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Section 9 BMP Construction <br />A. Total Nitrogen Removal Rates <br />Each BMP receives an assigned TN (total nitrogen) removal rate. <br />BMP Type <br />TN Removal Rate <br />Appropriate'Design Standards <br />Wet detention ponds <br />25% <br />NC and MD Design Manuals <br />Constructed wetlands <br />40% <br />NC and MD Design Manuals <br />Open channel practices <br />30% <br />NC and MD Design Manuals <br />Riparian buffers <br />30% <br />Co. Buffer Standards <br />Vegetated buffer strips with <br />level spreader <br />20% <br />-Orange <br />NC and MD Design Manuals <br />Bioretention <br />25% <br />NC and MD Design Manuals <br />Sand filters <br />35% <br />NC and MD Design Manuals <br />Other BMPs <br />varies <br />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 />The removal rate is NOT 25% plus 30 %. <br />B. Surety <br />The financially responsible parry 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 />27 <br />