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FLOOD SITUATION Flood Season and Flood Characteristics Storms characteristic of the watershed are usually of the thunderstorm or cloudburst type, which are of short duration and cover only a III1IAll area. Moisture-laden winds coming in contact with the mountain slopes, or convectional air currents over the plains area are forced upward causing rapid condensation with the resulting torrential precipitation. Storms of a general character are infrequent and there are few instances of storms of several days duration. Factors Affecting Flooding and Their Impact Obstructions to Floodflow. Obstructions, natural and man- made, within floodways impede floodflows, creating backwater and increased heights. Debris washing downstream during floods often collects against bridges or within restricted flow areas, reducing waterway openings and impeding the floodflow. This creates a dam- ming effect and, depending on the degree of clogging, causes greater backwater depths with increased overbank flooding. Also, a pro- nounced increase in flow velocities usually occurs downstream from an obstruction, thus extending the flood damage potential. Man- made obstructions on or over the floodway such as dams, levees, bridges and culverts can also create more extensive flooding than would otherwise occur. Manmade obstructions in the study area include two city bridges, a Colorado and Southern Railroad bridge and the Interstate 25 bridge. Table 1 lists descriptive structural data of these bridges and other bridges in the study area. Figure 1 shows typ- ical bridges in Walsenburg and vicinity. Flood Damage Reduction Measures. There is one structure in the study area that has significant water regulating facilities. 6