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downstream flooding and erosion. <br />Sheet flow. The site shall be graded in a manner that maintains sheet flow over as <br />large an area as possible. The maximum area of sheet flow shall be determined <br />based on the slope, the uniformity of site grading, and the use of easements or other <br />legally-binding mechanisms that prohibit re-grading and/or the placement of <br />structures within sheet flow areas. In no case shall the sheet flow length be longer <br />than 300 feet, nor shall a sheet flow area exceed 1.5 acres. Flow shall be directed <br />into an open channel, storm sewer, or other storm water management practice from <br />areas too long and/or too large to maintain sheet flow, all as determined by the <br />Mayfield Village Engineer. <br />4. Open channels: Unless otherwise allowed by the Mayfield Village Engineer, <br />drainage tributary to storm water management practices shall be provided by an <br />open channel with landscaped banks and designed to carry the 10-year, 24-hour <br />storm water runoff from upstream contributory areas. <br />5. Open drainage s. stems: Open drainage systems shall be preferred on all new <br />development sites to convey storn water where feasible. Storm sewer systems shall <br />be allowed only when the site cannot be developed at densities allowed under <br />Mayfield Village zoning or where the use of an open drainage system affects public <br />health or safety, all as determined by the Mayfield Village Engineer. The following <br />criteria shall be used to design storm sewer systems when necessary: <br />a. Residential storm sewers shall be designed such that they do not surcharge <br />from runoff caused by the 5-year, 24-hour storm, and that the hydraulic <br />grade line of the storm sewer stays below the gutter flow line of the <br />overlying roadway, or below the top of drainage structures outside the <br />roadway during a 10-year, 24-hour storm. Commercial storm sewers shall <br />be designed such that they do not surcharge from runoff caused by the <br />10-year, 24-hour storm, and that the hydraulic grade line of the storm sewer <br />stays below the gutter flow line of the overlying roadway, or below the top <br />of drainage structures outside the roadway during a 25-year, 24-hour storm. <br />The system shall be designed to meet these requirements when conveying <br />the flows from the contributory area within the proposed development and <br />existing flows from offsite areas that are upstream from the development. <br />The minimum inside diameter of pipe to be used in public storm sewer <br />systems is 12 inches. Smaller pipe sizes may be used in private systems, <br />subject to the approval of the Mayfield Village Engineer. <br />All storm sewer systems shall be designed taking into consideration the <br />tailwater of the receiving facility or water resource. The tailwater elevation <br />used shall be based on the design storm frequency. The hydraulic grade line <br />for the storm sewer system shall be computed with consideration for the <br />energy losses associated with entrance into and exit from the system, <br />friction through the system, and turbulence in the individual manholes, <br />catch basins, and junctions within the system. <br />d. The inverts of all curb inlets, manholes, yard inlets, and other structures <br />shall be formed and channelized to minimize the incidence of quiescent <br />