Floods in Utah,
* Magnitude and Frequency
GEOLOGICAL SURVEY CIRCULAR 457 Floods in Utah, Magnitude and Frequency By V. K. Berwick
GEOLOGICAL SURVEY CIRCULAR 457
Washington 1962 United States Department of the Interior STEWART L. UDALL, SECRETARY
Geological Survey THOMAS B. NOLAN, DIRECTOR
Free on application to the U.S. Geological Survey, Washington 25, D. C. CONTENTS
Page Page Abstract >...... -...... 1 Flood-frequency relations Continued Introduction. __-_----__------_----_-. 1 Regional flood frequency Continued Cooperation and supervision______1 Composite frequency curve ______13 Description of the area___-_-_____._-- 1 Regional analysis for summer Physical features.__------_----_-. 2 floods ______13 Colorado River basin ______2 Derivation of the mean annual flood._ 15 The Great Basin....______2 Basin characteristics .___._._____ 15 Snake River basin __-_-__-___-___ 3 Mean annual flood relations.______15 Major rivers______--._-_ 3 Major rivers in Utah..______18 Climatic features.______3 Cloudburst floods and mud-rock Flood - frequency relations ______3 flows. ...-.______Flood frequency at a gaging station __ 4 Application of flood-frequency Basic data ______4 curves ______Flood series ______4 Method ______Station frequency curve ______4 Use of flood-frequency analysis on Regional flood frequency...... ______4 major river s______._ Regional sampling...... 13 Limitations ______Selection of comparable floods ____ 13 Homogeneity ______13
ILLUSTRATIONS
Page Figure 1. Map of Utah showing location of gaging stations ______5 2. Map of Utah showing flood regions.....______12 3. Composite flood-frequency curves, regions A and B ______14 4. Composite flood-frequency curves, regions C and D ______14 5. Map of Utah showing hydrologic areas__-______.______16 6. Variation of mean annual flood with drainage area and mean altitude.______17 7. Colorado River, discharge of 50-year flood______---__ 18 8. Green River, discharge of 50-year flood______18 9. Duchesne River, discharge of 50-year flood.______18 10. Strawberry River, discharge of 50-year flood______-_--_ 18 11. Price River, discharge of 50-year flood___-___-______----_---_------_--_-_ 18 12. San Rafael River, discharge of 50-year flood.....______19 13. San Juan River, discharge of 50-year flood ______19 14. Bear River, discharge of 50-year flood. ______19 15. Weber River, discharge of 50-year flood ______19 16. Provo River, discharge of 50-year flood______.______19 17. Sevier River, discharge of 50-year flood ______19
TABLES
Page Table 1. Peak discharges at gaging stations used in frequency analysis 6 2. Main stem gaging-station data______20 HI Floods in Utah, Magnitude and Frequency
By V. K. Berwick
ABSTRACT records adjusted to a common time base is a This report presents a procedure for estimating the mag logical means for predicting future flood ex nitude and frequency of floods, within the range of the base pectancy anywhere within a homogeneous flood data, for any site, gaged or ungaged. From the relation of an region. This report (a) outlines the homoge nual floods to the mean annual flood, a composite frequency curve was derived for recurrence intervals of 1.1 to 50 years. neous flood regions in Utah and presents a For regions of similar hydrologic characteristics, curves were composite flood-frequency curve for each, and developed by multiple correlation to express the relation of (b) outlines hydrologic areas and presents mean aunual flood to drainage area and mean altitude. The record^ of gaging stations having 5 or more years of record graphs .showing the variation of mean annual were used as base data when the natural conditions of stream- flood with drainage area and altitude. flow are not affected by works of man. For major rivers where the flow is affected by diversion or regulation, separate analyses were made for each stream. The results may be applied to any area in Utah, except the Great Salt Lake COOPERATION AND SUPERVISION Desert and a small area of the State in the Snake River basin. This report was prepared by the Geological INTRODUCTION Survey, in cooperation with the Utah Depart ment of Highways and the U.S. Bureau of The proper design of structures in or near Public Roads. It was prepared in the office a stream should include the determination of of the Geological Survey at Salt Lake City, the magnitude and frequency of floods. Where Utah, under the direction of M. T. Wilson, flooding may endanger human life, structures district engineer. Technical guidance was should be designed to withstand the greatest furnished by the Floods Section, Water Re probable flood magnitude. In most instances, sources Division, Geological Survey, Wash however, the risk to human life from flooding ington, D. C. is small, and structures are designed to with stand a flood of some selected frequency of DESCRIPTION OF THE AREA occurrence based on economics. High, rugged mountain ranges are the most Floods result from the combined effects of distinguishing features of the Utah landscape. climatic events and physiographic charac Their presence influences the flow of air teristics of a basin. Some of the principal masses and therefore has a major affect upon physiographic factors affecting flood flows precipitation and resulting streamflow. The are: drainage area, altitude, geology, basin State is divided by the Wasatch Mountains, a shape, slope, aspect, and vegetal cover. high range that runs in a north-south direc tion. On both sides of this divide are extensive Flood data for an individual site show what deserts or arid regions with the exception of has occurred at that site during a definite a humid section in the northern mountain period of time but could lead to erroneous ranges. The Uinta Mountains, one of the few results in the prediction of future events, even mountain ranges in the Unites States with an at that site, if the record is not representative east-west axis, extend eastward from the of the long-term average. A composite flood- north-central part of the State almost to the frequency curve based on many gaging-station Utah-Colorado State line. FLOODS IN UTAH, MAGNITUDE AND FREQUENCY
