EVALUATION OF RIPRAP DESIGN PROCEDURES - USGS report

7/30/2019 EVALUATION OF RIPRAP DESIGN PROCEDURES - USGS report

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ROCK RIPRAP DESIGN FOR PROTECTION OF STREAMCHANNELS NEAR HIGHWAY STRUCTURES

VOLUME 2 ~ VALUATION OF RIPRAP DESIGN PROCEDURES

By J . C . Blodgett and C . E . McConaughy

U . S . GEOLOGICAL SURVEYWater-Resources Investigations Report 86-4128

Prepared i n cooperation withFEDERAL HIGHWAY ADMINISTRATION

CNoI< rmoo o

Sacramento, California1986


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UNITED STATES DEPARTMENT OF THE INTERIORDONALD PAUL HODEL, Secretary

GEOLOGICAL SURVEYDallas L . Peck, Director

For additional information,write t o :District ChiefU.S. Geological SurveyFederal Building, Room W-22342800 Cottage Wa ySacramento, CA 95825

Copies of this report can bepurchased from:Open-File Services SectionWestern Distribution BranchU.S. Geological SurveyBox 25425, Federal CenterDenver, CO 80225Telephone: (303) 236-7476


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CONTENTS

PageAbstract -- -- -- - - - ______ ] _Introduction - - -- - - - - 2R ev i ew of ripr a p d e s i g n t e c h n o l o g y - -- -- - - - -- ---- -- - 4Shear stress related to per m i s s i b le flow v e l oc i t y -- - --- - 5

S h e a r stress related to h y d r a u l i c r a dius a n d g r a d i e n t - - - - - - - - - - - - - 7C h a r a c t e r i s t i c s of riprap f ailure -- --- ____ - 9C l a s s i f i c a t i o n of f ailures ----- ____ 1 0

P a r t i c l e e r o s i o n --- __________________________________________ 10T r a n s l a t i o n a l slide - --- - _____ __ 1 5M o d i f i e d slump -- ---- _-- __ __ ___ _ ___ \&Slump ---------------------------------------------------------- 1 8

Hydraulics a s s o c i a t e d with riprap f ailures of selected streams -- 1 9Pi nole Creek a t Pi nole, C aliforni a ----___ - - -- - - - 21S a c r a m e n t o R i v e r a t Site E - 1 0 n e a r Chico, C a l i f o r n i a --- - - 2 1Hoh River at Site 1 n e a r Forks, W a s h i n g t o n -- -- - -- _--- ' 22Cosumnes River a t Site 3 n e a r D i l l a r d Road Bridge nearSloughhouse, C a l i f o r n i a - - --- ____ _- 24Truckee River at Spar k s , N e v a d a - -- --- - ----- ---__ 24

S u m m a r y of f actors c o n tr i bu ti n g to riprap f ailures --- - - - - - 28E v a lu a t i o n of riprap design procedures ----------------------------------- 28D a t a for c o m p a r i s o n of design procedures ---------------------------- 31E v a lu a t i o n of H y dr a u l i c E n g i n e er i n g Circular No. 11 (HEC-11) 34Description ---------------------------------------------------- 34

H y dr a u l i c f actors - - --- 35Riprap stability f actors ---- -- -- - - 37Appl i c a t i o n --- -- - --- ____ _ 38S u m m a r y discussion of Circular HEC-11 - -- -- --- - 41E v a lu a t i o n of H y dr a u l i c E n g i n e er i n g Circular No. 15 (HEC-15) 42D es c r i pt i o n -- -- --- _____ ___ _ _ 43H y dr a u l i c factors - ---- ______ _ __ _ 45C h a n n e l g e o m e t r y -- ----- - -- -- 47E s t i m a t i o n of stone size b a s e d on d e pth of flow (chart 27) ----- 47Ch annel bends ----- ------ _ - ___- -_ 49Appl i c a t i o n -------- ________ __ _ 50S u m m a r y discussion of Circular HEC-15 - ----- -- --- - - 50

E v a lu a t i o n of C a l i f o r n i a D ep a r t m e n t of T r a n s por t a t i o n "Bank a n dShore Protection" m a n u a l ---- --- - -- 57D es c r i pt i o n - - - - - - ---- 57Application ---------------------------------------------------- 61

E v a lu a t i o n of U.S. A r m y Corps of Engi neers, B u lle t i n E M - 1 6 0 1 62D es c r i pt i o n ---------------------------------------------- ---- 62Appl i c a t i o n - - -- - ---- 68

E v a lu a t i o n of "S edi m e n t a t i o n E n g i n e er i n g , " Am er i c a n Society ofCivil Engi neers ( M a n u a l No. 54) --- --- -- - 70Description ---------------------------------------------------- 70Appl i c a t i o n ------- - -----_ ____ _ ________ 7 1

E v a lu a t i o n of "Sediment T r a n s por t T e c h n o l o g y " (S im ons a n d Senturk) -- 72D es c r i pt i o n -- -- ---- ---- --- 72Application ---------------------------------------------------- 74

III


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E v a lu a tion of riprap design procedures (continued)Evaluation of U.S. Bureau of Reclamation, EngineeringMonograph No. 2 5 --------- ----_____ ________ ________ ________ y gDescription --- ---- -- ________________ _________________ 7 3Application -------- --------- -------- -- -__----- _________ g oRiprap specifications ---- __________ ___________ ____ ____ _ __ g jRock specifications - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - _ _ __ _ _ _ _ g jShape 8 2Durability 82Specific gravity - --------------- ______ ______ _ _ _ g2Riprap-layer specifications ------------ -- _____ ________ __ g2Thickness 84Method of placement -- ----- ----- ----- - ________ __ ______ 34

Toe construction ------- --- ------- ------- __________________ g 5Gradation of stone - ---- ---- -- ____________ _ __________ g 5Filter blankets - 8 7Development of a n ew procedure for estimating m e d i a n stone size - - - - - - - - g gSummary comparison of various design procedures - --------- ----- _______ 90Literature survey and references - - - - - - - - - - - - - - - - - - - - - ______ ______ 93

ILLUSTRATIONS

Figure Page1 . Photograph of erosion of rock riprap on left bank of Pinole Creek

at Pinole, California, following flood of January 4 , 1982 - - - - - 22 . Graph showing comparison of procedures relatin g velocity t o stoneweight (adapted from EM-1601) 63 . Three-dimensional free body diagram of forces acting on awater mass - --- ---- -- ______________________ _________ __ 74 . Sketches of classification of principal types of riprap failures - - 1 05-7. Photographs of riprap on left b a n k of Sacramento River at E-10

near Chico, California:5 . A reference line shows location of stones in December 1981(photographed March 4 , 1982) 1 26 . Showing initial effect of particle erosion (photographedFebruary 1 , 1983) 1 37 . Showing advanced stage of particle erosion (photographedMa y 2 , 1983) 1 3

8 . Sketch of typical riprap failure area in the shape of a horseshoe,caused by particle erosion - ----- ----- ___- ______ ____ 1 59-11. Photographs of riprap on Cosumnes River near Dillard Road Bridgenear Sloughhouse, California (photographed Ma y 3 1 , 1983):

9 . At site 2 , showing translationa l slide failure -------- - - 1 71 0 . At site 3 , looking downstream, showing modified slumpfailure - - - ---- ______ __ --- 1 71 1 . On left bank, at site 1 , showing slump failure - -- 1 9

1 2 . Photograph of damaged riprap on left b a n k of Pinole Creek atPinole, California, following flood of January 4 , 1982(photographed March 1982) 2 2

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Figure Page1 3 . Photograph of new riprap placed on left bank (upstream view) ofHoh River at site 1 near Forks, Washington (photographedAugust 1982) 2 31 4 . Photograph of damaged riprap on left bank (downstream view) of

Hoh River at site 1 near Forks, Washington (photographedDecember 1982) 2 31 5 . Photograph of damaged riprap on right bank of Truckee River atSparks, Nevada (photographed June 1 5 , 1983) --------------------- 2 51 6 . Water-surface profiles of Truckee River at Sparks, Nevada, forfloods of December 2 0 , 1981, a nd Ma y 2 7 , 1982 261 7 . Sketch of scour hole in riprap adjacent to obstruction onstreambank ------------------------------------------------------ 2 7

18-19. Graphs of comparison of procedures for estimating stone size:1 8 . On chan nel ba n k based on permissible velocities -------------- 2 91 9 . On channel bed using critical shear stress ----- ------------ 3 0

2 0 . Aerial photograph of Pinole Creek at Pinole, California, showingstudy reach (October 1 , 1982) 3 22 1 . Flow chart of riprap design procedure for HEC-11 ------------------ 3 52 2 . Graph of relationship of D 5 Q stone size on chan nel bottomto velocity a g a i n st stone --------------------------------------- 3 72 3 . Graph of relationship of stone size to velocity --------------- -- 382 4 . Sketch showing toe trench detail for riprap protection ------------ 4 1

