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City of Orono <br />Surface Water Management Plan May 2018 53 <br />MODELING METHODOLOGY AND MAPPING <br />1.The general procedure used in the runoff modeling aspects of this analysis has been <br />performed using the Sanitary and Storm Analysis (SSA) by Autodesk modeling <br />software. The typical analysis is based on Soil Conservation Service, Technical <br />Release No. 20 (SCS TR-20). The SCS procedure is based on a standard synthetic <br />rainfall hydrograph, which is modified by local parameters (i.e., rainfall, soil type, <br />time to peak flow, etc.) and is widely accepted among drainage engineers across the <br />United States. <br />2.For purposes of this report, typical 24-hour rainfall events of 2.87", 4.26” and 7.29” <br />have been chosen to analyze runoff/development interaction. These events are best <br />described as those having probabilities of occurring once every 2, 10 and 100 years, <br />respectively. <br />3.The probabilities of occurrence do not imply that a 2.87", 4.26” or 7.29” rainfall <br />cannot occur multiple times within the same year; they simply say that a 2.87" <br />rainfall will occur on the average once every 2 years, a 4.26" rainfall will occur on <br />the average once every 10 years and a 7.29” rainfall will occur on the average once <br />every 100 years. In other words, the 2-year rainfall has a 50 percent chance of <br />occurring in any given year. Similarly, the 10-year rainfall has a 10 percent chance <br />of occurring in any given year and the 100-year rainfall has a 1 percent chance of <br />occurring in any given year.