The absence of a usable water supply in the The geology of the Great Basin is complex arid regions of Utah has discouraged the col and is not discussed here except in general lection of enough streamflow records to de terms. The Wasatch Mountains are charac fine flood-frequency relations; therefore, the terized by rugged topography with high peaks Great Salt Lake Desert has not been included and deep canyons. The mountains are long in this analysis. narrow ridges which rise abruptly from the valley floors with relatively small foothill areas. Several streams, including the Bear, PHYSICAL FEATURES Weber, and Provo Rivers, are classed as relatively old and have cut through the high COLORADO RIVER BASIN mountain ranges in deep canyons. Many of the smaller streams in the area are younger The Colorado River basin within Utah con in origin, and for the most part are respon tains approximately 41,000 square miles, with sible for much of the more recent erosion. plateaus and mountains ranging in altitude from 3,000 to 13,000 feet above sea level. The High Plateaus consist of three parallel The Uinta Mountains bisect the Green River strips of tabletop formations that are com basin near the Utah-Wyoming State line, mon to both the Great Basin and the Colorado forming the northern boundary of the Utah River basin. The altitudes range from 5,000 portion of the basin. The Wasatch Mountains to 10,000 feet above sea level. The streams and High Plateaus form the western boundary. originating in the High Plateaus have steep Other mountain ranges and plateaus are in slopes leading to gently sloping valley floors. terspersed throughout the area. Some of the When a stream reaches the mouth of a can more prominent ranges are the LaSal, Abajo, yon, where the gradient is lower, the flow and Henry Mountains. Another orographic often spreads out on the valley floor. In some barrier in the Colorado River basin, known places, the surface flow disappears as the as the Book Cliffs, extends in a general east- streams cross the alluvial fans at the en west direction near the central part of the trance to mountain valleys. These conditions State. Drainage is predominately by streams are more prevalent in the Great Basin, but with deep canyons and relatively steep'slopes. apply to some degree in the Colorado River Drainage areas of streams used in the study basin. for the Colorado River basin range from 2 to 77,000 square miles. The vegetal cover in the Great Basin is similar to that in the Colorado River basin; The vegetal cover within the region varies however, in the northern part of both basins, considerably in kind and in growth depending the average annual precipitation is higher, largely upon the amount and seasonal distri and at some altitudes the forest is denser. bution of precipitation. Parts of the Colorado Drainage areas of streams used in this study River basin have an extremely shallow soil for the Great Basin range from 10 to 8,000 mantle with some large areas of exposed square miles. sandstone. Where these conditions exist the vegetal cover ranges from sparse to almost The modifications of peak discharge by nonexistent. different geological formations are noticeable in both the Colorado River basin and the Great Basin hydrologic areas. One example THE GREAT BASIN of this modification is in the Logan River basin where extensive limestone deposits That part of the Great Basin considered in have a retarding effect on major floods by this study (approximately 18,000 square absorbing some of the surface flow. In other miles) includes the Wasatch Mountains and areas, the exposure of impervious formations the western portions of the High Plateaus. hastens the rate of runoff. In general, the The Wasatch Mountains extend southward streams materially affected by geology are from the Utah-Idaho State line and connect widely scattered, and because the flood data with the High Plateaus to form a north-south do not define these effects sufficiently, it is divide ending near the southwest corner of considered impractical to separate these the State. FLOOD-FREQUENCY RELATIONS drainage basins from the larger hydrologic storms frequent the region, precipitation oc areas. curs in the form of snow. The precipitation during this season is the most consistent and becomes the major source of streamflow. In SNAKE RIVER BASIN the northern part of the State, most streams are fed chiefly from perennial snowfields and Flood-frequency relations for the Snake the amount of runoff is a function of the snow River basin in Utah, a small area in the north fall. The southern part of the State is also west corner of the State, are not included in subject to precipitation in the form of snow, this report. A flood-frequency report and snowmelt floods may be significant at (Thomas, Broom, and Cummans, 1961) for times. However, the annual maximum floods the Snake River basin includes that area of are usually caused by thunderstorms, and Utah. only on rare occasions is the annual maxi mum flood produced by a combination of rain and snowmelt. The second season is during MAJOR RIVERS the late summer and early fall when thunder storms occur. These storms have a different The discharges of the main stems of the source than the winter storms; the moisture, larger rivers are so modified by diversion usually following a high-pressure system, is and storage that only extreme floods would carried from the Gulf of Mexico by unstable reflect the general characteristics of geology, air masses over the arid regions of south topography, and other natural factors. Fur western United States. Occasionally, a storm thermore, the large rivers drain more than from the Gulf of California extends into the one flood region. Studies of frequency and State from the southwest, and some floods magnitude for the larger rivers are, there have been caused by this condition. fore, considered independently of the flood regions. The slopes of most streams, in Some of the thunderstorm activity develops cluding the main stem of the larger streams, over the desert areas without regard to are relatively steep and for most channel change in elevation and are triggered by con reaches, floods of 50-year recurrence inter