25-26. Flow charts for application of HEC-15 procedures:2 5 . Straight chan nel with side slopes flatter than 3:1 ----------- 442 6 . Curved chan nel with side slopes steeper than 3:1 ------------- 4 5

2 7 . Graph of applicability of chart 2 7 in HEC-15 to open chan nels ----- 4 82 8 . Graph of comparison of median stone size ( D 5 0 ) estimated on basisof shear stress ( HEC-15 procedures) a nd performance at fieldsites ----------------------------------------------------------- 542 9 . Flow chart for b a n k protection procedure in "Bank a nd ShoreProtection in California Highway Pra'ctice" ---------------------- 5 73 0 . Flow chart for application of U.S. Army Corps of EngineersManual EM-1601 in design of rock riprap ------------------------- 633 1 . Sketch of channel b a n k used to measure boundary resistance inthe form of M a n n i n g 's n a nd equivalent roughness k -------------- 6 63 2 . Graph of relationship of M a n n i n g 's n a nd equivalent roughness kfor chan nel banks ----------------------------------------------- 6 73 3 . Definition sketch of variables used by Simons and Senturk fordesign of bank protection --------------------------------------- 7 53 4 . Flow chart for Simons and Senturk bank protection procedure ------- 7 63 5 . Graph of curve to determine maximum stone size in riprap mixture - - 7 93 6 . Graph of relationship to determine median stone size basedon average velocity (adapted from USBR-EM-25) 803 7 . Graphs of comparison of stone gradations specified in differentdesign procedures ----------------------------------------------- 8 63 8 . Graph of comparison of median stone size ( D 5 o ) estimated on basisof mean velocity (HEC-11 procedures) a nd performance at fieldsites 8 9


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TABLES

T a b l e Pa ge1 . Agency, pu b li c a t i o n title, a n d a b b r e v i a t e d title of v arious rock

riprap design procedures ------------------------------------------ 32 . H y d r a u l i c properties a n d ch annel g e o m e t r y of streams as a fu nctionof ch annel slope -------------------------------------------------- 93 . G e o m e t r y of progressive ripr a p f ailures related to p a r t i c l e e r o s i o non the S a c r a m e n t o River at Site E-10 n e a r Chico, C aliforni a( surveyed M a y 2 , 1983) --- 144 . Hydraulic, riprap, and damage ch aracteristics of selected sites w i t hriprap f ailure -------------------- ----- ____ ___________ ____ 205 . Hydraulic properties of the J a n u a r y 4 , 1982, flood at crosssections 0. 2 a n d 3 on Pi nole Creek at Pin ole, C aliforni a ---------- 336 . Steepest suggested side slopes by design procedure a n d pl a c e m e n tm e th od - 537 . E s t i m a t e s of flow v e l oc i t y a n d shear stress related to ripr a pperforma nce ------------------------------------------------------- 558 . Particle-size g r a d a t i o n a n d riprap data - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 609 . S a f e t y factors for v arious sizes of riprap, Pi nole Creek at Pin ole,C aliforni a (cross-section 3 ) -------------------------------------- 7 7

1 0 . S u m m a r y of ripr a p criteria used in design procedures -- _______ 331 1 . C o m p a r i s o n of D 50 stone size d e t e r m i n e d by v arious ripr a p designprocedures -------------------------------------------------------- 911 2 . Riprap D 50 stone size design equ ations ----- ______--_-------------- 9 2

LIST OF SYMBOLSrmbol ________________________Term_______________a C o n s t a n tA AreaB C h a n n e l b otto m w i d thC Coefficient; Chezy resistance coefficientd D epthd M e a n depthd M a x i m u m depthm *D D i a m e t er of rock particlesD Spheric al di ameter of ston esF Froude n u m b erg Ac c eler a t i o n of g r a v i t yG Specific g r a v i t y of ston eSk Stone di ameter; e qu i v a l e n t roughness; c o n s t a n t

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LIST OF SYMBOLS (con tinued)

Term

K . Shape f actor of ston ek Stone size for stone of w pou nds per cubic foot rH L o n g a xis of stoneL L e n g thn M a n n i n g ' s rough ness c oe f f i c i e n tP W ett e d per i m e t erQ Total disch argeQ_ D es i g n disch arger C e n t e r l i n e radius of ch annel bendR H y dr a u l i c r a diusR, M e a n r a dius of outside b a n k of benddR M e a n r a dius of ch annel c e n t e r l i n eoS E n er g y slopeS_ F r i c t i o n slopeSF S a f e t y f actor for riprap on side slope with h o r i z o n t a l flowSF S a f e t y f actor for riprap on side slope w i th no flowS C h a n n e l bed slopeoS W a t er - s u r f a c e slopew rt T h i c k n e s s of riprap normal to face slopeT Top w i d th of cha n n el; w a t er - s u rf a c e w i d th of c h a n n e lv P o i n t velocity; k i n e m a t i c v i s c os i t yV V eloc i t yV A v e r a g e (mean) v e l oc i t y in cross sectionS iV M a x i m u m po i n t v e l oc i t y in cross sectionV V eloc i t y a g a i n s t ston eSw U n i t weight of stone; water-surface width a t u ps tr e a m en d of b e n d

W U n i t weight of ston eW Class w e i gh t of stoneW W e i gh t of stonesy D epth above b ou n d a r y c or r espo n d i n g to vz Side slope, ratio of horizontal to v er t i c a lY U n i t weight of w a t erY Specific weight of stoneS

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LIST OF SYMBOLS (continued)Term

AAcAyne\4ptt ,

Internal a ngle of channel bend, in degreesInternal a ngle which differentiates between a long and short bendMagnitude of superelevationStability factor for particle on plane horizontal bedAngle of repose of riprap; a ngle of bed slope; side slope angleAngle between horizontal and velocity vector at a pointConverts safety factor from nonhorizontal to horizontal side-slopeflow70 constant for broken rockActual or design shear on channel bedShear at bendCritical shear stressMean boundary shear acting over wetted perimeter; critical

shear stressAverage tractive force on side slope in vicinity of particleAngle of side slope to horizontal; angle of repose of riprapAngle of side slope

CONVERSION FACTORS

For readers who prefer metric units rather than inch-pound units, theconversion factors for the terms used in this report are listed below:Multiply Byft (feet) 0.3048ft (feet) 304.8ft/s (feet per second) 0.3048ft/s 2 (feet per square second) 0.3048ft 2 (square feet) 0.0929ft 3 / s (cubic feet per second) 0.0283inches 25.4I b (pound) 0.454lb/ft 2 (pounds per square foot) 4.882lb/ft 3 (pounds per cubic foot) 16.02mi (miles) 1.609ton 0.907

To obtainm (meters)m m (millimeters)m/s (meters per second)m/s 2 (meters per square second)m 2 (square meters)m 3 / s (cubic meters per second)m m (millimeters)kg (kilogram)kg/m 2 (kilograms per square meter)kg/m 3 (kilograms per cubic meter)km (kilometers)m e g a g r a m

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ROCK RIPRAP DESIGN FOR PROTECTION OFSTREAM CHANNELS NEAR HIG HW AY STRUCTURES

VOLUME 2--EVALUATION OF RIPRAP DESIGN PROCEDURES

By J.C. Blodgett a n d C.E. McConaughy

ABSTRACT

Volume 1 , "Hydraulic Characteristics of Open Channels," discusses thehydraulic a nd chan nel properties of streams, based on data from several hundredsites. Streamflow and geomorphic data were collected a nd developed to indicatethe range in hydraulic factors typical of open channels, to assist design, maintenance, a nd construction engineers in preparing rock riprap bank protection.Typical channels were found to have a maximum-to-mean depth ratio of 1.55 a nd aratio of hydraulic radius to m ea n depth of 0.98, which i s independent of width.Most stable channel characteristics for a given discharge are the slope, maximumdepth, a nd hydraulic radius.

In volume 2 , seven procedures n ow being used for design of rock riprapinstallations were evaluated using data from 26 field sites. Four basic typesof riprap failures *were identified: Particle erosion, translational slide,modified slump, and slump. Factors associated with riprap failure include stonesize, ba n k side slope, size gradation, thickness, insufficient toe or endwall,failure of the ba n k material, overtopping during floods, and geomorphic changesin the channel. A review of field data a nd the design procedures suggests thatestimates of hydraulic forces acting on the boundary based on flow velocityrather than shear stress are more reliable. Several adjustments for local conditions, such a s channel curvature, superelevation, or boundary roughness, ma ybe unwarranted in view of the difficulty in estimating critical hydraulic forcesfor which the riprap i s to be designed. Success of riprap i s related not onlyto the appropriate procedure for selecting stone size, but also to reliabilityof estimated hydraulic and channel factors applicable to the site.

