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ffiffi$"fl*tffiH,qt#PYUNIVEBSITY OF MINNESOTA

ANTHONY FALTS HYDRAUTICTABORATORYTORENZG. STRAUB,Director

Techniccl Paper No. 15, Series B

Straight Drop Spillway Stilling Basinby

Chcsles A. Donnelly qnd Fred W. BlcisdellHydraulic Engineers, USDA, AnS

November, 1954Study conducted by

uNrrED TATES EpARTMEiwr AcnlcutrunEAGRICUTTURAT RESEARCHSERVICE

SOIL AND WATER CONSERVATION NESEANCHBRANCH,r in coopercrtionwith the

Minnesots Agriculturcl Experiment Stationcrnd the

St. Anthony Folls Hydrculic Lcborctory

ST.

EDBUNIVERSITY OF MINNESOTA

ST. ANTHONY FALLS HYDRAULIC LABORATORYLORENZ G. STRAUB, Director

Technical Paper No. 15, Series B

Straight Drop Spillway Stilling Basinby

Charles A. Donnelly and Fred W. BlaisdellHydraulic Engineers, USDA, ARS

November, 1954Study conducted by

UNITED STATES DEPARTMENT OF AGRICULTUREAGRICULTURAL RESEARCH SERVICE

SOIL AND WATER CONSERVATION RESEARCH BRANCHin cooperation with the

Minnesota Agricultural Experiment Stationand the

St. Anthony Falls Hydraulic Laboratory

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ST.UNIVERSITY OF MINNESOTA

ANTHONY FALLS HYDRAULICLABORATORYTORENZG. STRAUB, Director

Techniccl Pcper No. 15, Series B

Straight Drop Spillway Stilling Basinby

Chcrles A. Donnelly qnd Fred W. BlqisdellHydrculic Engineers, USDA, ARS

November, 1954Study conducted by

UNITED STATES DEPARTMENTOF AGRICUTTUREAGRICUTTURATRESEARCHSERVICE

SOIT AND WATER CONSENVATION RESEARCHBRANCHin cooperation with the

Minnesota Agriculturcl Experiment Stcrtioncrrd the

St. Anthony Fclls Hydrculic Lcborctory

UNIVERSITY OF MINNESOTAST. ANTHONY FALLS HYDRAULIC LABORATORY

LORENZ G. STRAUB. Director

Technical Paper No. 15. Series B

Straight Drop Spillway Stilling Basinby

Charles A. Donnelly and Fred W. BlaisdellHydraulic Engineers. USDA. ARS

November. 1954Study conducted by

..

UNITED STATES DEPARTMENT OF AGRICULTUREAGRICULTURAL RESEARCH SERVICE

SOIL AND WATER CONSERVATION RESEARCH BRANCHin cooperation with the

Minnesota Agricultural Experiment Stationand the

St. Anthony Falls Hydraulic Laboratory

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4 g g g & aThis paper deseribes the development of the generalized desi-gn

ru.J.es for a nero sti.lling basin .for use lrith the straight drop spillway.This generallzed. still.ing. basin design'was devetoped because experieneein the field hacl shown hat 'there was' no 'satisfactory si;il l ing basi-n fo rthe straight drop spillway. Ho'r+ever, iln:ited field eq:erienee indicatesthat this ner"r design will adequately protect the dor.n:strear* eharrnel fromscour.

!'fater'fa11ing over the spillway crest falls onto a flat apron. Thenappe i-s broken up by floor blocks, ''rhich also prevent damaging seour ofthe dowrstream charurel banks" $cour of the domstreasr .char:nel bed i-s pre-vented. by _an end siJ.].. . Flaring r.ringtralls, triangular j-n elevaiicnr Fr-vent erosion of ift" a*to ill, For proper operaticn of the stil]ing basin,the contraction of the fl-ow at the ends of the spillway openi-ng must bepartial-ly suppressed.

The still ine.basin can be'used for. :on the crest, crest length, height of drop,

rt tTl

t

a rc-i-de "ange of discharge, heada-nd dolrrrstrearn tailwater fevel.

3n i:eportant f.endipg is that the stl11ing basin length computedfor the $dni.nun tailwater leve1 required for good perfo::aance may be in-adequate at hj-gher taiiwater levels* Dangerous scour of the downstreaunchannel may occu.r if the flappe is supported srrfficiently by h:igh tailwaterso that it lands beyond the end of the .stil1ing' basin. A method of cora-puting the stilling basin length for all tailwater l.evels i-s presented.

The design rules developed as a result of the laboratory test'swere carefully cheeked and verlfied. An exarnplesholrs how these nrles areapplied to the design of a field strueture.

1 T 1

A B S T RAe TThis paper describes the development of the generalized design

ruJ.es f or a new s t i l l in g basin .for use with the s t raight drop spillway. s general ized st i l l ing , basin design ,was developed because experience

in the f ie l d had shmm tha t 'there 'was ' no 'satisfactc:ry st i l l ing basin fo r.., e s t ra ight drop spillway. However, l imited f ield experience indicates_a t this new design wil l c:deq-qately protect the downstream channel from

scour .vJater fa l l ing over the spillway cres t fa l ls onto a f la t apron. The

nappe i s broken up by f loor blocks, ,ihich a:lsoprevent damaging scour ofthe dovmstream channel- banks. Scour of the downstream channel bed i s prevented by an end s i l l . :F'lar ing W i r : : g v m ~ l s , 't r ian gula r in elevation, prevent erosion of the dam f i lL For proper operat ion of the st i l l ing basin,t he contract ion of the flow a t the ends of the spillway opening must bepar t i a l ly suppressed.

The st i l l ing basin can be used for a wide range of discharge, headon the cres t , c res t length , height of drop, and dovmstreamtailwater leve l .

An p o r t a n t f ~ n d i ? i s that the s t i l l in g basin length computedfo r the minimum ta ihrater level required for good performance may be in -adequate a t higher tai lwater levels . Dangerous scour of the downstreamchannel may occur i f the nappe i s support-ed s tuf ic ient ly by high ta i lwaterso tha t it lands beyond the end of the Bti l l in g basin . A method of comput ing the st i l l ing basin length for a l l tai lwater l evels is presented.

The design rules developed as a resul t of the laboratory tes tswe re carefully checked and verif ied. An example ShOvlS how these r ule s areapplied to the design of a f ie ld structur e .

iii

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r i n X T r n i i t \ T T qu v l \ l u M v

A b s t r a c t . . . t . ' a t . ' . . . . . t . .List of I l lustrations . . r . . . ' . ' . . ' ' . .r . . .I r i s t o f T a b l e s . . . r . . f . , . . , . . . . .L i s t o f S S t i r b o l st I r . r . r . e ' . . . . r e . . t . t .F r o n t i s p i e c e . . . . . r ' . . . . . .

A P P S . . I A T U S A 3 ] l P R C C I D U ] i E. ' . ' i . ' . . ' , . . . o . . ,m:qm DTc-rTlT rt qI!rl" l l l luuir.l l i . t . e . . . . . . a . . t . .L e n E t h o f B a s i n . . . . . . . . . . . r . . . . ' II { a p p e T r a j e c ' ; o r y. . . . . . . ' r } r . } . .D i s t a n c e o F l o o r B l o c k s . ' . r . , . . . . . , . .D i s t a n e e t o E n d S i l l ; ' r . . , ' ' . . .T a i l w a t g r n e p t h . . ' . . i e r . . . . ' ' . . . ' I

Floor Bloek and End Sill lieight . . .Ffoor Block i5dth and Spaci [g . r . r . ] . . . ) . ' I . . . .S i d e l t a l l H e i g h t . . . . . . . . . r ' . . . . e . . . . a .f i i n g w a 1 l s . . . + . I a . . . , . . . . . . 'A p p r o a c h C h a r r n e l. . . . . . . .A e r a t i o n U n d e r l { a p p e} i . . . . . ' i , . . 'C h e c i i T e s t s . . ' . . .SUI'rlUlftJ. . . . ' . t l . . . . t

EXj{{PLE 0$' APPIJCATI0}'] . . + . . . . . . | . .' . . . . . . r . .' l f i k] i nry:.rnlrrrIJILJJ-JLJ6IdIJ1IJ/ . . . . . . . . . . . . . . . I | . . '

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CON TEN T SAbstractList of Il lustrationsList of TablesList of S;ymbolsFrontispieceINTRODUCTION .PREVIOUS HORKAPPARATUS AiiJD PROCEDUHETEST RESDLTSLength of Basin

Nappe TrajectoryDistance to Floor BlocksDistance to End Sil lTailwater Depth

Floor Block and End Sill HeightFloor Block vJidth and SpacingSide"t-Jall Height"HingwallsApproach ChannelAeration Under Nappe Check Tests

SU lvlHARYEXA1IPLE OF APPLICATION Bibliography . ..

iv

'. ..

..

..

Pagei i iv

vv i

vii i12355510

121317192324242526272935

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! r ! . 9 9 e g ! l g g I g g S , I g g g .FigureF r o n t i s n i e c e .i I r . . . 3 . . . c . r . o 1 . . o r . .

1 t e s t A p p a l ' a l u s . . , f r f . . r . . . . . * 1 .2 Design Chart for De'i:errni::ation of ** , . .: o ' . *3 The Bed Scour fs leeper than Necessary and the Bank'Ssour Is Excessive Beeasse of k:proper location orAbsence of the Floor Blocks ; . . r . . l r . r . .l+ Floor Blocks Located 0,8 dc fronr the Point',rlherethe NappeStrikes the Basin Floor and 1.75 dc from

the End of the Stilling Basin Give l{axj:nura Protectionto the Bed and Banks * | . . . r . . o a .5 Floor Blocks lo*ated. 0.75 de fron the End of theStiJ.ling Basin Are Ineffeetive; t'he Bed Scour j-sDeeper ihan l.lecessary, ancl the Fank $cour is Exces-

sive . . . . . r a r . . . . r . . . . I a6 Determination of ?ailwater Depth . . . . ) * ., c .7 effect of Taihraier Ievel on Downstrearn cour . r ,n ^E SeourNear End of St i l l ing Basin . ' I r ' | . r . I9 Effect of Proportion of Basin i""Iidth 0ccupied byFloor Blocks on $cour of Fed arrd Banks r . . . .". ,

10 Straight Drop Spil}*ay Stilling Basin . . . . . . .11 Example of StraightDrop Spilhray Stilling Basin De-signed. for illgh Taj-},rater' . . . . r . . . . .. . c12 Hr,anpJ-e f StraightDrop Spillway Stilli:rg Basin De-"signed for Nc:raa1Tailwater . . . . . . . . ' t ..f

a t t

a f *

a l t

Pageviii

11

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22^ O1 0

32

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a c l

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1III I

Tft.LV

! I E T g T T A E I E gFloor Block and find $i11 Height . . . . r . , . . . . . ' r . 18F l o o r B l o c k ' ' r r l : - d t h a n d S p a c i n g . . r r ? r ' . r . . r I f , , 2 A$iderrlall Height ?gsts c c . r . . . . . . . . . ' s * r r. 23S u i ? m a r y o fh e c l c T e s t s. ! . . . . . . . . . . . r . , ' 2 6

L IS T o F IL L U S T RAT ION S- - - - - _ .... _- - - - -FigureFrontispiece .. .. .. .. .. .. .. .. .. .. .. . . .I

23

Test Apparatus .. .. .. .. .. . Design Chart for Determination of x . . . . aThe Bed Scour Is Deeper than Necessary and the Bank .Scour Is Excessive Because of Improper Location orAbsence of the .Floor Blocks ................. . . .4 Floor Blocks Located 0.8 dc from the Point 1rnlerethe Nappe Strikes the Basin Floor and 1.75 dc from

the End of the Sti l l ing Basin Give Maximum Protectionto the Bed and Banks .. .. .. .. .. 5 Floor Blocks Located 0.75 dc from the End of theStill ing Basin Are Ineffective; the Bed Scour isDeeper than Necessary, and the Bank Scour is Exces-6789

101112

TableNo.II IIIIIV

sive .. . . . . . . Determination of Tailwater Depth .. Effect of Tailwater Level on Downstream Scour . Scour Near End of Still ing Basin .. .. .. .. .. ..Effect of Proportion of Basin Wdth Occupied by .Floor Blocks on Scour of Bed and Banks .