vection currents. The amount of moisture val are confined within their natural banks. available and the ambient temperatures play important roles in the summer storms. An important factor influencing the paths of CLIMATIC FEATURES thunderstorms is the proximity of the storm to mountains. Thunderstorms usually affect The climate of the State of Utah ranges relatively small areas; therefore, a flood at widely from arid to humid over rather short a site will often be caused by a high-intensity distances. The average annual temperature storm covering only a part of the drainage is about 48°F. Extreme temperature obser basin above that site. vations recorded are a maximum of 116°F at St. George and a minimum of -50°F at Wood ruff. These temperature variations have a FLOOD-FREQUENCY RELATIONS considerable effect on the accumulation of snow. The annual snowfall ranges from less Methods developed by engineers of the than 6 inches at St. George to 346 inches at Water Resources Division of the Geological Silver Lake in the Wasatch Mountains. Survey and others were used to compute magnitude and frequency relations for Utah is a region of relatively low rainfall streams in Utah. Steps taken in the analysis with an average annual precipitation of about were: (a) computation of a flood-frequency 12.6 inches. During an average year there curve for each gaging station, (b) the com are only 57 days on which 0.01 inch or more bination of individual curves to define regions of precipitation falls anywhere in the State. of homogeneous flood-frequency relations, and (c) computations of the mean annual flood Variations in precipitation during two sea from the related drainage basin character sons of the year have a major effect on floods. istics. During the winter months, when Pacific FLOODS IN UTAH. MAGNITUDE AND FREQUENCY
FLOOD FREQUENCY AT A GAGING STATION regard to the water year or any other selec ted time unit. There is a statistical relation BASIC DATA between recurrence intervals computed by the two methods, as shown in the following Streamflow records from 167 gaging sta table by Langbein (1949): tions (fig. 1) with 5 or more years of record were used in the analysis. Selection of records from 118 stations was based on the Recurrence intervals, in years condition that the flood peaks resulted from Annual flood Partial- duration natural flow. Records from the remaining 49 series series stations are for the major rivers and are af 1.16. 0.5 fected by diversions or storage. 1.58. 1.0 2.00. 1.45 The effect of diversions or storage on flood 2.54. 2.0 5.52. 5.0 flow may be inconsequential for extreme 10.5... 10 floods but may be significant for lesser 20.5... 20 floods. If peak flow was not affected by more 50.5... 50 100.5... 100 than 10 percent, it was assumed that the streamflow record could be used without ad The annual flood series was used in this justment. Table 1 contains a list of the gag report. As will be noted from the table, re ing stations for which records were used in currence intervals are essentially the same the flood-frequency analysis. The table shows in both series for periods greater than 10 the maximum known flood and other pertinent years. For those desiring information on the data for each station. basis of the partial-duration series, it is suggested that results be computed by meth To place all records on a comparable ods described in this report and conversion basis, it is desirable that they be adjusted to made by use of the table. the same time period. The period 1938 57 was used for 71 stations in flood regions A and B (fig. 2). Except in flood regions C and D, STATION FREQUENCY CURVE the order of magnitude of the. known peaks in shorter records was adjusted to this base by The annual peak discharge for each station correlation with p<^aks at nearby stations. was listed in order of magnitude with no. 1 as the largest. The time scale, designated as the recurrence interval (T) and plotted as FLOOD SERIES the abscissa on special probability paper (Gumbel, 1941; Powell, 1943), was fitted to Two methods for analyzing floods are by the data by the formula the annual flood series and by the partial- duration series. In an annual flood series, T = n~^~ + 1 only the maximum instantaneous discharge for each water year is listed. In a partial- where a is the number of years of record and duration series, all floods equal to or greater a is the relative magnitude of the event with than an arbitrarily selected base are listed. the largest as 1. Corresponding peak dis It should be noted that, in the annual flood charges were plotted as ordinates, and a series, no consideration is given to secondary smooth curve was then drawn on the basis of floods, which in some years may be greater the plotted points. The use of extreme value than the maximum for other years. On the probability paper tends to cause the plotted other hand, in the partial-duration series, points for many stations to fall in a straight some water years may have no flood as great line. as the selected base discharge.
The recurrence interval in an annual flood REGIONAL FLOOD FREQUENCY series is defined as the average interval of time within which the given flood will be A composite frequency curve based on rec equaled or exceeded once as an annual maxi ords for many gaging stations is a better tool mum. The recurrence interval in a partial- for estimating the magnitude of future floods duration series denotes elapsed time without FLOOD-FREQUENCY RELATIONS
114* Itt*
2$ 0 2$ SO = STATUTE MILC8
Figure 1. Map of Utah showing location of gaging stations. p Table 1. Peak discharges at gaging statiMs used in frequency analysis
Maximum floods Flood Mean Drainage Period Areal Ratio region Station alti Gaging station area of Q Dis to and No. tude 2.33 hydro - (sq mi) record Date areal (feet) (cfs) charge logic Q2.33 (cfs) area (cfs)
COLORADO RIVER BASIN Tributaries between Dolores River and Green River
