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flooding, none drain areas large enough to affect the peak flows in the main channel of Uncompahgre River. Natural obstructions to floodflows within the study reach exist such as brush, small trees, and other streamland vegetation. Some manrnade features such as bridges, culverts, fences, and buildings exist which may act as obstructions by themselves or in combination with floating trees, brush, or transported rocks to cause interrup- tion to the flow. During floods, these obstructions impede flood- flows and cause backwater conditions that may increase the flood heights upstream of the obstruction and velocities downstream of the obstruction. The bridge heights, in combination with high velocity flow in the steep channel grades, carry debris through the openings without structural damage to the bridges. 2.4 Flood Protection Measures In the wider valleys of Ridgway, an attempt has been made to confine flows to the present newer channel by the gra9ing of berms with native alluvial material. These berms provide adequate protection during normal flows, but consist of permeable material that allows water to seep through during flows higher than normal. When these conditions last several days, water will cover areas lower than the 50- and 100-year flood elevations. Erosion of berms during major flows may flood low-lying areas. 3.0 ENGINEERING METHODS For the flooding sources studied in detail in the co~unity, 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 500-year period (recurrence interval) have been selec- ted as having special significance for flood plain management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10, 2, 1, and 0.2 percent chance, respec- tively, 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 in- creases when periods greater than 1 year are considered. For example, the risk of having a flood which equals or exceeds the lOO-year flood (1 percent chance of annual exceedence) in any 50-year period is approxi- mately 40 percent (4 in 10), and, for any 90-year period, the risk in- creases 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 ele- vations will be amended periodically to reflect future changes. 5