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<br />CURREJo;T FLOOD SITUATION <br /> <br />the structure involved. A pronounced increase in flow velocities <br />usually occurs downstream from obstructions, thus extending the flood <br />damage potential. <br />Natural obstructions to floodflows occur mostly in the <br />upper reaches of the Purgatoire tributaries where trees, brush and <br /> <br />Developments in the Plood Plains <br />Plate 2 is an index map for Plates 3 through 15 which <br />show the flooded areas along the Purgatoire River and tributaries <br /> <br />considered in this report. <br />The flood plains abound with co~,ercial, industrial, <br />residential developments, and public utilities. Transportation <br />facilities within the flood plains include portions of u.s. Highways <br />85-87 (Interstate 25), leO, 350, and State lIighways 12, 23Sand <br />239, along with numerous county roads and city streets. Rail lines <br />crossing the flood plains inClude the Colorado and Wyoming (C&W) <br />Railway, the Colorado and Southern (CSS) Railway, and the Atchi- <br />son, Topeka, and Santa Fe (AT&SF) Railway. <br />Starkville residents have abandoned many dwellings <br />along the entrenched Raton Creek partly beeau~e of past destructive <br />floods. No new developments were noted along this stream. Devel- <br />opment along the other tributaries appears to be gaining momentum <br />without considering the flooclways and rlrainag" ways. Continuing <br />economic development within the study ,)rea is "xpected and pressures <br />leading to intensified flood plain use will undoubtedly accompany <br />~"r.h <'l..velop""'nt. Futur.. d..v..lopm..~t_ coulrl ;nel,,<'l~ impr"p..r US" "f <br />certain floodplain areas because of the "false sense of security.' <br />provided by the Trinidad Dam. <br /> <br />other vegetation encroach into channel areas during periods of low <br />flows. The denSity of such growth is especially pronounced at <br />channel bends. Man-made obstructions include landfills, sometimes <br /> <br />originating ~s random refuse dumps or often as levees construoted <br />with the intent of protecting isolated properties. Flooding in the <br />vicinity of well-intended levees may be ~re Severe than would have <br />occurred under natural conditions, ShOuld a levec~-of random con- <br /> <br />Obstructions to FloodflowS <br />Obstructions, natural and OIan-made, within the flood- <br />ways impede floodflows, crt'sting backwater and increased heights. <br />Debris washing downstream during floods often collect against bridges <br />or within restricted flow areas, redUCing the waterway openings and <br />impf'ding the floodflow. This crellte. a damming effect a"d, depend- <br />ing on the degree of clogging, causcs greater backwater depths with <br />increased overbank floocling. water pressure on emba.nkrnents and <br />bridges can then result in stress damage or total destruction of <br /> <br />struction--fail, large volumes of floodwater may be released <br />unexpectedly into normally habitable areas. A major obstruction to <br />floodflow On Fishers Peak Arroyo is the closed conduit running under <br />the business district. It is rough and eroded, creating considerable <br />turbulence within the conduit. Turbulence decreases the capacity for <br />floodflow and causes further deterioration of the floor and walls. <br />As erosion of the floor continues, the sidewalls will be endangered <br />by undermining. Past experience indicates the conduit IldS serious <br />cloggingprob~ems, causing d=ging ov",'bank flow throughthebusi- <br />ncss district. Other man-made obstructions arc the approach ombank- <br />mcnts for the various r<.lilroild.."d vehicular bridges distributed <br />throughout the study reaches. Table 3 l{ets descriptive structural <br />information for the bridge crossings, and representative photographs <br />are shown in Figures 6 through 12. <br /> <br />B <br /> <br />" <br />