9-1810 Onion Creek near Moab, Utah______18.8 1950-55; 5,810 677 Aug. 29, 1951 2,100 3.10 D9 1959 1820 Castle Creek above diversions, near Moab, Utah- 7-. 58 1950-55; 9,480 34 June 7, 1952 23 .67 D8 1957-59 1830 Courthouse Wash near Moab, Utah ______150 1949-55 4,810 2,210 Aug. 5, 1957 12,300 5.57 D9 1840 Mill Creek near Moab, Utah ______76 1914-19; 7,170 1,070 Aug. 21, 1953 5,110 4.78 D9 1949-59 1855 Hatch Wash near La Sal, Utah ______370 1950-59 6,550 939 Aug. 4, 1959 3,210 3.42 D8 1860 Indian Creek near Monticello, Utah. ______4.5 1949-57 9,620 21 Aug. 6, 1955 122 5.84 D8 1865 Indian Creek above. Cottonwood Creek near 31.2 1949-59 7,130 93 July 20, 1955 582 6.26 D8 Monticello, Utah. 1870 Cottonwood Creek near Monticello, Utah ______115 1949-57 7,210 340 July 10, 1953 2,140 6.29 D8 1875 Indian Creek above Harts Draw, near 257 1949-57 6,580 2,100 Aug. 30, 1957 3,120 1.49 D9 Monticello, Utah. Green River basin
2185 Blacks Fork near Millburne, Wyo ______156 1939-59 10,270 1,140 June 7, 1957 2,530 2.22 Al 2200 East Fork of Smith Fork near Robertson, Wyo,.-.- 53 1939-59 10,250 497 June 13, 1953 1,200 2.41 Al 2205 West Fork of Smith Fork near Robertson, Wyo___ 37.2 1939-59 9,790 325 May 30, 1950 920 2.83 Al 22fiO Henry's Fork near Lonetree, Wyo______- _ 56 1942-59 10,270 522 June 13, 1953 1,860 3.56 Al 2265 Middle Fork Beaver Creek near Lonetree, Wyo __ 28 1948-59 10,480 329 June 12, 1953 663 2.02 Al 2275 West Fork Beaver Creek near Lonetree, Wyo____ 23 1948-59 10,680 302 June 13, 1953 417 1.38 Al 2330 Carter Creek near Manila, Utah ______19 1948-54 10,280 229 June 3, 1952 153 .67 Al 2340 Carter Creek at mouth, near Manila, Utah ______MO 1946-55 9,010 565 June 4, 1952 928 1.64 Al 2640 Ashley Creek below Trout Creek near 27 1943-54 9,930 343 May 19, 1948 630 1.84 B4 Vernal, Utah. FLOOD-FREQUENCY RELATIONS
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O 0 u i 0 E 0 5 0 S T3 0* EH J*1 a pH THE GREAT BASIN Bear River basin 10- 115 Bear River near Utah-Wyoming State line______176 1942-59 9,770 1,060 June 6, 1957 2,800 2.64 Al 120 Mill Creek at Utah-Wyoming State line and 60 1942-48; 9,320 398 June 7, 1957 690 1.73 Al Mill Creek near Evanston, Wyo. 1 1949-59 160 Sulphur Creek near Evanston, Wyo______80.5 1942-59 7,930 291 Apr. 23, 1952 1,220 4.19 Al 190 Bear River near Evanston, Wyo._-----_------_ 715 1913-56 8,130 1,680 June 14, 1921 3,690 2.20 Al 205 Bear River near Woodruff, Utah ______870 1942-59 7,930 1,790 Apr. 28, 1952 3,010 1.68 Al 210 Woodruff Creek near Woodruff, Utah ____------_ 65 1937-43; 7,900 244 May 25, 1950 528 2.16 Al 1949-59 230 Big Creek near Randolph, Utah _____------_---_ 52.2 1939-44; 7,370 164 July 11, 1957 337 2.05 Al 1949-59 265 Bear River near Randolph, Utah ------_-----_- 1,640 1943-59 7,470 2,380 May 8, 1952 2,660 .12 Al 270 Twin Creek at Sage, Wyo-__--_-----_.------246 1943-59 7,180 490 Mar. 18, 1947 649 .32 Al 320 Smiths Fork near Border, Wyo ______-_-_-__-__ 165 1942-59 8,270 579 June 7, 1957 1,500 .59 Al 350 Smiths Fork at Cokeville, Wyo_--_-.----_------275 1942-52 7,810 706 May 4, 1952 1,320 .87 Al 395 Bear River at Border, Wyo_-__-____-_------_ 2,490 1937-59 7,390 3,160 May 11, 1952 3,680 .16 Al 400 Thomas Fork near Geneva, Idaho ______------45.3 1939-51 7,170 134 May 18, 1950 418 .12 Al 405 Salt Creek near Geneva, Idaho------37.6 1939-51 ,390 128 .do---.-- 382 2.98 Al 410 Thomas Fork near Wyoming-Idaho State line____ 113 1949-59 ,290 284 .do---.-- 869 3.06 Al 425 Thomas Fork near Raymond, Idaho_-_-_-_------202 1942-52 .090 404 May 19, 1950 1,070 2.65 Al 475 Montpelier Creek at irrigators weir, near 50.9 1942 59 7,370 160 May 18, 1950 224 1.40 Al Montpelier, Idaho. 585 Bloomington Creek near Bloomington, Idaho.---- 22.1 1943-47 7,860 105 June 2, 1943 184 1.75 Al 690 Georgetown Creek near Georgetown, Idaho..---- 22.2 1911; 7,830 104 June 8, 1912 162 1.56 Al 1914; 1939-56 *In excess of. Equivalent records combined. 10 FLOODS IN UTAH, MAGNITUDE AND FREQUENCY It's 2 ^S CMCO COCMCMCO CM CM CM CM ^ ^Tf rf rfrf £ ^ *lJf « TJ d> OOO rHcOCOOO CM C- CO in C-CO^J* c- COCO O I bD'TT- COtO COCOOO'^t1 CO CM rH CO rHrfin t- rfOO O VI FH <«J » » » * « ^ rH CM rH rf rH rH 3 r< ° S °~ coco pcoinc- c- oo co co «-HCMCM coincM sfTt* inTfino « i rf T^ rt* cM^in ^cM^in l-lg COCO COCOCOCO CO CO CO CO CO CO CO rHCOCOCO X J2 d> 2 *j rHCO OOOOCOrf in CM CM CMCO COrfOO OOOOCO *^ cd CMCM rHrHCMCM rH CM CMCM rH CM rHrHCM Q TJ 4) 3 &-^^^ ^ao)^^FnO^ 1 ^^^3 o-FH clftcd gaadrnFH ^* bO^^ ^* <;S S r-Jo 1 5S Isi,. S-S5 6 5 I Sgi § I s" rt U ^"t^ rt r-i^-^ r-l * «i±! 3 .5 i^ 0^ JS In I « £ a g S» l^.s ? s t B. aS = fis .ra o bo 1"! 1 2 "§ till t 1 i ||3 1 ss rH bo cd 1 S£-£jai l*"! s 5 1 *3!| | | 2 O 2^^ fnffl^^ ^Snfe^ U g fernFnO O dJ fi fc.0 rt d)d3cd -« oi > .tJ ft > cdcdO h FH§ ad)^ d)ri!>S -Hd).^;1^ o ., d)d). J rt U-a - 6i -2|8 &""% | 0-« S«5 i ^§ ?| g-S5 gl|l ||5 ||1 « |i! g£ d 5|S2 |^|» CSg « 56 B §| I S 84*5 S |o-ag a £-3 |S=S i 4s o|S|SS?58sS £03 g^^ g si OS SWJJ PQ J Q S SmO J Km inm mooin m o m in moo in om a rt*c- coincoco co co CM in ooorH CM inc- rHo ft. 0000 CO O O O rH rH CM CM CMCOCO CO COCO -u O rHrHrH rH rH rH rH rHrHrH rH rHrH cd ^^ 1 O 55 rH Tributaries between Weber and Jordan Rivers J.T:14.1 x «JR Holmes Creek near Kaysville, Utah ______2.49 1950-59 7,580 28 ___ do ...... 36 1.27 B4 1420 Farmington Creek above diversions, near 10 1949-59 7,470 77 May 20, 1958 282 3.67 B4 Farmington, Utah. 1440 Stone Creek above diversions, near Bountiful, 4.48 1950-59 7,050 36 May 5, 1952 82 2.27 B4 Utah. Jordan River basin 1 J.KK lt«J 1745 Sevier River at Hatch, Utah ______340 1911-28; 8,480 621 Mayi/ 26,* 1922 1,490 2.40 D6 1939-59 1835 Sevier River near Kingston-, Utah ______1,110 1914-59 7,790 830 Mar. 4, 1938 3,000 3.61 D6 1950 Clear Creek at Sevier Utah 169 1912-19; 7,690 365 Aug.o 17,* 1955 611 1.67 D6 1934-58 2060 Salina Creek at Salina, Utah ______298 1914-19; 7,810 481 July 27, 1953 2,650 5.51 D6 1942-55 Pavant Valley 2325 Chalk Creek near Fillmore, Utah ______60 1914; 8,020 261 May 4, 1952 509 1.95 D6 1943-59 Parowan Valley 941 "=» fe»nff»T* (~1 r'e'e'\f ne*f*r PaiT»wan TTtah ^t i 'its* HCX" % in* 2S 0 25 90 = STATUTE MILES 4 40- 3T 110* 112" ^ A 111' Figure 2. Map of Utah showing flood regions. FLOOD-FREQUENCY RELATIONS 13 than a curve based on records for any one necessary to show that all of the records are site. To combine records, the relative mag from a region of similar flood-frequency nitude