... . .. .

Straight Drop Spillway Stilling Basin . . . . . . . . . . . . .Example of Straight Drop Spillway Still ing Basin De-s igned for High TailT,vater .. .. . . . . . . . ..Example of Straight Drop Spillway Still ing Basin De- .signed for Nonnal Tailwater . . . . . . . . . . . . .

L IS T o F TAB LE S;

Floor Block and End Si l l Height .. . Floor Block vJidth and Spacing . .. Si deHa ll Height Tests ..

. . . . . . . . ...SUffi,uary of Check Tests . . . 0 .. 0 . . . .

v

Pagevi i i

39

11

12

1315161622283233

Page18202326

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ar nX ?1 q

Y"T

Xd

ciisiaace frora tailwater'surfaee to floor of stilliirg basin^ t ocrj-rica1 depth = lf (r;/L)'/E = Q/l) n

acceleratlon due to ;ravi-tYspecific head n'approach to crest = depth plus veloclty head : (3/2) decrest length = stilling basin rridthminimruii stilling basin length = *o * 1,

* *"dischargeerj-tica1 velocityhorizontal distance from crest to point where upper surfaee of free-falling nappe strikes still.ing basin floorhorlzontal distance frorn crest to point at which avel:age of uppersurfaces of free-falling and tangent neppes strikes stilling basinf ' tnnr . = (u + - ) /2 "-F' r\fj]distance to floor blocks fron point at which average of upper su.r*faces of free-falling ancl angent nappes strikes stilling basin floordl.stance from upstreara face of fLoor blocks to enci.of stilling ba.sinhorizontel d"istance from crest to upper surface of free-failing nappehoriaontal dist;rnce.fron cfest to Uppe-rsurface of suhi:rergednappehorizontal distance from e_re6t o point at,whj.eirupper sr.rrface of free-falling nappe plunges .inio tailwaterhorizontal distance from crest tc point where upper surface of tangentnaplre strikes stilling basin floor

"y vertical d.istanee fron cyest to stilling basin floor (y is negative)y* vertical dlstance frpm crest to upper surface of free-falling nappetr (y- is positive abovd the crest and negative below the crest]Jr, vertical clists.nce from crest to t,arl-tra,ter surface (n is positiveu 1.rhenire tai}w&ter.surfaeeis above the crestr negativeirhen the tail-r,rater surfaee is belor'r the crest)

\n

d2dc

xa

x cxn

yn

1 1 S T o F SYMBO L Sdistance from t aihrater ' surface to f loor of s t i l l ing basincr i t ic al depth = 3 (Q/L)2/g = (2/3) H, ' .acc elerat ion due to grc:tvity specific head in ' approach to crest = depth pius velocity head := (3/2) dccres t length = s t i l l ing basiriwidthminimum st i l l ing ' basin length =x t x. + xa D cdischargecr i t ica l velocityhorizontal distance f romeres t to point where upper surface of freefa l l ing nappestr ikes .s t i l l ing basin f loorhorizontal distance . from cres t to point a t 'timch average of uppersurfaces of free-fal l ing and tangent nappes strikes s t i l l ing basinf loor ' = (x + x )/2"F '1'distance to f loor blocks from point at lfhich average of upper surfaces of free-fal l ing and tangent nappes s t r ikes s t i l l ing basin f loor

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STFAIG}{T !]lfiF $i}II,,]"'i";AYSTILLI}{0 BJTSIN-X.

II{fR,O}UCTICl'J

The straight drop spilh.;ay is, just as the name 3mplies, a straigirt:;erfal1 r.reir. Ti-te water flor'ring over" the spillway fa1ls onto a horizcnt-a1 apron. ?he energ;in ihe '.+e-te:' s dissipatedby rieans of blacks, si-l ls,anC 'i;aik;ate::. lhe wets:" is discha.rged fron the stilling h,asin int,o theCc',.i-nsireamchannel in such a inarlner as 'bo prevent deiaaging so1:.r,

The straight drop spil l i"ay is useC as ar1 erosion control- struc-+.ure in gu1Iles, as a .::r 'edeconircl structure in drainage ditches, as anirriaa' j-on drop and cireck stnrcture, an.,i. s l spil}"'ry for eertir d.a:s.

A generalized clesign for a s'braight, drop spil lr 'av sti l l ing ba.sinr+a-sdevelcpecl" as a result of ihe tesi,s Cescri-bed in this paper. The de-sign is applicablc to relative heights of fal-l rar;ing fron I.O y/a^ to^ J l -t> y/a^ and r,o crest len;ths ;reatcr than 1.5 d^. l{ere y is the ver-9 Vtical distence bet'..,'een he crest and the stilling basin floor, asrd d" isthe critical dei:i}: af f1ow. '

: : l '

fhi-s e:ryerimental in'restigation'"+as begun early ::n7951- at ti:e re;quest ojl the En,qineering Council of the Soil Conserraticir Serrice, U. S.DeparLnent af Agriculture. :t wa.s corai:leied late tn Ig53. Tire study wasnad.e by the steff of 'Lhe Ag.ricul-tural Research Sers'j-cu-"t* lo"*ted. at theSi. An'bl:any FalJ-; I{rdraulic T,ab*ratcry, eniversii;y of I'Ij*nesotae }iinneapo-lis. There the Agricultural Research,$ervice, the iriLnesota AgriculturalE4perinent Station, and the St. Anthony Fells li-v-Cra'u1ic taboratory eoopr-ate in .L,hesolu'tion cf pro'cIens eoncerning consenration h)'draulics. Thestuclies uhich are being cono.uci;eciby the hSricr,-Itural Research Service atthe St. A:rthon], lalls Hyd:'iutic Laboratory of the Lnivei'sity of l.{ii'rnesotaare eC,rainistered by the l"ie.tcrshed ii'rdrclogy Secticn of tha Soil and t',aierConservation -"eseaz'chBranch. The testi:rg and much of the analysls rrasdone h,;r Cha::"les A. Doane11y, iSdraii l ic Engineer, Agricultural I'eseareh

" i t *+ ^" , r + . , *a1 : - ] .CSgafCh Se- ; - f * ^^ - r^*^-+ r ;^ r , - r^ t )^JigI'l-CUIAUv n_ aCe repor! llo . Lll-)U4-)1.:-q).4-,"The stuCiesat the Si;. Ani;hony lalls H]'d:'aul-ic La'boraiory, initiated

by Soil Conservation $ervice - Resee-rch on January 1, 19110, :ere t:'e-ns-ferred to Agricultural Research Serrrice on Janua"ry 7, L95\.

...

-xSTRAIGHT DROP SPILUJAY STILLING BASIN

IWffiODUCTION

The stra ight drop spi lhmy i s , ju s t as the name :Luplies, a s t ra ighto-.-er fall vJe i r . The ,,-rater floHing OVel" th e spi lhmy fa l l s onto a horizonta l ap ron . The energy in the lpJa te r i s dissipated by means of blocks, s i l l s ,an d t a ihmter . The 'Hater i s discharged from the s t i l l ing basin into thedo';m stream channel in such a manner as to prevent daJrtaging s.cour.

The s t ra ight drop spilhJCW is used as an erosion control s t ruc-ture in gull ies , as a grade control structure in drainage ditches, as ani r r igation drop an d check s t ructure, and as a sp i lhJay for ear th dams.

A general ized design for a s t ra ight drop spilhJay s t i l l ing basinHa s developed. as a resul t of the tes ts described in this paper. Tl:l..e desiE;n i s appl icable to relat iVe heights of f a l l ranging from 1 .0 yld toc15 y l d and to cres t lengths greater than 1.5 d c Here y i s the ver-ct i c a l distance betv,een the cres t and the s t i l l ing basin f loor , 2,ndt he c r i t i ca l depth of .floH.

dc i s

This e).rperitnental investigatior1vJasbegun ear ly in 1951 a t the re..:.que s t of the Engineering Council of the Soi l ' Conser-vatiem Service, U. S.Depart.ment of Agriculture. I t vIas completed la te in 1953. The study Hasmade by the s ta f f of the Agricultural Research S e r v i c e ~ H < - located a t theS t. lmthony Falls I tJdraul icLaboratory , Universi ty of Hinnesota, l'hnneapol i s . There the Agricultural Research Service, the Hinnesota AgriculturalExperiJnent Station , and the St. Anthony Fa:lls Hydraulic Laboratory cooperate in the solut ion of problems concerning conservation hydraulics. Thestudies Hhich are being conducted by the Agricultural Research Service a tt he S t. Anthony Falls Hydraulic Laboratory of the University of Hinnesotaare administered by the Vatershed Hydrology Section of the Soi l and itJaterConservation Research Branch. The tes t ing and much of the analysis wasdone by Charles A. Donnelly, Hydraulic Engineer , Agricultural Research

- l ~ A g r i c u l t u r a l Research Service Report No. hl-504-52 ., ~ - l ~ T h e studies a t the s t . Anthony Falls Hydraulic Laboratory, in i t ia ted

by Soi l Conservation Service - Research on January 1 , 1 9 L ~ o , 'were t rans-ferred to Agricultural Research Ser-vice on JanuaI"J 1, 1954.

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STF,AIGIN'NftCP SIJI1,1,I,,.iAYTIILI$G E/r.SITf-,.

Ii']TN.ODUCTION

?he siraight drop spillnayis, ju-st es the name i*rpli-es, a straightoverfall r'reir. ?he weter flowing over the spill',ray falls onto a hor"izont-a1 e.pron. ?he energrin the ruater :-s

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2 -ljeiwj-ce. This paper Irlas coilpiled by h-Ln and Fred 1'.'{.Blaisdellt F::ojectSupe:.:risor, r+ho is also responsible fc:' the nappe trajectorSr anaiysis'The iechnical content anC presentation of the paper heve been criticalll're1;ier.reCb;'!r. Alrrin G. ,'-nclerson, Assisi;:nt Professor of hydr;ul5-cs, 5t.:inihon;' Fall-s Hydr:ir}-c l,aboratory, and lir, i'i. I'1. Cu1p, Head, lesign Sec-*i nn in,,i "rpnri n^' ;t-i ri ci nn Sni 1 Conservation Service. iiditofj-al prepara-U - L U I I , f , l J . * I - l g U I L l t L uL v l u l v r r t v v +4tion r.ras with ihe assistance of the Laboratory staff . The thanks of 'r,heam*,horsgo -boall whohave so generously contributed construciive co*:nents.

This repori is broken d.olnrinto a munber of sebtions. Tntroduc-tory sections describe ;+revious work, the tesi progf&:rl, e.Ild he apparatusand.procedure used in conducting the tests. The results of the tests aresuriimarized1n the fo:rn of design rules and equations '

PA,trVIOUS$I0Ri{lxcessive scour ai; the outlet of a number of s'i;ralght d.rop spill-

i^ia;Zs ocated at the i"Jhiting Field itlaval Au:ciliary Air $ration, i"iilton,Florida, resultecl in a reclu-est for model stuCies of th.is outl-et and forrecoiulenclation of a better outlet design. The i'ihiting Field strtrctureshad been designecl according to the eriteria presented in the.,paper entitl-edrtHJ-d.raulicDesign of Dron Strrrctures for Gu11yControlrt l1]". I'todel stud'-ies eonducted j-n 19bB with a downstreem channel fosned of sanC verifiedthe excessive sQoi:r observed at lihiting Field l2].

A serj-es of tests was ccnducted in l-950 using the outlet desi-gnmentioned. n the previous paragraph to see if r"ringrtalls triangular in eie-vation r"ronld red.u.cethe scow. Triangul-ar r"ringwalls had previously beenfound effeciive in controlling bank scour i3, hl. Tirese tests T{ere spon-sorr.r.t "!r:ritarr"ion Ii f of the Sbil Conserva-bion Service. They slrorred hatr,rhile the bank scour r,rasreduced ihrough the u-se of the trrangular wing-walls, not enough energy r,rasd.issipated. in the outlet to reduce t]:e bedscour to a iolerab]-e aaount.

lr satisfactory stilling basin incorpora.ting floor bloeks and anend sill r+asdevelopeclthrough ieodel studies foruse ai iihiting Field 12]-Subsequent experience at ifhiting l,-ie1d has shoun the complete absence ofscour in the channel downstrea:nfrom this stilling basin and. has verified'the laborator;r stuclies. The instrtctions frorn ihe Engineering Councilrsere to develop generalized. d.esign rules for thls stilling basin'

-t'-i,h_,*b"r"in brackets refer io bibLiography l-isted on Page J$.

