[R.O. 1993 § 545.130; Ord. No.
99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
A.
Stormwater 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 concentrations are significantly decreased, resulting
in an increase in both the rate and volume of runoff.
C.
These modifications may create harmful effects on properties downstream.
Therefore, to minimize these effects, the following minimum stormwater
detention requirements have been established.
[R.O. 1993 § 545.140; Ord. No.
99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
A.
A complete set of stormwater detention plans and calculations shall
be provided for all construction projects that increase stormwater
runoff.
B.
Plans for the construction of public improvements shall be submitted
to and approved by the Building Official.
C.
The required stormwater detention plans for private improvements
shall be submitted, along with the building plans, to the Building
Official for review and approval. An escrow for stormwater detention
will not be accepted.
[R.O. 1993 § 545.150; Ord. No.
99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
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 one-hundred-year (frequency) runoff coefficients
shall be used. Differential runoff rates shall be evaluated by equation:
|
R = (Cd x I-100) - (Cu x I-100)
|
|
Where:
| |||
|
R
|
=
|
Differential runoff rate
| |
|
Cd
|
=
|
Runoff coefficient for development conditions
| |
|
Cu
|
=
|
Runoff coefficient for underdeveloped conditions
| |
|
I-100
|
=
|
Intensity for 100-year storm
| |
|
"C" values shall be determined from the following table:
|
|
Suggested Runoff Coefficients
| |
|---|---|
|
Surface Conditions
| |
|
"C" Value
| |
|
0.10 — 0.15
|
Tall grass, brush
|
|
0.15 — 0.20
|
Parks, golf courses, farms and 1-acre single-family residences
|
|
0.35
|
Single-family residences on lots of not less than 15,000 square
feet
|
|
0.45
|
Single-family residences on lots of not less than 10,000 square
feet
|
|
0.47
|
Single-family residences on lots of not less than 7,500 square
feet
|
|
0.51
|
Single-family residences on lots of not less than 6,000 square
feet
|
|
0.90
|
Gravel surfaces
|
|
0.95
|
Asphalt and concrete surfaces
|
|
1.00
|
Buildings and other structures
|
[R.O. 1993 § 545.160; Ord. No.
99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
Volume of detention for areas less than twenty (20) acres shall
be evaluated according to the "Simplified Volume Formula." For areas
larger than twenty (20) acres, other methods may be used upon the
approval of the City Manager and/or Planning and Zoning Board. The
"Rational Formula" method will generally be accepted for areas greater
than twenty (20) acres.
|
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 a determined for use with differential
runoff rates
|
|
The design volume of detention shall be determined from the
following Table:
|
|
Calculated Volume
|
Design Volume
|
|---|---|
|
1 cubic foot through 500 cubic feet
|
500 cubic feet
|
|
501 cubic feet through 4,999 cubic feet
|
Round up to nearest 500 cubic feet
|
|
5,000 cubic feet through 9,999 cubic feet
|
Round up to nearest 1,000 cubic feet
|
|
10,000 cubic feet through 49,999 cubic feet
|
Round up to nearest 5,000 cubic feet
|
|
50,000 cubic feet through 99,999 cubic feet
|
Round up to nearest 10,000 cubic feet
|
|
100,000 cubic feet and above
|
Round up to nearest 25,000 cubic feet
|
[R.O. 1993 § 545.170; Ord. No.
99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
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 Superintendent
of Utilities.
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.
a.
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 Building Official.
Maximum depth of detention in open channels shall be four (4) feet.
Minimum flow-line grade shall be one-half of one percent (0.5%).
b.
For trapezoidal sections, the maximum side slopes of the detention
area of the channel shall not exceed one-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.
c.
The entire reservoir area of the open channel shall be seeded,
fertilized and mulched, sodded or paved.
d.
The hydraulic elevations resulting from channel detention shall
not adversely affect adjoining properties.
4.
Permanent Lakes.
a.
Permanent lakes with fluctuating volume controls may be used
as detention areas, provided that the limits of maximum ponding elevations
are no closer than thirty (30) feet horizontally from any building
and less than two (2) feet below the lowest sill elevation of any
building.
b.
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.
c.
Maximum fluctuation from permanent pool elevation to maximum
ponding elevation shall be three (3) feet.
d.
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.
a.
Detention will not be permitted in primary parking lots. The
primary parking lot will be considered to be the most accessible eighty
percent (80%) of total parking for a facility.
b.
In non-primary parking lots, detention will be permitted to
a maximum depth of twelve (12) inches.
c.
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.
d.
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.
e.
The minimum freeboard from the maximum ponding elevation to
the lowest sill elevation shall be one (1) foot.
6.
Other Methods. Other methods of detention, such as seepage pits,
french drains, etc., will not be approved.
7.
Retention Areas. In drainage basins where sinkholes provide
the only outlet for storm sewer runoff, and those basins are greater
than three hundred twenty (320) acres in area, a retention area capable
of storing the first inch of runoff is required for and development
within the basin. It shall have a four-inch outlet pipe to insure
eventual discharge.
[R.O. 1993 § 545.180; Ord. No.
99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
A.
Analysis of all elements of design shall be performed by a professional
engineer licensed in the State of Missouri. 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.
[R.O. 1993 § 545.190; Ord. No.
99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
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 one-year storm. The maximum discharge
shall be designed to take place under total anticipated design-head
conditions. The design-head storage volume is not to be considered
as part of the volume of detention required.
C.
