7.3.4 Allowable Best Management Practices

Intro

7.3.4 Allowable Best Management Practices

Neuse River buffers required by the Division of Water Quality may not be used for compliance with nitrogen reduction requirements; however, additional 50-foot buffers (including locally required buffers) may be used as nitrogen control BMPs. New development outside of the Neuse basin may use UTBs for nitrogen control. The following best management practices may be utilized for nitrogen reduction:

(A) Wet detention ponds;

(B) Constructed wetlands;

(C) Grass Swales;

(D) UTBs;

(E) Bioretention;

(F) Dry Detention

(G) Proprietary BMPs;

(H) Others, as approved by the Town of Cary and the North Carolina Division of Water Quality

The total nitrogen (TN) BMP removal rates are provided in the table below.

TABLE 7.3-2: BMP TYPES, TN REMOVAL RATES, AND DESIGN STANDARDS
BMP Types	TN Removal Rate Based on Current Literature Studies	Design Standards
Wet detention ponds	25%	NC and MD Design Manuals
Constructed wetlands	40%	NC and MD Design Manuals
Grass Swales	20%	NC and MD Design Manuals
UTBs	30%	Neuse UTB Rule (15A NCAC2B.0233)
UTBs with a greenway	20%	Neuse UTB Rule (15A NCAC2B.0233)
Vegetated filter strips with level spreader	20%	NC and MD Design Manuals and other literature information
Bioretention	25%	NC and MD Design Manuals
Sand filters	35%	NC and MD Design Manuals
Dry Detention	10%	NC and MD Design Manuals
Proprietary BMPs	Varies	Per manufacturer subject to DWQ approval
Other BMPs	Varies	Subject to DWQ approval

Staff may modify the effective removal efficiency for a BMP based on deviations from design standards. For example, where there are significant impacts to buffers, no nitrogen removal credits will be allowed.

If more than one BMP is installed in series on a development, then the removal rate shall be determined through serial rather than additive calculations. For example, if a wet detention pond discharges through a UTB, then the removal rate shall be estimated to be 47.5 percent. The pond removes 25 percent of the nitrogen and discharges 75 percent into the buffer. The buffer then removes 30 percent of the nitrogen discharged from the pond, which is 22.5 percent. The sum of 25 and 22.5 is 47.5. The removal rate is not 25 percent plus 30 percent.

Cary, NC Land Development Ordinance On-line VIC Version - This site last updated 3/4/2008

Note: This is not the official version of the Cary Land Development Ordinance