2Scrvice. This paper was co:r;1piled by him and Fred vI. Blaisdel l , Proj ec tSupervisor , 1-,ho i s a lso responsible for the nappe t ra jec tory a.."lalysis.The technical content and presentation of the paper have been cr i t ica l lyreviewed by Dr . Alvin G. Anderson, Assistant Professor of Hydraul ics , St .Anthony Falls Hydr2,ul ic l,aboratory, and Er . H. H. Culp, Head, Design Sec t ion , Engineer ing Division, Soil Conservat ion Service. Editoria l preparat i on was v-Jith the assistance of the Laboratory staff . The thanks of theauthor s go to a l l 1oJho have so generously contributed cons t ructive comments.

This report i s broken down into a number of sections. Introductory sec t ions describe previous 1{ork, the t es t p rograLil, and the apparatusand procedure used in conducting the tes ts . The results of the tes ts aresummarized in the form of design rules and equations.

PREVIOUS ~ i O R K Excessive scour a t the out le t of a number of s t r a ight drop sp i l l -

wa ys located a t the ~ ; ] h t ing Field Naval Auxiliary Ai r Station, IYli l ton,Flor ida , resulted in a request fo r model studies of thi s outle t and forrecommendat ion of a bet ter out let design . The ~ ' . } b t ing Field structureshad been designed according to the cr i te r ia presented in the paper enti t ledtlHydraulic Design of Drop Structures fo r Gully Controli' [1)". Model stud-ies conducted in 1948 v-rl th a d01oJl1stream channel fOl'l1led of sand verif iedthe excessive scour observed a t ~ ' J h i t i n Field [2).

A series of tes ts vlaS conducted in 1950 using the outlet designmentioned in the previous paragraph to see i f Hinglvalls t r iangular in elevat ion 1ilOuld reduce the scour. Triangula r wi ngr.rJ'all s had previously beenfound effect ive in controll ing bank scour [3, 4]. These tes ts were sponsored by Heg ion I I I of the Soil Conservation Service . They shov-Ted tha tv-lhile the bank scour was reduc ed through the use of the t r iangul ar l.nngwa l l s , not enough ener6J vIaS dissipated in the out le t to reduce the bedscour to a to lerable amount.

A sa t i s f actory s t i l l ing basin incorporating floor blocks and anend s i l l 1,l a S developed through model studies for use a t Vhi t ing Field [2].Subsequent experience at lJhit ing Fie ld has shmm the complete absence ofscour in the chan.."lel dOv-Tl1stream from this st i l l ing basin and has verif iedthe laboratory studies. The instruct ions f r om the Engineer ing Councilwere to develop general ized design rules fo r th is s t i l l ing basin.

' ~ t ' u I t l b e r s in brackets r efer to bibl iography l i s ted on Page 35.

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APPARATUS ND PROC,EDIIREThe test apparatus is shown in Fig. 1 The waterfor the e>peri-

nents r.rasobtained from a constant-Ieve1 tank installed in the Lacoratorylrain supp\r channel. ldater entered the constant-level tank through a 12-in. butterfly valve which was hrydraulically operated from the nain floor.The exit was through a 6;i-n. pipe. The quantity of water was controlledby a 5-in. gate valve, diseharged i-nto a stlIIlng pool, and passed undera solid baffle which sewes to quiet and d:istribute the flow before itgoes to the 1.0-ft type H-flume whlch was used for measuring the quantityof water.

Waterfronr the ll-flurne drops into a stilling pooland passes undera soli-d baffle which se]:ves to quiet and distribute the fLow in the 6-ftwide, lO-ft 1ong, and 2-ft deep approach channel. The approach charmelr+as made of steel- staj-r stringer channels bolted together. For most ofthe tests, the approachto the crest was e concrete channel 1eve1 with the

N-R-5 7i:4

Fig. I -TestApporotus

3

APPARATUS AND PROCEDURE

The t es t apparatus is shown in Fig. 1 The water for the experiments was obtained from a constant-level tank instal led in the Laboratorymain supply channel. Hater entered the constant-level tank through a 12-in . butterfly valve which was hydraulically operated from the main f loor .The exi t was through a 6-in. pipe. The quantity of water was controlledby a 6-in. gate valve, discharged into a s t i l l ing pool, and passed undera solid baffle which serves to quiet and distribute the f ~ o w before i tgoes to the 1.O-ft type H-flume which was used for measuring the quantityof water.

Water from the H-flume drops into a s t i l l ing pool and passes undera sol id baffle which serves to quiet and distribute the flow in the 6-f twide, 10-f t long, and 2-f t deep approach channel. The approach channelwas made of s teel s ta i r str inger channels bolted together. For most ofthe tes t s , the approach to the crest was a concrete channel level with the

Fig. 1 - Test Apparatus

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I4

crest and. equal to the srest lengt'h i-n bottour width" The eoncrete channelsides had. a 1 an 2 slope, ..The outlet char:nel was 5 ft widen 10 ft longtand 2.5 ft ddep. The hea&rall between the tllo ebannels was provided r+ithan opening fsr insertion of itre models" Ae adjustable gate at the do'r*a-streefl end of the exit charurel contrsls the tail*rater level.

A poi-nt gage r,rasattached to a carrj-age wh-ichran op rollers alongthe top of tire char:nel in sueh a nar:ner that 1eve1s enprhere ire the approachcharxrel ar:d in the test section eould be readily obtaiqed. Tbis gage wasused in setting ihe nodel-s to the eorrect el-evation and in detetrtdnir:tg thelevel-s of the rsater surfaee and the sand bed" ?he modeLs lrere :nade of p1y-sood and.white pine, With the use of, }rmber, clu"nges were :aadernrith verylittle efforb. CIrdiaary coacrete sand pasoiag an 8-mesb screen was usedfor the strean bed d.or,nrstrean rom the spillway. fbe eroeisn of the sand.bed nas used as a $ea$ure of the effieieney of the outlet.

In eaeb experS*rent, a stiIlfurg basia was i-astalled and the strearnbed was filled, sith sand to an elevation above the top of the end si11.Ttre bank slope was approximaiely I on 2, ?he stream bed was flooded sothat the initial rr.rsh of iraterLhrough the stiLl-ing basin roou"ldnot erodethe strean bed exeessively" fhe gate valve tn thp supply l!ne' tras thenopened to give the desired. &ischarge, and. the tailtraiez' Level was ad.justedto give the carrect depth. A flot* photograph was takes dr:ri*g itre test.After the water had rr:n through thE nodel for tsio hoursr the valve waselosed and the water allosed to di!'ain* fhe water 3eve1 in the outlet lrasmeasured at irlteryals during the dralnage proeess by measrsof the point'gage" Zero elevation was as*r,red to be the elevation of the top of, *?reend. sj.11" tr'ihite wocl yar"a tras placed on the water 1j:re at l-in, isrtervaSsto define the eontours" The eodSqred bed was photographed t's raeord thescour"

Ihe data obtained fusing each test consj.sted of the stn:cture di-xxensiaas, flsw da*a, netes, end photographs. The photographs provided theprincipal mear:s of recording and analyaing the perforraance of the slme*trJ.re" Since only oae featnre of the sii3li$g basirr was changed for eachtesto the phatographs provid.ed a record. of the eff*c* of eaeh change and.serued to define the opti-fi$xt d.inensionr"

4

crest and equal to the cres t length in bottom width" The concrete channelsides had a 1 on 2 slope . ..The outlet channel was 5 f t wide, 10 f t long ,and 2.5 f t deep. The headwall between the t wo channels was provided lvithan opening for insertion of the models. 1m adjustable gate a t the downstream end of the exi t channel controls t he tailwater level .

A point gage was attached to a carriage which ran on rol lers alongthe top of the channel in such a manner that l evels anywhere in the approachchannel and in the te s t section could be readily obtained. This gage wasused in set t ing the models to the correct elevation and in determining thelevels of the water surface and the sand bed. The models were made of ply-wood and white pine . l-li th the use of lumber, changes were made with veryl i t t le effort . Ordinary concrete sand passing an 8-mesh screen was usedfor the stream bed downstream from the spillway. The erosion of the sandbed was used as a measure of the efficiency of the outlet .

In each experiment, a s t i l l ing basin was instal led and the streambed was f i l led vIith sand to an elevation above the top of the end s i l l .The bank slope was approximately I on 2. The stream bed was flooded sothat the in i t i a l rush of water through the s t i l l ing basin would not erodethe stream bed excessively. The gate valve in t l1.e supply l ine. vTaS thenopened to give the desired discharge, and the t a i l ~ a t e r level was adjustedto give the correct depth. A flow photograph was taken during the t es t .After the water had run through the model for t wo hours, the valve wasclosed and the water allowed to dfain. The water level in the outlet wasmeasured a t intervals during the drainage process by means of the pointgage. Zero elevation was assumed to be the elevation of the top of theend s i lL White wool yarn was placed on the wate r line a t l - in . intervalsto define the contours . The contbured bed was photographed to record thescour.

The data obtained during each te s t consisted of the structur e di-mensions, f lo v1 data, notes, and photographs. The phot ographs provided theprincipal means of recording and analyzing the performance of the st ruc-ture. Since only one feature of the s t illlllg basin was changed fo r eachtes t , the photographs provided a record of t he eff ect of each change andserved to de f ine t he optimum dimensi ons.

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TESTHESI]ITSThe vesults of the tesis rnade to deterciline the di:irensions of the

strai-ghtdrop sprllway si;ilIing basinwill be pnesented separe'ue1.v- or eachelenent comprising the basin. Tire o::Cer cf' presen'r,e"tj-on is not the orderin t+i:ich the tests r,lere condueted. In fact, it lqas neeessary to siu$r solreelei;:ents several tjmes because a cha:r"{e in one elel,rent woulC, affect theperfo:taa-nce of other elernentsr ffid tire best result coulcl only be obtained.aftei" ea"ch elenent had i-ts optjmurn djmensions. ,

'^-rth of Basi-nc TTLft i.ras recognized before the tests ',+ere begun thet the point at

rrhich the nappe hit the stilling basin floor nould provide one of the di-nensions f*r ti:e de-b.eminatj.on of the basjn l-ength. Other C:r,rensians d.e-ten::ining the basjn length e-re the distance from the nappe to the f'1corblocks and. the distance fronr the flocr blocks to the end of the basin.Ea-ch of ihese di:aensions wil-l .l:e discussed. in turn,

Nappe lraJectorya

?he equaiion first used rlurjrg these studies to givg t?:.e rajec-tory of the r-r;':irel urfaee of the free-fc.l1ing nappe is

"+,],-H ^ | / a t a PUr{ O - U .J -U) zx( _ _ ) +ITl t;::ez-e yn i-s the vertical disi;ance, x* is tJre horj-zontal distence fromI It:e crest to the upper surface of the napper and H is the total head..f . i is equat ion r , ' is der ived fron data presented b,- D: ' . A" T. Ippen [5] for--:-e free overfell, si:rce '-}: s,r'a'i..ht drcp spil-1 i,;e;r s assu:ieclto l"rave hea;prcach che.nnel level r'-ith 'i;he spi,J-lwa;r erest.

Thrs equation pro.red satisfactcr;:.' '::rti1 tests T.lere ;..ui'L .r:-th the--:-l--'i:,ier 1evel close to tjre spi1h"'e.y crest, lror t]:ese tesis the nappei'd -ci ial]- freel;r Lrut l.ras sup*,rorted by the high tai}i:ite::" ?he result'*=s i:r::i ice na,:pe hit Lhe sirean ted aioi"jl:rstr+ernof the stillinl: basin and.

