[Ord. No. 1250, 10-15-2002]
Storm water runoff and the velocity of discharge are considerably
increased through development and growth of the City. 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. These modifications may create harmful
effects on properties downstream. Therefore, to minimize these effects,
the following minimum storm water detention requirements have been
established.
[Ord. No. 1250, 10-15-2002]
A complete set of storm water detention plans and calculations
shall be provided for all construction projects that increase storm
water runoff.
[Ord. No. 1250, 10-15-2002]
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 100-year (frequency) runoff coefficients
shall be used. Differential runoff rates shall be evaluated by equation:
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R
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=
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(Cd X I100) - (Cu X I100)
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Where
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R
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=
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Differential Runoff Rate
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Cd
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=
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Runoff Coefficient for developed conditions
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Cu
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=
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Runoff Coefficient for undeveloped conditions
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I100
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=
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Intensity for 100-year storm
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"C" values shall be determined from the following table:
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Suggested Runoff Coefficients
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|---|---|
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"C" Value
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Surface Conditions
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0.10 - 0.15
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Tall grass, brush
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0.15 - 0.20
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Parks, golf courses, farms and one (1) acres single-family residences
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0.35
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Single-family residences on lots of not less than 15,000 sq.
ft.
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0.45
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Single-family residences on lots of not less than 10,000 sq.
ft.
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0.47
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Single-family residences on lots of not less than 7,500 sq.
ft.
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0.51
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Single-family residences on lots of not less than 6,000 sq.
ft.
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0.90
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Gravel surfaces
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0.95
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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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[Ord. No. 1250, 10-15-2002]
Volume of detention can be evaluated according to the "Simplified
Volume Formula", or other method with approval of the City Engineer.
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Total volume of detention shall be computed by the equation:
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V
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=
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R X A X tc (min.) x 60 (sec./min.)
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V
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=
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Total volume of detention (cu. ft.)
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R
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=
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Differential Runoff Rate
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A
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=
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Area of project in acres
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tc
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=
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Time of concentration as determined for use with differential
runoff rates
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The design volume of detention shall be determined from the
following table:
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Calculated Volume
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Design Volume
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|---|---|
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1 cu. ft. thru 500 cu. ft.
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500 cu. ft.
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501 cu. Ft. thru 4,999 cu. ft.
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Round up to nearest 500 cu. ft.
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5,000 cu. ft. thru 9,999 cu. ft.
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Round up to nearest 1,000 cu. ft.
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10,000 cu. ft. thru 49,999 cu. ft.
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Round up to nearest 5,000 cu. ft.
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50,000 cu. ft. thru 99,999 cu. ft.
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Round up to nearest 10,000 cu. Ft.
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100,000 cu. ft. and above
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Round up to nearest 25,000 cu. ft.
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[Ord. No. 1250, 10-15-2002]
A.
The following conditions and limitations shall be observed in the
selection and use of method of detention.
1.
General Location. Detention facilities shall be located within
the parcel limits of the project under consideration with the following
exceptions:
a.
No detention or ponding will be permitted within public road
rights-of-way without specific written approval of the City.
b.
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.
2.
Dry Reservoirs. Wet weather ponds or dry reservoirs shall be
designed with proper safety, stability and ease of maintenance features.
Maximum side slopes for grassed reservoirs shall not exceed one (1)
foot vertical for three (3) horizontal (3:1). In no case shall the
limits of maximum ponding elevation be less than two (2) feet vertically
below the lowest sill elevation, nor should the maximum limits of
ponding be designed closer than ten (10) feet from a building unless
waterproofing of the building and pedestrian accessibility are properly
mulched, sodded or paved. A minimum of one (1) foot of freeboard is
required above the spillway. The outlet structure shall be concrete
or other equivalent material. Spillway areas shall be paved with a
minimum of six (6) inches of concrete.
3.
Open Channels. Normally permitted open channels may be used
as detention areas provided that the limits of the maximum ponding
elevation are not closer than thirty (30) feet horizontally from any
buildings with habitable areas below ground level, and less than two
(2) feet below the lowest sill elevation of any building. In no case
should the maximum limits of ponding be designed closer than ten (10)
feet from a building unless waterproofing of the building and pedestrian
accessibility are properly documented. No ponding will be permitted
within public rights-of-way without specific written approval of the
City. Maximum depth of detention in open channels shall be four (4)
feet. Maximum flow line grade shall be one-half percent (0.5%).
For trapezoidal sections, the maximum side slopes of the detention
area of the channel shall not exceed one (1) foot vertical for three
(3) horizontal (3:1). For design of other typical channel sections
the features of safety, stability and ease of maintenance shall be
observed.
