[R.O. 2014 §530.010; Ord. No. 1029, 8-28-2003]
A.
Storm
water runoff and the velocity of discharge are considerably increased
through development and growth of the City.
B.
Prior
to the development of the land, surface conditions provide a higher
percentage of permeability and longer time of concentration. With
the construction of buildings, parking lots, etc., permeability and
the time of concentration are significantly decreased, resulting in
an increase in both the rate and volume of runoff.
[R.O. 2014 §530.020; Ord. No. 1029, 8-28-2003]
A.
Generally. Systems shall be designed to protect against
flooding of property of all classes and maintain the required level
of service for public facilities. Storm sewer systems shall be designed
as a coordinated unit and may include any or all of the following
elements:
B.
Streets And Enclosed Storm Sewers. Enclosed storm sewers
shall be used to collect and convey drainage on, across and through
public street rights-of-way. Outfall drains shall extend at least
sixty (60) feet to the rear of the front building line or twenty (20)
feet past the back line of the structure, whichever is greater.
C.
Open Channels. Open channels are acceptable only to carry
storm water runoff from tributary areas exceeding one hundred (100)
acres or from smaller tributary areas otherwise requiring an enclosed
storm sewer pipe forty-eight (48) inches in diameter or larger, except
enclosed drainage structures shall be provided where open channels
cross public right-of-way.
D.
Side Ditches. Side ditches are generally not acceptable and may be used to convey drainage along public right-of-way only in rural areas when designated by the City. Culverts and appurtenant drainage facilities shall be designed to permit their incorporation into a future enclosed storm sewer system when possible. Side and rear property line ditches shall be by City approval only and shall be designed in accordance with Subsection (H).
E.
Ultimate Development. Compute the estimated ultimate density
and impervious surfaces of the area. Information may be obtained from
the City Engineer's office of the Planning and Zoning Department.
F.
Street And Pipe Design Criteria.
1.
Velocity. Discharge velocity shall be controlled
to prevent both erosion and siltation at and immediately downstream
from the point of discharge. Energy dissipating structures shall be
used if required.
2.
Inlets. When calculations indicate that curb capacities are exceeded at a point, no further allowance shall be made for flow beyond that point and basins shall be used to intercept flow at that point. All flow shall be picked up by an inlet, except as noted in Subsection (F)(8) — Inlets. Paved gutters may be used to intercept flow and drained to an approved outfall on approval of the engineer.
3.
Maximum depth. Drainage water must not exceed the
depth of curb at any intersection.
4.
Coefficients of friction.
The coefficients of friction allowed for the various kinds of
pipe are as follows:
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Portland Cement Concrete
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N=0.013
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Corrugated Metal
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N=0.023
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Corrugated Metal with Paved Invert
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N=0.021
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Smooth Flow Corrugated Metal
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N=0.013
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Vitrified Clay Pipe
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N=0.013
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Asbestos Cement
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N=0.012
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5.
Pipe sizing.
Manning Formula. Pipe sizes in integrated underground
systems will normally be determined in accordance with the Manning
Formula, V=1.5/N r ⅔ s ½.
Minimum pipe size. The minimum size storm sewer
shall be one (1) inch in diameter except under roadways where fifteen
(15) inches shall be a minimum.
Velocity. All storm drainage systems shall
be designed so as to maintain a minimum velocity of flow of three
(3) feet per second and a maximum velocity of fifteen (15) feet per
second when flowing full.
6.
Location. Storm drainage lines shall generally be
located in the parkway area and shall be placed as shown on the approved
submitted plans.
7.
Depth. All storm drainage lines shall have a minimum
cover of eighteen (18) inches where practical. Cover may be decreased
to avoid conflicts or on short laters. Special bedding or protection
shall be required where cover is decreased below eighteen (18) inches.
8.
Inlets.
a.
Location. Provide inlets to maintain a reasonable
level of vehicular and pedestrian traffic service.
b.
General. Contain all flow within street curbs during
the following design storms.
c.