.' 5TEST RESULTS

The resul ts of the tes ts made to determine the dimensions of thes t raight drop spillway s t i l l in g basin wil l be presented separate l y fo r eachelement comp r i s ing the bas in . The orde r of presentation i s not the orderi n which the tes ts "rere conduc t ed. In f2.c t , it iv-as necessary to study someelements s evera l times be cause a change in one element would affec t thepe rforraance of other elements , and the bes t resul t could only be obtaineda f te r each element had i t s optimum dimensions.

Length of BasinI t was recogn ized before the tes ts iv-ere begun tha t the po in t a t

vJhich the nappe h i t the s t i l l ing basin f loor would provide one of the di-mensions for the determinatlon of the basi,,- l ength . Other dimensions de-te rm ining the basin length are the distance from the nappe to the f l oorblocks and the di s tance from the f loor "910cks to the end of the basin.Ea ch of these dimensions will be discussed in turn.

Nappe TrajectoryThe equation f i rs t used during these studies to giVE? the t ra jec

t ory of t he upper s urface of the f ree-fa l l ing nappe is

: : e r

ynH = 0.46 - 0.105

i s t he vert i c a l d is tance, xn i s the horizontal distance from:.e cres t to the upper surface of the nappe, and H is the t o t a l head

:':Lis equatio n 1ms de rived from data presented by Dr. A. T. Ippen [5] for~ : : e f ree ove r Ial l , since the s t raigh t drop s pilhmy i s assuI!led to have t he-J roach channe l level Hith the spil11vay crest.

This equation proved sa t is fac tory ul1til t e s ts He re run u-rith the";.ai - c l eve l close to the sp i lhray cres t . For these tes ts the nappe

" :lot fa ll f r ee ly but i'la s supporte d by the high t a i luat e r . The resu l tt: E..t the n a ~ : h it the s t ream bed dO'l-ffis t ream of t he s t i l l in g basin and

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b

sconred a deep hoLe there, The fact that greaier $oo1rr was obtainecl r'ritha higher tailwater 1evel had a;r important effect on the stilling basin de-sign. Although this diseovery is conirar'' to the widely held' opinion thathigher tai-h.rater w'i1l car:se less $e011ro occur, it is an entirely logicalfinding. The r+aier in the free-falli-ng nappe is assumed bo heve a con*stant horizontal velocit;' and a vertical- velocity accelerating under theeffect of gravity. }-fter the free-falling nappe plunges into the tail--gater, gravity is no longer effective and the zubmerged- appe rrill con-tinue in the direction the free-fal1ing nappewas tlaveling when it enteredthe tailwater. Near ihe crest the upper surface cf the free-fa1lirtg nappetrajeciary has a relatively fla!=slope, and il tne tailt"rater ]evel is af--so close to the crest, the subraerged.nappe w'i11 cont|mre cn this flatslope. The result is , of course, ihat the submergednappe trajectory isrorell downstreen of the free'falling nappe trajectory uhen the tailwaterl-evel- as ru-gn.

One qualification to the statement regard,ing the plunging nappeshould. be made- i,Ihen the tailwater level ls considerably above the crestof the spillway, the nappe does not plunge through the iailwater but 'rfloatsrton or close to the surface of the tailwater. For ihis conditionr the nappedoes not attack the bed downstrean from the stilling basin''" Duri:rg thetests it rsas noticed. that the rrsurfaee nappert occulred' r"rhen he d.epth ofthe tailwater above the spillway crest leve1 e.xceeded wo-thirds the crj-ti-ca1 depth approximately. Therefore, it i-s eoncluded that the effect ofhigh tailwater levels on the positi-on of the nappe need not be consideredin the determination of the stilling basin lengti: when the tailwater levelexceeds the spillway-lcrest elevation'p1us tr^ro-thirds the criticd- depNh,but the effect of Sailwater on the nappe t:'ajeciory must be considered fortai-lwater elevatisns lower tharr the spillway crest elevation plus tr'ro-1;hirdsthe critical d"epth. T]1otherword.s, the ma-t'i-mi:mailrraier level that wi1laffect the stilling basin length 5-s that leve1 uhich is ttro-i,hirds thecriiical. depth gbcve the spilh+ay crest level'

After discovering that the taitwai,er lerrel in-fluenced the nappetrajectory, -bests trere made consideri-ng that the nappe is free-f a| ii:g; iothe tailr+ater level anC coni,inues on a iangent be}'onC that point- Thesetests resultecl j-n consid.erably tr-ongerbasins w*en ihe tai-},rater r+ashigh.In fact, the expei.jments sborsedhat the basins '*rere longer than Ir-ecessary

6

scoured a deep hole there. The fact t hat greater scour was obtained 'I,l i tha higher t ai lwater level had an important ef f ect on the s t i l l i ng basin des ign. Although th is discovery i s contrary to the 'Iridely held opinion t hathigher tail i mter vlil l caus e less scour to oc cur , it i s an ent i re ly lo gi ca lfinding. The water in the free-fa l l ing nappe i s assumed to have a constant horizontal veloci ty and a vert ic a l veloci t y accelerating under theeffect of gravi ty . After the fre e-fal l ing nappe plunges into the tail;;,.vmter, gravity i s no longer effect ive and the SUbmerged nappe 'Hill cont inue in the direction the f r 'ee-fal l ing nappe was t r a v e l ~ n when it enteredt he tai lvmter. Near the cres t t he upper surface of the free-fa l l ing nappet rajec tory has a relat ively f l a t . slop$, .Sllldif the tai lwater level i sso close to the cres t , the submerged nappe wil l continue on this f l a tslope. The resul t i s , of course, that the submerged nappe t ra jec t ory i s,\-1811 downstream of 'the free""falling nappe t rajectory when the ta i lwaterlevel i s h igh.

One quali f icat ion t o the s ta t ement regarding the plunging nappeshould be made. Hhen the ta i lwater level i s considerably above the cres tof the spi l lway, the nappe does not plunge through the ta i lwater bu t lIfloats tlon or close to the surface of the ta ihrate r . For th i s condition, the nappedoes not at t ack t he bed dOvmstre am f rom the s t i l l ing bas in . t hetes ts it vlas noticed t ha t the tlsurface nappe lt occur red when the depth oft he t a i lwa te r above t he spilhray crest level exceeded two- thi rds the c r i i cal depth approximatel y . Therefore , it i s concluded t hat the effect ofhigh ta i lwater levels on the pos i t ion of t he nappe need not be consideredin the detenninatiQn o f the s t i l l i ng basin length when the t a i l ~ . , r a t e r levelexceeds the spi l lway :" cres t elevation 'plus two-thirds the cr i t i c a l depth ,but the effect of t a i l w a t e r on the naplle t ra jec to ry must be cons idered fo r1_ . '.ta i lwater elevations lower than the spilhray cres t eleva tion p lus t ,vo-thi rdsthe cr i t i ca l depth. In othe r wordS, the maxitlllUTI taih.,rater l evel tha t wil laffec t the s t i l l i ng basin length .i s t hat level uhich i s two - third s thecr i t i ca l depth ?-bove the spi l hmy cres t level .

After di scover in g that the tailwa t er l evel in fluenced t he nappet rajec tory , t es ts were made ,cons i dering that the nappe i s f re e- fa l l ing tothe t ailwater level and cont inues on a tangent beyond that ~ Theset ests r esulted in considerably longer basins ifhen the t a i l ~ . , r a t e r vIas high .In fac t , the experiment s showed t hat the bas ins were longer than necessary

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Tsc they were shos"tenedsuccessivel;r utrltt th*t were so short that theirperfor',nance becar;repoorr .As a result cf ihese iests, the *ptiraunr trajec-tory cf ihe 'r:pper nappe sr.rrface for use in determi:ring one of the elements*aki-ng up ihe basin length ''*as foend to be mid,*ay between the free-fal1ingcnd f ho c r . i ' - a r . ^od t fa j ee tO f i eS .i j v v e v f 4 v e .

?his mean trajectory "r+a-sused for al l subsequent tests and wasfor:nd i;o give entirely satisfactory results, It 1s proposecl only for usei:r de'r,err:rining the length of the straight d.rop spill;"ray stilling 'basi:r andshould not be used ilor cther purposes until it i.s confir:aed for such usen

?he eqr.ration of the upper surface of the free-fallilg nappe pro-posed. or use is

This equei,ion is a reerrangement and a subsiitution ofthe general equaiion for free-felling nappes presentedT+ ^-.^'r.:^^ +^ ]Lg free overfal-l only.V c l i P f J U J L U ! i - !

v- o,ho6 /:.r*5 - f+.:aef'%

0.l+06+ ,B,l:gS - J+.368\ rl / d .

i nri

f r \

3de/2 for H in[ 6 ] .y Blaisde1*l

At lhe point r"'here the upper surface of the free-falling nappestrikes the stil]ing basin f1oor, Ea,. (1) becone" '

( 2 )

where x,, is -r,hehaviaantal distance frorn the crest to the upper surfaceof the free-fa1ling nappe at the elevatien of -r,hestilling basi:n flaor,

The eqriationfor the upper "u"f*"" of the submerged napps irajec-tory abave the tai}.,ra-ter leve1 is the same as that for the free-fa1lingnappe. The poi:rt at which the upper nappe plunges into the 'uailueter is

0.1+06 I e \

I'F = -

j!.U

4 = 3

*-here x, is the1-.the surface of ihe

harizontal distan-eeupper nappe plunges

frorn the erest to the point at whichinto ihe tailnater and y+ is the

U

7so they were shortened successively unt:Ll they were so short tha t the i rperformance became poor . As a resul t of these t es t s , the optimum t ra jectory of the upper nappe surface for use in deierminingone of the elementsmaking up the bas in length li as f ound to be midway between the f ree-fa l l ingand the submerged t ra jec tor ies .

This mean t rajectory was used for a l l subsequent tes ts and wasfound to give ent irely satisfactory re su l t s . I t is proposed only for usein determining the length of the s t ra ight drop spilhray s t i l f ing basL'1 andshould not be used for other purposes unt i l it is confirmed for such use.

The equation of the upper surface of the free-fal l ing nappe pro .posed for use i s

xn = - - 40386yn (1)

This equation is a rearrangement and a subs t i tu t ion of 3dc/2 for H int he gene ra l equat ion for f ree-fa l l ing nappes presented by Bla isde l l [6].I t applies to the free overfall onlye

At the point where the upper surface. of the free-fal l ing nappe..s t r ikes the s t i l l ing basin f loor, Eq. (1) becomes

(2)

where i s the horizontal distance from the cres t to the upper surfaceof the f ree-fa l l ing nappe a t the elevat ion of the s t i l l ing basin f loor .

The equation for the upper surface of the submerged nappe t ra jectory above the t a i l'dater level i s the same as tha t fo r the f ree-fa l l ingnappe. The point a t which the upper nappe plunges into the ta ihra ter i s

(3)

\-lhe re i s the horizontal distance from the c res t to the point a t whicht ne surface of the upper nappe plunges into the ta i lwater and Yt i s the

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Bvertical distance from the crest to the ta-ilwaier surface. It is necessaryto keep the signs correct, renembering -i;hat yt is positive above crestelevation and negative below, fhis is il-lustratedinthe definition sltetchof Fig. 2, The equation of the trajectory of the upper nappe surface belowtailwater leve1 1s

a.69r + 0.228 x"/a")z - tvota")0.185+ 0.l+56 xrla")

nhere *rr" j.s the horj-aontal distance from the erest to ihe upper surfaceof the submergednappe. Fquation (l+) lras obtained. by considering that theportion of the nappe trajectory above the tailr^rater 1evel has the saaeequation as does the free nappe trajectory, trhile below the tailwater levelthe nappe trajectory has a slope equal to

a (yrld") = - 0 . 1 8 5 - a , \ 5 6 TLl.

xns i l , 1

. / 1 1 \o. \xt lcc/

Equation (5) ean be obtainedAt the point where

jectory strjkes the stil-1ing

by rearranging andthe upper surface ofbasin fIoor, Eq. (l+)

differentiating Eq. (3).the subnrerged nappe tra-becomes

t \ t

\ o /xml_.1

a

Values of *" have been computed for a wideresults are presen'i;eo.graphically in Fig. 2.

