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I I I I I I I I I I I I I I I I I I I The main area of concern in La Junta and Otero County is the lOO-year floodplain in the North La Junta area, Most of the structures in this area have basements that were constructed before the community entered the NFIP, If structural and non-structural mitigation activities can be implemented it will ultimately reduce flood losses in the community. B. Flood Situation 1. Flood Season and Flood Characteristics Storms over the upper Arkansas Basin can be of the general type characterized by low- intensity, long-duration rainfall over a large area, and a short duration rainfall and high intensity over a small area. The former is most prevalent during the autumn, winter and spring seasons, Thunderstorms are most active during July and August, although the two largest and most destructive floods of record occurred during the month of June when general storms concentrated in a series of intense cloudbursts over the foothills and plains areas. Floods on the Arkansas River are of two general types. "spring floods" which result from melting snow and are often augmented by storm runoff represent one type. "summer floods" which result entirely from storm runoff represent the other type, The spring floods are characterized by . comparatively moderate rates of flow of long duration with large volumes of runoff. The summer floods are characterized by high peak rates of discharge with relatively smaller volumes of runoff. 2. Factors Affecting Flooding and Their Impact Obstructions to Flood flow: Obstructions, natural and rnanmade, within f100dways impede flood flows, 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 flood. flow. This creates a damming effect and, depending on the degree of clogging, ciwses greater backwater depths with increased over-bank flooding. Also, a pronounced increase in flow velocities usually occurs downstream from an obstruction, thus extending the flood damage potential. Manmade obstructions on or over the floodway such as dams, levees, bridges and culverts can also create more extensive flooding than would otherwise occur. Flood Damage Reduction Measures: Levees have been constructed along the Arkansas River by local interests but are substandard in design and material. These levees are generally ineffective in preventing large floods and in some instances such as during the June 1965 flood, damages might have been less without the existence of these levees. 14