Further identification of chan nel properties a nd the development of a n ewprocedure for estimating stone size are presented in volume 3 , "Assessment ofHydraulic Characteristics of Streams at B a n k Protection Sites."


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INTRODUCTION

The need to e v a l u a t e the v arious procedures b ei n g used to design roc k riprap has b ee n i ndic ated by the diverse results that m a y be o b t a i n e d d e p e n d i n g o nthe procedure used a n d assumptions concern i ng hydraulic a n d geomorphic con ditions a t a site. F a i lu r e a t a site is usually attributed to excessive hydraulicforces a c t i n g on the b a n k a n d c ausi ng d i s pl a c e m e n t of the ston es th at comprisethe riprap (fig. 1). However, other factors, such as i m p r o p e r g r a d a t i o n orpl a c e m e n t of stone, i n a d e q u a t e a s s e s s m e n t of p r o b a b l e m or ph olo g i c changes, orf ailure of the o r i g i n a l b a n k m a t e r i a l m a y contribute to the riprap failure.

Methods av ailable to protect highway structures from s t r e a m f l o w h az ardsi nclude armori ng, retards , a n d spurs. Ar m or i n g i s the surf aci ng of a ch annelbed or ba n k s , or a n e m b a n k m e n t slope; retards a re a p e r m e a b l e or impermeablestructure in a ch annel to deflect flow; and spurs are l i n e a r structurespr oj ec ti n g into a ch annel to induce d e pos i t i o n a l o n g the bank. A description ofthese ( a n d other ) pr ot ec t i v e measures (cou ntermeasures) and a n e v a lu a t i o n oftheir per f or m a n c e at v arious field i n s t a l l a t i o n s i s given in a report b y Bricea n d B l o d g e t t (1978).

FIGURE 1 . Erosion of rock riprap on left bank of Pinole Creek at Pinole, C alifornia, following flood ofJanuary 4,1982. Note deposition of displaced riprap in channel be d (photographed March 1982).


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This study presents an analysis of various procedures commonly used forarmoring a bank by placement of flexible rock revetment, also known a s riprap orriprap lining, to protect highway structures from damage ca used by channelerosion. The study was funded by the Federal Highway Administration (FHWA) aspart of their effort to develop streambank stabilization measures in the federa l l y coordinated program of Highway Research, Development, a nd Technology.Dr. Roy E . Trent i s the FHWA's technical representative for the study.

Procedures for design of riprap have been prepared by a number of agencies,such as FHWA, U.S. Army Corps of Engineers (USCE), U-S. Bureau of Reclamation(USER), a nd California De p a rt m e n t of Transportation (CALTRANS, formerly California De p a rt m e n t of Public Works, Division of Highways). The various design procedures have been identified in table 1 , using an abbreviation for brevity.

The purpose of this research effort i s to evaluate inconsistencies orpossible deficiencies in the procedures described in HEC-11 a nd HEC-15 fordesign of rock riprap. Other procedures for riprap design, such a s "HydraulicDesign of Flood Control Channels," (EM-1601) a nd the "Bank and Shore Protectionin California Highway Practice'" m a n u a l (Cal-B&SP) were also evaluated. All ofthe design procedures included in this study were reviewed with the intent ofproviding information needed for interim a nd long-term a pproaches for the practical a nd functional design of riprap.

Table 1 . Agency, publication title, and abbreviated title ofvarious rock riprap design procedures.Agency Title and date of

riprap design procedureProcedure *

abbreviationFederal Highway Administration (FHWA)

California Department ofTransportation (CALTRANS)

U.S. Army Corps of Engineers(USCE)

American Society of CivilEngineers (ASCE)

Simons and SenturkU.S. Bureau of Reclamation

(USER)

Oregon De p a rt m e n t ofTransportation

Hydraulic Engineering Circulars:Use of riprap for bank protec- HEC-11tion (Searcy, 1967),Design of stable channels with HEC-15flexible linings (Normann, 1975)B a n k and shore protection in Cal-B&SP

California highway practice(1970)

Hydraulic design of flood control EM-1601channels, EM 1110-2-1601 (1970)

Sedimentation Engineering, Manual Man-54No. 54 (Vanoni, 1975)

Sediment transport technology Simons-STT(1977)

Hydraulic design of stilling USBR-EM-25basins a n d energy dissipators(Peterka, 1958, E n g i n e e ri n gmonograph No. 25)

Keyed riprap (no date given) ODOT


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Procedures for riprap and ch annel d es i g n h ave been developed usi ngtheoretic al concepts to defi ne the m a g n i t u d e of h y d r a u l i c stress (force) in theb ou n d a r y zone a t which m o v e m e n t of i ndividu al ston es becomes im m in en t. Theseprocedures were then e m pi r i c a l l y confirmed or extended to prototype conditionson the basis of l a b or a t or y flume study data. HEC-15 summarizes the status ofsome of these procedures a n d i ndic ates tha t v er y few ac tu al da ta poi nts a rea v a i l a b l e for the study of flow a t bends. Al th ou gh the lack of ac tu al da ta wasnoted in reference to es t i m a t i o n of stresses a t ch annel bends, it applies tom a n y of the other procedures for d e s i g n i n g b a n k protection. A s i m i l a r observ ation w as a lso m a d e in a report, "Practic al Riprap Design," by M a y n o r d (1978).

It wa s d e t er m i n e d th at the best a p pr o a c h for e v a l u a t i n g the v arious ripr a pdesign procedures would be to utilize pr ot o t y pe data. Three sources of da tawere used: ( 1 ) F i e l d surveys made s pec i f i c a l l y for this project, ( 2 ) the o n g o i n gU.S. Geologic al Survey s t r e a m - g a g i n g p r o g r a m , a n d ( 3 ) reports that included e t a i l e d t a b u l a t i o n s of hydraulic a n d ch annel data. F i e l d surveys for thisproject were m a d e at 26 sites in Washi ngton, Arizona, Oregon , C aliforni a, a n dNev ada. M a n y of the sites, referred to as pi l o t study sites, were selectedb ec a u s e rock riprap h ad been insta lled. D a t a o b t a i n e d as p a r t of the stream-g a g i n g pr ogr a m genera lly were from sites without ripr a p but were selected topr o v i d e flow a n d ch annel data.

R E V IE W OF RIPRAP DESIGN TECHNOLOGY

A number of approaches h ave b ee n developed to relate the m a g n i t u d e a n dd i r ec t i o n of forces ac ti ng on the b ou n d a r y of a ch annel w i th the p a s s i v e forcesth at tend to pr e v e n t er os i o n of the b ou n d a r y m a teria l. These approaches ca n bec ategorized as follows:

o R el a t i o n s h i p of permissible v e l oc i t y to p a r t i c l e size for cohesive a n dn o n c oh es i v e soils li ni ng the ch annel (HEC-11, Cal-B&SP, EM-1601, M an-54,USBR-EM-25) .

o R el a t i o n s h i p of per m i s s i b le v e l oc i t y to grasses or other ch annel li ni ngs(HEC-15).

o R el a t i o n s h i p of b ou n d a r y shear to the size of p a r t i c l e s tha t comprise thec h a n n e l b ou n d a r y (HEC-15, EM-1601, Simons-STT) .

All of these approaches assume u niform a n d subcritic al flow conditions inthe reach, a l t h o u g h the procedures in EM-1601 consider supercritic al flow also.The ch annel sh ape is u s u a l l y assumed to be t r a p e z o i d a l w i t h a c o n s t a n t crosssection a n d bed slope.


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In the h y d r a u l i c a n a l y s i s of a site for design of riprap to pr e v e n t scour,the e v a lu a t i o n g e n e r a l l y a s s u m e s u n i f o r m or gr a du a l l y v a r i e d flow con ditions .U n i f o r m flow has a c o n s t a n t d e p t h for all cross sections in a reach. V a r i e dflow conditions occur if the d e p t h of flow ch anges along the l e n g t h of cha n n el.The e v a lu a t i o n of a site is g e n e r a l l y b a s e d on a design d i s c h a r g e a n d u n i f or mflow con ditions . A large m a g n i tu de ch ange in ch annel size or g r a d i e n t m a y causethe state of flow to change. The depth i s less a n d the v e l oc i t y g r e a t e r insupercritic al flow th an in subcritic al flow. Abrupt ch anges m a y cause hydraulicdrops or jumps. A h y d r a u l i c drop will occur where flow ch anges abruptly fromsubcritic al to supercritic al, a n d a hydraulic jump will occur where flow ch angesfrom supercritic al to subcritic al. Severe turbulence accompan ies a hydraulicjump. If supercritic al flow occurs in a reach b etw ee n two reaches of sub-critic al flow, a h y d r a u l i c drop m a y occur a t the u ps tr e a m en d of the critic alr e a c h a n d a h y d r a u l i c jump m a y oc c u r a t the d ow n s tr e a m end.