0.69r + a.228 (*r/a")z - (:r/d")0.185+ a,b56 (xr/a*)

The clistanee from the -;erest at which the average of the irppersurfaces of the free and. submerged nappes strikes the floor *" is usedto deteruTine part of the stilling basin length. The eque.tj-on or t. is

= k * * T (r)range of cqnditions. lhese

8vertical distance from the crest to the tClihrater surface. I t i s necessaryto keep the signs correct , remembering that yt i s posit ive above cres televation and negative below. This i s i l lust ra ted in the definition sketchof Fig . 2. The equation of the t ra jectory of the upper nappe surface belowtai lwater level is

xnsdc

20.691 + 0.228 (x t / d ) - (y /d )c n c (4)

' t ~ h e r e xns i s the horizontal distance from the crest to the upper surfaceof the submerged nappe. Equation (4) Ivas obtained by considering that theportion of the nappe t ra jectory above the tai lwater level has the sameequation as does the free nappe t ra jectory , vJh,ile below the tail 'tfater levelthe nappe t ra jectory has a slope equal to

d (Yt/dc)d (xt/dc )

Xt0.185 - 0.456 -dc (5)

Equation (5) can be obtained by rearranGing and different iat ing Eq. (3).At the point where t he upper surface of the submerged nappe t ra -

jectory str ikes the s t i l l ing basin floor, Eq. (4) becomes0.691 2+ 0.228 (x t / d ) - (Y/d )c c (6)

The distance from ~ c r e s t a t vlhich the average of the uppersurfaces of the free and submerged nappes str ikes the f loor x i s usedato determine pa r t of the s t i l l ing basin length . The equation for x i sa

x =----a 2Values of x have been computed for a wide range of conditions. Thesearesults are presented graphical ly in Fig . 2.

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x.,/dc2 3 4 5 6 7 8 9 1 OI

oI

2

3

4

4 sc6

7

I

9t o

YtG+o.7

+o.65

+o.6+o.55+O.5

+o.4+o.3+o.2+o . lo

Fig. 2 - DesignChort for Determinoi ion f x^

Lowernoppe(No oilwoter)

Toilwoter levelA-Tongentr point I,\ of submergenceIN'*x!:1,*oia trojectoryree noppetrojeclory

o1 4 6 7 8 9 10

Ytde

9

2 +0.7

3 Lower nappe(No toi 1wate r)

6

7

+0.65

+0.6

+0.5

+0.4

8a n g e n t at point

..L Mean of submergencede trajectory - + ~ ~ r + - ~ t - - - - ' ' I r l - ~ d - ~ ~ d - ~ - ' " +0.3\" Submerged nappe

9Free nappe , traje ctory - - ; ' ~ l - t r - - - t ' ~ H r + ~ + - ~ - - " I I r l - " I n trajectory --\ : \ +0.2

" ~ X a / d : - - - - ; r - - + - + - ~ ~ r l - ~ - ~ ~ ~ - P ~ ~ ~ ~

Fig. 2 - Design Chart for Detenni nation of xa

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10" htrhen y/dc, vr/d"r or Tt/d* exeeeds 0"728r tire valae ur:.der

ihe rad.ical 5:: Xqs" (f), ie), end (3) necan:es egative and the racis i: fthe equation aye i:aaginary, I{owever, si-nee the stilliag basj:r i-ength isnot affected b;r ihe tailwaier levels which exceed bro--Lhirds the criticaid.epth approximately, tVrlA*> 0"6?r approxj:aaielylr ihis li:,:itaii-on isnot Smportaat here. &eferring *o Fig. ?, it is suggested that the cnrvefor v./a = a.7 be r:,sed ghen y,ld exceeds +A,7. ?h-1s ule was usedL ' c ' r ' ' cduring the labovatory studies with entj-rely satisfactory resr:1ts,

Distance to Floor Bl*cksSufficisnt distanee is requ:ired betneen the point at rrhish the

rrpper nappe strikes the basin floor and the flcsr blocks tc pertri:it thestream io besome approximately para13e1 to the f1*ar l:efore it reachesthe b1-ocks* The dete:xrir:aiion of ihe *pti-num d:imension proved to be large-ly a sratter of the judgroent af the sbseffer. It was deslred to makethis diri,ension as sma11 as possible. ifowever, if the floor bl*cks are tosclose to the nappe, a l:-igh boil is ihrorE"s. ff of the blocks, lu'her: hedistance between the r:pper nappe and the fl-oor blocks0"5 d^, they proved to be largely ineffeciive, This

? was less tha*shorm by ihe

se?eve bar:k ercsion in F'ig. 3a where the floor blecks are lacated ai thepoint r'rhere the upper nappe strikes the stilllng basi:r floor; that is,where *b = 0. The scour in Fig. 3a is as severe as that in Fi-g, 3b whereno floor blocks are used. l.Ihen *b = 0.5 d" the boil caused by the fSoorblocks r'ras higher than was eonsidered desirable. i',then % r+as neveasedto O.Bd"r the appearanee of the water sarfaee was satisfactory, A$shown in Fig, [, the bed sconr was decreased and the bl-soks ga're sufiicientprotecii-on ie the bar:irs so they stood at' thcir angle of repose, The d:i-s-tance q = 0.S d^ Eas used. for all subscquen* tests sith resalts thath Fwere complete}-y satisfectary,

?1:e early tests to detersrine % were rsade using d pair oflongitudinal si1ls in the stilli*g basiR. Later it rsas diseovered thatlongitudinal sills are r:nne*essary if the floor bloeks are proper'3-y p:"opar-tioned and spaced" i'Io fuviher siudy'aras tade'bs redeierTiline %. Hcvever,the tests madewith 5 * *.8 d* *irsued this d"istance *c give sailsfa*'barystil1i-ng basin perf cr:rrranc,

10

y/dc ' y or Yt/dc exceeds 0.728, the value underthe radical in Eqs. (1), (2), and (3) becomes negative and the roots ofthe equation are imaginary. However, since the st i l l ing basin length i snot affected by the ta i lwater levels which exceed two-thirds the cr i t i ca ldepth approximately, (yt/ c > 0.67, approximately), this l imita t ion i snot important here. Referring to Fig. 2, it i s suggested that the curvefor Yt/dc = 0 .7 be used when Yt/dc exceeds +0.7. This rule was usedduring the laboratory studies with ent i re ly satisfactory resul ts .

Distance to Floor BlocksSufficient distance is required behveen the point a t which the

upper nappe s tr ikes the basin f loor and the f loor blocks to penni thestream to become approximately para l le l to the floor before it reachesthe blocks. The determination of the optimum dimension proved to be largely a matter of the judgment of the observer. I t was desired to makethis dimension as small as possible. However, i f the f loor blocks are tooclose to the nappe, a high boil i s throvm off of the blocks. Hhen thedistance between the upper nappe and the f loor blocks was less than0.5 d, they proved to be la rgely ineffective. This i s shovm by thecsevere bank erosion in Fig. 3a vlhere the f loor blQcks are located a t thepoint where the upper nappe s tr ikes the s t i l l ing basin f loor; tha t i s ,where = O. The scour in Fi g . 3a i s as severe as that in Fig. 3b 1fhereno f loor blocks are used. "men = 0.5 dc the boil caused by t he f loorblocks viaS higher than was considered desirable. vihen i.Jas increasedto 0 . 8 d , the appearance of the 'tvater surface was sat isfactory. Ascshovm in Fig. 4, the bed scour was decreased and the blocks gave suf f ic ie ntprotection to the banks so they stood at the i r angle of repose. The dis-tance = 0.8 dc wa s used for a l l subsequent tes ts with results tha twere completely sat isfactory.

The ear ly tes ts to determine xb uere ma.de using ;Ef pair oflongitudinal s i l l s in the st i l l ing basin. Lat e r i t w a s discovered thatlongi udinal s i l l s are urmecessa.ry i f the f loor blocks are properly proportioned and spaced. No fur ther study was made to redetermine YD ' However,the t es t s made with Yo = 0 . 8 dc showed this distance to give sat isfactoryst i l l ing basin performanc e.

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1 1

the Fl oo r Blocks l,ocated at , the Point ivherelJappe Strikes the Bas-,rt Floor

b. No Floor Blocks

Fig. 3 - The BedScour s Deeper hon Necessory nd the BonkScour sExcessiveBecouseof lmproper Locotion or Absenceof the Floor Blocks

a. Floor Blocks Located a t th e Point ,,,hereth e upper Nappe Strikes the Basin Floor

b. No Floor Blocks

Fig. 3 - The Bed Scour Is Deeper than Necessary and the Bank Scour IsExcessive Because of Improper location or Absence of the Floor Blocks

11

,

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t aI L

Fig. 4 - Floor BlocksLocoted 0.8 d" from the PointWhere he NoppeSti ikes he BosinFloor ond | .75 d^'from the End of the 5ti l l ing BosinGive MoximumProtection o the Bd dcn d Bonks

The recomrnended istanee from the point at r+hich the average ofthe upper surfaces of the free-fa1ling and submergednappes strikes thestilling basjn floor to the upstram face of the fLoor blocks is

x . - 0 . 8 dD C ( B )

Distance to End Sil1If the distance between the floor blocks and the end siIL is

too short, neither the blocks nor the end si1I are fu1ly effective. ?hisis evidenced both by excessj-ve scour of the channel bed and strong eddj-esrvhi-eherode the banks and the dam filI.

The ninimirn distance between the floor blocks and the end sil]x thatprevents this excessive scourwas found to be L.75 d . Di-stancesc ' Cgreater than 1.75 dc had little beneficial effect on the scour pattetr.Values of x, as great as 2.7t d were tested. Figure I showsthat thec - efloor blocks are located too close to the end sil1 when x" = 0.75 d"'I,Jlren x * !.75 d , as jn Fig. l+, the floor blocks are fu\y effective.c c

12

Fig. 4 - Floor Blocks Located 0.8 de from the Point Where the NappeStrikes the Basin Floor and 1.75 de from the End of the Stilling BasinGive Max imum Protection to the Bed and Banks

The recommended distance from the point a t which the average ofthe uppe r surfaces of the free-fal l ing and submerged nappes s tr ikes thest i l l ing basin floor to the upstream face of the f loor blocks is

(8)

Distance to End Si l lI f the distance between the f loor blocks and the end s i l l i s

too shor t , nei ther th e blocks nor th e end s i l l are fully effective. Thisis evidenced both by excessive scour of the channel bed and strong eddieswhich erode the banks and the dam f i l l .

The minimum distance between the floor blocks and the end s i l lx tha t prevents this excessive scour l-;as found to be 1.75 d. Distancesc cgreater than 1.75 d had l i t t l e beneficial effect on the scour pattern.cValues of x as great as 2.75 d were tes ted. Figure 5 shows that thec cf loor blocks are located too close to the end s i l l when x .. 0.75 d

c c~ J h e n x = 1.75 d, as in Fig. 4, the f loor blocks are fu l l y effect ive.c c

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Fig . 5 - F loorBlocks ocoted 0.75 dc f rom he Endof rhe St i l l i ngBos inAre lneffective; the Bed Scour ls DeEper thon Necessoryr ond th l BonkScour s Excessive

Here agai-n the judgment of an exlerienced observer is essenti-alto the deterndnation of the nin-inu'r value of x^ that gives the rnostsatisfactory performance.

It is recommended, as a result of these tests, that the rrinimundlstance between the upstream face of the floor blocks and the exlt of thestilling basin x. be

1 n f 't " l ) d

val-ues of *" greater than r.75 dc may be used if , for sorae eason, thebasj-n must be lengthened. The lengi;hs *u and % should not be variedappreci-ably fron the values given i-n Eqs. (?) ana (B).

Tailwater DepthA sufficj-ent depth of tailwater is required so that the water

leaving the stilling basin has no opportun-ity to plunge and scour a holein the strean bed; the water surface (tailwater level) in the downstream

1 o \r )

Fig. 5 - Floor Blocks Located 0 .75 de from the End of the Stilling BasinAre Ineffective; the Bed Scour Is Deeper than Necessary, and the BankScour Is Excessive

13

Here again the judgment of an ex:.='erienc ed ob server is essentialto the determinatio n of the rninir,wn value of x that gives the mostcsatisfactory performance.