of floods must be computed on a di- characteristics. A statistical test is applied mensionless basis. The ratio of each flood to the slope of the individual frequency curves to the mean annual flood, or simply the flood to determine whether or not the deviations ratio, is a means of satisfying this premise. from an average slope are due to chance Floods used in computing composite flood- alone. The selection of the flood regions frequency curves are expressed as the me shown in figure 2 was based on the results of dian flood ratio for each recurrence interval. the homogeneity test with consideration given to geographic differences. REGIONAL SAMPLING COMPOSITE FREQUENCY CURVE Sampling for a flood-frequency study can be accomplished by two methods time and When the requirements of the homogeneity areal sampling. Time sampling involves the test were satisfied, the stations were grouped collection of records at a few long-term sta together and flood ratios for several recur tions, while areal sampling requires a large rence intervals were computed. Ratios of number of stations to evaluate the various floods from each record for the selected re characteristics of the region, without regard currence intervals were listed, and the me for the length of record. Streamflow records dian flood ratios were obtained from this at the present time do not satisfy the re tabulation. Each median flood ratio was quirements of either method, since the long- plotted to its corresponding recurrence in term records or the density of stations nec terval and a curve was fitted smoothly through essary to complete a statistical approach do these points (figs. 3 and 4). These curves, not exist. An analysis of existing records with the flood discharge expressed in terms will be helpful until such time as additional of the ratio to the mean annual flood, were information can be obtained. To combine based on all of the significant discharge rec records from all the stations, it is advanta ords and represent the most likely flood- geous to place the flood on a comparable frequency relation for each respective re basis as to time and physical characteristics gion. Ratios to. the mean annual flood for any of the drainage basin. recurrence interval are obtained from the composite flood-frequency curve. At any desired recurrence interval, the magnitude SELECTION OF COMPARABLE FLOODS of the discharge is estimated by multiplying this ratio by the mean annual flood. The peak flows of a stream at a given point integrate all of the flood characteristics of the drainage basin. A suitable index for the REGIONAL ANALYSIS FOR SUMMER FLOODS effect of physical features of a basin is the mean annual flood. Theoretically, a peak For flood regions C and D, the length of discharge with a recurrence interval of 2.33 Streamflow records range from 5 to 10 years years is the same as the arithmetical mean with a few records of longer periods for of an infinitely long series. An estimated large drainage basins. Floods in these re value of the mean annual flood(^2.33)may be gions are generally caused by summer thun obtained graphically from the flood-frequency derstorms. The extension of records and the curve for each individual station. All of the determination of the order of magnitude of floods are placed on a dimensionless basis floods by correlation methods are unsatis by dividing the recorded floods by the mean factory in these regions because each flood annual flood; thus, flood records for all gag occurrence may be entirely independent of ing stations within the region are comparable any other flood. A flood may occur on one irrespective of flood magnitude. stream while no flow would be experienced on a nearby stream. HOMOGENEITY If a station record of 1,000 years were available, it would be reasonable to assume Before a group of stations can be combined that 20 floods having a recurrence interval on a regional basis, a test of homogeneity is of 50 years or more may be experienced. 14 FLOODS IN UTAH, MAGNITUDE AND FREQUENCY 1.5 2 345 10 20 30 40 50 RECURRENCE INTERVAL, IN YEARS Figure 3. Composite flood-frequency curves, regions A and B. o O O 7i _» 4 < LU 1.1 1.5 2 345 10 20 30 40 50 RECURRENCE INTERVAL, IN YEARS Figure 4. Composite flood-frequency curves, regions C and D. FLOOD-FREQUENCY RELATIONS 15 Within regions C and D, there are records of The above five characteristics were tested 690 annual peaks. If the same analogy is ap for effect on the mean annual flood in order plied, then 2 percent of these annual peaks to determine which were significant. Curves should be 50-year floods or greater. From were derived by a multiple correlation of the an envelope curve drawn on a plot of annual mean annual flood with the two most signifi floods versus drainage area to exclude 2 cant characteristics, drainage area and mean percent of the floods, it is possible to obtain altitude of the basin. Use of the remaining an estimated 50-year flood for each of the three characteristics did not significantly 47 stations. The estimated flood selected improve the statistical correlation. Slope, from the envelope curve is used as a guide shape, and aspect probably have some effect to extend the individual flood-frequency on the mean annual flood, but the effect, if curves. Although it is difficult to meet all of any, cannot be adequately defined on the basis the conditions set forth in an accepted re of available streamflow records. gional analysis, the two composite frequency curves (fig. 4) should be a suitable substitute for predicting peak discharges until additional MEAN ANNUAL FLOOD RELATIONS hydrologic information is available. The relationship of the mean annual flood to basin characteristics was correlated from DERIVATION OF THE MEAN ANNUAL FLOOD the records of 188 gaging stations. Bound aries of the selected hydrologic areas (fig. 5) To use the regional flood-frequency curve, are somewhat arbitrary; however, the pres a means must be provided for the determi ent program of data collection will aid in the nation of the mean annual flood. verification of these boundaries. For each hydrologic area, a family of curves was drawn (fig. 6). The