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35. Flood frequency.-Floods, i. e., discharges exceeding bankfull stages, may be expected more frequently in particular reaches because of the varying channel capacities. A curve of probable flood occurrences was constructed for Purgatoire River at Trinidad by standard methods, using the period of record from June 1922 through June 1942. Reliable and historical flood records indicate that five floods exceeding a discharge of 30,000 cubic feet per second have occurred during a period of 76 years. An inspection of the probable flood occurrence curve for Trinidad indicates that five or six floods equaling or exceeding this magnitude may be expected per 100 years (chart No. 6, appendix B). As a period of 21 years and 17 years elapsed between the 1904, 1925, and 1942 major floods, and five major floods occurred which exceeded 30,000 cubic feet per second during 76 years of record, it was concluded that the probable flood occurrence curve, constructed from the period of record from June 1922 to June 1942, is representative of flood occurrences per 100 years.
36. Design flood criteria.--The maximum flood of record at Trinidad occurred in September 1904 when a peak discharge of 45,000 cubic feet per second and a volume of approximately 50,000 acre-feet were estimated. Frequency investigations at Trinidad indicated that a flood of this magnitude would occur about once in 100 years. It is considered that the design of flood-control improvements should be based on providing adequate protection against a flood of this magnitude if found economically feasible. Such a degree of protection would avoid inducing a false sense of security which might be engendered by improvements designed to provide flood protection for floods of lesser magnitude. Plans investigated and recommended are discussed in paragraphs 60 to 78.
37. Marimum probable flood.-The maximum probable flood was developed from consideration of the maximum probable rainfall occurring when minimum loss rates prevailed in the drainage area under consideration. A peak discharge of 127,000 cubic feet per second and run-off volume of 175,000 acre-feet were computed for these conditions. The maximum probable flood hydrograph is shown on chart No. 9, appendix B. The development of the maximum probable flood is given in appendix B.
38. Extent and character of flooded area-General.---Although there are areas subject to overflow in other portions of the main valley and on the tributaries, the most important are along the main stem upstream from the head of the canyon section, near Alfalfa, to the Sopris Dam site. The probable overflow area is indicated on maps Nos. 2 to 7, inclusive,' appendix A. In this portion of the flood plain, which includes that in Trinidad, there is a total of 5,000 acres of which 370 acres are urban lands. Between Alfalfa and the mouth of the river, there are some cultivated areas, but they are relatively unimportant. Flood damages in this portion of the valley are not extensive, and crop damages are limited largely to the vicinity of Higbee and the agricultural area below the mouth of the canyon. Major floods cause some damage to the irrigation ditches, diversion dams, and the bridges. There are no important overflow areas in Long Canyon.
39. Extent and character of flooded area—Trinidad.--An area of 370 acres in Trinidad is in the overflow plain. This contains rail
1 Not printed.
road tracks and yards, residences, city parks, and business and industrial establishments. There are also several highway, railroad, and city street bridges, streets, sewers, water and gas mains, wire lines, and other improvements of an urban nature within the overflow area. Building in the flood plain has been curtailed for several years. The residences erected in recent years to accommodate the increase in population, as well as other buildings, have been constructed well above the overflow limits. Although it is expected that the normal growth of Trinidad will continue, it is believed that the availability of land free from flood menace will limit future development within the flood plain. The overflow areas are well served by highways and railroads at present. The design of any future transportation facilities will probably take into account the flood history of the stream and make increased construction allowances for the safety of the structures.
40. Ertent and character of flooded area--Agricultural and improvements. The flood plain of Purgatoire River between mile 163.3 and Alfalfa, excluding Trinidad, consists of a narrow strip of agricultural land. This area contains about 4,630 acres, of which 1,050 acres are cultivated by irrigation and dry farming methods. Pasture an grazing lands comprise 1,130 acres, and the remainder of the flood plain is river area and waste land. Farm units vary from small garden plots to portions of large ranches. Practically all tillable sections of the flood plain which can be reached by gravity canals are under irrigation. Because of the low annual rainfall and the small amount of subsurface water available, water for irrigation is in demand and the entire normal flow of Purgatoire River has been allotted by court decree. There is usually insuíficient water in the river to supply decreed rights. Seven low concrete dams and several brush and rock dams for the diversion of irrigation water have been constructed in that portion of the river which flows through the areas under consideration. None of these dams provides any storage, the structures serving merely to raise the water to the elevation of the irrigation canal headgates. At the present time, several diversion structures are not in operation because of damages sustained during the flood of April 1942, and reconstruction plans are as yet in the preliminary stages. The irrigation ditches with decreed rights drawing water from that part of Purgatoire River in the area under consideration, prior to April 1942, with the priorities and amounts of water decreed to each, the length of ditch, and the acreage under each ditch, most of which is not in the flood plain, are shown in table No.3, appendix C. The use of both power farm machinery and horsedrawn equipment is common, although hand labor is used in the cultivation of some of the crops. The area overflowed and pertinent data thereto are shown in table No. 7.