I t is recommended, as a result of these tests, that the minimumdi stance between the ups tream face of the floo r blocks and the exit of thes t i l l ing basin x bec

x :> 1. 75 dc c (9)

Values of x greater than 1.75 d may be used i f , for some reason, thec cbasin must be l engthened. The lengths xa and should not be variedappreciably from the values given in Eqs. (7) and (8) .

Tailwater DepthA sufficient depth of tailwater is required so that the water

leaving the s t i l l ing basin has no opportunity to plunge and scour a holei n the stream bed; the water surface (tailwater level) in the downstream

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Ii+

channel siiould be at approxi*iately the same level as the lrater surface inthe sti1-ling basin, This requ:|res the detersrination of a m:inj*ruro eqr:r-ired.taillrater depih for the stilling basin.

The niai:au:n pe:nissrble depth cf the tailwater above the sfiiS:ingbasin floor de was found to be 2.T5 de, The tai},rater depih over theend"sill is L.75 dc, since the end.s'il1 is g,h d" iiigh. The resultingveloci-ty a.t the exii of the stilling basin is

^ ^ t= J . l l 1

r.rr., fnr tr:r"imr.q rlalge5 Of H And the corresponding valaes of d" ,3 i + 5 6 f t

t-I OalY o*cT;IL . I 2 . L l Z

t 1 -

u

1!

a,67c.

J . l 4

a , v v

4 ' ) ot . o {2 r L 7

? ? ?

r ' ^ a

l+.AOr, LR

I Tf ncr v s

It is apparent 'Lhat the adopted rainj-*r'umaifs{ater depth results in veloc-ities at the stilling basin exit ihat are, in most cases, gveater thancan be tolerated by the ordinary mate-r'ials comp:'iring natural dor"rnstrear,'rchannels. Therefore, ii: is assrmiedthat e:ry ati;enpt to furLher decreasedZ lsould be of no practical value since argr decrease in dZ *"+o;.:,1dau.sehigher exit veloci--uies, Even r,rith tlie present velociiies, $osrewideningof the dcrqnstream channel near the stilling basin e6r:.be expecied, Thisis tolerated because! t does not endanger the outle* or the dare i11, srrdfurther increases j-n ihe minimr:::i tailisater reqr:-treneni l-rill urrnecessarilyincrease the cost cl ihe oi:.tlet.

The resalts of the tests to deterraine the nrinj:nurn aceeptabletailr^later level are presented in Fig" 6. The tests -r'Iereconducied asnoted in the sectian deseribing rrApparatus and Procedurerr using tailwaterleveis varying frorn an excessivel], high to an excessively 1ow level. /rtthe conelusion of the tesis to deterraine the tail"r'=ater depth, the phcta-graphs of the bed contours Tirerestudied and gfven a. rati-ng of good, falr,or poor as each indjvidual case r'rarranteci. T;pical photographs ar* showr inFig, 7. In Fig. 7a the tai}aa"ter le.rel a*a$ xcessil-e1;'high (Ct = A.Lil d.).

14

chaIl.l1el should be at approximately the Si3me level as the vrater surface inthe s t i l l ing basin. This requires the determination of a minmtull requiredtai lwater depth for the s t i l l ing basin.

The minimum permissible depth of the ta i lwater above the s t i l l ingbasin f loorend s i l l i s

d2 was found to be 2.15 dc '1.75 d, since the end s i l l i scThe ta i lwater depth over the0.4 d hi ah. The resul t ingc 0

veloci ty a t the exit of the s t i l l ing basin i s

v

or, for various values ofH =

d =cv

10.672.65

21.333.74

M1. 75 = 3.24 d 1/2c

H and the corresponding values of3

2.004.58

42.675.29

3.335.91

d ,c6 f t

4.00 f t6.48 fps

I t isi t i e s

apparent that the adopted minmtull tai lwater depth resul ts in veloc-a t the s t i l l ing basin exit tha t are, in most cases, greater than

can be tolerated by the ordinary materials comprising natural dOvmstream.channels. Therefore, it i s assumed tha t any a t t emp t to further decreased2 would be of no pr act ica l value s ince any decrease i n d2 would causehigher exit veloc i t ies . Even "Ji th t he pr esent veloel. t i e s , some ~ , r i d e n i n g of the d01IDstrearn channel near the s t i l l ing basin can be expected. Thisi s tolerated because it does not endanger the outlet or the dam f i l l , andfurther increases in the minmum tai lwater requirement l;ull unnecessarilyincrease the cost of the outlet .

The resul s of the tests to detennine the minirnum acceptableta ihra ter level are presented in Fig. 6. The t es ts ~ e r e conducted asnoted in the sect ion describing IIApparatus and Procedure" using tai lwaterlevels varying from an excessively high to an excessively low level . Atthe conclusion of the t es t s to determine the tai lwater depth, the photo-graphs of the be d contours were studied and given a rat ing of good, fa i r ,or poor as each individual case vmrranted. T;ypical photographs are sho"t,m inFig. 7. In Fig . 7a the ta i lwater level was excessively h:igl (d2 = 2.40 dc ) '

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r 5

2 .40

2 .3

2 .20

9 t , nO 62 .O0

lr.o t?.o r5.o r4.o l5.o 16.0

Fig. 6 - Determinot ion f Toi lwoterDepih

Both the banks and the bed look good; eonsequent\y this scour patterrr wasgiven a rating of trgood.tr In Fig. ?b the tailwater depthwas thatadoptedfor design purposes (dZ - 2.15 dc). Herealso the scourpattern was givena rati:ng of trgood.n In Fig. 7c the tailwater depth was a 1ittle l-ow(d^ - 2.O0 d ). The banks are eroded more than is felt desirable andz c 'the bed scour is greater than is shor,nan Figs. Ja and 7b. This scourpattern wasgiven arating of trfairrrt sinceit isnot very bad, but it doesleave sonethingto be desired. The tailwater 1eveI for F*g. Za was exces-sive\y Iow (d2 . 1.p0 d")r the scour of the banks ean be seen to beexcessive, and the bed is deepened considerably. This scour pattern wasgiven a rating of npoor.rl

The results of the analysis of the photographs, when plotted inFig. 5 aga5nst the relative height of the drop, show that the nrinjmr:ndesirable depth of the tailwater U, j-s 2.I5 dc and that UZ/d" isindependent of the relative height of drop. Although there is the possi-bility that some scour patterns would be rrgoodrrwhen de is less than2.I5 d"t tailwater depths lower than 2.15 dc would.ordinarily give on\yrfairtr or rrpoorrr scour patterns ard it is unr,rise to decrease the desigatail-water depth below the recorunended value

d2 - 2.15dc (ro1Ad.ditional verification of the fact that Ur/U" should be a

constant is presented bv Nazir Ahmad 7]. Mr. Ahmad found that R./q2/3

15

2.40 I I u0 PERFORMANCE Check test,Good 0o Good.3 0 o Fair0 0 0 A Poor

2.20 00 00 p0 0 0

0 0'"

0!:J. 0 .6. 0 0

de 2. 10

0 () 0 '"2.00

'"08 0 0

1.90 000 0 0 '"

rr", "'''' '" '"'"

.'" '" '"'".800.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0

y/d cFig. 6 - Detenninotion of Toilwoter Depth

Both the banks and the bed look good; consequently this scour pattern wasgiven a rating of IIgood." In Fig. 7b the tailwater depthwas that adoptedfor design purposes (d2 2.1.5 de). Here also the scour pattern was givena rating of "good." In Fig. 7e the tailwater depth was a l i t t l e low(d2 - 2.00 de). The banks are eroded more than is fe l t desirable andthe bed scour is greater than i s shown in Figs. 7a and 7b. This scourpattern was given a rating of "fa ir ," since i t is not very bad, but i t doesleave something to be desired. The tai lwater level for FJ g. 7d was exces-sively low (d2 - 1.90 dc ) ' the scour of the banks can be seen to beexcessive, and the bed is deepened considerably. This scour pattern wasgiven a rating of "poor."

The results of the analysis of the photographs, when plotted inFig. 6 against the relat ive height of the drop, show that the minimumdesirable depth of the tailwater d2 i s 2.1.5 de and that d2/de isindependent of the relat ive height of drop_ Although there is the possi-bi l i ty that some scour pat terns would be "good" when d2 is less than2.1.5 d, taihrater depths lower than 2.1.5 d would ordinarily give onlye c"fair" or "poor l l scour patterns and i t is unwise to decrease the designtailw&ter depth below the recommended value

Addit ional verificat ion of the fact that d /d2 c( 10)

should be aconstant is presented by Nazir Ahmad [7]. Mr. Ahmad found that R/q2/3

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I O

a. trGoodnScour PattemIligh tailwater (a2 ' 2.110d")' Banks andbed are good.

c. trFairrr scour Patterallailwater s1i-ght1y low (d2 = 2'0O dc)'Banks scoured a little and a slightincrease in bed scour.

b. trGoodilScour PattersrFqig.n tailwater (d2 = 2.15 dc). Banks andbed show l-1ttl-e scour.

d. rrPoorlr Scour PatternTailwater excessively low (d2 = 1'90 d'g).Scour of both banks and bed is excessive.

Fig. 7 - Effect of To i woter Level on DownstreqmScour

F ig . 8 - ScourneorEndof S t i l l i ng Bos in

16

a. "Good" Scour PatternHigh tailwater ( d2 2.40 dc ) . Banks andbed are good.

c. "Fair" Scour PatternTaihrater sl ightly low ( d2 c 2.00 dc ) .Banks scoured a l i t t le and a slightincrease in bed scour.

b. "Good" Scour PatternDesign tailwater ( d2 2.15 dc ) ' Banks andbed show l i t t l e scour.

d. "Poor" Scour PatternTailwater excessively low ( d2 1.90 dc ).Scour of both banks and bed is excessive.

Fig . 7 - Effect of Tai Iwater Level on Downstream Scour

Fig. 8 - Scour near End of Stilling Basin

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is a ccnst;nt, where A is t?:e dei:th from ii:e water surface to ihe bottomof il:e $cour hcle (see Fig. E) and q is 'i,hedischarge per unit width ofthe siilling basin. Since

e = vcdc = G% ac = gT/zu*t/,elru

shere o is assr::ncc.

R = d 2 * o d " = t 2 . L 5 * o ) d "to be a constant, see Fig. 8),

p

\:t= - - +

--e/ J -1 c

d 2 * o d " d ' + o d " d Zr^L/z^s/2,,2/3 _r/i^\ 6 6 * c J 6 * c

?lri.s alge=*raic manipulati-on incJ.ice.tes t}:at R/qZl3 and do/d^ are varia-z ' cbles rrlt.ich di-ffer in absolute aagni-tude only b; ' the application of cerbainconstan'ts and fu:";l'iec' verifies tlle finchng that d,/d. is a constant,

The cheek tests plotteci in Fig. 6 showed tf,lt a tailwater depthof 2.15 a^ gave eniirely satisfactory scour pat:-.errrs.

Floor Block *:rd En{_Sill HeightThe heighis of ihe floor bloeks and the end sill greatly 1nflu-

ence the perf*r:rance of the sti-l-}ing basin and help to deter:nine ihe amountof scour in the downstreem channel. it e'as found that the floor blocks-or5naril;' influenced tlre arouni of bank erosi-on r.rhile the primary- effectof end sill- heiglht r'ras on the depth of bed scour,

The tests nade to det+:Ttine the floor block and end si1l heightsare sumile.:'ized in Table I. .$-ilter ca.refully considering these data, theopti:m:.m floo:. blcck height uas tel:en as C,E d,- and the end sill heighto .h dc .

