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<br />PURGATOIRE (PICKET WIRE) RIVER, COLO.
<br />
<br />37
<br />
<br />. (First endorsemenN
<br />
<br />OFFICE, D,V,SION ENGINEER,
<br />SOUTHWESTERN DlVISION,
<br />Dallas, Tex., March 15, 191,3.
<br />
<br />To the CHIEF OF ENGINmERS, UNITED STATES' ARMY.
<br />L The Purgatoire (Picket Wire) River, with a watershed area of
<br />3,390 square miles, is the most important tributary of the Arkansas
<br />River in the. State of Colorado. Its topographic relief varies Over
<br />10,000 feet in elevation, ranging from the 14,000,foot peaks of the
<br />frontal ranges of the Rocky Mountains to the relatively flat, rolling
<br />prairies at an elevation of around 4,000 feet above sea level in south,
<br />eastern Colorado. The climate is semiarid, but despite that fact the
<br />basin at times suiTers severe flood damage especially in the spring
<br />months when the run,ofi from melting snow is augmented by general
<br />rainfall. Other severe floods arc, also experienced as a result of
<br />precipitation of almost e1oudburstproportions eoncentrated over
<br />eomparatively small areas, which in conjunction with the extremely
<br />steep slopes readily produce "flash" floods with high peak rates of
<br />discharg-c bu I. small volume, '
<br />2. The principal area subject to overflow in this basin is a 35-mile'
<br />alluvial reach that extends downstream from the mouth of Long
<br />Canyon, and includes not only severalthousand acres of well-developed
<br />agriculturallund that is under irrigation, but also includes a portion of,
<br />Trinidad, the largest city in the baSIn (population 13,223). The urban
<br />area consists of about 370 acres of residential, business" and industrial
<br />property, as well as roads, railroads, and bridges, all with a total vallie
<br />of nearly $5,000,000. In the major flood of April 1942, which had a
<br />peak flow of approximately 36,000 cubic feet per second, the dama~es
<br />sustained by the city alone amounted to around $400,000, includmg
<br />the loss of two city bridges and the failure of the approach to a,rail"
<br />r~lad bridge. The flood of September 1904 was of considerably
<br />greater magnitude (45,000 cubic feet per second) and estimates o,f
<br />the, damage caused by that flood varied from $350,000 to $500,000,,'
<br />including the complete destruction of four bridges, It is believed
<br />witho!,!t, question that a, ~'epetition . of that, flood under present,day
<br />condItIons would result In vastly Increased losses, not only in the,
<br />directly evaluable damages but 'also in other widespread although
<br />intangIble losses. '. ,
<br />3, Investig\ttion was made of all possible means of providing flood
<br />protection, n?~ only, for Trinidad p"t als? for the agri?ultural areas
<br />and commumtles. The cost of constructwn of the deSIred improve-
<br />ments was, however, found in general to be prohibitive. Further,
<br />more, even when consideration was given to the eonstruction of
<br />dual-purpose works for the conservation of water for irrigation use
<br />in combination with flood protection it was found that the anticipated
<br />conservation benefits added to the prevented flood losses were wholly
<br />incommensurate with the estimated costs.
<br />4. The most feasible plan of improvement was that ror an unlined,
<br />floodway, part leveed and part with reinforced concrete cantilever'
<br />retaining walls, which was designed to insure safe passage through
<br />Trinidad of all flood flows up to the maximum of record, or 45,000-
<br />CUbIC feet per seeond. On the basis of the invoiceable benefits the,
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