This article sets forth requirements to protect public safety through the proper design and operation of stormwater management basins. This article applies to any new stormwater management basin.
A trash rack is a device intended to intercept runoff-borne trash and debris that might otherwise block the hydraulic openings in the outlet structure of a structural stormwater management measure. Trash racks shall be installed upstream of such outlet structure openings to ensure proper functioning of the structural stormwater management measure in accordance with the following:
The trash rack should be constructed primarily of bars aligned in the direction of flow with a maximum bar spacing of approximately 1/2 the diameter or width of the hydraulic opening it is protecting. Transverse bars aligned perpendicular to flow should be sized and spaced as necessary for rack stability and strength.
The trash rack shall not adversely affect the hydraulic performance of either the outlet structure opening it is protecting or the overall outlet structure.
The trash rack shall have sufficient net open area under clean conditions to limit the peak design storm velocity through it to a maximum of 2.5 feet per second.
The trash rack shall be constructed and installed to be rigid, durable, and corrosion resistant, and shall be designed to withstand a perpendicular live loading of 300 lbs./ft. sq.
An overflow grate is a device intended to protect the opening in the top of a stormwater management measure outlet structure. If an outlet structure has an overflow grate, such grate shall meet the following requirements:
The overflow grate shall be secured to the outlet structure but removable for emergencies and maintenance;
The overflow grate spacing shall be no more than two inches across the smallest dimension;
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 lbs./ft. sq.
Structural stormwater management measures shall include escape provisions as follows:
If a structural stormwater management measure has an outlet structure, escape provisions shall be incorporated in or on the structure. "Escape provisions" means the permanent installation of ladders, steps, rungs, or other features that provide readily accessible means of ingress and egress from the outlet structure.
Safety ledges shall be constructed on the slopes of all new structural stormwater management measures having a permanent pool of water deeper than 2 1/2 feet. Such safety ledges shall be comprised of two steps. Each step shall be four 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 to 1 1/2 feet above the permanent water surface. See § 296-26 for an illustration of safety ledges in a stormwater management basin.
In new stormwater management basins, the maximum slope of the interior and exterior of an earthen dam, embankment, or berm shall not be steeper than five horizontal to one vertical in accordance with N.J.A.C. 7:8-6(c)3.
An emergency drawdown method for detention basins is required where the permanent pool will be more than 2 1/2 feet deep. This drawdown method must consider downstream or off-site stability at the outfall in accordance with the Standards for Soil Erosion and Sediment Control in New Jersey.
A variance or exemption from the safety standards for stormwater management basins may be granted only upon a written finding by the appropriate reviewing agency (municipality, county or Department) that the variance or exemption will not constitute a threat to public safety.