T]:ree thi-ngs were looked for in cornparing the data of Tabl.e T:(i) fiie ;,-aJri;iiur?eni;h oj scou:: should. not be excessive and should. be ire1l

^ 1 . JA a . L > T u- = = COnStantt / z t l zn- / / *" / J

17

i s a const2nt, 1-There R i s the depth from the "Jater surface to the bottomof the scour hole (see Fi g . 8) and q i s the discharge per uni t width oft he s t i l l i ng basin. Since

q = v d = r::::.ld d = ;:;1/2d 3/2c c V6Uc C 0 Cand

R = d2 +0 d = (2.15 + a)dc c-here a i s assl.lliled to be a consta..'1t (see Fig. 8),

R2/3q ( 1/2d 3/2)2/3g c

= a 2.15 + a+ - - - - = - - - - - - - -1/3 1/3g , g constant

This a lgebraic manipulat ion indicates tha t R/q2/3 and d2/dc are var ia-bl e s 1'Thich di f fe r in absolute magnitude only by the applicat ion of cer ta incons t ants and fur t her veri f ies the f i ndi ng that d2

/dc

i s a constant.The check t es t s plo t t ed in Fi g . 6 sho-tied tha t

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1BTable I - FL00R BLOCKANDEI\IDSIIT I{EIGHT

' i lepl"h oi Soorr( nehas )

Test T,ong. i11No. Helght'"

:__^ (42u . ) o co . )75o.562o.552o.6000 60 00.5000.lr500 . o z t99 0 . 625100 0.800101^

I U'10 ' -t a.5?51o)r')* 0.625109 a.625106 0.137107 o.)J37108 0 . 137

. t loor t j lock Lnd JfllHeiqht* Heleht*

MaxlnunDlstancefrom End

Depth of BasinNext to Next tolYlngrail End 5111

-2 + -2-1+ -l +-0 .5 . _o.8^ /v . ) a . )_o.t _0.5_0.t _o.5^ /r i . ) 4 , ) -4 .5 - 1 0{ . ) - U. - l-1 .3 -1 .1

( nches),36z . o o2.662.66z . o o2.55z .ooz . c oz . o oL.ooL.ooLr. oI .oo! . 00L 'ooL .ooL.ooL ooL.OOI .oolr .ooL.00h.00L.oolr.coL.ooL.oolr ,colr. CC-t)r .ooL.oo3 . 3 3j . t l) . ) 13 . i 3l . l ?

BankEro sion8L86a t88R A9093aA97

10 911 011 1l1611 7

o.6?5o.625o,375r .25oo.625

0.?50o.375^ (A )o.562o.750o.750o.3750. t62u 5oz0.6000.6000.5000, l+50o . 6 2 5o.6250.80 0o .625o,625a , 6 2 5o . 6 ? 5o , 5 2 '0 1170 .h i70 . 1 1 7o.5250 .L3?v . ) l )1.25a1 . 2 5 oo 5oo0 60 00 . 8 0 0o. 8oo0 600

o .? too.375o.J1 5o,379o.375o .3?5o .375a.37ia.3750. L000 . 3000 . 1000 . 3000. 3000 . 313o , 6250. Lcoo . 3 1 2v ' j t >a . 3 7 5c. 31 2o . 3 7 to,3r20.137A.)r37o.25aa,250o.625a , i75o . 3 7 50.1000,lr000.5o0c 10 0ii . lco

-2+-1+-1+{ . o

- 1 . 0- l . o- t . J-1 rL !- ! . U- l r u-1 .3-1 .0_0.7-.'t 0- t . c- t . o- t . 0- 1 . . o-I.2-7.2-1.2- 0 . 6- t . o- r . c

- -1.c-0. l r-1 .0- 2 . O

-t .0-r.2-1 . C- 1 . 1-0 . q:- 0 . 7-0.8-0 .7

-1 .O-1 .O- t , 1- t J . o-0 .7-1 .0- 2 . 0- J . o

- 2 . C

-1.2

- i . t

-0. ?

-o .8

-7.2-0 .7*0.5-1 0-0.5o .0-0.1r

- t . o

GoodGooCGoodGoodGoodCoodGoodFairPoor.PoofFairFairFairGooCFairPoorPocrPoorCoo.lPoor

Lr5Jt5L0)))33205506lr2)h

PoorPoorGoodGoodGooCGoodPoorGoodPoorGoodGoodGoodGoodGoodGooC

118 A . 62512lr 0. 500r2 5 c,5ooL2 6 0.800\27 0 . 8C Cr2 e 0.600l n Le rns oi e c

i l ):Two row5 cf b locl ts

00Llr255t0o)6

away from the downstrean end of the outlet; (2) the scour a-1ong he wing-wa11s and at the end of the basin should be a rninj.num; and (3) tne Uanterosj.on should have a ttgoodtrrating, a rrfairrr or rrpoorrrrating i-ndicatingthat ed.dies had washed away an excessive anount of bank material. A studyof Table I shows that Tests No. 87, BB, 89, 100, 116, and 127 best nreetthese criteria. Thetests were:

Test No.

relative floor bl-ock and end si1l heights for these

ftelative floor blockBT BB 89

height o.562 o.75o a.75a100 116 L27

o.BooL.25a o.Boo0.1+00O.625 0.1100elative end sill height o.375 O.375O.375

18Table I - FLOOR BLOCK AND END SILL HEIGHT

IJepth of Sr:our( i nches)Max i mum

Di st anc eTes t Long. Si l l Floor Block End Si l l d f r om End Next to Next to BankNo . Height " Height* Hei!lht* ( i nches) ,Depth of Basin Wngwal l End Si l l Er osion84 0.7 50 0.750 2. 66 -2+ 4 - 2+ -2 Poor85 0. 375 0 375 2. 66 -1+ 5 -1+ - ) + Poo r86 0. 562 0 375 2.66 - 1+ 2 -0. 5 -0 . 8 Good87 0 . 562 0.562 0. 375 2. 66 -0 .6 3 -0 . 5 -0 .5 Good88 0. 562 0.750 0. )75 2.66 -0.6 ) - 0. 5 ...{l .6 Good89 0 .75 0.7 50 0 375 2. 66 -1.C 5 -0 . 5 - 0.6 Good90 00375 0375 0 375 2.66 -1.C 4 - 0 . 5 - Poor91 0. 562 0.562 0. )75 2. 66 -1 .0 0 -0 . 5 -1. 0 Good92 0.562 0. 562 0 375 2.66 ..(J. 3 3 -o .S ..(J . 3 Poor9) 0.600 0.600 0. 400 4.00 - 1. 8 5 -1 . ) -1 . 3 Good90 0 .600 0.600 0. )00 0.00 -1.3 ) -1 . 0 -1.2 Good95 0. 500 0. 500 0.)00 0.00 -1 .l ) - 1. 2 - l . h Good96 0 500 0. 500 0. )00 L.OO -1.u 3 -1.2 -1.u Good97 0. 450 0. u50 0 300 4.00 - ] .L 2 - ). 2 - 1.1: Good98 0.625 0.625 0. )13 4.00 -1. 1: 0 -1.C - J.L Gooc99 0.625 0.625 0.625 4.00 - 1.3 5 - 1.1 ..(J . e Good100 0. 800 0 .800 0. 4Co 4.00 - 1.C 5 - 0. 5 -c.5 Good101'''''' 0.625 0. 'll2 L.oo -0 . 7 0 -0 .7 -0.7 Good102** 0.625 0.-)75 4.00 - LO 6 - 0 . 1l ' -0 .6 Good103 "-1' 0.625 0.625 0375 4. 00 -1.0 4 ..(J . B ...{l . R Good

104':1-* 0.625 0.625 0 312 L.OO -1. 0 2 - 0. 7 -0. 8 Good105 0.625 0.625 0.375 4.00 - ).0 3 - 0.8 -0 .8 Good106 0.437 0.L37 0 312 4.00 -1.0 0 -0. 7 -0 . 8 Fai r107 0. 437 0. 0'17 0.037 4.00 - 1.2 S -1.0 - Cl . P Poor108 0. 437 0. 437 0. 037 4.00 -).2 5 -1.0 -O. P. Poor109 0.625 0.625 0. 250 0.00 -1.2 0 -1.1 - 1.2 Fair110 0.625 0.437 0.250 L.oo -0. 6 0 -0. 6 - 0. 6 Fair111 0 375 0. )75 0.2, 0 4.00 -1.0 4 -0 .7 - 0. 7 Fair116 1.250 1.250 0.625 0.00 - 1. 0 0 -0. 5 -0 . '0 Good117 0.625 1.250 0. 175 0.00 - 1.2 - 1.0 -1.0 Fair118 0.625 1. 250 0 375 h. oo - 2. 0 5 - 2.0 - 0. 5 Poor12L 0. 500 0.500 0.)00 ) . )3 - 30 5 -3 0 0.0 Poer125 0.600 0 .600 O.uOO ) .33 -0 . 4 -0 -O . L -o. h Poor126 o.BOO 0.800 0 500 3 3'1 -0.1 C -O . L - 0.1: !!a i r127 O. ACO O. eoo c.Loo 3. 13 - 1. 0 5 -0 . 5 -O . S Good128 0.600 0.600 -: 300 3. J l - 2. 0 6 - 2.C -1.0 PoorIn terns of dc

; : : r ows of blocks

away from the downstream end of the outlet; (2) the scour along t he wing,valls and at the end of the 'iJasin should be a minimum; and (J) the bankerosion should have a "good" rating, a "fair" or "poor" rating indicatingthat eddies had washed away an excessive amount of bank material. A studyof Table I shows that Tests No. 87, 88, 89, 100, 116, and 127 best meetthese criteria. The relative floor block and end s i l l heights for thesetests were:Test No. 87 88 89 100 116 127Relat ive floor block height 0.562 0.750 0.750 0.800 1. 250 0.800Relative end s i l l height 0.375 0.375 0.375 0.400 0.625 0.400

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1 A

l,Jith the exception of Test No. EJ, all floor blocks were twice as high asthe end sill, Both higher and lo"reer ratios of floor block height to end.sil1 height produced either a greater depth of scour nerb to the end sil1and r,ringwalls, or poorer ba"nk erosion, or both. Alsoo irith the exceptionof Tests No. BT and 115, all end sill-s were approxfurately 6,l+ d" highand all floor blocks were 0.8 dc high. fhe lesser height was selectedbecause the tailuater depth was greater for these tests than was later de-ter:rlined to be the best min-imumdepth. tihen the tailwater depth was re-duced, there was a greater opportunity for the strean to scour the bed. inplunging over the high end sill than in passing over the lower end sill.

Although longitudinal sills were employed during most of thetests summarized in Table T, subsequeni tests r+'ithout longitudinal si11sand with floor blocks and end si1l heights of 0.8 dc and g.b de.r r-spectively, showed that these heights were eompletely satisfactory.

As a result of the tests made to deternine:reights, it is recommendedhat:

the height of the floor blocks be 0.8the height of the end. si11 be 0.1+ d^

the block and silln

Floor Block '[.trj-dthand SpacingThe actual width and spacing of thE floor bloeks, as well as the

:roportion of tbe stilling basin,ridth occupied by the bloeks, have beenfound to have an importarrt effeet on the perforn:anee i-n previous sti3.1ingbasin studles. fhe straight drop spil*lway stil1.ing basinwas no exceptioniIf the floor blocks are too wide, they do not break up the stream intomall enough segments to di-ssipate the high-veloeity fI-ow in a short dis-tance. If the bloeks occupy too nruch of the stilling basin width, theircor,rposi-te ction becomesmore like a solid sill than like individual blosks.Fina11y, if the blocks oceupy an insufficient proportion of the basin widththey become -neffective. There is, therefore, an optimura spacing and pro-::ortion of the basin width occupied by the blocks that must be dete:nrined.The primary effect of varying the floor block w:dth and spaci-ng is on theanount of bank erosion obtai-ned.

?he tests made o deterniine the block width and spaclr are sum-narj-zed in Table 11. The same things were looked for in Table II when

19

Hi th the exception of Test No. 87, a l l f loor blocks were twice as high asthe end s i l l . Both higher and lower rat ios of floor block height to ends i l l height produced either a greater depth of scour next to the end s i l land wingwalls, or poorer bank erosion, or both. Also, with the exceptionof Tests No. 87 and 116, a l l end s i l l s were approx::iJnately 0.4 d highcand a l l f loor blocks were 0.8 d high. The lesser height was selectedcbe cause the tai lwater depth was greater for these tes ts than was la te r de-te rmined to be the best minimum depth. itJhen the tai lwater depth was re duced, there was a greater opportunity fo r the stream to scour the bed inplunging over the high end s i l l than in passing over the lower end s i l l .

