
Single-Family Lots:
(a)
Average roof area: two thousand five hundred (2,500)
square feet.
(b)
Average drive area: eight hundred (800) square
feet.
(c)
Average impervious area per lot: three thousand
five hundred (3,500) square feet.
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If gutter downspouts are directed to drain toward lawn areas,
seventy-five percent (75%) of the roof area shall be considered disconnected.
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(2)
Duplexes And Patio Homes:
(a)
Average roof area: two thousand five hundred (2,500)
square feet.
(b)
Average drive area: one thousand six hundred (1,600)
square feet.
(c)
Average impervious area per lot: four thousand
five hundred (4,500) square feet.
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If gutter downspouts are directed to drain toward lawn areas,
seventy-five percent (75%) of the roof area shall be considered disconnected.
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(3)
Multi-Family, Commercial And Other Areas:
The amount of impervious area contained in multi-family, commercial,
office, and manufacturing developments shall be determined based upon
the site plan for the development.
3.
Vegetative Filter Strips: Vegetative filter strips consist either
of areas of undisturbed vegetation in good condition, including trees,
grass, sod or other vegetative cover which meets the objectives for
this BMP, or areas where new vegetation has been established. Vegetative
filter strips shall be provided in areas of sheet flow only. The hydraulic
loading for filter strips shall not exceed 0.05 cubic feet per second
(cfs) per lineal foot of filter strip length for the fifty percent
(50%) AP (two (2) year) storm (equal to the runoff per unit width
from a four hundred (400) feet length of impervious area).
The minimum width of the filter strip shall not be less than
twenty percent (20%) of the length of the sheet flow from the upstream
impervious surface, and in no case shall be less than six (6) feet.
The slope along the width of the filter strip shall not exceed 4:1
twenty-five percent (25%).
Typical details for vegetative filter strips are shown in Figure
WQ1.
4.
Grass Swales: Grass swales may be provided to convey runoff
from vegetative filter strips and impervious areas to BMPs designed
for capture and temporary storage of runoff. Design criteria for grass
swales shall be as follows:
a.
Maximum side slopes: 4:1.
b.
Maximum longitudinal slope: five percent (5%).
c.
Minimum longitudinal slope: one percent (1%).
d.
Maximum velocity: Two (2) feet per second for peak flow from
the fifty percent (50%) AP (2-year) storm.
e.
Grass swales shall be lined with sod or seeded and covered with
suitable erosion control blanket and mulch.
f.
Typical details for grass swales are shown in Figure WQ2.
5.
Extended Dry Detention Basins: Extended dry detention basins
may be provided to capture and provide temporary storage for the required
water quality capture volume. Extended dry detention basins shall
be placed outside of the primary watercourses which allow off-site
flows to pass through the development (i.e., "off-line") where possible.
a.
Design criteria for extended dry detention basins shall be as
follows:
(1)
Volume: Minimum volume shall be one hundred twenty-five
percent (125%) of the required water quality capture volume (WQCV).
Detention basins for water quality may be combined with detention
basins for flood control. Effects of the WQCV may be considered in
the design for flood control.
(2)
Drain Time: The WQCV shall be released over a minimum
period of twenty-four (24) hours and a maximum period of seventy-two
(72) hours.
(3)
Outlet structure: Outlet structures shall consist
of a perforated riser pipe, outlet pipe and gravel filter material
as shown in Figures WQ3 and WQ4. The minimum allowable riser pipe
diameter is eight (8) inches. The riser pipe shall be connected to
an outlet pipe of equal of greater diameter. The outlet pipe shall
have adequate capacity to carry the maximum rate of flow from the
riser pipe. Material for the riser pipe shall be Schedule 40 PVC,
ductile iron, or corrugated, galvanized metal.
A removable cap shall be provided at the top of the riser pipe.
The cap shall have a one (1) inch diameter hole for air relief.
The outlet pipe shall be bedded in firmly compacted clay, free
of stones. For dams exceeding ten (10) feet in height, an anti-seep
collar shall be provided around the pipe.
Number of rows of perforations, number of perforations per row,
and diameter of perforations for the riser pipe shall be specified
on the plans. Perforation pattern shall be determined based upon orifice
calculations to provide for release of the WQCV over the specified
time. Perforations shall meet the following requirements:
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Minimum perforation diameter:
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1/4 inch
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Maximum perforation diameter:
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1 inch
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Minimum number of holes per row:
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4
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Maximum number of holes per row:
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8
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Minimum row spacing:
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4 inches
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Maximum row spacing:
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12 inches
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(4)
Freeboard: Where the basin is to be utilized as
a water quality BMP only, twelve (12) inches minimum freeboard shall
be provided above the WCQV.
(5)
Forebay: It is preferred that a forebay be provided
to dissipate energy from incoming flows and to trap settleable sediment
entering the basin. The forebay should be separated from the remainder
of the basin by an earth dike. The top of the dike shall be set six
(6) inches above the stage of the WQCV. Outflow from the forebay to
the basin shall be through a gravel filter as shown in Figure WQ5.