Arterial streets. Limit gutter flow width to prevent
encroachment on the center twenty-four (24) feet of street during
runoff occurring at sixty-three percent (63%) of peak design rates
of 10-year storms or fifty-five percent (55%) of peak design rates
of 25-year storms.
G.
Open Channels.
1.
General. Open channels shall be sized to carry design
rates of flow without significant damage or erosion to the channel.
Channels shall be fenced, sloped or otherwise protected to prevent
injury to the public.
2.
Connections. Pipe culverts, box culverts and other
structures entering channels shall not project into the normal waterway
area.
3.
Velocity. Channel design shall include lining or
treatment of the invert and sides as required to minimize erosion
in accordance with the following table:
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Mean Flow Velocity
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Type of Lining
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3 F.P.S. and less
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Seeded
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3 — 8 F.P.S
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Sod
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8 — 15 F.P.S.
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Riprap or Concrete
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Overt 15 F.P.S.
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Concrete paved
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Lining materials having equivalent erosion control properties
to those shown in the foregoing table may be used in lieu thereof.
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4.
Capacity. Open channels shall be sized to carry
design flow rates with one (1) foot of freeboard.
5.
Sections. Channel sections shall be compatible with
the type of lining and maintenance practice to be used. Side slopes
shall not be steeper than two (2) horizontal to one (1) vertical.
Channels lined with sod, grass or other vegetative ground cover and
having slopes steeper than three (3) horizontal to one (1) vertical
are not readily susceptible to mowing. Friction factors used in design
shall consider type of lining.
6.
Natural channels. Natural channels of adequate capacity
and having stable banks and invert may be used without modification.
H.
Side Ditches. Side and rear property line ditches in residential
developments may be used only as follows:
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For grass lined ditches:
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Maximum velocity — Three (3) feet per second
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Maximum volume — Three (3) cubic feet per second
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Maximum water depth — Six (6) inches
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Enclosed storm sewer requirements may be waived by the City
on request.
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For ditches with a concrete lined invert:
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Maximum velocity — Five (5) FPS
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Maximum water depth — One (1) foot
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For ditches which are totally concrete lined:
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Maximum velocity — Twelve (12) feet per second
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Maximum water depth — One (1) foot
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I.
Design Details.
1.
Plan. The plan view of all storm sewer details shall
indicate the proper location of the storm sewer, appurtenances, size
of line, capacity and other details relating to the storm drainage
system. The plan shall show sufficient detail to include exact locations,
proper ties into existing permanent reference points, property angles
and distances from other utilities to be placed or presently in the
street right-of-way. Easements shall be a minimum of fourteen (14)
feet for enclosed structures and twenty (20) feet for open paved channels
where they cross private property or as designated by the City Engineer.
2.
Profile. The profile of all storm sewers shall show
the necessary slope, existing and proposed street grades, locations
of angles and appurtenances, and proper elevations for existing outfall
ditches. The profile shall indicate the size of line and the capacity
of each line as determined by the design engineer. The total area
draining to each basin and the Q that will be required to be dissipated
at that point shall also be indicated on either plan or profile.
[R.O. 2014 §530.030; Ord. No. 1029, 8-28-2003]
A.
When
required by the City, a complete set of storm water detention plans
and calculations shall be provided for new developments which increase
storm water runoff, quantity or velocity.
1.
Method of evaluation. Differential runoff evaluation
consists of the determination of the rates of runoff before and after
development, determination of required volume of detention and verification
of adequacy of discharge and control structures. The 50-year (frequency)
runoff coefficients shall be used. Differential runoff rates shall
be evaluated by equation:
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R =
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(Cd x I) —
50
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(Cu x I)
50 Where:
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I =
50
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Intensity of 50-year storm
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"C" values shall be determined from the following table:
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Runoff Coefficients
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"C" Value
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Surface Conditions
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.10
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Tall grass, brush
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.20
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Park, golf courses, farms, and one (1) acre single-family residences
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.35
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Single-family residences on lots of not less than fifteen thousand
(15,000) square feet
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.45
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Single-family residences on lots of not less than ten thousand
(10,000) square feet
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.47
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Single-family residences on lots of not less than seven thousand
five hundred (7,500) square feet
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.51
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Single-family residences on lots of not less than six thousand
(6,000) square feet
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.90
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Graves surfaces
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.95
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Asphalt and concrete surfaces
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1.00
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Buildings and other structures
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Use the Chart included as Page A-7 to find time of concentration
(t) then use Chart included as Page A-6 to determine intensity (I).
