A.
This article sets forth requirements to protect public safety through
the proper design and operation of stormwater management BMPs. This
article applies to any new stormwater management BMP.
B.
The provisions of this article are not intended to preempt more stringent county safety requirements for new or existing stormwater management BMPs. County stormwater management plans and ordinances may, pursuant to their authority, require existing stormwater management BMPs to be retrofitted to meet one or more of the safety standards in § 179-30A, B and C for trash racks, overflow grates, and escape provisions at outlet structures.
A.
A trash rack is a device designed to catch trash and debris and prevent
the clogging of outlet structures. Trash racks shall be installed
at the intake to the outlet from the stormwater management BMP to
ensure proper functioning of the BMP outlets in accordance with the
following:
(1)
The trash rack shall have parallel bars, with no greater than six-inch
spacing between the bars;
(2)
The trash rack shall be designed so as not to adversely affect the
hydraulic performance of the outlet pipe or structure;
(3)
The average velocity of flow through a clean trash rack is not to
exceed 2.5 feet per second under the full range of stage and discharge.
Velocity is to be computed on the basis of the net area of opening
through the rack; and
(4)
The trash rack shall be constructed of rigid, durable, and corrosion-resistant
material and designed to withstand a perpendicular live loading of
300 pounds per square foot.
B.
An overflow grate is designed to prevent obstruction of the overflow
structure. If an outlet structure has an overflow grate, such grate
shall meet the following requirements:
(1)
The overflow grate shall be secured to the outlet structure but be
removable for emergencies and maintenance.
(2)
The overflow grate spacing shall be no less than two inches across
the smallest dimension.
(3)
The overflow grate shall be constructed and installed to be rigid,
durable, and corrosion-resistant, and shall be designed to withstand
a perpendicular live loading of 300 pounds per square foot.
C.
Stormwater management BMPs shall include escape provisions as follows:
(1)
If a stormwater management BMP has an outlet structure, escape provisions shall be incorporated in or on the structure. Escape provisions include the installation of permanent ladders, steps, rungs, or other features that provide easily accessible means of egress from stormwater management BMPs. With the prior approval of the municipality pursuant to § 179-30, a freestanding outlet structure may be exempted from this requirement;
(2)
Safety ledges shall be constructed on the slopes of all new stormwater management BMPs having a permanent pool of water deeper than 2 1/2 feet. Safety ledges shall be comprised of two steps. Each step shall be four feet to six feet in width. One step shall be located approximately 2 1/2 feet below the permanent water surface, and the second step shall be located one foot to 1 1/2 feet above the permanent water surface. See § 179-32 for an illustration of safety ledges in a stormwater management BMP; and
(3)
In new stormwater management BMPs, the maximum interior slope for
an earthen dam, embankment, or berm shall not be steeper than three
horizontal to one vertical.
A variance or exemption from the safety standards for stormwater
management BMPs may be granted only upon a written finding by the
municipality that the variance or exemption will not constitute a
threat to public safety.
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Elevation View-Basin Safety Ledge Configuration
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