HomeMy Public PortalAboutCity of Ferguson3/26/2021 Ferguson, MO Code of Ordinances
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Sec. 7-225. - Stormwater quality—Best management practices.
Purpose and intent. Two (2) consequences of construction and development are increased runoff created by the
changed properties of ground surface and the rate of discharge of this increased runoff. These are both of great
relevance to stormwater. The natural condition of the land before development is in relative balance with the
natural capacity of the receiving creeks. Normally the undeveloped conditions provide greater permeability and
longer times of concentration. By modification of the ground surface from the irregular, pervious, and
vegetation-covered, the areas are changed to more impervious, more quickly drained and., in some cases,
denuded of vegetation. It is the policy of the city to protect and promote the public, health, safety and general
welfare. The management of stormwater will reduce the erosion on land and creek, channels, will reduce the
possibility of damage to public and private property, and will assist in the attainment and maintenance of water
quality standards, and will preserve the environmental quality of the watercourses in the city.
Requirements—Creekbank setback. New development along natural watercourses shall have residential lot lines,
commercial or industrial improvements, parking areas and driveways setback a minimum of twenty-five (25) feet
from the top of the existing stream bank. This is not meant to prevent access across creeks. Except as otherwise
provided herein or as allowed by the city, the section of land between a natural watercourse and lot lines shall be
designated as common ground and drainage easement to be maintained by the trustees or homeowners
association of the subdivision within all types of residential developments. In lieu of the common ground
requirement, the section of land between the natural watercourse and twenty-five (25) feet from the top of the
bank can be (i) private property with an easement dedicated to the trustees or homeowners association
requiring maintenance of the areas as the responsibility of the trustees or homeowners association, (ii) private
property subject to a conservation easement in favor of an organization or land trust dedicated to environmental
protection or land preservation such as Trailnet, Inc., the Open Space Council for the St. Louis Region or similar
organization, (iii) set aside in a stream bank mitigation program approved by the United States Army Corps of
Engineers under the provisions of the Clean Water Act. All developments shall maintain a setback minimum of
fifty (50) feet from the top of the existing hank to any building structure requiring a construction permit that
necessitates excavation or also requires a foundation. Commercial and industrial areas shall have creek areas
dedicated as drainage easements. Notwithstanding the foregoing setback requirements, an applicant may obtain
a variance to develop within the stream buffer area from the city provided the applicant is able to demonstrate
that the water quality and stream morphology will not be adversely affected or adequate mitigation is provided
to offset such adverse affects. Mitigation may include, but not be limited to the following: (i) installing additional
erosion and sediment control; (ii) if the stream has the potential for lateral movement, installing stream
stabilization practices within the stream; (iii) for those sites where vegetation does not exist, establishing
vegetation; and/or (iv) additional tree planting.
Post construction—Water quality. In order to preserve the quality of water in natural streams it is important to
provide a mechanism to remove contaminants on the site prior to water entering the natural water course.
Typically called post construction BMPs (best management practices), these methods identify a critical water
quality volume that will need to receive a treatment to remove certain contaminants. These improvements
whether structural or non-structural will remain in place after the construction is completed. The concepts
introduced in the following paragraphs are taken from the Georgia Stormwater Management Manual, Volume 2,
August, 2001. Nothing in the following paragraphs shall change or replace any of the city's detention ordinances.
Water quality volume (WQv). The water quality volume (denoted as the WQv) is the storage needed to
capture and treat the runoff from ninety (90) percent of the recorded daily rainfall events. In numerical
terms, it is equivalent to 1.14 inches of rainfall multiplied by the volumetric runoff coefficient (Rv) and site
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area. The WQv is directly related to the amount of impervious cover created at a site. A minimum WQv of 0.2
inches per acre shall be met at all sites.
Redevelopment projects can appeal to the planning and zoning commission to reduce or eliminate WQv if it
can be shown that there are stormwater enhancements being provided.
As a basis for determining water quality treatment volume the following assumptions may he made:
The water quality volume WQv for offsite areas is not required. The following equations are used to
determine the storage volume, WQv (in acre-feet of storage):
WQv = [(P)(Rv)(A)]/12
P = 1.14 inches of rainfall
Where: WQv = water quality volume (in acre-feet)
Rv = 0.05 + 0.009(I) where I is percent impervious cover
A = area in acres
Measuring impervious cover: The measured area of a site plan that does not have vegetative or
permeable cover shall be considered total impervious cover.
