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030 <br /> Section 9 BMP Construction <br /> A. Total Nitrogen Removal Rates <br /> Each BMP receives an assigned TN (total nitrogen)removal rate. <br /> BMP Type TN Removal Rate Appropriate Design Standards <br /> Wet detention ponds 25% NC and MD Design Manuals <br /> Constructed wetlands 40% NC and MD Design Manuals <br /> Open channel practices 30% NC and MD Design Manuals <br /> Riparian buffers 30% Orange Co. Buffer Standards <br /> Vegetated buffer strips with 20% NC and MD Design Manuals <br /> level spreader <br /> Bioretention 25% NC and MD Design Manuals <br /> Sand filters 35% NC and MD Design Manuals <br /> Other BMPs varies subject to Orange County and <br /> NCDENR-DWQ approval <br /> 1. If more than one BMP is installed in series on a development,then the <br /> removal rate shall be determined through SERIAL rather than additive <br /> calculations. <br /> a) Example: <br /> If a wet detention pond discharges through a riparian buffer,then the TN <br /> removal rate would be 47.5%. This number is reached by assuming that <br /> the pond removes 25%of the TN and discharges 75%into the riparian <br /> buffer. The buffer then removes 30%of the nitrogen discharged by the <br /> pond, which is 22.5%. This totals 47.5% (25%+22.5%=47.5%). <br /> Theremoval rate is NOT 25%plus 30%. <br /> B. Surety <br /> The financially responsible party may be required to file with Orange County an <br /> improvement security,performance bond or letter of credit in the form of an escrow <br /> account or other instruments satisfactory to the County attorney in the amount deemed <br /> sufficient by the County to cover all costs of repairing or installing the required <br /> Permanent BMPs according to requirements of this Ordinance, and the approved <br /> Stormwater Management Plans. The amount of such surety requirement shall be <br /> determined by the County. Such surety shall be valid until the work is completed in <br /> accordance with the Stormwater Management Plan and until same is released by the <br /> County. Applicable surety shall be forfeited upon violation of this Ordinance and shall be <br /> used to establish protective cover on the site,to control the velocity of runoff, and/or <br /> prevent off-site nitrogen export. Any monies in excess of the cost of providing protective <br /> measures shall be refunded to the applicant. Surety shall be released when the County has <br /> certified that the requirements of this Ordinance have been met. Surety amounts will be <br /> determined by rates set in the Orange County Planning Fee Schedule. <br /> 24 <br />