The entire reservoir area of the open channel shall either be
seeded, fertilized and mulched; sodded; or paved. The hydraulic elevations
resulting from channel detention shall not adversely affect adjoining
properties.
4.
Permanent Lakes. Permanent lakes with fluctuating volume controls
may be used as detention areas provided that the limits of maximum
ponding elevations area no closer than thirty (30) feet horizontally
from any building and less than two (2) feet below the lowest sill
elevation of any building.
Maximum side slopes for the fluctuating area of permanent lakes
shall be one (1) foot vertical to three (3) feet horizontal (3:1)
unless proper provisions are included for safety, stability and ease
of maintenance.
Maximum fluctuation from permanent pool elevation to maximum
ponding elevation shall be three (3) feet. Special consideration is
suggested to safety and to limiting accessibility of small children
in design of permanent lakes in residential areas.
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.
5.
Parking Lots. 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.
In no case should the maximum limits of ponding be designed
closer than ten (10) feet from a building unless waterproofing of
the building and pedestrian accessibility are properly documented.
When detention is being effected on parking lots by means of
retaining walls or curbs, these retaining walls and curbs must be
constructed or reinforced concrete.
The minimum freeboard from the maximum ponding elevation to
the lowest sill elevation shall be two (2) feet.
[Ord. No. 1250, 10-15-2002]
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.
Routing calculations shall be submitted in legible tabulated form.
Proof of adequacy of the volume of detention and sizing computations
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 this 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. Projects over
two hundred (200) acres in area shall provide documented verification
of adequacy according to scope and complexity of design.
[Ord. No. 1250, 10-15-2002]
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.
The design-head storage volume if not to be considered as part of
the volume of detention required.
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 that
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 the 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 25-year frequency rain for drainage
areas less than one (1.0) square mile and from a 100-year frequency
rain for drainage areas one (1.0) square mile or greater.
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Street Classification
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Allowable Depth and Inundate Areas of 100-Year Storm
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Local and Collector
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Residential dwellings, public, commercial and industrial buildings
shall not be inundated at the ground line. Water depth over the gutter
flowline shall not exceed eighteen (18) inches.
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Arterial
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Residential dwellings, public, commercial and industrial buildings
shall not be inundated at the ground line. Depth of water at the street
crown shall not exceed six (6) inches to allow operation of emergency
vehicles. The depth of water over the gutter flowline shall not exceed
eighteen (18) inches.
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[Ord. No. 1250, 10-15-2002]
A.
Responsible storm water management should consider all available
alternatives including construction of on-site detention basins, improvements
to the conveyance system, or by making a payment in lieu of providing
improvements. This regulation allows the City to make a permit decision
as to whether it is more protective of the community to have the developer
construct on-site detention and/or make conveyance system changes,
or to buyout when downstream conditions permit.
1.
Conveyance Improvements. Whenever a storm water detention basin
is required and the City Engineer determines that the receiving storm
water conveyance system should be improved to accommodate the additional
storm water, the developer may provide to the City sufficient funds
or material and labor to construct the improvements to the conveyance
system. The decision to accept or allow additional drainage to the
conveyance system shall rest with the City. The funds for improvement
to the conveyance system shall be in an amount determined by the developer's
engineer upon acceptable review by the City Engineer to be reasonable
and adequate to accommodate the improvement, and shall be spent solely
for that purpose by the City.
2.
Buyout Option. The developer of a tract of land (through his/her
engineer) shall determine the detention volume necessary for the tract
of land being developed. This volume shall be determined using the
methods outlined in Part 6 of this Article. Whenever the storm water
volume analysis presented by the developer and accepted by the City
shows that detention has no reduction in downstream flooding, the
developer may pay to the City in lieu of constructing storm water
detention facilities the amount of three dollars ($3.00) for each
cubic foot of detention volume. Whenever the storm water volume analysis
presented by the developer and accepted by the City shows that detention
will increase downstream flooding, the developer is required to pay
to the City in lieu of constructing storm water detention facilities
the amount of three dollars ($3.00) for each cubic foot of detention
volume. This money shall be used by the City to construct and maintain
conveyance systems within the city limits, and does not offset the
cost by the developer to construct downstream conveyance improvements
within the drainage basin where the storm water detention facilities
would have been constructed.
A waiver of the detention facility construction may not be allowed
if the City Engineer has made a finding that there is a significant
drainage problem below the property where the waiver is proposed to
occur even though the detention facility is marginally efficient.
The City Engineer may consider in making such determination that the
discharge of any additional waters at an increased rate onto the properties
below is not desirable due to the significant drainage problems that
exist on subservient properties.