In several design procedures, the shear stress ( al so referred to as thet r a c t i v e force, Chow, 1959) is used as a q u a n t i t a t i v e i n d i c a t o r of the forcesa c t i n g on the ch annel bed a n d ba n k s . The m a g n i tu de of shear stress i s d e p e n d e n ton the depth of flow a n d ch annel gradient; therefore, v alues of shear stress ina reach with supercritic al conditions m a y be less th an those for subcritic alflow.

Shear Stress R el a t e d to Per m i s s i ble F low V eloc i t y

The m a x i m u m per m i s s i b le v e l oc i t y i s the highest m e a n v e l oc i t y th at will notcause erosion of the bou ndary. Procedures for d es i g n of ch annels b a s e d . o nper m i s s i b le v e l oc i t y a re described in EM-1601, HEC-11, and USBR-EM-25. Chow( 1 95 9 ) p r e s e n t s a s u m m a r y of several design procedures th at a re b a s e d on m a x i m u mper m i s s i b le ( me an ) v e l oc i t y for c h a n n e l s w i th v eg e t a t i v e lin in gs . Figure 2( ad a pted from EM-1601) shows a c o m p a r i s o n of design curves used to relateper m i s s i b le v e l oc i t y to ston e size. The Isbash (USER) procedure gives thelargest stone size for a given velocity. The curve for an i s o l a t e d cube, whichis n ot g e n e r a l l y used for riprap design , is given for comparison.


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6000 I I I I~T1

ISBASHW=2.44x10' > V 6

EXPLANATIONFOREIGN. PROTOBONNEVILLE FLUMEBONNEVILLE CLOSURECOLUMBIA RIVER PROTOLO S ANGELES DIST. PROTOZUIDER ZE E PROTOPASSAMAQUODDY, MODELSTILLING BASIN, MODELCHANNEL. MODELMcNARY DAM. MODEL

1 . Specific weight o f rock = 16 5 Ib/ft* 2 . For stone protection below energy dlssipatorsus e as average velocity across en d sill.

L UL U

O CL UL U

QLUZo -C O

L U_J


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S h e a r Stress R el a t e d to H y dr a u l i c R adius a n d G r a d i e n t

W a t e r flows in a c h a n n e l as a f u n c t i o n of gr a v i t y a n d develops forces th ata ct in m a n y directions d e pe n d i n g on the a m o u n t of turbulence, but w i th thepr i m a r y v ec tor in the d i r ec t i o n of flow. F r i c t i o n forces a ct in the opposited i r ec t i o n to the flow and, when considered over an are a, a re referred to asb o u n d a r y shear. The forces a c t i n g on a b ody of w a t er a re shown in figure 3 .F or u n i f or m flow, the pressure forces a re equ al a n d a ct in opposite directions,a n d a ll a c c e l e r a t i o n s a re zero. A n a l yz i n g the flow c o n d i t i o n s h o w n in figure 3a n d c o n v e r t i n g forces to shear stresses gives the equation :

T 0 PAL = \AAL sin 6 ( 1 )F or small a n gles , sin 6 = ta n 6 = S 0 . By d i v i d i n g b y PAL a n d s u bs ti tu t i n g S 0 ,the a v e r a g e s h e a r stress on the b ou n d a r y is g i v e n by:

T =where TQ = m e a n b ou n d a r y shear a c t i n g over the w ett e d per i m e t er

P = wetted per i m e t erL = l e n g t hy = u nit weight of w a t erA = cross-sectional a r ea6 = a n g l e of bed slopeR = h y dr a u l i c radius

S 0 = ch annel bed slope

(2)

Note: dm is measured in the vertical and is a closeapproximation (0.5 percent) as dm = dm cosefor channel slopes less than 0.1.FI , F2 = Forces of static pressureW=Weight of water

FIGURE 3 . Three-dimensional free body diagram of forces acting on a water mass.


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O th er a n a l y t i c a l procedures (Chow, 195 9 ) derive the shear stress (tractiveforce p er u nit of wetted a r ea ) for gr a du a l l y v a r i e d flow conditions in which S 0is replaced by the e n e r g y slope S , g i v i n g the equation :

T vP*s ("\\0 j * ^ ' - * V . - J )

In u n i f or m flow, the e n e r g y slope (Se) a n d w a t er - s u r f a c e or bed slope ( S 0 ) a reequal; in g r a d u a l l y v a r i e d flow, the difference b etw ee n the e n e r g y a n d w a t e r -surf ace slopes is g e n e r a l l y sm a ll a n d either v alue ca n be used in e s t i m a t i n gshear stress. The e n e r g y slope is less th an the water-surface slope w h e n flowis contracti ng. This suggests th at use of the water-surface slope will givelarger th an actu al v alues of energy slope w h e n es t i m a t i n g the shear stressu nless the reach is e x p a n d i n g or is at a bend. In a n e x p a n d i n g reach, e n e r g ylosses a re u s u a l l y a s s u m e d to be 50 per c e n t of the ch ange in v e l oc i t y head.

For wide cha n n els , where the m e a n d e p t h ( d a ) is a p pr ox i m a t e l y equal to theh y dr a u l i c radius, the av erage shear stress m a y be d e t e r m i n e d b y the e qu a t i o nfrom Chow ( 1 95 9 ) :

TO = Y^SO ( 4 )To d e t e r m i n e the m a x i m u m stress at a cross section, the m a x i m u m depth (dm) i ssubstituted for m e a n depth. Equation 4 may then be m o d i f i e d to e s t i m a t ecritic al shear stress n e e d e d for riprap design procedures b y HEC-15.

TO = Vdm S 0 ( 5 )B ou n d a r y shear is a lso a fu nction of ch annel velocity, a n d e qu a t i o n 4 m a ybe expressed in ter ms of m e a n v e l oc i t y (Va ) a n d M a n n i n g ' s rough ness coefficient

n . U s i n g the f o l l o w i n g procedure, the Chezy equation V a = C^RSe may ber e a r r a n g e d a n d m o d i f i e d for bed slope so th at RS = RS 0 = (V /C) 2 a n d substitutin g in equation 3 gives:

T O = Y( V /C) 2 ( 6 )aThe relationship b e t w e e n M a n n i n g ' s n a n d the Chezy C ca n be expressed by theequ ation:

= 1.486 R 1 / 6 ( 7 )n

where C is a c o e f f i c i e n t tha t v aries with the h y d r a u l i c radius ( R ) a n d roughness ( n ) of the channel. The b ou n d a r y shear on the w ett e d per i m e t er i s given bythe equation :

YV a n ( 8 )T o ~2 .

F or wide cha n n els , the h y dr a u l i c r a dius a n d m e a n d e p t h a re a s s u m e d to be approximately equ al (table 2). G r o u p i n g c o n s ta n t s a n d simplifyi ng yields:

_ 28 - 2 V" ( 9 )T 0 ^ 0.333a


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Table 2 . Hydraulic properties a nd channel geometry of streams a s a functionof channel slope (adapted from table 1 in volume 1 of this report).

Average value of variablefor sample ( N variesfrom 44 to 763) Water-surface slope (ft/ft)0.001-0.005 All slopes

M a xi m um point velocity, V (ft/s) 1 9.77Average velocity, V (ft/s) 3.97aMaximum depth, d ( f t ) 1 50.4mAverage depth, d ( f t )aFroude number, FV /Vm ad /dm aT/d (T=top width)

mR/d (R=hydraulic radius)a

14.80.221.561.6119 . 40.979

4.94

4.10.451.621.6827.81.03

7.4

4.10.681.711.7319.50.965

16.74. 410.36.90.361.611.5519.80.975

1 Maximum value for sample.

Application of the hydraulic factors in equations 8 a nd 9 indicates thataccurate estimates of M a n n i n g 's n a nd velocity are needed. The hydraulic radiusor m ea n depth ma y be defined by measuring the cross section.

There are difficulties in applying the concepts of permissible velocity orshear stress to determine the riprap material required to resist erosion. Permissible velocities (such as given in figure 2 ) a nd shear stresses are usuallyexpressed as mean values for the cross section. Estimates of shear stress basedon gradient are n ot considered reliable because in localized areas of turbulence, the gradient m a y be negative, a nd at channel banks, the gradient alongeach ba n k m a y be dissimilar." The problems in a n a l y z i n g boundary stresses basedon shear stress are discussed in detail in later sections of the report. Theactual p o i n t values that effectively contribute t o erosion of the ba n k materialare difficult to determine and are estimated from relationships establishedusing laboratory data. These data are then extended to accommodate the magnitude of hydraulic conditions that occur in the field.

