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impermeable membrane would actually produce less runoff volume than actual impervious areas such as asphalt pavement. o Area B - Assume 100 for open water o Area C - Assume 80 for open space in good condition (grass > 75%) o Composite curve number 98 Time of Concentration: The time of concentration was estimated based upon TR-55 methods. The time of concentration was calculated from an approximate high point on the existing landfill area to the location where Little Johns Creek exits the landfill property. The method by which the time of concentration was calculated was obtained from the TR-55 manual and is also included in i Attachment B. The calculation is found in Table A. TR-20 Model Parameters: 1 Same as current landfill conditions. Post-Development Conditions Results The results of the pond/pump system sizing are shown on Table C. A pond having surface dimensions of approximately 239 feet by 564 feet (135,000 total square feet) would be required based upon the pumping arrangement and the assumed design elevations. The TR-20 results for post-development conditions are included as Attachment E. 3. Intermediate Phasing Developing the stormwater detention pond and pump system in phases is not recommended for the Austin Road landfill. Phasing stormwater ponds is often preferred for landfill sites because projected flows into stormwater facilities can be much lower during intermediate landfill stages than at final closure. However, phasing the pond is not particularly beneficial at this site for two reasons. First, the post-closure flows are only moderately higher than intermediate landfill stages (approximately 45% increase) so that the volume requirements I of the intermediate facilities would not be much lower than the volume requirements for the ultimate facilities. Second, phasing the pump system would likely be less cost effective than constructing all of the pump facilities at one time.