A five (5) minute time of concentration is the minimum permitted.
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2.
Volume of detention.
Total volume of detention shall be computed by the equation:
V=R x A X tc (min.) x 60 (sec./min.)
V=Total volume of detention (cu. ft.)
R=Differential Runoff Rate
A=Area of project in acres
tc=Time of concentration as determined for use with differential
runoff rates
NOTE — Sites larger than twenty (20) acres may use alternate
formulas upon approval of the City Engineer. Detention for any revised,
upgraded or changed site shall be designed assuming the site was undeveloped.
3.
Method of detention. The following conditions and
limitations shall be observed in the selection and use of method of
detention.
a.
General location. Detention facilities shall be
located within the parcel limits of the project under consideration
with the following exceptions:
(1)
No detention or ponding will be permitted within public road
rights-of-way without specific written approval of the City Engineer
and State authority, if applicable.
(2)
Location of detention facilities immediately downstream of the
project will be considered by special request if proper documentation
is submitted with reference to practicality, feasibility, proof of
ownership or right-of-use of the area proposed, and provisions are
made for perpetual maintenance.
b.
Dry reservoirs. Wet weather ponds or dry reservoirs
shall be designed with proper safety, stability and ease of maintenance
feature and should completely drain soon after the end of the storm.
c.
Open channels.
(1)
Normally permitted open channels may be used as detention. No
ponding will be permitted within public rights-of-way without specific
written approval of the City Engineer. Maximum depth of detention
in open channels shall be four (4) feet. Minimum flow line grade shall
be one-half percent (0.5%).
(2)
The entire reservoir area of the open channel shall be seeded,
fertilized and mulched, sodded or paved.
(3)
The hydraulic elevations resulting from channel detention shall
not effect adversely adjoining properties.
d.
Permanent lakes.
(1)
Permanent lakes with fluctuating volume controls may be used
as detention areas.
(2)
Maximum fluctuation from permanent pool elevation to maximum
ponding elevation shall be three (3) feet.
(3)
Special consideration is suggested to safety and limiting accessibility
of small children in design of permanent lakes in residential area.
(4)
The entire fluctuating area of the permanent reservoir shall
be seeded and fertilized and mulched, or sodded or concrete paved.
Any area susceptible to or designed as overflow shall be paved with
concrete.
e.
Parking lots.
(1)
Detention will not be permitted in primary parking lots. A primary
parking lot will be considered to be the most accessible eighty percent
(80%) of total parking for a facility.
(2)
In non-primary parking lots, detention will be permitted to
a maximum depth of twelve (12) inches and designed to self drain soon
after the storm.
(3)
When detention is being effected on parking lots by means of
retaining walls or curbs, these retaining walls and curbs must be
constructed of reinforced concrete.
(4)
Other methods. Other methods of detention such
as seepage pits, french drains, etc., will not be approved.
4.
Verification of adequacy.
a.
Analysis of all elements of design is always performed by the engineer.
The following outline is provided to ascertain that certain critical
elements of design are in workable compliance to the aims of design.
b.
Runoff calculations shall be submitted in legible tabulated form.
Proof of adequacy of the volume of detention and sizing computation
for low-flow structure shall also be submitted. Features of stability
and safety will also need to be documented if the scope of the project
requires special attention in the area of design.
c.
Spot elevations shall be included in sufficient detail on the site
plan so that the final direction of water flow can be determined,
and so that the volume of detention can be ascertained.
5.
Control structures.
a.
Detention facilities 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.
The design-discharge (Q) for the low-flow outlet shall not exceed
the existing runoff for the 1-year storm. The maximum discharge shall
be designed to take place under total anticipated design-head conditions.
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. The low-flow pipe
shall be provided with a bar-screen on a minimum 2:1 slope to reduce
blockage by debris.
d.
