[Ord. No. 662 §1, 4-3-2006]
A. The
requirement of stormwater detention shall be evaluated for all projects
submitted to the City and/or City Engineer for review and approval
and may be required, if deemed necessary. Detention facilities shall
be provided and designated in accordance with the requirements of
this Section.
B. Developments
that have a differential runoff of less than two (2) cfs for the 15-year,
twenty (20) minute event and developments with three (3) acre or larger
lot sizes shall be exempt from detention requirements unless there
are known stormwater problems or special conditions downstream from
a project.
C. If
there are known stormwater problems or special conditions downstream
from a project, detention shall be required.
D. Detention
requirements shall be at the discretion of the City Engineer for projects
that have a differential runoff of two (2) cfs to five (5) cfs for
the 15-year, twenty (20) minute event. Projects that have a differential
runoff of greater than five (5) cfs for the 15-year, twenty (20) minute
event shall be required to provide detention per the following requirements:
1. For developments located entirely within one (1) watershed the stormwater
detention requirements shall be based on the entire platted acreage
of the development, including future phases. Reduction in the detention
requirements for developments with several subwatersheds shall not
be permitted. Detention requirements for developments located in two
(2) or more major watersheds shall be at the discretion of the City
Engineer.
2. The post-developed peak flow from the development may not exceed
the existing peak flow for the 2-year, twenty-four (24) hour event
and the 10-year, twenty-four (24) hour event for developments larger
than ten (10) acres. For developments less than ten (10) acres, the
post-developed peak flow from the development may not exceed the existing
peak flow for the 2-year, twenty (20) minute event and the 15-year,
twenty (20) minute event. The City and/or the City Engineer, at his/her
discretion, can create stricter detention standards for watershed
that are known to have stormwater management problems. The design
engineer shall be made known of any stricter standards during the
preliminary plat process or during the preliminary site improvement
plan process for single lot developments.
3. The existing and post-developed peak flows shall be determined using
Technical Release 55 (TR-55) for developments ten (10) acres or greater
in size. All assumptions that are required for the TR-55 method shall
be approved, in writing, by the City and/or the City Engineer, prior
to submitting construction plans to the City for review and approval.
For developments less than ten (10) acres, flow rates shall be determined
using the method set forth in Section 50.30.1 "Flow Quantities" of
the St. Charles County Highway Department Design Criteria for the
Preparation of Improvement Plans dated February 2002.
E. When
existing detention facilities are going to be used to accommodate
additional runoff from building or parking lot expansions or subdivision
additions, the facilities shall be retrofitted to meet the current
detention requirements for the drainage area that is a tributary to
the facility. Projects that cannot meet this requirement due to physical
constraints shall be evaluated on a case-by-case basis.
F. All
variances to the stormwater detention requirement must be approved
in writing during the preliminary plat process or the preliminary
site improvement plan process for single lot development.
[Ord. No. 662 §1, 4-3-2006]
A. For
rates of runoff (pre-developed and post-developed) for developments
larger than ten (10) acres, the 2-year, 10-year and 100-year, twenty-four
(24) hour inflow hydrographs shall be determined by using Technical
Release 55 (TR-55) "Urban Hydrology for Small Watersheds" from the
Natural Resources Conservation Service, formerly Soil Conservation
Service (SCS). The inflow hydrograph shall be developed based on the
actual flow and timing characteristics upstream of the detention facility.
The rainfall distributed shall be Type II.
B. The
rates of runoff (pre-developed and post-developed) for developments
less than ten (10) acres shall be determined by the Rational Method
for the twenty (20) minute rainfall intensity. The 2-year, 15-year
and 100-year, twenty (20) minute inflow hydrographs shall be determined
as directed in Figure 50-5 of the St. Charles County Highway Department
Design Criteria for the Preparation of Improvement Plans dated February
2002.
C. For
developments larger than ten (10) acres, stormwater shall be detained
on-site or off-site, as approved, and released at a rate not to exceed
the release rate from the development under pre-developed conditions
for the 2-year and 10-year, twenty-four (24) hour events. Note that
stormwater pipes, downstream from the control structure, shall be
sized to carry the runoff from the 15-year, twenty (20) minute design
storm for the total tributary upstream watershed. No reduction in
outfall pipe size shall be permitted because of detention.