CHARACTERISTICS OF RIPRAP FAILURE

Inadequate recognition of the type of erosion process that i s occurring orimproper riprap design may lead to failure of the riprap, as shown in figure 1 .Types of erosion that can be successfully controlled by riprap include chan neldegradation, ba n k erosion, scour, a nd changes in alinement associated withmeandering, branching, and braiding of streams. The rate of channel erosionvaries with time, but i s primarily a function of the magnitude of streamflow.Other factors that affect channel erosion are stream control works, sand andgravel pit operations, a n d land-use developments.


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A discussion of ( 1 ) geomorphic factors a n d the classification of streamproperties, ( 2 ) a s s e s s m e n t of s t a b i l i t y as related to s e d i m e n t t r a n s p o r t a n dhydraulics, a n d ( 3 ) effectiveness of cou ntermeasures for hydraulic problems a tbridges i s given by Brice a n d B l o d g e t t (1978). A n o v e r v i e w of s t r e a m b a n k stabil i z a t i o n m e a s u r e s is given by Brown (1985).

C l a s s i f i c a t i o n of Failures

During this study, the au thors identified four b a s i c types of riprapf ailure along streamb anks: Particle erosion , t r a n s l a t i o n a l slide, m o d i f i e dslump, a n d slump. The cause of each type of f ailure is d i f f e r e n t a n d certai nriprap design procedures will be n e e d e d th at consider each type of failure. Asketch of e a c h type of f ailure is shown in figure 4 .

P a r t i c le E r os i o n

P a r t i c l e e r o s i o n is the t r a n s p o r t of riprap ston es to the ch annel bed nearthe i n s t a l l a t i o n or to a po i n t downstream. Particle erosion i s considered them os t common type of fa ilure, a n d the m e c h a n i c s of i m p e n d i n g m o v e m e n t are docum e n t e d in the literature. A m a th e m a t i c a l a n a l y s i s of p a r t i c l e e r o s i o n ispr es e n t e d by Simons a n d S e n t u r k (1977). The sketch in figure 4 shows a na d v a n c e d sta ge of f ailure c aused by p a r t i c le erosion. Displaced ripr a p u s u a l l ycomes to rest on the b e d n e a r the eroded a r eas a n d for some distance downstream.A m o u n d of d i s p l a c e d riprap on the ch annel bed i ndic ates th at the t r a n s p o r tc a p a b i l i t y of the s t r e a m is i n s u f f i c i e n t to move a ll of the eroded riprap fromthe site. This s i t u a t i o n occurred on Pi nole Creek a t Pin ole, C a l i f o r n i a (seefig. 1 ) . A d e tr i m e n t a l effect of the m o u n d i s the t e n d e n c y to confi ne flows ofh i gh v el oc i t y b etw ee n the m ou n d a n d the toe of the emb ankment, c ausi ng a ddit i o n a l b a n k a n d bed erosion.

STONES TOO LARGEFOR TRANSPORT

RIPRAPLAYER

PARTICLE EROSION

BASE MATERIAL

Mound of displaced riprap. Particle erosion resultsif flow shear stress or velocities are excessive.If displaced stones are not transported from theeroded area, the channel be d will show a mound.

FIGURE 4. Classification of principal types of riprap failures.

1 0


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Lower part of riprap separates from upper part, and movesdownslope as homogeneous body. The to e may not showa bulge if channel be d is scoured. Translational slideusually occurs if side slope is too steep or toe of riprapis undermined. FAULT

TRANSLATIONAL SLIDE

Failure plane

Riprap moves downslope along a failure planethat lies at or above base material. Failureplane is at a flatter slope than original ripraplayer. This type of failure is usually causedby excess hydrostatic pressure in ripraplayer or shear along filter blanket. fD isp acedoc k riprap

Filter blanket atsurface of basematerial (not shown)

Riprap moves downslope along a failure planethat lies in base material. Failure zo ne isdish-shaped. This type of failure is usuallycaused by excess hydrostatic pressurein base material.

Displacedrock an dbase material

Failure zon e in base material'BASEMATERIAL

SLUMP

FIGURE 4. Classification of principal types of riprap failures (Continued).1 1


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A m e th od to study the s t a b i l i t y a n d m o v e m e n t of i ndividu al particles(stones) c o m p r i s i n g the riprap layer was d e v e l o p e d for the S a c r a m e n t o R i v er a tsite E - 1 0 n e a r Chico, C aliforni a, b y p a i n t i n g a red stripe along the top face ofthe riprap layer duri ng the low flow season (fig. 5). A n y m o v e m e n t of in dividua l ston es w as noted followi ng flood events. The i niti al f ailure by particlee r o s i o n in F eb r u a r y 1 9 83 is illustrated in figure 6 . At the time of the photograph, the red stripe h ad b ee n d es tr o y e d in the area shown but was still visiblen e a r the upper end of the rod. F ol l ow i n g a number of flood events b etw ee nF eb r u a r y a n d M a y 1 9 83 duri ng which the entire b a n k a n d riprap was subject toi nu ndation, p a r t i c l e e r o s i o n pr ogr es s ed to the condition shown in figure 7 . Allbut the largest ston es were subsequently t r a n s p o r t e d from the eroded area.

The sc arp a t the upslope end of the f ailure (fig. 6) is not related to aslump f ailure as described by Schuster a n d Krizek (1978). A slump i s a m a ssm o v e m e n t of m a t er i a l a l o n g a slip surface w i th the termi nus of the upslope faceof the failure d e s i g n a t e d a scarp. The scarp observed with f ailures c aused byp a r t i c le er os i o n is related to a n g u l a r i m p i n g e m e n t of flow. P r o g r e s s i v e scourdu r i n g eddy a c t i o n of the s tr e a m f l ow a t flow expansions also ten ds to erode theexposed b a n k a f t e r the protective layer of riprap has b ee n da m a ged. Duri ng thei n i t i a l stages of f ailure related to p a r t i c le erosion , the height of the sc arpface is small.

Steel fencepostLevel ro dat top of bank

Displacedrock

FIGURE 5 . Riprap on left bank of Sacramento River at E-10 nearChico, California. A reference line shows location of stones inDecember 1981. Flow is from left to right. Note displaced stonenear steel fencepost (photographed March 4,1982) .

1 2


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FIGURE 6. Riprap on left bank of Sacramento River at E-10 near Chico,California, snowing initial effect of particle erosion. Survey ro d atapproximate location of original bank line. Scarp face (cross-hatchedarea, between 15-foot mark on rod and tripod leg) is approximately1 foot high (photographed February 1 , 1983) .

FIGURE 7. Riprap on left bank of Sacramento River at E-10 near Chico,California, showing advanced stage of particle erosion. Survey rodsheld at approximate location of original bank line. Note only thelargest stones, as compared with the well-graded distribution infigure 5, have not been displaced (photographed May 2, 1983) .

13


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A n ex ample of pr ogr es s i v e ripr a p f ailure a n d i ncreased height of the scarpc aused by the p a r t i c le er os i o n i s given in table 3 . D a t a for f ailures at threesites on the S a c r a m e n t o R i v e r at E - 1 0 n e a r Chico were collected in Ma y 1983.The larger f ailure was first observed in J a n u a r y 1983, a f t e r the i niti al damageoccurred duri ng a flood in D ec e m b er 1982. This f ailure subsequently i ncreasedin size, a n d the other two fa ilures occurred duri ng floods that overtopped theb a n k in J a n u a r y a n d M a r c h 1983. All of these fa ilures exhibit the geometricsh ape of a horseshoe, shown in figure 8 , duri ng the b e g i n n i n g p h a s e of riprapfailure. The riprap f ailure on Pi nole Creek at Pi nole represents the a d v a n c e dsta ge of p a r t i c le erosion , in w h i c h the i ndividu al areas of f ailure a re combin ed. A s i g n i f i c a n t c h a r a c t e ri s t i c of the Pi nole Creek f ailure i s that the n e wside slope is flatter th an the o r i g i n a l side slope of 2.4:1, thus l e a v i n g a nexposed face (scarp) th at is e a s i l y eroded.

The probable causes of p a r t i c l e e r o s i o n are:o M e d i a n size ( D 5 0 ) stone not large e n ou gh to resist the shear stress of thestream.o Ab r a s i o n or remov al b y impact of i ndividu al stones. F or this a n d theprevious situ ation, i ndividu al ston es are removed, a n d in time, the cumulative effect results in f ailure of the riprap.o Side slope of the b a n k so steep that the angle of repose of the ripr a p ise a s i l y exceeded, c ausi ng i nstability of the i ndividu al stones.o G r a d a t i o n of riprap m a y be too u n i f or m (all stones n e a r the m e d i a n size).

W i th ou t e n ou gh s m a l l e r d i a m e t e r ston es th at tend to fill the voids a n dpr o v i d e lateral support for larger m a t e r i a l , f ailure m a y occur eventhough the m e d i a n size is adequ ate a n d the b a n k side slope i s n ot toosteep.

T a b l e 3 . Geometry of progressive riprap f ailures related to particleer os i o n on the S a c r a m e n t o R i v e r at Site E - 1 0 n e a r Chico, C a l i f o r n i a( surveyed M a y 2 , 1983).