Al hough longitudinal s i l l s were employed during most of thete s ts summarized in Table I , subsequent tes ts without longitudinal s i l l sand vn th f loor blocks and end s i l l heights of 0.8 d and 0.4 d, re-c cs pec t ively, showed tha t these heights were completely satisfactory.

As a result of the tes ts made to determine the block and s i l l:te i ght s , it i s recommended that:

the height of the f loor blocks be 0.8 dcthe height of the end s i l l be 0.4 dcFloor Bl ock "lhdth and Spacing

The actual 'It.Jidth and spacing of the f loor blocks, as well as thepropor t ion of the s t i l l ing basin width occupied by the blocks, have been~ u n d to have an important effec t on the performance in previous s t i l l ingbas in studies. The s t ra ight drop spillway s t i l l ing basin was no exception.If the floor blocks are too wide, they do not break up the stream intosma l l enough segments to dissipate the high-velocity flo'ltJ in a short distance. I f the blocks occupy too much of the s t i l l ing basin width, thei rcompos i te action becomes more l ike a solid s i l l than l ike individual blocks.Final ly , i f the blocks occupy an insufficient proportion of the basin widththey become ineffect ive. There i s , therefore, an opt::iJnum spacing and propor t ion of the basin width occupied by the blocks that must be determined.The pr::iJnary effect of varying the f loor block width and spacing i s on theamount of bank erosion obtained.

The tes ts made to determine the block width and spacing are sum-a r i zed in Table I I . The same things were looked for in Table I I when

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comparing the effect of the floor block width and spacing as were lookedfor in Table I when comparing the effect of the floor block and end s111height. A study of.Table II shows that the block width has no detectableeffect on the bed or bank scour. However, j-t will be shown n the sectionon ttsidewall heighttt 'that wid,e blocks cause a tr:igh boil in the basin andthus requi-re tr-igher sidewalls. This indieates that thene is a limit tothe lridth of the i-ndlvidual bl-ocks that nrust be taken into account r+hendesigning stilling basins.

Table II - Ft00R BLOCK TDTHAI\D SPACING

Basi,r Wldth0ccupledBlock bv Blocks longryicth* (ier cent) si l tso.zt 50.0 2o.25 L1 .7 2o.25 111.7 20 . 2 t 5 0 . 0 0o.25 50.0 2o.?5 5o.o L0 . 3 7 5 5 0 . 0 00.15 50 .0 0o.5o 5o.o oo . ! t 60.0 o0.3o lro.0 0o .L5 5o.0 0o .5L 60.0 oo .5o 55.o o0.110 5O.0 0o .Lo 5o.o 00.!0 53.J o

Depth of Scour----L!"sbgs)Marin'rn Next to At End of BankDepth lflgell Basln ErosionTestN o,1881891 0 n

( t nc ie" )lr.col+ .OOt+.OOlr.OOL.ool+ .OOL.ooL.oolr.oo1r.00L.oo5 . t J3 . 3 3J . t t2.672,67h.00

^ _a . J v2 . 3 02 . 3 02 . O2 . ) O2 . 3 02 . J Oa .wZ . J Uz . J v4 . L )z . L >z . L t2 . 002 . 30

_x-2.50< > v2 . 5 o2 . 5 02 . 5 0u ( a2 . 2 02 . 2 02 , 2 0z . z )

-1.5- L . )- r. )_r.5_.t-. )- r . )_ L . )_ L . ,

- t . )

-r.7t - 1 . )_2. l r- L . )-o.8

GoodPoorPoorFairFairFairGood0oodGoodGoodPoorGoodGoodGoodGoodGoodOood

1q)' l s 119Ut9 i196198201z v ata 1zoi'*2C,5206

-1.2 ^ a-o.8- 1 ? 4 .8- 1 . 2 -O .8- 1 . 5 { .8- \. J l t t 7-0 .1 -o . l- \ . j - L . t- 1 . 3 4 .8-1.3 -O.8-1 .8 4 .8-1.5 -o.8-o.8 -7..2- L . 1 - L . a-1.0 :L.2-o.3 4.8{. ) I . t . t*--In terlF ol o^

#Blocks "r" O.l t d" long; all otherblock lengths are eoual to thetrr id th

Although the I,r'idth of individual blocks is not criticalr it isessential that a sufficient nrunberof floor bloeks be used to break up thenappe intoturbulence.

small strea.ms. These smal1 streans then decay into isotropicThe d.istance required for this decay to talce plaee depends

to a large erbent on the size of the streams into which the nappe is bro-ken by the floor blocks. Beeause of these consideratj-ons, it is recom-mended that the floor blocks be o.h dc wide. A variation of 0.15 dcfrom tlr-ls ljrnit is per:n:issible in order to fit the blocks i-nto the stillingbasin and avoid, od.d.d.j-:rnensions. However, it is stressed that the recom-mended floor block width is O.h d. and that this di:nension should beheld as closely as is practically possJ-b1e.

20

comparing the effect of the floor block ,iLdth and spacing as were lookedfor in Table I when comparing the effect of the floor block and end s i l lheight. A study of, Table II shows that the block width has no detectableeffect on the bed or bank scour. However, i t will be shown in the sectionon "sidewall height" 'that wide blocks cause a high boil in the basin andthus require higher sidewalls This indiaates that the:re is a l imit tothe width of the individual blocks that must be taken into account 1vhendesigning st i l l ing basins.

Table II - FLOOR BLOCK { [ D T H AND SPACING

Basi:! Width Depth of ScourOccupied ( inches)Test de * * Block by Blocks Long Maxi:num Next to At End of BankWidth*o . (inches) .JL (per cen t) S i l ~ ~ ! : t L W i n ~ g Bas in Erosion188 4.00 2.30 2.50 0.25 50 .0 2 -1.5 -1.2 -0 . 8 Good189 4. 00 2 30 2 50 0.2 5 41. 7 2 - 1. 5 -1. 2 -0.8 Poor190 4. 00 2.30 2.50 0.2$ 41.7 2 -1 . 5 -1 . 2 -0.8 Poor191 4.00 2. 30 2 50 0.2 5 50.0 0 -1.5 -1 . 2 -0.8 Fair192 4. 00 2.30 2. 50 0.2 5 50.0 2 -2.0 -1.6 -0 . 8 Fair193 4.00 230 250 0.25 50.0 4 -1 . 5 - 1. 3 -0.3 Fair194 4. 00 230 2.50 0.375 50.0 0 -0 . 5 -0 . 3 -0 3 Good195 4 .00 2.30 2.50 0 .75 50 .0 0 -1 . 5 -1 . 3 -1 . 3 Good196 4. 00 2.30 2.50 0.50 50 .0 0 -1 . 5 -1.3 -O.B Good19 8 4.00 2. 30 2.$0 O . ~ S 60.0 0 -1.3 -1 . 3 -0 . 8 Good199 4. 00 2. 30 250 0.30 40 .0 0 -2 . 0 -1.8 -0 . 8 Poor201 3.33 2. 15 2.20 50.0 0 -1 . 5 - 1. 5 -0 . 8 Good202 3.33 2.15 2.20 0. 54 60. 0 0 -1.7 -0.8 -1.2 Good203 3.33 2. 15 2.20 0.50 55 0 0 - 1. 5 - 1. 2 -1 . 2 Good204** 2.67 2.00 2.25 o . ~ o 50 . 0 0 -2 . 4 -1.0 -1.2 Good205 2.67 2.00 2.25 0.40 50 .0 0 -1.3 -0 .8 -0 . 8 Good206 4. 00 2.30 1 . 75 0.40 53.3 0 -0 . 8 -0 . 5 -0.3 Good

*I n terms of dc**Blocks are 0.75 d lo ng;c al l ot her bl ock l eng t hs are eoual to their width

Although the width of individual blocks is not cr i t ical , i t isessential that a suff icient number of floor blocks be used to break up thenappe into small streams . These small streams then decay into isotropicturbulence. The distance required for this decay to take place dependsto a large extent on the s i ze of the streruns into which the nappe is broken by the floor blocks. Because of these considerations, i t is recom-mended that the floor blocks be 0.4 d wide.c A variation of 0.15 dcfrom this l imit is permissible in order to f i t the blocks into the st i l l ingbasin and avoid odd dimensions. However, i t is stressed that the recom-mended floor block width is 0.4 d

cand that this dimension should be

held as closely as is practically possible.

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The floor bloeks for the tests were ordlnarily square in p1an.However, for Test $c. 20L (see Table II), the floor blocks were 0.1+dcwide by 0.7, d" 1ong, and the ssour depths are somewhat greater thanthose for coapanion Test No. 205 where the floor blocks were g.h ds wid.eby 0.L dc 1ong. The miaimu:* perarissible distance betr*een the floorblocks and the basin erlt is quite short and thj-s entire. distance is re-quired for energy dissipati-on. If sone of this space is occupied by ex-tra long floor blocks, it will not be available for energy dissipation,and adCi-tional $cour in the downstream char:ne1 will result. It is there-fore recommend.ed hat the floor bloeks be square in plan with the dimen:-sions indj-cated in the preced:i::g paragraph.

Study of ?ab1e fT al1so shows that the proportion of the basinridth occupied by the floor bloeks should be between 50 and 60 per cent ofthe stilllng basin r^ridth. This is also shoi.rn n Fig. 9r where the con-tours of the scour are al:nost the same 1n Fig, 9b as they are in Fig. pe.For these photographs the portion of the basin !flidth occupied by the blocksis 50 and 60 per cent, respeetively. In Fig. !a, where the proportion isl+0 per ceni, the maximrm depth of seour is greater and the bank erosionis much more seYere. '

' The tests show that longitu*tnal sills are nej-ther si-gn:ificantlybenefi-ci-al nor har,nful- as regards the hydraulic perforxrance of the still-ing basin. The longitudinal si1ls may be used to strengthen the stiDi-r:gbasin floor j- f ihis proves desirable, If longitudinal si1ls are used,they should. be located to pass through a floor b1ock. Their height should.be deternined by structural requirements.

Summarizing: ?he width, spacing, and length of the floor blocksshoul-d be approxi-:nately 0.h dc with an extrerne variation of + 0.15 d"iit is not necessarythat the floor blockT'fiidth and spaeing be exactly equal,but the blocks should oeeupy between 50 and 50 per cent of the stillingbasin width; a half spaee should be located next to the basin sidewall;and longitudinal sil1s passing througb the floor bloeks may be used forstrrrctural pu.rposes rs-ithout detrjmental hydraulic effects.

21

The f loor blocks for the tes ts were ordinari ly square in plan.However, for Test No. 204 (see Table I I ) , the f loor blocks were 0.4 dcwide by 0.75 d long , and the scour depths are somewhat greater thanct hose for companion Test No. 205 where the f loor blocks were 0.4 dc wideby 0 .4 d long. The minimum pennissible distance between the f loorcblocks and the basin exi t i s qui te short and this entire distance i s re-qui red for energy dissipat ion. I f some of th is space i s occupied by ext ra long f loor blocks, it wil l not be available for energy disSipation,and add i t ional scour in the dOiVllstream channel will resul t . I t i s there-fo r e recommended that the f loor blocks be square in plan with the d i m e n ~ s io ns indicated in the preceding paragraph.

Study of Table I I also shows that the proportion of the basinwidt h occupied by the f loor blocks should be between 50 and 60 per cent ofth e s t i l l ing basin width. This i s also shown in Fig. 9, where the cont ours of the scour are almost the same in Fig. 9b as they are in Fig. 9c.or these photographs the port ion of the basin width occupied by the blockss 50 and 60 per cent, respectively. In Fig. 9a, where the proportion i so per cent, the maximum depth of scour i s greater and the bank erosions much more severe.

The tes ts show that longitudinal s i l l s are nei ther s ignif icant lyenef i c ia l nor hannful as regard