D. For
developments less than ten (10) acres, stormwater shall be detained
on-site or off-site, as approved, and released at a rate not to exceed
the release rate from the development under pre-developed conditions
for the 2-year and 15-year, twenty (20) minute events. Note that stormwater
pipes, downstream from the control structure, shall be sized to carry
the runoff from the 15-year, twenty (20) minute design storm for the
total tributary upstream watershed. No reduction in outfall pipe size
shall be permitted because of detention.
E. The
volume of detention may be provided through permanent detention facilities
such as dry basins or ponds, permanent ponds or lakes, underground
storage facilities or in parking lots. The design engineer shall make
every effort to locate the detention facility at or near the lowest
point of the project such that all of the on-site runoff shall be
directed into the detention facility.
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Flows from off-site upstream areas shall be bypassed around
the detention facility to ensure that the proposed detention facility
will function as designed and will provide effective control of downstream
flows with development in place. If off-site flows are approved by
the City and/or the City Engineer to be directed into a detention
facility, the design engineer must provide an analysis showing that
the detention basin will adequately release stormwater under both
existing and future developed off-site conditions. Modifying the release
rate to accommodate off-site flows may reduce or eliminate the effectiveness
of the detention facility, because it will no longer control the increased
volume of runoff during the critical time period of the watershed.
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F. Detention
basin volume shall be based on routing the post-developed 2-year,
10-year and 100-year, twenty-four (24) hour inflow hydrographs for
developments larger than ten (10) acres or the 2-year, 15-year and
100-year, twenty (20) minute inflow hydrographs for developments less
than ten (10) acres through the detention facility while satisfying
the appropriate allowable release rate. The routing computations shall
be based on an application of the continuity principle (i.e., level
pool routing).
G. Design Of Underground Basins.
1. Adequate access for basin maintenance and inspection shall be provided.
A means of visual inspection from the ground surface of the low-flow
device, overflow weir and outlet structure is necessary. Access also
shall be provided to allow for cleaning of the low-flow device from
the ground surface.
2. The basin should be of sufficient volume and spillway capacity to
pass/contain the 100-year, twenty-four (24) hour event.
H. The
design engineer must submit the following for review of a detention
facility:
1. Pre-development and post-development drainage area maps. The drainage
area maps along with calculations for pre-development and post development
shall be used to determine the differential runoff for each basin
within the development.
2. Watershed specific time of concentration for each detention basin.
3. Elevation versus discharge tables of curves for all design frequencies.
4. Elevation versus storage tables of curves for all design frequencies.
5. Inflow calculations and data for all frequencies.
6. Maximum ponding elevation with the low-flow blocked for the 100-year,
twenty-four (24) hour event if the development is larger than ten
(10) acres, or the 100-year, twenty (20) minute event if the development
is less than ten (10) acres.
7. Hydraulic gradeline computations for pipes entering and leaving the
basin for all frequencies.
8. If the embankment contains fill material, a geotechnical report may
be required.
9. Site plan showing appropriate design information.
10. Structural calculations for the outlet control structures. Structural
calculations shall be provided for cast in place structures. The calculations
shall be certified by a registered Missouri professional engineer.
Shop drawings shall be provided for precast structures prior to installation.
11. Cross sections defining size, shape and depth of the detention basin
shall be required. At a minimum, three (3) sections, one (1) at each
end and one (1) in the middle of the basin shall be required. These
sections shall be used to compute the as-built volume of the basin
and thus must be tied to a known physical structure or baseline.
I. All
ends of pipes discharging into a dry basin or pond shall be connected
with the low-flow pipe or control structure by means of a paved swale.
The paved swale shall be non-reinforced concrete, six (6) inches thick,
with a minimum two percent (2%) slope to the center and a minimum
two-tenths percent (0.2%) longitudinal slope. Paved swales shall be
a minimum of six (6) inches deep and four (4) feet wide or one and
three-tenths (1.3) times the diameter of the pipe entering the basin,
whichever is greater, and be keyed to a structure or channel. The
bottom of the basin shall be sloped a minimum of two percent (2%)
towards the concrete swale.
