5.6 The importance of baseflow, inlet and outlet design in ponds.5.1 Applicability of BMPs for Cold Climate.5 Preliminary considerations for design sheets based on cold climate performance.4.3 Water quality credits for onsite snow management.3.2.6.4 Snow Storage and Winter Vegetation Maintenance.3.2.6.1 Alternatives to sodium chloride (NaCl).3.2.5 Filtration, hydrodynamic structures and treatment trains.3.2.4 Wetland and biological-based systems.2 Challenges in engineering and design - complicating factors for cold climate design.1.4 Wetland, open space and biological impacts.Any MTD that includes an underground storage chamber used to attenuate peak discharges, when located in residential areas, shall be subject to the same restrictions as underground detention in residential areas per the PFM § 6-0303.6.Sizing computations must be included on the plan. In no case shall the treatment capacity be less than the minimum required by Chapter 124 of the County Code or an instantaneous flow rate of 0.7 cfs per acre, irrespective of the land cover. The design procedure can be found in Chapter 11 of the Virginia Stormwater Management Handbook (July 2013) in Section 11.5.3. Any MTD that is sized using a flow-based method shall be designed to treat the water quality peak flow rate calculated using the Modified CN method.The plan shall also include the manufacturer's installation requirements and maintenance recommendations, noting the intervals and methods for inspections and routine maintenance. The manufacturer or manufacturer's representative shall verify, in the form of a letter or certification on the plan, that the size and proposed configuration of the MTD meets the manufacturer's sizing, design, and installation requirements for the product.In accordance with the PFM § 6-0402.5, the Director may preclude the use of any BMP otherwise allowed, or require more stringent conditions upon its use, for a specific land-disturbing activity based on a review of the stormwater management plan and project site conditions provided such limitations are based on site-specific concerns. Pursuant to the PFM § 6-1320.2A, individual manufactured BMPs will have different site constraints and limitations and must be feasible and appropriate for use on a specific site.A private maintenance agreement must be executed prior to plan approval.
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