2 2 5 111/12.8 11111 .5 ~:t 1.0 ~; IIIF8 1111/ . 2 ~.;; IIIII~~ S.i.i; 1111/3 .2 11111,i!i, Lu;.; IIm36 n~: lil_ .: "11,,-0 I' t- f:~~ t Il'~ I 2.0 1- ~ L. ~ I-.... Lo. 1.1 11:ll "'L.~ 111111.8 111111.8 111111.25 111111.4 111111.6 111111.25 111111.4 111111.6 MICROCOPY RESOLUTION TEST CHART MICROCOPY RESOLUTION TEST CHART NATlON~1 BURf AU Of "TANDARP;; 1%) A MlltlNAt f<llRIAIf Ol ',TANDAmh 1%' A " ,',' ORVtILLE A. £A!!IS 4s$ociat~Iri:igotion Engineer-;.DiVision 'ofIrrigar,iori Bureau of AlZricul.turol Enllineerinrl ·",­ • '(;; ""0 UNITIIDSTATES DEPARTI•.ffiNT OF AGRICULTURE WASHINGTON, D.C. THE SILT LOAD OF TEXAS STREAMS 1 13,Y ORVILLE A. FAllIS, a880ciaCe ~rrlgation engineer, Div.i8icn ot Irrigation, B1!TCaU of AgricuZturaZ Engvneering CONTENTS Page Page. Intr;Muction__ _______________ _____________ 1 Silt survey of Medina Reservolr-Contd_ :R'Ti,~.;lqus silt Investigations in Texas________ 3 Nature of silt In'MedIna,Reservolr__._.. 26 Thill. andmethods__________________________ 4 Silt in Lake Worth_______________________ 30 8exotElIng equlpment.__________________ 5 Reserv.,ir effective as slit trap___________ 32 M1l od ,of samplIng_____________________ 7 Cbaract4lr of siltln Lake Worth_________ 33 Laboratoly methods____________________ 8 Lake Kemp________________________________ 37 Bed sIlt____ ~____________________________._._ 8 Silt In the old reservoir at Clsco_____________ 39 Dlstribution of silt throughout streexo cross The AustIn Reservoir on the Colorado River tosection ,elocity and__________________ relationship of quantity.___________ of silt 11 of Texas at Austw______________________ 40 Determination of the SUtload______________ 46 Velocity and silt-percentage curves____ ~_____ 13 Prevention of sllting_________________________ 49 Mean percentega of silt, by weight, In cross ReIiloval of slit from reservoirs______________50 sections_______________________________ 15 81lt problems oUhelower Rio Grande Valloy_ 60 Graphical comparison of dlscharge andsUt Chemical composition of'BrazllB Riverwnter. 52 ~ntaga..------------------------------ 17 Summaryand conclusions___________________ 52 echaulcal analyses ofsuspendedsIlL_______ 19 Literature cited____________________________ 66 .ReIstionship between percentages of silt by Appendix__________________________________ 67 weight and by volume after settlementior 7 days__________________ ___________________ 19 81l:asc:r~~<!n~lh;~~e~r&e;r:h:=d Silt survey of Medin& .Reservoir____________ 20 States OeOlOgl~8urvey------------- 57 EffecttlIled of with flood clear ente.ting water a_________________ reservoir partly 23 INTRODUCTION Qne of the most important and valIDlble of the natural resources of Texas is its water supply. The development of all other resources, the extension of agricultural areas, and the growth of cities are largely dependent upon properly controlled supplies of water suit­ able as to quality and sufficient in quantity. A stream that periodi­ cally overflows its banks, submerging fertile bottom land, is a menace to life and health and a significant factor in retarding the progress of the community "ihich it .should serve. In Texas, where stream flow is generally intermittent, being sup­ plied by ,rainstorms of varying mtensities occurring at irregular intervals, the full utilization of the water resources mlWt necessarily include provision for the impounding of flood water in stora~e reservoirs. The function of such reservoirs is twofold-to retam the water for future use and to play a very important part in .flood control by .materially reducing the peaks of .floods that in~date iertile areas adjacent to the streams. 1 Prepared in cooperation with the Texas Board of Water Engtneers. 181593-33--1 1 TEOHNIOAL BULLETIN 382, U.S. DEPT. OF AGRIOULTURE I:; Run-off,erosion, and transpOrtation of eroded ~dweathered ma­ ,terial are affected by many diverse factors, the more important Qf which~l:'e amount and intensity of rainfall, topography, structure and texture of the soil, and amouni; and'!lharacter of the surface cover. The majol' portion of tl.1e eroded and weatheredmatenal car­ ried into a reservoir by the supply stream is deposited all soon as the velocity of the inflow is sufficiently diminished. Such deposits reduce the storage capacity. and, in some instances, where proper allowance .for the accumuIatJon was not made, the usefulness of the reservoirs has been destroyed. A vital requirement of a successful storage reservoir is that its, life-the interval between its initial use and the time when its ca· pacity is reduced by the deposition of silt to the extent that it n() lon~er serves the purpose for which it was designed-shall be of sufficient length to return the cost plus a reasonable profit. 