use of the curves is limited by BASIN CHARACTERISTICS the range of the basic data. Characteristics of the basin as they affect For all hydrologic areas except 3, and 9, floods are numerous, and many are difficult the mean annual flood was found to be directly to define or separate. The ones selected and related to drainage area and mean altitude of investigated in this study are as follows: the basin. In area 3 the effects of limestone deposits are known to change the character 1. Drainage area. Noncontributing area is istics of streamflow when compared with that normally deducted from the total. from surrounding drainage basins where limestone deposits are not present or pre 2. Mean altitude. The mean altitude of a dominant. Also, in area 3, mean altitude was basin is the average altitude above a given not used as a parameter because of relatively point. few gaging-station records. In hydrologic area 9, where the peak discharge generally 3. Slope. The slope of a stream is the total results from summer storms, the mean an fall between any two points divided by the nual flood is directly related to the drainage stream length between those points. The area; however, it is inversely related to the points selected were at 0.2 and 0.8 of the total mean altitude of the basin. One condition stream length above the site. which could contribute to this fact is the marked change in vegetal cover with respect 4. Shape. The shape of a basin is approx to altitude. Vegetal cover is almost nonex imated by the length-width ratio. istent at the lower altitudes, whereas at the higher altitudes the cover is sufficient to re 5. Aspect. The aspect of a basin is the tard runoff. In area 9, winter precipitation in compass direction of the main stream and is the form of snow at the higher altitudes aids a measure of the usual storm pattern in the the growth of a vegetal cover. Frequently, area as well as exposure to prevailing winds summer storms are triggered before reach and sunshine. This characteristic is signifi ing the higher altitudes, thus causing larger cant when considering the peak discharge that floods to occur at lower altitudes. results from snowmelt. 16 FLOODS IN UTAH, MAGNITUDE AND FREQUENCY 25 0 2MP"I STATUTE MILES no* __l 37« 114" 11 112* 'N 111* Figure 5. Map of Utah showing hydrologic areas. FLOOD-FREQUENCY RELATIONS 17 2000 § 1000 X 1000 0 ^ 2 i>"ff x 8 10° X AREA 3 o _l Li_ 10 100 _J < 3000 Z Z 1000 Z >,,. \ 1000 UJ 10.000 -.'* 100 300 1000 1000 10 100 1000 5000 DRAINAGE AREA, IN SQUARE MILES Figure 6. Variation of mean annual flood with drainage area and mean altitude. 18 FLOODS IN UTAH, MAGNITUDE AND FREQUENCY MAJOR RIVERS IN UTAH Inmost reaches of the major rivers, nearly complete development of the water siipply for " EXPLANATION I Gaging station * - _ a o 1° c < Q * O i ftrt I S fr 1 10 20 30 40 50 o uj 80 DISTANCE, IN MILES, ABOVE MOUTH u!'s> I 5 Figure 10. Strawberry River, discharge of 50-year flood. L i * 50 2 EXPLA NATION 4 120 150 180 210 240 270 300 33 1 i _ Gaging station. i DISTANCE, IN MILES, ABOVE UTAH-ARIZONA STATE LINE NearWe 'ellington City c r town Figure 7. Colorado River, discharge of 50-year flood within the 30 \ yNearV State of Utah. Vellingtor ^^ i S. « EXPLANATION 3-JL GreenRiver> 3"- 20 X CL « S > Gaging station C. « 2 8S9 n= ' a k^ o C ity or town a ^ 50-YEARFLOOD,THOUSANDSIN c « OFCUBICPERSECONDFEET «l X CLx«> r= \j « 20 40 60 80 100 120 14 o « > ST, DISTANCE, IN MILES, ABOVE MOUTH c o « o 1 Figure 11. Price River, discharge of 50-year flood. ill irrigation and power is reflected by the ab sence of homogeneity with the tributary 200 300 400 500 streams. The major rivers are not homo- DISTANCE, IN MILES, ABOVE MOUTH geneoiis among themselves; therefore, the use 8. Green River, discharge of 50-year flood below of a composite ratio curve for estimating Wyoming -Utah State line. flood frequencies similar to methods used for PMro*-H- the smaller streams is unsatisfactory. Each major river was treated individually for es EXPLAr4ATION e5o^o'"re01 5 1 timating the magnitude and frequency of K i floods. Table 2 contains a list of gaging sta City o r town 50-YEARFLOOD,iNTHOUSANDS 3OFCUBICFEETPERSECOND ! tions for which records were analyzed on the - above basis. The data shown for each station &In* are drainage area, length of annual peak rec ord, and estimated discharge for the 50-year S Tabioniir flood. The discharge for the 50-year flood at .c u a each gaging site was plotted against the cor Q !;! responding river miles above the mouth (figs. Liiil 7 17). With the regimen of the rivers affected by regulation and diversion, it was impracti 20 40 60 80 100 120 14 cal to compute the magnitude of the floods for DISTANCE, IN MILES, ABOVE MOUTH recurrence intervals of less than 50 years. Figure 9. Duchesne River, discharge of 50-year flood. FLOOD-FREQUENCY RELATIONS 19 ,N 10 a i £85m nio *~*~* GreenRi EXPLANATI 3N ^*^ 8 1 50-YEARFLOOD.INTHOUSANDSOF Gaging stat »n 5 tj \ 8 , CUBICFEETSECONDPER PINear EXPLANATION (flowsCreekhalk intoreservoi I a 1 Gaging station , "» 6 a ~~\ Coalvllle' \ City or tov m 1 CastleIr -1 4 tl1 :! 1 _ Reservoir,Echoc 1|in \ Res*Rockyport nU.S. Am ly Engin vers esti mated rr aximum 2 - disch urge unc er propo sed rese rvoir ^ regul >tion for flood co ntrol. 0 20 40 60 80 100 120 140 DISTANCE, IN MILES, ABOVE MOUTH 10 20 30 40 50 60 70 Figure 12. -San Rafael River, discharge of 50-year flood below DISTANCE. IN MILES, ABOVE MOUTH Perron Creek. Figure 15. Weber River, discharge of 59-year flood and U.S. Army Engineers'estimated maximum discharge under proposed flood-control project below Rockport Reservoir. ! iia 8 i gs 0 ft | * S ii z «- is a I s LJ x " p 1- 1C ^ I I i tt O ". io ^ ft oa .,^4 EXPLAf ATION 0 i Gaging, station r 00 120 140 160 180 200 220 240 a i DISTANCE. IN MILES, ABOVE MOUTH City or town -1- 13. San Juan River, discharge of 50-year flood below Shiprock, N. Mex. S 10 IS DISTANCE, IN MILES, ABOVE MOUTH EX PLANATI DN "0 10-ea 16. Provo River, discharge of 50 -year flood below Deer b ging stati on a Creek Reservoir. a C ity or tow n 50-YEARTHOUSANDSFLOOD,INOF o Jo»ooo*M Gunnison CUBICFEETSECONDPER fc e I EXPLA NATION £ ^ c I - Gagini J 1 » to I 'i « Crtyeir town 3FLOOD,THOUSANDS50-YEARINOF s iFi -CM % *§CUBICFEETPERSECOND -I *1 % LJ b U-lL? r I } \ "TO 1 1 '5M 1 40 80 120 160 200 240 C i -4 -* DISTANCE, IN MILES, ABOVE MOUTH J Figure 14. Hear River, discharge of 50-yearflood. 