Failure

site

123

Ne w side slopein a r ea oferosion

(feet/ feet)

0.54.62. 5 9

L e n g th (feet) a tM a x i m u mw i d th 1(feet)

5.510.511.0

i nterv alsof

0

122544

25

122443

stated(in percent )w i d th 250

11.52240

75

919.530

90

71614

H e i gh tofsc arp 3

(feet)

1.61.92. 4

is m a x i m u m slope dista nce, m e a s u r ed per pe n d i c u l a r to shorelin e.2 L e n g th i s the distance across fa ilure, m e a s u r ed p a r a l l e l to shoreli ne

The zero per c e n t l e n g t h is at the downslope end of the failure. The100 per c e n t length i s at the face of the scarp, as shown in figure 8 .

3 M a x i m u m height of scarp. The height decreases to zero at the0 per c e n t w i d th in terva l.

14


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TOP OF BANK

present waterline

y-'-V:'--V-.--.-'.'.- :.-\:'.''.V-"-.-J

RIPRAP LAYER

BASE MATERIAL

FIGURE 8. Typical riprap failure area in the shape of a horseshoe, caused by particle erosion.

T r a n s l a t i o n s ! Slide

A tr a n s l a t i o n a l slide is a riprap f ailure c aused by the d o w n s l o p e m o v e m e n tof a m a s s of stones th at consists of a s i n g l e or c los el y related u nits w i th thef ault lin e u s u a l l y on the horizonta l plane (Schuster a n d Krizek, 1978), as shownin figure 4 . The riprap is u n d i s tu r b ed except at the fault lin e a n d a bulge a tthe toe. If the m o v i n g m a s s is n ot gr e a t l y deformed, it m a y be c alled a b loc kslide. The i niti al ph ases of a t r a n s l a t i o n a l slide a re i n d i c a t e d b y cr ack s inthe u pper p a r t of the riprap b a n k th at e x t e n d p a r a l l e l to the cha n n el. Them o v e m e n t of tr a n s l a t i o n a l slides i s c o n t r o l l e d by (1) v a r i a t i o n s in shears tr e n g th a l o n g the i n t e r f a c e b etw ee n the riprap a n d the b a s e m a t e r i a l , a n d(2) s t a b i l i t y of the riprap at the j u n c t i o n po i n t w i th the ch annel bed. Atr a n s l a t i o n a l slide is i n i t i a t e d if the ch annel bed scours a n d u n d e r m i n e s thetoe of the riprap layer, or p a r t i c le e r o s i o n of the toe m a t e r i a l occurs, reducin g the support of the upslope materi al. In either case, the shear r e s i s t a n c eof the i nterf ace b etw ee n the bed m a t er i a l a n d riprap ma y be i n s u f f i c i e n t toresist tr a n s l a t i o n a l movement. The trans lation al slide m a y progress downslopei n d e f i n i t e l y if e r o s i o n of the riprap m a t e r i a l at the toe (which constitutes thebulge shown in fig. 4) con tinues . C o n t i n u e d d o w n s l o p e creep of the riprap m a ya lso occur if the base m a t er i a l u n d er l y i n g the riprap is s u f f i c i e n t l y s a t u r a t e dw i th w a t er a n d the shear resistan ce along the i n t e r f a c e is less th an the gravit a t i o n a l force.

1 5


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A t r a n s l a t i o n a l slide w i th the f ault lin e loc ated high on the e m b a n k m e n tsuggests th at extensive ch annel bed scour or p a r t i c l e e r o s i o n u n d e r m i n e d the toeof the e m b a n k m e n t materi al. In this situa tion , the slide would occur only w h e nthe mass of ripr a p wa s sufficiently large for d o w n s l o p e forces to exceed thes h e a r s t r e n g t h a t the in terface. The occurrence of t r a n s l a t i o n a l slides is a lsor e l a t e d to the presence of excess h y dr os t a t i c (pore) pressure in the basem a t er i a l th at causes reduced frictional resistance of the riprap a t the in terface. Excess pore pressure m a y develop duri ng periods of high precipitation,floodin g, or rapid fluctu ation of w a t e r levels in the stream. The presence of afilter b l a n k e t pl a c e d on the b a s e m a t e r i a l pr ob a b l y w ou ld n ot pr e v e n t this typeof f ailure a n d m a y a c tu a l l y p r o v i d e a po t e n t i a l f ailure pla n e. Figure 9 showsa n e x a m p l e of tr a n s l a t i o n a l slide f ailure duri ng the w i n t er of 1 9 8 2- 83 on theCosumnes R i v er at site 2 n e a r Sloughhouse, C aliforni a.

The probable causes of t r a n s l a t i o n a l slide f ailure are:o B a n k side slope too steep.o Loss of f ou n d a t i o n support a t the toe of the riprap c aused b y scour ord e gr a d a t i o n of the ch annel bed, or by particle erosion of the lower partof the riprap.o Presence of excess hydrostatic (pore) pressure th at reduces the frictional

r e s i s t a n c e a l o n g the i n t e r f a c e b etw ee n the riprap and base material.

M o d i f i e d Slump

The riprap f ailure referred to as a m o d i f i e d slump is a mass m o v e m e n t a l o n ga n i n t e r n a l slip surface. Slumps a re described by Schuster a n d K r iz e k (1978) asr o t a t i o n a l slides a l o n g a conc ave surf ace of rupture. The m o d i f i e d slump i sdifferent, however , from the v arious types of slumps discussed by Schuster a n dK r iz e k bec ause the f ailure p l a n e is loc ated in the riprap, an d the u n d er l y i n gm a t er i a l s u p p o r t i n g the riprap does n ot fail. As a result, the surf ace of therupture is n ot conca ve, but is a r el a t i v e l y flat pla n e. This type of f ailure iss i m i l a r in m a n y respects to the t r a n s l a t i o n a l slide, but the geometry of thed a m a g e d riprap (fig. 4 ) is similar in shape to i niti al stages of f ailure c ausedb y p a r t i c le erosion . The n ew side slope w i th i n the m o d i f i e d slump area i s fla tter t h a n the slope of the i nterf ace between the b a s e m a t e r i a l a n d the riprap.M a t er i a l th at is d i s l o d g e d from the f ailure area u s u a l l y comes to rest on theb a n k just downslope from the fa ilure, as shown in figure 10, similar to w h a toccurs in a typic al slump f ailure on hilly terr a in . The displaced ston es m a ycause i ncreased turbulence of flow a n d eddy action a l o n g the b a n k in the a r ea ofthe slump. The secondary currents m a y then cause addition al riprap f ailure byp a r t i c le er os i o n of s m a l l e r m a t e r i a l s , es pec i a l l y those exposed a t the scarp.A n i n t e r e s t i n g f a c t o r c o n c e r n i n g m o d i f i e d slump f ailures i s tha t the m e d i a nston e size ( D 5O ) ma y be adequ ate for the site, but m o v e m e n t of certai n (key)ston es ( possibly due to poor gradation) lea ds to a loc alized f ailure of the riprap.

16


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End ofslide area

Scarp

Fault line

FIGURE 9. Riprap on Cosumnes River at site 2 near Dillard Road Bridgenear Sloughhouse, California, showing translational slide failure(photographed May 31 , 1983) .

Survey rodat face oforiginal riprap Survey rodon top ofdisplaced riprap

FIGURE 1 0. Riprap on Cosumnes River at site 3 near Dillard Road Bridgenear Sloughhouse, California, looking downstream, showing modifiedslump failure (photographed May 31 , 1983) .

1 7


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The p r o b a b l e causes of m o d i f i e d slump failures are:o B a n k side slope is so steep that the ripr a p i s resti ng very n e a r the a n gle

of repose. A n y imb alance or m o v e m e n t of i ndividu al ston es creates as i t u a t i o n of i n s t a b i l i t y for other ston es in the riprap.

o C er t a i n stones, critic al in supporti ng upslope riprap, are dislodged bys e t t l e m e n t of the submerged riprap, impact, abrasion, or p a r t i c l eerosion . The loss of support provided by the k ey stones results in thedownslope m o v e m e n t w i th i n a loca l area n e a r the p o i n t of the dislodgedstones. This cause of f ailure may be reduced in frequency if the riprapm a t er i a l i s of pr oper size gr a da tion .

