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DRAFT description of existing facilities. Finally, a field investigation was performed to catalog information such as culvert sizes, drainage pan locations, curb and gutter locations and general drainage characteristics. Flow Resistance Resistance for both overland flow and conveyance elements are expressed in terms of Manning's n. The overland flow units are characterized by one n value for pervious areas and another for impervious. For this study, an n value for pervious areas of 0.35 and an n value of 0.030 for impervious areas was used. For conveyance elements the following n values were used: Natural drainageways Road side ditches Gutters & pans Roadways CMP pipe RCP pipe n=0.050-0.060 n=0.025-0.030 n=0.020 n=0.020 n=0.025 n=0.014 Rainfall Loss Parameters Rainfall loss in the overland units is based on a specified amount of surface detention and infiltration loss simulated by Horton's equation. The amount of surface storage was set at 0.10 inches for impervious areas and 0.30 inches for pervious areas. The parameters for Horton's equation were generally taken as an initial infiltration rate of 4.5 inches per hour, a minimum infiltration rate of 0.60 inches per hour and a decay rate constant of 0.0018. To reflect future developed conditions, the precentage of impervious area was increased based on the land use guidelines in the Breckenridge Master Plan. Precipitation The precipitation was derived from the NOAA Atlas II - Volume III, Colorado. A one hour storm was used, with the time increment of each rainfall intensity being five minutes. From the NOAA Atlas, the ratio of various maximum intensities for specified durations were used to distribute the storm. The intensities were arranged such that the maximum rainfall rate occurred in the 30 to 35 minute time increment. The one-hour rainfall volumes are listed in Table 2.11. Table 2.12 lists the hyetographs for the 2-, 5-, 10-, 25-, 50- and 100-year storms. 21