The top of the gravel filter shall be set equal to the stage of the
WQCV.
The volume of the forebay shall be a minimum of ten percent
(10%) and a maximum of twenty percent (20%) of the WQCV. The volume
of the forebay is considered to be part of the required WQCV, not
additional volume.
(6)
General Construction Requirements: The optimal
length to width ratio for a water quality detention basin is four
(4). The length to width ratio should be no less than two (2). The
minimum allowable length to width ratio is one (1). Side slopes, dams
or dikes, and retaining walls shall meet the requirements of the Stormwater
Detention Design section.[2]
(7)
Overflow spillways: Where the basin is to be utilized
as a water quality BMP only, a spillway or outlet structure meeting
the requirements of the Stormwater Detention Design section and capable
of passing the peak flow from a one percent (1%) AP (one hundred (100)
year) storm for the drainage area upstream of the basin shall be provided.
The lowest point on the spillway or outlet structure shall be set
at the top of the WCQV.
(8)
Trickle Channels: Trickle channels shall be provided
to provide grade control and to minimize chronic wet areas. Trickle
channels shall be constructed of six (6) inch stone or other porous
medium. A typical trickle channel cross section is shown in Figure
WQ4.
A typical plan and section for extended dry detention basins
are shown in Figure WQ6.
6.
Extended Wet Detention Basins: Extended wet detention basins
may be provided to capture and provide temporary storage for the required
water quality capture volume. Extended wet detention basins shall
be placed outside of the primary watercourses which allow off-site
flows to pass through the development (i.e., "off-line") where possible.
a.
Design criteria for extended wet detention basins shall be the
same as for extended dry detention basins, with the following exceptions:
The volume of the permanent pool should not be less than one
(1) to one and one-half (1.5) times the WQCV.
A bench area (littoral zone) with a width of ten (10) feet shall
be provided as shown in Figure WQ7. It is preferred that emergent
aquatic vegetation be provided in this zone.
It is recommended that a minimum of twenty-five percent (25%)
of the WQCV be provided in the upper eighteen (18) inches of depth.
A maximum of fifty percent (50%) of the permanent pool volume shall
be provided in the upper eighteen (18) inches of depth.
Depth of the principal portion of the permanent pool shall be
a minimum of four (4) feet.
It is preferred that a forebay meeting the same requirements
as specified for dry detention basins be provided.
Where perforated riser pipes are not encased in gravel, only
corrugated metal or ductile iron pipe may be used.
Typical details for extended wet detention basins are shown
in Figure WQ7.
7.
Sand Filters: Runoff from fueling plazas, vehicle maintenance
areas, solid waste storage or transfer areas, and other areas having
potentially high concentrations of contaminants shall be passed through
a sand filter prior to discharge to receiving waters.
Total impervious area draining to a sand filter will generally be one (1) acre or less. Sand filters shall be provided with a sedimentation chamber and a filtration chamber. Design of sand filters shall be based upon the Austin, Texas, first flush filtration basin (full sedimentation design) as described in Debo and Reese pp. 596-598 [Reference Subsection (F)(2) of this Section]. A schematic cross section of a sand filter is shown in Figure WQ8.
8.
Other Structural BMPs: Constructed wetlands, porous pavements
and other structural BMPs for which detailed design criteria can be
documented in generally accepted literature can be provided in addition
to, or in lieu of, the BMPs described above, provided the objectives
of this Section can be met. The use of infiltration basins and trenches
is discouraged due to possible adverse impacts on groundwater.
E.
Operation And Maintenance.
1.
The City provides no maintenance of water quality BMPs located
on private property. Maintenance must be provided by the owner of
the property upon which the BMP is located. Maintenance includes,
but is not limited to, removal of debris, control of vegetation, removal
of accumulated sediment when the WQCV volume has been reduced by twenty-five
percent (25%) or more.
2.
Extended detention basins and wetlands or other "capture and
storage" BMPs shall be located within a single lot or property, within
a designated drainage easement. Where BMPs are located in common areas
or adjoining off-site areas, the property upon which the BMP is located
shall remain in the ownership of the developer or property owners'
association.
3.
Where a property owners' association is formed, restrictive
covenants which provide for collection of fees for maintenance of
the BMPs shall be filed in the office of the Polk County Recorder
of Deeds. Restrictive covenants must be approved by the County legal
counselor prior to filing of the final plat.
F.
References.
1.
American Society of Civil Engineers, Manuals and Reports of
Engineering Practice No. 77 (WEF Manual of Practice FD-20), Design
& Construction of Urban Stormwater Management Systems, Chapter
12. American Society of Civil Engineers, New York, NY, 1992.
2.
Debo, T.N. and Reese, A.J., Municipal Stormwater Management,
Chapter 13, Lewis Publishers, Boca Raton, FL, 1995.