J. Railroad
tie walls cannot be used where water will be in contact with the railroad
tie wall.
K. Permanent
detention ponds or lakes are to be designed to minimize fluctuating
lake levels. Maximum fluctuation from the permanent pool elevation
to the maximum ponding elevation shall be three (3) feet.
L. The
maximum side slopes for dry basins or ponds and the fluctuating area
of permanent ponds or lakes shall be three to one (3:1) (three (3)
feet horizontal, one (1) foot vertical) without fencing. A fencing
detail shall be provided on plans if side slopes require fencing.
M. Dry
basins or ponds and the fluctuating areas of permanent ponds or lakes
are to be lined with placed revetment, sod or commercial erosion control
blanket and kept mowed. Type of material proposed shall be noted on
plans.
N. Control
structures and overflow structures are to be reinforced concrete.
O. The
outflow pipe shall be sized for the developed flow rate.
P. An
emergency overflow is required for all detention basins. The emergency
overflow shall pass the 100-year, 24-hour storm event for developments
larger than ten (10) acres or pass the 100-year, 20-minute storm event
for developments less than ten (10) acres.
Q. In
basins with concrete walls or rock blanket covered slopes, the bottoms
should be paved or provisions should be made for mowing equipment
to reach the bottom (ramps, etc.).
R. Retaining
walls and any required safety features must be designed and constructed
as regulated by the Foristell Building Department.
[Ord. No. 662 §1, 4-3-2006]
A. The
maximum depth of water in a dry detention basin or pond shall not
exceed six (6) feet. Projects that need a deeper basin to attain the
required detention volume due to physical constraints may be evaluated
on a case-by-base basis. The design and construction of dams greater
than eight (8) feet or as directed by the City and/or City Engineer
must be sealed and certified by a professional engineer registered
in the State of Missouri with demonstrated expertise in geotechnical
engineering.
B. Parking
lots used for automobiles shall have a maximum depth of eight (8)
inches of water.
C. Parking
lots used for trucks or truck trailers shall have a maximum depth
of water of twelve (12) inches.
[Ord. No. 662 §1, 4-3-2006]
A. The
maximum ponding elevation shall be calculated based on a routing of
the 100-year, twenty-four (24) hour design storm for developments
greater than ten (10) acres and the 100-year, twenty (20) minute design
storm for developments less than ten (10) acres.
B. The
limits of maximum ponding in dry basins or ponds and permanent lakes
or ponds shall not be closer than thirty (30) feet horizontally to
any building and not less than two (2) feet vertically below the lowest
sill elevation of any building. Lowest sill building elevations for
lots adjacent to the detention basin shall be provided on the site
improvement plans.
C. The
limits of maximum ponding in parking lots shall not be closer than
ten (10) feet horizontally from any building and not less than one
(1) foot vertically below the lowest sill elevation of any building.
Lowest sill building elevations shall be provided.
D. A minimum
of two (2) feet of freeboard shall be provided from the top of the
basin to the maximum ponding elevation.
[Ord. No. 662 §1, 4-3-2006]
The detention basin, access roads or paths, control structure
and outfall pipes are to be located in common ground dedicated to
the subdivision trustees.
[Ord. No. 662 §1, 4-3-2006]
The subdivision trust indentures shall provide for maintenance
responsibility, operation, ownership and funding for the basin(s).
Maintenance, operation, ownership and funding for detention basins
on privately owned sites shall be the responsibility of the owner.
The City of Foristell shall not be responsible for maintenance, operation
and ownership of the detention basins, access roads or paths, control
structures and outfall pipes.
[Ord. No. 662 §1, 4-3-2006]
The low elevation of the detention basin shall be above the
15-year, twenty (20) minute hydraulic elevation of the receiving channel
or pipe system.
[Ord. No. 662 §1, 4-3-2006]
Dams with a height of thirty-five (35) feet or greater require
approval from the Missouri Department of Natural Resources.