Slump

A slump is a r o t a t i o n a l - g r a v i t a t i o n a l m o v e m e n t of m a t e r i a l a l o n g a conc avesurf ace of rupture. This type of f ailure i s u nli ke a m o d i f i e d slump in th at thef ailure zone is dish-sh aped rather th an a relatively flat plane (fig. 4). Thecause of the slump f ailure is related to shear f ailure of the u n d er l y i n g basem a t e r i a l tha t supports the riprap. As discussed by Schuster a n d Krizek (1978),the rupture may n ot occur simultaneously over the f ailure area, but propagatesfrom a loca l poin t. The displaced mass, i ncludi ng the riprap, moves downslopeb ey o n d the origi nal f ailure a r ea on to the surf ace of the ripr a p (fig. 11). Thepr i m a r y feature of a slump f ailure i s the loc alized d i s p l a c e m e n t of base material a l o n g a slip surface, which is usu ally c aused by excess pore pressure thatreduces f r i c t io n a l o n g a fault line in the base materi al. The scarp at the headof the slump is loc ated in both the base a n d riprap m a t e r i a l a n d m a y be a l m o s tvertic al. W i th progressive slump f ailures along the face of the riprap, theareas of i nstability may enlarge u ntil the entire b a n k has fa iled a n d a n ewlower g r a d i e n t b a n k slope i s presen t. As with a m o d i f i e d slump, once a failurehas occurred, d i s p l a c e d rock in a n a r ea of slump ten ds to crea te turbulence tha taccelerates the ac tion of particle erosion.

The probable causes of slump fa ilures are:o N o n h o m o g e n e ou s base m a t e r i a l with la yers of impermeable m a t e r i a l th at a ct

as fault p l a n e s when subject to excess pore pressure.o Side slope too steep, and g r a v i t a t i o n a l forces exceed the i nerti a forces

of the riprap a n d base m a t e r i a l a l o n g a friction pla n e.o Too much overburden a t the top of the slope; m a y be c aused in p a r t by the

riprap.

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H y d r a u l i c s As s oc i a t e d w i th Riprap Failures of Selected Streams

The hydraulics of streams a s s oc i a t e d w i th the four types of ripr a p f ailure( discussed in the pr ec ed i n g section ) were documented for this study at fivesites. T a b l e 4 lists the hydraulic properties a n d r i p ra p m a t e r i a l for three ofthe four types of failure. Shear stress was d e t e r m i n e d b y a p p l y i n g equation 5 .The depths used a re from surveyed pe a k w a t er - s u rf a c e e l e v a t i o n s for one specificflood, even though several periods of h i gh flow m a y h ave contributed to theobserved riprap f ailure a t some sites. All of the f ailures studied occurred inreaches where the ch annel was contracti ng. A ratio termed "flow c o n t r a c t i o n"wa s computed for e a c h site in order to ha ve a common basis for c o m p a r i s o n a m o n gthe v arious sites. The flow c o n tr a c t i o n i s the ratio of flow area at the m o s tcontracted section in the study reach to the area at the largest u ps tr e a m section. The c o n tr a c t i o n of flow m a y be c aused by a lateral constriction, as onP i n o l e Creek, or a v e r t i c a l constriction, as on the S a c r a m e n t o River at E-10.

The r a tio of p a r t i c l e sizes D 85 to D 50 (sizes at w h i c h 85 a n d 50 per c e n t ofthe particles, by number, are fi ner t h a n the i n d i c a t e d size) is used to i ndic atethe size g r a d a t i o n of the riprap materi al. The r e c o m m e n d e d ratio, from da tagiven in HEC-1 1 , i s about 1.4. If the r a tio is too large, p a s s a g e of flowthrough the voids in the ripr a p i s r e l a t i v e l y e a s y a n d m a y u n d e r m i n e the riprapor b a s e m a t e r i a l , u lt i m a t e l y c a u s i n g a m o d i f i e d slump failure. The design D 5 0th at would be obtai n ed from procedures outli n ed in HEC-1 1 , HEC-15, Ca l-B&SP, a n dEM-1601, i s pr es e n t e d for c o m p a r i s o n w i th the m e d i a n ( D 5O ) ston e size used ineach f ailed i nstallation. The C a l - B & S P procedure ga ve the largest size D 5 0 fora ll but on e site. For the Truckee River at Spar k s , Neva da , the D 5 0 from theE M - 1 6 0 1 p r o c e d u re s wa s larger than, tha t from the C al-B&SP methods bec ause of thecombi ned effect of depth a n d ch annel slope.

Displacedrock riprapat to e of slump

Approximaterupture plane

Area of slump failurean d displaced basematerial

To p edge of scarp

FIGURE 11 . Riprap on left bank of Cosumnes River at site 1 nearDillard Road Bridge near Sloughhouse, California, showing slumpfailure (photographed May 31 , 1983) .

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T a b l e 4 . Hydraulic, ripr a p, a n d damage ch aracteristics of selected sitesw i th riprap failure.

P i n o l e S a c r a m e n t o H oh River Cosumnes River TruckeeP a r a m e t er Creek a t R i v er at at site 1 , a t site 3 R i v er

Pi nole, CA E - 1 0 n e a r cross sec- n e a r D i l l a r d at(cross- Chico, CA tion 3 , n e a r Road Bridge Sparks,

section 3 ) Forks, WA n e a r Slough- NVhouse CAHydraulic :D a t eDisch arge ( ft 3 /s)W a t er - s u r f a c e slopeM a n n i n g s nM e a n v e l oc i t y (ft/s)M a x i m u m depth (ft)D epth of flow abovetoe of d a m a g e dripr a p (ft)M e a n depth (ft)Curv ature angle ()

Curv ature r a dius (ft)Flow contraction 2Shear stress 3 (lb/ ft 2 )Froude n umber , F

R ev e t m e n t materi al:D 85 (ft)D 5 0 (ft)DIS ( f t )Ratio D 8 5 /D 5 0Specific gr a vity, G SD es i g n side slope, zD 5 0 (ft) computed fromfollowi ng proceduresHEC-11HEC-15C al-B&SPE M - 1 6 0 1

C ause of failure:

1-04-822,2500.00540.0307.77.7

7.74.9661500.872.590.61

0.840. 600.421.402.852: 1

0. 434 0. 9 8 / 5 0.50.850.60Particlee r o s i o n

1-27-83^8,0000.0005560.0336.734.5

13.020. 2114 , 2 8 00.630. 9 00.26

0. 6 60.510.311.292 . 602 : 1

0.30( 6 )0.700.23Particlee r o s i o n

10-22-8222,0000 . 00140.0357. 9 419.1

19.13.5280.59 9 10.761.670.76

2.51.30.581 . 9 22.591 . 2 : 1

0. 40( 6 )1 . 40.40T r a n s l a -t i o n a lslide

3-13-8326,1000.000700.0304.0831.0

10.218.6994581.250.8120.17

1.000.780. 501.282 . 9 21 . 8 : 1

0. 20( 6 )0.30. 1 9 5

M o d i f i e dslump

3-13-837,3400.00300.0355. 1 917.5

17.510.5186460.883.270.28

1.140.710.461 . 6 12 . 6 81 . 8 : 1

0. 20( 6 )0. 40. 8 2

Particleerosion

x M a i n ch annel discharge.2 R a t i o of a p pr o a c h to contracted section , as described in text.3 M a x i m u m shear stress in cross section, T o ^ d m S o -4 D 50 from ch art 27 and appropri ate adjustments.5 D 50 from chart C-l.6 Ch art 27 of HEC-15 is n ot applic able to depths above 16 f t .

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Damaged rockriprap bank

FIGURE 1 2. Damaged riprap on left bank of Pinole Creek at Pinole,California, following flood of January 4, 1982. Note deposition ofdisplaced riprap from upstream locations in channel bed (photographed March 1982).

F a i l u r e of the ripr a p a t this site wa s i n i t i a t e d b y d i s p l a c e m e n t of in div i du a l ston es ( p article erosion ). Af t er repeated periods of high w a ter , theriprap li ni ng wa s eroded to the o r i g i n a l b a s e m a t e r i a l ; however , there w as noe v i d e n c e of b a s e m a t er i a l f ailure at the site. T h e g r a d a t i o n of the riprap(r a tio of D 85 /D 5O = 1.29 ) is close to the recommended ratio of 1.4 given inHEC-11 and HEC-15 a n d is w i th i n the range specified in EM- 1 601 . F a i l u r e of theriprap is attributed to the rock size b e i n g too small, a n d side slope of theb a n k b e i n g too steep.

Hoh River at Site 1 n e a r Forks, W a s h i n g t o n

The procedure used for riprap design at this site (figs. 13 a n d 14) is n otknown. Particle erosion occurred at two loc ations duri ng the first severalfloods after the riprap was i nstalled duri ng summer 1982.

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FIGURE 1 3 . New riprap placed on left bank (upstream view) of Hoh Riverat site 1 near Forks, Washington. Riprap w as damaged by modifiedslump at a location near truck tires during flooding in autumn of 1982(photographed August 1982).

FIGURE 1 4. Damaged riprap on left bank (downstream view) of HohRiver at site 1 near Forks, Washington. Site is near bulldozer (topof fig. 1 3) and about 40 0 feet upstream from the foreground of siteshown in figure 13. Damage is attributed to particle erosion by im pinging flows that overtopped bank during flood of December 3, 1982,and extends about 4 feet below top of bank (photographed December 1982).

