SW6191002_SWM NARRATIVE_10/30/2019

Anniversary
E N G I N E E R S
FT. BRAGG SOF STORMWATER MANAGEMENT NARRATIVE
The purpose of this land disturbance is to construct a military operations support facility. The
facility will include a 17,000 square foot building, approximately 100 parking spaces, and stormwater
quantity and quality control measures (SCMs). The applicable runoff from the proposed development
will be captured and routed into primary and secondary SCMs which include a bioretention pond and a
stormwater management pond.
The bioretention pond has been designed to treat and retain the runoff for the 981h percentile
rainfall event, which surpasses the state -required design storm volume. The proposed building and
primary parking lot/roadway will generate the majority of runoff. Due to topographical constraints, the
bioretention pond footprint involving the use of engineered media is relatively large at approximately
6,500 ft' which allows a shallow (1.5') depth from top of media to top of embankment. A 1.5' depth
ensures that state -required retention volume is met as is the minimum separation between the seasonal
high water table (SHWT) and the engineered media invert. The engineered media infiltration rate has
been designed to drain at a 2'.n/hr minimum. Because the infiltration rate of the native soils (0.41 in/hr,
Hydrologic Soils Group B) is lower than that of the native soils, an underdrain has been proposed. The
underdrain design calls for seven (7) cleanouts to be placed no more than 10' from adjacent cleanouts.
Additionally, the area of the pond which will be subjected to regular infiltration will be a lower
percentage than a volumetrically similar bioretention pond with greater depth, so maintenance
demands will be reduced.
The site topography and SHWT elevation demanded a shallow bioretention pond. Because of
this, the outlet control structure (OCS) could not be designed solely to allow the bioretention pond to
meet the minimum drawdown requirements set by the state. To meet those requirements, a detention
pond is proposed immediately downstream of the bioretention pond OCS. The OCS of the detention
pond has been designed to ensure that the minimum drawdown time and design storm outlet velocities
fall within the NCDEQ parameters.
Some portions of the post -developed site stormwater runoff will not be captured due to
topographical constraints. However, the overall condition of the site regarding stormwater runoff
quality and quantity will be improved. The overall site post -developed conditions peak discharge rates
and the hydraulic volumes for all rainfall events will be less than those of the pre -developed conditions.
990 Hammond Drive, Suite 900, Atlanta, Georgia 30328
Phone: [phone] Fax: [phone] loweengineers.com