0 40 80 120 160 200 240 280 DISTANCE, IN MILES, ABOVE MOUTH Figure 17. Sevier River, discharge of 50-yearflood below Piute Reservoir. 20 FLOODS IN UTAH, MAGNITUDE AND FREQUENCY Table 2. Main stem gagwfl-station data Drainage 50-year Station Gaging station area Period of flood No. (sq mi) record (cfs) COLORADO RIVER 9-1635__ Colorado River near Colorado-Utah State line___. 17,900 1951-59 61,000 1805__ Colorado River near Cisco, Utah______. 24,100 1914-17, 76,000 1923-59 3350__ Colorado River at Kite, Utah _._._.__...__.___.. 76,600 1947-58 123,000 GREEN RIVER 2255__ Green River near Linwood, Utah ______14,300 1929-59 23,000 2345._ Green River near Greendale, Utah ______15,100 1951-59 24,000 2610__ Green River near Jensen, Utah ______25,400 1904, 1906, 59,000 1947-59 3070__ Green River near Our ay, Utah______35,500 1948-55, 71,000 1957-59 3150__ Green River at Green River, Utah______40,600 1895- 70,000 1905, 1906-59 DUCHESNE RIVER 2740__ Duchesne River near Hanna, Utah ______78 1922-23, 1,640 1946-59 2770.. Duchesne River at Hanna, Utah ...... 230 1954-59 2,860 2775.. Duchesne River near Tabiona, Utah __.___.._.... 352 1919-59 2,550 2795__ Duchesne River at Duchesne, Utah ______660 1918-59 4,500 2950__ Duchesne River at Myton, Utah ....._...... _.... 2,750 1900-59 14,000 3020._ Duchesne River near Randlett, Utah______3,920 1943-59 18,000 STRAWBERRY RIVER 2850._ Strawberry River near Soldier Springs, Utah..... 212 1943-56 3,050 2885.. Strawberry River at Duchesne, Utah______. 1,040 1909-10, 3,600 1914-59 PRICE RIVER 3115__ Price River near Scofield, Utah.._...... 163 1918-20, 1,320 1926-27, 1929-31, 1939-44, 1946-59 3130__ Price River near Heiner, Utah.______455 1935-59 12,500 3135__ Price River near Helper, Utah______530 1905-06, 12,500 1908-11, 1913-34 3140__ Price River near Wellington, Utah ______850 1950-58 21,800 3145__ Price River at Woodside, Utah.______1,500 1909, 30,000 1946-59 FLOOD-FREQUENCY RELATIONS 21 Table 2. Main stem gagmg-sfation data Continued Drainage 50-year Period of Station Gaging station area flood No. record (sq mi) (cfs) SAN RAFAEL RIVER 9-3280.. San Rafael River near Castle Dale, Utah.-..-... 927 1948-59 7,100 3285.. San Rafael River near Grten Rivtr, Utah - --, 1,690 1909-18, 27,000 1946-59 SAN JUAN RIVER 3680.. San Juan River at Shiprock, N. Mex __,---_____, 12,900 1928-59 99,000 3795.. San Juan River near Bluff, Utah...... 23,000 1915-17, 85,000 1927-59 BEAR RIVER 10- 440__ Bear River at Harer, Idaho...... _, .,..,..... 2,780 1914-59 4,450 905.. Bear River near Preston, Idaho.______. 4,300 1890-98, 7,650 1900-02, 1904-16, 1944-59 1180,. Bear River near Collinston, Utah ,___.._._..__. 6,000 1890- 11,600 1959 WEBER RIVER 1295.. Weber River near Wanship, Utah______.______320 1951-55, 1958 1305. Weber River near Coalville, Utah. 438 1927-59 1320. Weber River at Echo, Utah-_____. 732 1927-58 4,800 1335. Weber River at Devils Slide, Utah. 1,100 1905-55 6,500 1360. Weber River near Morgan, Utah . 1951-55 8,000 1365. Weber River at Gateway, Utah.-.. 1,610 1890-93, 9,800 1895-99, 1901, 1921-59 1370. Weber River at Ogden, Utah ______1951-58 9,200 1410. Weber River near Plain City, Utah 2,060 1905-59 10,000 PROVO RIVER 1595. Provo River below Deer Creek Dam, Utah 560 1954-59 2,870 1605. Provo River near Wildwood, Utah______590 1939-49 3,030 1610. Provo River at Vivian Park, Utah ______600 1912-59 3,200 1630. Provo River at Provo, Utah ______680 1903-05, 2,550 1934, 1937-59 SEVIER RIVER 1915. Sevier River below Piute Dam, near Marysvale, 2,440 1912-59 2,700 Utah. 1940. Sevier River above Clear Creek, near Sevier, Utah 2,700 1914-16, 2,700 1939-55 22 FLOODS IN UTAH. MAGNITUDE AND FREQUENCY Table 2. Main stem gaging-station data Continued Drainage 50-year Station Period of Gaging station area flood No. record (sq mi) (cfs) SEVIER RIVER Continued 10-1955__ Sevier River at Sevier, Utah. ______2,850 1917-29 2,900 2050_. Sevier River near Sigurd, Utah ______3,340 1915-59 2,500 2170__ Sevier River below San Pitch River, near 4,880 1918-59 2,700 Gunnison, Utah. 2190.. Sevier River near Juab, Utah ______5,120 1912-59 2,200 2240_. Sevier River near Lynndyl, Utah ______6,270 1914-19, 1,740 1943-59 2280.. Sevier River near Delta, Utah ______7,380 1913-19 1,600 2315__ Sevier River at Oasis. Utah ______8.080 1913-27 1.580 CLOUDBURST FLOODS AND MUD-ROCK FLOWS Some drainage basins are subject to more cloudburst floods than others in the same Cloudburst floods and mud-rock flows are general locality because of physical features common to the southern parts of both the such as topography, aspect, vegetal cover, Colorado River basin and the Great Basin in and other contributing factors. One example Utah. These phenomena occur at less fre is Pleasant Creek near Mount Pleasant where quent intervals in other areas of the State. a large alluvial cone covering more than 12 Most of the annual flood peaks in the southern square miles is evidence of frequent cloud regions occur during the summer storm burst flooding. period, as previously described, and are in cluded in the flood-frequency relations for A cloudburst flood may occur without pro that region. In contrast to the annual floods ducing a mud-rock flow. Although mud-rock observed in the snowmelt regions, occasional flows may be associated with cloudburst peak flows resulting from cloudburst storms floods, the presence of certain soil conditions in the central and northern parts of the State are required to produce them. Mud-rock may have yields of 5,000 cfs (cubic feet per flows are flows of mud, rock, debris, and second) per square mile or more from small water, mixed to a consistency similar to that drainage basins. This is not to imply that of wet concrete. A wide variety of these these excessive flows occur more frequently flows has been observed from those carry in any one section of the State; instead, it is ing a small load of sediment to others moving to acquaint the reader with the