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A shorter levee on the southern bank has been built between Main Street and Cedar Street to separate the Millrace from main stem flow. These levees, along with a diversion dam upstream of U.S. Highway 385, serve to protect the shopping center and other flood plain developments from the usual seasonal high waters. The levees are not adequate, however, to protect against the flood levels that would occur during a 100-year event and, therefore, were not considered in the hydraulic analyses. 3.0 ENGINEERING METHODS For the flooding sources studied in detail in the community, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this study. Flood events of a magnitude which are expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, or SOO-year period (recurrence interval) have been selected as having special significance for flood plain management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and SOO-year floods, have a 10, 2, 1, and 0.2 percent chance, respectively, of being equaled or exceeded during any year. Although the recurrence interval represents the long term average period between floods of a specific magnitude, rare floods could occur at short intervals or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood which equals or exceeds the 100-year flood (1 percent chance of annual exceedence) in any 50-year period is approximately 40 percent (4 in 10), and, for any 90-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials based on conditions existing in the community at the time of completion of this study. Maps and flood elevations will be amended periodically to reflect future changes. 3.1 HydrOlogic Analyses Hydrologic analyses were carried out to establish the peak discharge- frequency relationships for each flooding source studied in detail affecting the community. Flood hydrographs and peak discharges for the 10-, 50-, 100-, and SOO-year flood events on the North Fork Republican River were based on statistical analyses of streamflow records of several drainage basins in the Same geographical region as the North Fork Republican River basin. A frequency-discharge curve for floods on the North Fork Republican River was established using log-Pearson Type III methods outlined by the U.S. Water Resources Council (Reference 5). The hydrologic analysis is described in greater detail in the Technical Addendum of the COE Flood Plain Information Report (Reference 6), where the analysis is referred to as the Upper Republican River Basin Regional Frequency Study. 8