Mile 162.3 to mile 122..
Irrigated.... 1,040 Irrigated 400
10 Nonirrigated. .730 1,050
* Not printed.
41. Extent and character of flooded area-Crops.--The principal crops in order of their importance in the overflow area are sugar beets, alfalfa, corn, beans, small grains, truck garden, and hay. The average total annual value of the main crops, based upon present conditions, is $48,300 or $46 per acre for 1,050 acres. The distribution, average annual yield and value, and total value for the most important crops, when not damaged by flood, are shown in table No. 5, appendix C.
42. Extent and character of flooded area-Property values.--The value of all public, industrial, business, railroad, and utility properties was determined from information furnished by officials of the political subdivisions concerned, and by owning corporations or parties. The value of lands and other improvements was determined by field appraisals. The estimated total value of property in the flood plain is $5,673,500. The value of railroad property in the flood plain amounts to $3,450,500, in Trinidad, and $278,300 outside of Trinidad, which is about 65 percent of the total valuation in the overslow area. The total property in the city of Trinidad flood plain, excluding railroad and irrigation, amounts to $1,520,700. The agriculturalland values in the flood plain are $118,300.' Irrigation headworks, prior to the flood of April 1942, were valued at $166,500. The classification and estimated value of property in the flood plain are shown in table No. 6, appendix C.'
43. Flood damages-General.--Flood damages have been classified as tangible and intangible. Tangible flood damages bave been compiled as direct and indirect losses. A distinction has been made between preventable and nonpreventable, also between recurrent and nonrecurrent damages. At the time the inventory of property in the flood plain was made, estimates were also made of the damages that would be sustained by each unit of property in Trinidad in the event of the recurrence of floods reaching the elevation of those of September 30, 1904, and of July 22 1925, and in the event of the occurrence of a flood reaching an elevation 5 feet higher than that of 1904, which would have an estimated maximum discharge of 112,000 cubic feet per second. Immediately following the flood of April 1942, a survey of damages from Sopris Dam site to the mouth of Purgatoire River was made.
44. Flood damages-- Trinidad.--The greatest losses from floodwaters have occurred at Trinidad and the tangible losses at that city have consisted principally of damages to residential, business, industrial, municipal, and public-utility properties. An account of the 1904 flood, the most disastrous known at Trinidad, published in the United States Geological Survey Water Supply and Irrigation Paper No. 147, gives the estimated losses in the city as $350,000. Local interests have estimated the losses in the city from this flood at $500,000. The flood reached an estimated peak discharge of 45,000 cubic feet per second. During this flood communication, transportation, and public utility installations were severely damaged and services were interrupted. Several brick and stone structures adjacent to the river were destroyed, and four bridges in the city were swept entirely away.
45. The flood of April 1942, the second largest of record, produced a peak discharge about 9,000 cubic feet per second less than the 1904
1 Not printed.
flood. The resulting damages were similar but of less magnitude. Public utility, communication, and transportation installations were again severely damaged. Two city bridges were completely destroyed, two others were in danger of collapsing, and if the remaining two had failed, vehicular traffic across the river would have been disrupted. Railroad facilities were severely hampered by the failure of one bridge approach and erosion of the stream embankment. A flood damage survey for this fiood indicated that the direct and indirect damages were $400,000. A tabulation of flood damages in Trinidad resulting from the April 1942 flood is given in table No. 7, appendix C.
46. Floods of comparatively small volume such as the one of July 1925, which produced the third largest peak discharge of record, do not cause very extensive damages in the urban area. Flood damages for this flood were estimated as $85,000. Discharges of less than 15,000 cubic feet per second do not cause damage by overflow within the city, although damage by lateral erosion occurs at these lesser discharges.