Overflow spillways will be required on all detention facilities which
have storage volumes of one thousand (1,000) or more cubic feet.
e.
The overflow opening or spillway shall be designed so that the combination
flow of the low-flow outlet and flow over the spillway will not exceed
the total peak runoff for the improved area. The total peak runoff
is to be determined from a 50-year frequency rain.
f.
The overflow spillway shall exit into a natural or improved drainageway.
If the drainageway does not provide for public access, then topographic
detail along with a profile of the centerline of the drainageway shall
be provided from the overflow spillway to the point of public access.
This detail shall show all topography within ten (10) feet of the
centerline of the drainageway, centerline profile, typical cross section,
and capacity of the drainageway.
g.
If the capacity of the existing drainageway is inadequate to carry
the total peak runoff, necessary improvements to the drainageway may
be required to provide for the total peak runoff.
6.
Easement. Two (2) types of easements shall be provided
for storm water detention.
a.
Access easement. All detention reservoirs with the
exception of parking lot and roof detention shall be enclosed by an
access easement. The limits of the easement shall extend ten (10)
feet beyond the maximum anticipated ponding area. The limits and designation
of detention facilities shall be shown on the project plans of final
plat.
b.
Drainage easement. A minimum ten (10) feet wide
drainage easement shall be provided within the reservoir area, connecting
the tributary pipes and the discharge system, along the most direct
possible routing of a piping system for possible future elimination
of detention. The limits of the drainage easement shall be shown on
the project plans of final plat.
7.
Maintenance.
a.
Detention facilities are to be built in conjunction with the storm
sewer installation and/or grading. Since these facilities are intended
to control increased runoff, they must be fully operational soon after
the clearing of the vegetation. Silt and debris connected with early
construction shall be removed periodically from the detention area
and control structure in order to maintain full storage capacity.
b.
The responsibility of maintenance of the detention facilities in
commercial and industrial development projects shall remain with the
general contractor until final inspection of the development is performed
and approved and a legal occupancy permit is issued. After legal occupancy
of the project, the maintenance of detention facilities shall be vested
with the owner of the project or property.
c.
The responsibility of maintenance of the detention facilities for
multi-family development projects where the detention facilities are
located on the site of the multi-family units shall remain with the
general contractor until final inspection of the development is performed
and approved and a legal occupancy permit is issued. After legal occupancy
of the project, the maintenance of detention facilities shall be vested
with the owner of the project or property.
d.
The responsibility of maintenance of the detention facilities for
multi-family development projects and single-family development projects
where the detention facility is located on private property within
a subdivision shall be maintained by the developer of the subdivision,
the trustees, or property owner of the subdivision, whichever is applicable.
e.
If the trustees or owner fail to provide a reasonable degree of maintenance
and the facilities become inoperative or ineffective, and after written
notice that the facilities have become inoperative or ineffective
from the City Engineer and the trustees or owner fail to take action
to remedy the situation within the time stated in the notice, Marshfield
public works crews may perform remedial work and assess the trustees
or owner for the cost of repair and maintenance.
8.
Off-site detention concepts. Storm water detention
facilities designed and constructed off-site outside the limits of
the proposed development will be considered for approval. This approval
is contingent upon documentation being furnished to verify that drainage
easements have been obtained for the channel area from the proposed
development to the detention facility and including the detention
area. The drainage easements must clearly set out provisions for maintenance.
[R.O. 2014 §530.040; Ord. No. 1029, 8-28-2003]
A.
At the discretion of the City, payment for increased storm water runoff may be required in lieu of on-site detention facilities required in Section 530.030.
C.
Payment
shall be on the basis of the square footage of constructed impervious
(non-absorbent) surfaces such as buildings, parking lots and driveways
but shall not include any improvement built on a public right-of-way.
No credit will be given for existing development except as previously
paid for under this specification.
D.
Payment
shall be made at the rate of ten cents ($0.10) per square foot of
impervious surface constructed. Payment for gravel and rocked surfaces
shall be made at the rate of five cents ($0.05) per square foot.
