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• due to local topographical or other physical conditions, land area limitations, and inaccessibility to an existing drainage system for outlet control; the planning commission may allow the subdivider or owner of the property, as an option to on-site detention,to provide payment of a one-time stormwater payment-in-lieu of fee. Unless otherwise directed, the planning commission shall determine the stormwater payment-in-lieu of fee based on a prorated formula of$2,500.00 per acre for all multifamily, commercial, and industrial development and $250.00 per lot for single-family detached residential(for lots one acre or larger the fee shall be $500.00 per lot). The requirement for detention or stormwater payment-in-lieu of fee shall apply to all development including previously approved preliminary plats, and development with staged (phased)construction, in which case it shall apply to the entire development. In instances where city staff determines that a proposed development will create a flooding problem downstream, the planning commission will require detention or improvement of the downstream system as a condition for approval of the development. Stormwater payment-in-lieu of fee is to be deposited with the city prior to final plat approval unless otherwise directed by the planning commission. (4) Funds generated from the payment of stormwater payment-in-lieu of fee shall be used for the specific purposes of better management of and improvements to the downstream drainage systems to which the payment applies. (5) Stormwater may not be diverted from one major watershed to another. (6) If any area or lot is within a designated floodplain or floodway, the final plat shall have a floodplain statement indicating the panel number, date and 100-year floodplain contour and/or floodway limit of the flood insurance rate map (FIRM) applicable to the area. In order to protect the public interest, floodways in every subdivision shall be kept free of incompatible urban development. Floodways, as defined by the current FIRM, or as modified by a detailed engineering analysis accepted by the Army Corps of Engineers, Federal Emergency Management Agency (FEMA), and city staff, shall be either designated on the plat as a drainage easement, or at the option of the landowner,dedicated to the public. (7) During construction of the subdivision and during the maintenance irrevocable letter of credit period, the subdivider shall provide all necessary maintenance and erosion control measures to keep ditches and drainage systems free of debris and sediment. The submitted subdivision construction documents should include the appropriate details and specifications pertaining to erosion control. Erosion control measures shall include temporary or permanent seeding, sodding, mulching, staked straw bales, silt fences, temporary diversion ditches, silt basins, terracing and ditch checks. (b) Facility design specifics (1) Facilities for storm drainage shall be of adequate capacity, and designed in accordance with not less than a one in ten-year storm design frequency for single-family detached residential subdivisions, and one in 25-year storm design frequency in multifamily, commercial, and industrial subdivisions (except in the city center commercial area, where one in 50-year design will be used). Developments where the upstream drainage area contributing runoff is less than 200 acres may be designed using the rational method for calculating runoff. Developments where the upstream drainage area contributing runoff is between 200 and 2,000 acres shall be • designed using the U.S. Soil Conservation Services TR-55 method of calculating runoff. For developments where the upstream drainage area is greater than 2,000 acres, the U.S. Army Corps of Engineers HEC-1 program shall be used. A professional engineer licensed in the state shall prepare all such calculations. Provisions shall be made for stormwater emergency - 174 - Batesville Subdivision Regulations