47. In addition to the tangible damages resulting from the major floods at Trinidad, the intangible losses not susceptible of evaluation because of the nature of the losses, were large. These intangible losses included such items as human suffering, loss of life, injury and exposure during floods, inconvenience during the period of rehabilitation, interruption of traffic inside and outside the flooded areas, impairment of sanitary conditions, and interruption of utility services to unflooded areas. A greater portion of the intangible losses would be sustained during major floods. There is an unconfirmed report of 1 drowning due to the failure of a railway bridge near Trinidad during the flood of 1886, and of the loss of 22 lives in another flood. It is probable that the latter catastrophe occurred cnlcf the tributary streams above the city, and apparently resulted from an intense local storm which produced a sudden rise in the canyon, as it was stated that bridges and houses were also destroyed. A daily newspaper printed in Trinidad, following the flood of September 30, 1904, carried a comment that the most remarkable thing about the flood was the absence of fatalities among human beings. There was no loss of life during the flood of April 1942, but as the peak discharge of this flood occurred during hours when the majority of the people were asleep and it was not great enough to menace seriously the residential section, the danger of accident and drowning was substantially less.
48. Flood damages, agricultural.- The major portion of the agricultural area lies between Trinidad and Alfalfa. The United States Geological Survey Water Supply and Irrigation Paper No. 147 gives the estimated losses for the 1904 flood in this portion of the valley as $25,000 and states that fences and hay were washed away, cattle and hogs drowned, and minor damages sustained. Agricultural damages sustained as a result of the April 1942 flood were estimated as $130,200. A tabulation of the agricultural losses for the April 1942 flood is given in table No. 7, appendix C.
49. In the agricultural areas of the valley between the mouth of Long Canyon and Alfalfa, the losses resulting from large-magnitude floods have been small in comparison with those at Trinidad. result of the April 1942 flood, channel capacities have increased and * Not printed.
floods of about equal magnitude will cause flooding in each section of the river, in Trinidad, above, and also below the city. The losses in the agricultural areas of the flood plain result principally from damage to crops, farm improvements, roads, and bridges. Losses also include the deposition of sand, gravel, and debris on cultivated land. Except for the interruption of through rail and highway traffic in a northeast-southwest direction, flooding in the agricultural areas between Long Canyon and Alfalfa causes little intangible loss.
50. Flood damages, caving banks.-Caving banks have been an item of loss along Purgatoire River. The velocity and turbulence of the water in the stream cause a large amount of caving even when high stages do not prevail. As the greater part of sloughing along a caving bank occurs at less than bankfull stages, a large portion of these losses would be nonpreventable, except by constructing bank-protection works.
51. Flood damages, irrigation.-Damage to the irrigation structures is dependent not only on the peak discharge, but also on relative volume of run-off for a given discharge. Large volume floods with sustained recession flows result in larger damages than small volume floods of comparable peak discharge. A comparison of damages during the flood of July 1925, one of small volume, which amounted to $17,000, and those of April 1942, a large volume flood, which were $90,000, for respective peak discharges of 33,000 and 35,800 cubic feet per second at Trinidad, indicates that peak discharge only is not a measure of damages. The destruction or damage of small temporary headworks, which are used to divert water into irrigation ditches, is not considered herein as a flood damage. Since such damage normally is to be anticipated, it is more properly classified as a maintenance cost to the irrigation project. These structures are damaged or destroyed by small flows, and such damage or destruction may occur several times during a single year.
52. Eristing projects. There has been no project authorized by Congress for flood control, navigation, or other stream improvements on Purgatoire River or its tributaries to be constructed by the War Department.
53. Improvements, Trinidad.- Subsequent to the destructive flood of September 30, 1904, the city of Trinidad and the Atchison, Topeka & Santa Fe Railway Co. cooperated in straightening the channel and constructing reinforced bank protection walls at certain critical localities in the city. These improvements are reported to have cost about $62,000. The Works Progress Administration completed a bank protection project in Trinidad during 1936. This work consisted of approximately 430 linear feet of rock-filled crib dikes. The total cost of the project was $6,930, of which $4,068 were Federal funds. Approximately 250 feet of wall were washed out by the flood of July 17, 1938. This break has been repaired by the Works Progress Administration by driving wood piles and attaching heavy wire, behind which rock was dumped. The total cost of the work was $3,000, of which $2,000 were Federal funds. Repairs to bank protection damaged during the April 1942 flood were estimated to have cost $10,800.
54. Improvements, irrigation.-- Irrigation projects along Purgatoire River have been described in paragraphs 17 and 18 and relevant data are given in table No. 3, Appendix C.