011e of the important problems to be solved in properly design­ ing a storage reservoir is that of estimating the silt load of the supply stream so that capacity can be provided to take care of the deposited material long enough to insure the economic soundness of th~ project. The beEt'basE for estimating the silt load is a long­ term record of silt measurements made at or near the reservoir site. However, when the construction of a new reservoir is contemplated such records are seldom available and it is rarely practicable til) delay the project several years while one is being made. The result is that estimates are based on short-term or fragmentary records at the site, or on records taken in another locality where conditions are assumed to be comparable. Either procedure may lead to serious error. In compliance with numerous requests from engineers practicing in the State and in recognition of the n.eed for definite information cOlicerning the silt load of Texas streams that might assist the State Board of Water Engineers in passing upon applications for permits to store flood water, an investigation was inaugurated in 1924 and is still being carried on under a cooperative agreement between the­ Bureau of Agricultural Engineering, United States Department of Agriculture and the Texas Board of Water Engineers. Thefol­ lowing report is the first progress report of the studies thus far made. The following organizations have assisted in the collection of water samples and other associated work: United State~ Geological Sur~ vey, International WaterComlpission, Walker-Caldwell Water Co., Wichita County water improvement district no. 1, and the city of Waco. Stream-flow records and drainage-basin areas havebeen fur­ nished by the water resources branch of the Geological Survey. R. G. Hemphill,2 irrigation eng,meer, had general supervision of the .investigations until May 1930. The writer has been inimmediate eharge of most of the field, office, and laboratorv work since the in­ ,ception of the project. Sin~ September 1930 F. J. Fricke, junior civil engineer, has assisted with the laboratory work and the pro­ tracted computations required in connection with the ta,bulation of the daily records. • ltalIllnumbers in parentheses refer to T.iterntul'e Cited, p. 56. THE SILT. LOAD (IF TEXAS STREAMS 1~~/.• 'Jl., '~itEVIOUS. SILT INVESTIGATIONS IN TEXAS During 1899, 1900, 1901, and 1902, J. C. N aO'le directed a silt investigation which. included Brazos River at Jones Bridge near College Station .and ·Wichita. .River at Wichita Falls (13, 14,15).3 On Brazos:Riyer at Jones Bridge, samples were. t-llken atirregular in­ tervals of from 1 to 90 days 'from May 29,1899, to December 31, 1902. The samples were placed in glass tubes and after 7 days' settlement the percentage of silt by volume was determined from the heights of the prisms of silt in the lower parts of the tubes and the clear­ water columns above. A Jew of the samples were allowed to remain in the tubes for 11 months. The percentages of silt by volume were determined after 3D days and also at the end of the 11-month period. At the end of 30 days there was an additional shrinkage of 10 per­ cent over that of '"{ days and at the end of 11 months an additional shrinkage of 15 percent was noted. There was no evidence that .shrinkage had ceased at the end of the ll-month period and it seem£' reasonable to suppose that still greater subsidence would have belm found if the period had been extended. Using the percentage of silt by volume after 7 days' settlement and assuming that the percentage of silt determined at irregular intervals would, with some modification based on the color of the muddy water, apply to i;he intervening period, an estimate was made of the volume of silt passing the section. A 25 percent reduction was ap­ plied for the purpose of estimating the> volume after settlement for 1 year (see table 1). TABLE 1.-SuftI,mary of silt mea8urement8, Braz08 Rive,., a.t Jone8 Bri'dge 1 Total dis- Silt, 1 week's set- Silt, 1 year's set­ 1'ime charge tlement tlement Acre-feet Acre-feet Percent Acre·feet P~anl Aug. 1 to Dec. 31, 1899__________ J _____.____________ 1,165,300 10,090 0.866 7,567 0.649 :ran. 1 to Dec. 31, 100(L__________________________ 8,806,986 115,782 1.315 86,837 .986 Jan. 1 to Dec. 31, 100L___________________________ 976,602 12, 328 1.262 9,246 .1147 Jan. 1 to Dec. 31, 1002_____________________________ 3,362, 991 ~O, 190 1.1:15 30,142 ---.8IlII Total·____________________________________ 14,311,8791 178,390 1.241l 133,792 .1135 1 U.S.Dept.Agr., Oft'. :Expt. Stas. Dul.l33 (16, p. 105). • For 41 months. Daily samples taken since June 1924 at several stations on the Brazos River indicate that the color of the water depends upon the color of the soil where the flood originates and not upon the charge of silt in suspension. They indicate further that serious error may result from extending the periods between samplings. On Wichita River at Wichita Falls (19, 14) samples taken at irregular intervals of from 2 hours to 228 days, between May 21, 1899, and February 15,. 1902 were treated in the same manner R..C; those from Brazos River at Jones Bridge.