fact that these large amounts of mud, rock, and other debris. events have been observed. Although cloud Some flows have just enough water to lubri burst storms may occur on many days in one cate the mass of moving material and usually season and may be distributed over a rather travel at a relatively slow velocity. Because wide area, the high-intensity rainfall is lim of infrequent observation of these flows, it is ited to very small areas, often less than 1 difficult to estimate the probable recurrence square mile. Observations of rainfall inten interval at any given site. It has been noted sities from unofficial observers have shown by Wooley (1946) that the total discharge of that as much as 7 inches fell in less than 1 debris and water can exceed 2,000 cfs per hour during one of these storms. The prob square mile. ability of a cloudburst or high-intensity rain fall recurring in the same small drainage The cost of replacing bridges, culverts, area during consecutive years is rather re and sections of highway and other economic- mote. Experience shows that in some drain factors will determine whether design should age basins, the frequency of floods may vary be for rare floods of the above type which from three in one year to one in each of three may have a recurrence interval in excess of consecutive years to none in several years. 50 years. LIMITATIONS 23 APPLICATION OF FLOOD-FREQUENCY CURVES 6. Estimate the magnitude of the flood by multiplying the mean annual flood (step 4) by The application of flood-frequency curves the flood ratio (step 5). to any drainage basin within the scope of this report required analysis of the basin char A complete frequency curve for a site may acteristics vfrhich effect peak discharge. be derived by repeating steps 5 and 6 for The significant characteristics are those various recurrence intervals. most readily available from topographic maps. USE OF FLOOD-FREQUENCY ANALYSIS ON MAJOR RIVERS METHOD To determine the magnitude of the flood for a 50-year recurrence interval at a given To obtain the magnitude and frequency of location on a major river, scale the mileage floods for any basin within the study area, of the main stem from a known reference except on major rivers, the procedure is as point as indicated on each graph (figs. 7 17). follows: Use of the main stem analysis will require some knowledge of local conditions and the 1. Determine the drainage area above the operation of any reservoirs involved. The site by planimeter or other acceptable pro Weber River is the only major stream that cedure. has a specific flood-control plan. The pro posed magnitude of the controlled flow is 2. Locate site on figures 2 and 5 and de shown(fig. 15) with the flood experience prior termine the flood region and hydrologic area to the initiation of this plan. involved. 3. Determine the mean altitude for drain LIMITATIONS age basin above the selected site, unless ba sin is in hydrologic area 3. In this report the The results of this study may be used to mean altitude was determined by using a grid estimate the magnitude and frequency of system of rectangular coordinates as an floods, with recurrence intervals up to 50 overlay on topographic maps. From a list of years, for any drainage area in the State with the average altitudes of each square, an ar the exception of the Great Salt Lake Desert ithmetical mean was computed. The squares and the Snake River basin. were of equal size and small enough to give a representative sampling, but not of a known The relationships of the hydrologic fea scale. Contour maps of the Army Map Series, tures of the drainage basins were defined scale 1:250,000 were used. from the records on streams with natural flows. To obtain the magnitude of any flood 4. Determine the mean annual flood for the on any regulated stream, adjustments should site from hydrologic area curve selected in be made for manmade development. Also, the step 2 (fig. 6). One exception is that to de varying effects of geology and vegetal cover termine the mean annual flood of the Dirty were not given detailed consideration in this Devil River near Kite, Utah (9-3335), use hy study. An investigation and evaluation of drologic area 8 curve. This exception applies these factors may be justified in some spe only to the main stem and should not be used cific areas. for the tributary streams of the Dirty Devil River outside of area 8. The results obtained from the flood- frequency study are limited by the basic data 5. Determine the flood ratio to the mean available. The degree of confidence that can annual flood for the selected recurrence in be placed in the results is closely related to terval from the appropriate flood-frequency the quantity of data used in defining the var curve obtained in step 2 (figs. 3 and 4). When ious curves. Extrapolation of curves beyond the drainage area above the site is near an the limits shown is not recommended, and other flood region or lies in two regions with values obtained by such practice may be widely different flood ratios, it is suggested greatly in error. that a weighted average of the two flood ra tios be used. 24 FLOODS IN UTAH, MAGNITUDE AND FREQUENCY SELECTED BIBLIOGRAPHY Linsley, R. K., Jr., Kohler, M. A., and Paulhus, J. L. H,, 1949, Applied hydrology: New York, Bodhine, G.L., and Thomas, D.M., 1960, Floods McGraw-Hill Book Co. in Washington, magnitude and frequency: Powell, R. W., 1943, A simple method of esti U.S. Geol. Survey open-file report. mating flood frequency: Civil Eng. v. 13. Fisher, R. A., 1954, Statistical methods for Thomas, C. A., Broom, H. C., and Cummans, research workers: New York, Hafner Pub J. E., 1961, Magnitude and frequency of lishing Co. floods in the United States, Part 13, Snake Gumbel, E. J., 1941, The return period of River basin: U.S. Geol. Survey Water- flood flows: Annals Math. Statistics, v. 12, Supply Paper 1688 (in press). no. 2, p. 163-190. Woolley, Ralph, R., 1946, Cloudburst floods Langbein, W. B., 1949, Annual floods and the in Utah, 1850-1938: U.S. Geol. Survey partial-duration flood series: Am. Geo- Water-Supply Paper 994. phys. Union Trans., v. 30, p. 879-881. INT.DUP..D.C.62-^1*6