Sizing of a low-flow pipe shall be by inlet control.
D.
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.
E.
Overflow spillways will be required on all detention facilities that
have storage volumes of one thousand (1,000) or more cubic feet.
F.
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 twenty-five-year frequency rain
for drainage areas less than one (1.0) square mile and from a one-hundred-year
frequency rain for drainage areas one (1.0) square mile or greater.
G.
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 center line 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 center line of the drainageway, center line profile, typical
cross section and capacity of the drainageway.
H.
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.
[R.O. 1993 § 545.200; Ord. No.
99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
A.
Two (2) types of easements shall be provided for stormwater detention.
1.
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.
2.
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.
[R.O. 1993 § 545.210; Ord. No.
99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
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.
[R.O. 1993 § 545.220; Ord. No.
99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
It may be advantageous in some situations to delay the building
of the permanent detention facilities until after the completion of
the other improvements. In these situations, temporary detention facilities
must be provided. The permanent or temporary detention facilities
must be constructed and be functional before proceeding with any other
construction.
[R.O. 1993 § 545.230; Ord. No.
99-2407, 12-14-1999; Ord. No. 2006-2722 § 1, 5-9-2006]
A.
Off-Site Detention. Stormwater detention facilities designed and
constructed off site or 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.
B.
Regional Detention.
1.
Detention facilities designed and located to provide detention
on major drainage channels will be considered as a regional detention
facility and must be one hundred sixty (160) acres or greater. The
facility must provide a detention volume for a one-hundred-year storm
for the entire drainage area and must be designed with a variable
control outlet structure that has one (1) year maximum outlet to opening.
The regional detention facility must be designed with a low flow concrete
channel through the limits of the basin. Upon conceptual approval
of the location and final approval of the design and construction,
the City of Aurora may accept the responsibility for the maintenance
of the regional facility. Drainage and access easements will be required
giving the City of Aurora the authority to gain vehicular access to
the facility from the public street.
2.
As additional development occurs upstream of the regional facility,
payment in lieu of constructing detention may be allowed, provided
the regional detention basin has been designed for full development
of the basin, or if modifications are made to the regional facility
by the developer to provide for the additional volume of detention
required for the new development. Easements must be provided along
the drainage channel from the new development to the regional facility
and the channel must be constructed to carry the peak rate of runoff
from the one-hundred-year storm for the entire basin upstream from
the regional facility.
[R.O. 1993 § 545.235; Ord. No.
2006-2722 § 1, 5-9-2006]
A.
Notwithstanding any other provisions of Article III, if development upon a site requires stormwater detention and if during the design of the required stormwater improvements a determination is made that channelization or other improvements can be shown to be a viable alternative to detention, then the developer may make application to the City to participate in an alternative program consisting of channelization and payment in lieu of a detection facility.
B.
If payment in lieu of detention is approved, it is still the developer's
responsibility to obtain design approval from the Planning and Zoning
Director, construct any required drainage improvements to collect
and convey additional or concentrated stormwater flow from the site
to an existing watercourse, drainage easement, storm sewer and to
procure any necessary easements.
C.
The City will evaluate each written request for payment in lieu of
detention based on the following criteria.
1.
Size of the site in relation to the stormwater generated.
2.
Size of the site in relation to the size of the drainage area
and location in the drainage area.
3.
Impact on the properties downstream of the site.
4.
Location of the site with respect to floodplains, streams and
large watercourses.
5.
Location of the site with respect to environmentally sensitive
areas such as sinkholes.
D.
Approval of a payment in lieu of detention is not automatic and will
be considered on a case-by-case basis upon written request to the
Planning and Zoning Director. The request shall include a letter signed
by the developer and submittals prepared by a registered professional
engineer showing calculations of the normally required stormwater
detention basin volume and proposed stormwater drainage conveyance
from the site to an existing watercourse, drainage easement or storm
sewer. Evaluation of the request will be made by the Planning and
Zoning Director and by the City Engineer.
E.
The amount of payment in lieu of detention shall be based upon the
agreed upon volume calculated for detention resulting from the proposed
development improvements, in the case of non-agreement the calculations
of the City Engineer shall prevail. When the development is a vacant
land subdivision and is zoned residential and consists of two (2)
or more lots, then the calculations of the impervious area per lot
shall use three thousand five hundred (3,500) square feet for single-family
zoned lots and four thousand five hundred (4,500) square feet for
duplex and patio home zoned lots. When the development is a vacant
land subdivision or site zoned for multi-family, commercial, office
or industrial use, the calculations of the impervious area shall be
determined based upon the maximum impervious areas allowed by zoning
for the development. The actual dollar value of the payment in lieu
of detention shall be determined using three dollars ($3.00) per cubic
foot of calculated detention volume for the first one thousand (1,000)
cubic feet plus two dollars ($2.00) per cubic foot of detention volume
required above one thousand (1,000) cubic feet.
F.
Payment in lieu of detention funds may be used by the City for any
drainage improvements at any location within the corporate limits
of the City.
G.
An agreement for payment in lieu of detention shall be fully executed
by the City and developer and the payment shall be received by the
City before the pertinent building permit can be issued or before
the pertinent final plat is recorded.
H.
The order of appeal from this decision by the Planning and Zoning Director as stated in this Section 545.250 and the applicable parts contained therein shall be from the decision of the Planning and Zoning Director directly into the Circuit Court of Lawrence County, Missouri. Administrative rules of the State of Missouri shall apply.