Multiple drainage areas: When a project contains or is divided by multiple drainage areas, the WQv
volume shall be addressed for each drainage area.
Offsite drainage areas: The WQv shall he based on the impervious cover o the proposed site. Offsite
existing impervious areas may be excluded from the calculation of the water quality volume
requirements.
BMP treatment: The final WQv shall be treated by an acceptable BMP(s) from the list presented in Section
2.
Subtraction for non-structural practices: When non-structural practices arc employed in the site design,
the WQv volume can be reduced in accordance with the conditions outlined in Section 3.
Extended detention for water quality volume: The water quality requirements can be met by providing a
twenty-four-hour draw down of a portion of the water quality volume (WQv) in conjunction with a
stormwater pond or wetland system.
Acceptable urban BMP options. This section sets forth five (5) acceptable groups of BMPs that can be used to
meet the water quality volume criteria (WQv). The design and selection of these BMPs should comply with the
Georgia Stormwater Management Manual, as prepared by the State of Georgia.
Where the city's criteria or requirements are more stringent, then they shall govern. Adapting to local Missouri
environment and natural conditions should be expected but shall be as approved by the city or a higher authority.
The acceptable BMP designs are assigned into six (6) general categories for stormwater quality control (WQv):
BMP Group 1 - stormwater ponds.
BMP Group 2 - stormwater wetlands.
BMP Group 3 - infiltration practices.
BMP Group 4 - filtering practices.
BMP Group 5 - open channel practices.
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See Section 1.3 of the Georgia Stormwater Management Manual for additional BMP options. Section 3 of the
Georgia manual has detailed descriptions of each control.
To be considered an effective BMP for stand-alone treatment of WQv, a design shall he capable of:
Capturing and treating the required water quality volume (WQv).
Removing eighty (80) percent of the TSS.
A combination of BMPs and/or credits is normally required at most development sites to meet all three (3)
stormwater sizing criteria.
BMP Group 1. Stormwater ponds: Practices that have a combination of permanent pool, extended
detention or shallow wetland equivalent to the entire WQvs include:
P-1 micropool extended detention pond.
P-2 wet pond.
P-3 wet extended detention pond.
P-4 multiple pond system.
P-5 pocket pond.
BMP Group 2. Stormwater wetlands: Practices that include significant shallow wetland areas to treat
urban stormwater but often may also incorporate small permanent pools and/or extended detention
storage to achieve the full WQv include (Modification of existing wetland areas will require a Corps 404
permit):
W-1 shallow wetland.
W-2 ED shallow wetland.
W-3 pond/wetland system.
W-4 pocket wetland.
Wetlands shall not he used for control of water quantity (i.e. the flood protection volume).
BMP Group 3. Infiltration practices: Practices that capture and temporarily store the WQv before allowing
it to infiltrate into the soil over a two-day period include:
I-1 infiltration trench.
I-2 infiltration basin.
Infiltration practices will he allowed on sites where it is proven that infiltration will work. Percolation rates
shall be determined for proper use.
BMP Group 4. Filtering practices: Practices that capture and temporarily store the WQv and pass it
through a filter bed of sand, organic matter, soil or other media are considered to be filtering practices.
Filtered runoff may be collected and returned to the conveyance system. Design variants include:
F-1 surface sand filter.
F-2 underground sand filter.
F-3 perimeter sand filter.
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F-4 organic filter.
F-5 pocket sand filter.
F-6 bioretention.*
F-7 proprietary filtering system.
* May also be used for infiltration.
A maintenance agreement and maintenance schedule shall be required.
BMP Group 5. Open channel practices: Vegetated open channels that are explicitly designed to capture
and treat the full WQv within the dry or wet cells formed by creekdams or other means include:
Dry swale.
Wet swab.
Filter strips.
Open channel practices shall he designed with the proper plantings. Wet swales shall be designed to
drain out over time.
When section not applicable. The provisions of this section shall not apply to the extent that any owner has
obtained a variance from such requirements as allowed under subsection (b) of this section. Additionally, the
provisions of this section shall not apply to the extent that the new construction or development is merely
replacement of existing structures which were destroyed by fire or a natural disaster as long as:
No subdivision of land takes place;
The original structure's footprint, location on the lot and size is not changed; and
No new construction or additions are added beyond mere replacement or rebuilding of the destroyed
structure.
(Ord. No. 2008-3349, § 1, 3-11-08)