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Riprap damage occurred at cross section 3 , loc ated n e a r the trailer shownin figure 13, duri ng October 1982. The damage is attributed to ( 1 ) ch annel bedscour tha t u n d e r m i n e d the toe of the ripr a p a n d c aused m o d i f i e d slump; ( 2 ) po orsize gr a d a t i o n of the riprap th at allowed erosion of the supporti ng smallerm a t e r i a l in the riprap; and ( 3 ) a steep side slope that reduced the a m o u n t offorce required to displace i ndividu al stones. The ratio of D 85 /D 50 was 1.92(table 4) , a n d exceeds the recommended r a tio given in a ll design procedures.Most of the larger ston es still in pos i t i o n at the site were a t a prec arioussta te of ba la nce. Crac ks along the top of the e m b a n k m e n t p a r a l l e l to thec h a n n e l were observed in N o v e m b er 1 9 8 2 after the first flood of the w i n t ers e a s o n 1982-83. These cr ack s i ndic ated the m a s s of riprap was u nstable a n d n e a rthe a n g l e of repose.

Riprap damage on the left b a n k at a n upstream loc ation (cross section 2 ,n e a r the bulldozer shown in figure 13) duri ng the flood of December 3 , 1982, isa t t r i b u t e d to p a r t i c l e erosion. The d a m a g e d riprap shown in figure 14 was overtopped about 3 ft ( 0.9 m) duri ng the flood. Most of the damage occurred nearthe top of the b a n k n e x t to the low e l e v a t i o n access road. R i p r a p e r o s i o n m a yh ave b ee n c aused by irregular patterns of o v e r b a n k flow in the v i c i n i t y of thelow b a n k access road.

Cosumnes River at Site 3 n e a r D i l l a r d RoadBridge near Sloughhouse, C aliforni a

The riprap at this site (fig. 10) wa s constructed to p r e v e n t lateral m i g r a tion of the cha n n el. The design procedure i s n ot known. A m o d i f i e d slumpf ailure about 15 ft (4.6 m) wide was noted about 1 m o n th after floodi ng a n d6 months after construction of the riprap. The ripr a p is subject to i m p i n g i n gflows. Individu al pieces of ripr a p in the slump area were displaced downslope,w i th the toe of the slump e n d i n g up 13 ft (4.0 m) b elow the top of the bank.The f ailure is attributed to f ailure of the i nterf ace b etw ee n the base m a t e r i a la n d riprap a n d possible excess hydrostatic pressure in the base m a teria l. Thel o c a t i o n of the ripr a p failure, w h i c h is about 21 ft (6.4 m) above the ch annelbed, i ndic ates th at stresses n e a r the top of the b a n k m a y be more critical th anstresses defi ned for the c h a n n e l bed.

Truckee River a t Sparks, N e v a d a

The ripr a p at this site was p l a c e d to p r e v e n t lateral m i gr a t i o n of thech annel toward the right b ank . The a ge of trees and brush growi ng along thech annel i ndic ates th at the riprap was i nstalled more th an 10 years prior to thesite survey. The ch annel is curved 18 a t the site, a n d flows are impin gin g.The riprap shows evidence of overall f ailure by particle erosion a t the outsideof the b e n d (fig. 15).

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Site surveys in Ju ne 1 9 83 i ndic ated some rocks h a d r e c e n t l y b ee n d i s pl a c e d(fig. 15). This d a m a g e is attributed to the flood of M a r c h 13, 1 9 83 (table 4).F i e l d surveys in M a y 1 9 8 2 showed extensive riprap damage ha d occurred duri ng a ne a r l i e r flood. S tr e a m f l ow records show a large flood ( disch arge 8,690 ft 3 /s or245.9 m 3 /s) on D ec e m b er 20, 1981. This flood caused p e a k sta ges a t the site tobe about 6 ft (1.8 m) higher th an the sta ge a t the time of the survey in M a y1 9 8 2 ( me asured disch arge 3,880 ft 3 /s or 109 . 8 m 3 /s).

A r i g h t - b a n k h i gh w a t er profile of the D ec e m b er 1981 flood was surveyed inM a y 1982. However, n o m a r k s could be fou nd on the left bank. The right-bankprofile, shown in figure 1 6 , d e m o n s t r a t e s the n e e d for c areful surveys to determ i n e the h y d r a u l i c p r o p e r t i e s of a cha n n el, a n d the d i f f i c u lt y in a p p l y i n g ripra p d es i g n procedures b a s e d on shear stress to sites w i th ben ds . For exa m ple,in the v i c i n i t y of the c h a n n e l ben d, the r i g h t - ba n k p r o f i l e of the D ec e m b er 20,1981, p e a k ha d a n e g a t i v e gr a dien t. The w a t er - s u r f a c e ele v a t i o n i n c r e a s e d from97.8 to 9 8 . 3 ft (2 9 . 8 to 30.0 m) in a reach 90 ft (27. 4 m) long, which i ncludedthe area where riprap w as da m a ged.

Top f b a n k

FIGURE 1 5 . Damaged riprap on right bank of Truckee River at Sparks,Nevada. Damage is attributed to particle erosion by impinging flowat channel bend. Area of damage is limited to upper 6 feet of bank(photographed June 1 5 , 1983) .

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99

98

D


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As an a l t e r n a t i v e , the slope of the w a t er surf ace surveyed in M a y 1 9 8 2 w asa p p l i e d a t the stage of the D ec e m b er 1981 flood. This a p pr o a c h y i e l d e d a shearstress of about 2 .2 lb/ft 2 (10.7 k g / m 2 ) , w h i c h is lower th an the stress listedin table 4 for the M a r c h 13, 1983, flood. F or the M a r c h 13, 1983, flood, procedures in E M - 1 6 0 1 i ndic ate a D 50 of 0.8 ft (0.2 m) , which is 0.1 ft (0.03 m)larger th an the D 50 for the m a t er i a l insta lled. E v e n if the riprap h ad m e t thed es i g n requirements of EM-1601, it pr ob a b l y w ou ld not h ave pr o v i d e d adequ atepr ot ec t i o n because ( 1 ) the ac tu al w a t er - s u r f a c e slope is pr ob a b l y steeper th anestimated, a n d ( 2 ) the v e l oc i t y m a y h ave b ee n greater th an estimated.

There wa s also loc alized d i s pl a c e m e n t of i ndividu al stones on the shorewardside of a 1 - f t - d i a m e t e r tree th at is 7 ft (2.1 m) from the top of the bank, asshown in figure 1 7 . The d i s pl a c e m e n t of riprap near the tree is attributed toloc alized shear stresses th at exceed the critic al shear stress of the stones.The f ailure a r ea w as o b l o n g in shape, about 5 ft (1.5 m) long, 1 ft (0.3 m)wide, a n d 1 ft (0.3 m) deep. Similar loc alized scour of riprap on c h a n n e l b a n k sin the v i c i n i t y of bridge piers ha s b ee n n o t e d at several sites.

The study of riprap f ailure on the Truckee R i v er illustrates tha t care mustbe exercised in s e l e c t i n g the slope to be used for design of riprap. Bed slope,a v e r a g e w a t er - s u r f a c e slope, local w a t er - s u r f a c e slopes in areas of turbulence,a n d energy slope differ considerably. The study also illustrates tha t addit i o n a l pr ot ec t i o n m a y be n e e d e d in the v i c i n i t y of piers and v e g e t a t i o n w h i c hcause loca l stresses greater th an those es t i m a t e d from design procedures.

tree

TOP OF BANK

>?-i'ifsij' ^BASE MATERIAL

ROCK RIPRAP(D50= 0.71 feet)

S c ou r hole at upslope side of obstruction

FIGURE 1 7. Sketch of scour hole in riprap adjacent to obstruction on streambank.

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S u m m a r y of F a c t o r s Contributing to Riprap Failures

C er t a i n hydraulic factors are a s s o c i a t e d with each of the four types ofriprap f ailure (particle erosion , t r a n s l a t i o n a l slide, m o d i f i e d slump, a n d trueslump), as i ndic ated by the field data collected a t five sites (table 4). Thespecific m ec h a n i s m c ausi ng f ailure of the riprap i s difficult to determi ne, a n da n u m b e r of factors, ac ti ng either i n d i v i du a l l y or combined, m a y be involved.Several reasons for riprap f ailures a re identified a n d grouped below:

o P a r t i c l e size was too sm a ll because:a . Shear stress wa s u nderestimated,b . V eloc i t y was u nderestimated.c . I n a d e q u a t e a l l o w a n c e was m a d e for ch annel curva ture,d . D es i g n ch annel c apacity was too low.e . D es i g n d i s c h a r g e wa s too low.f . I n a d e q u a t e a s s e s s m e n t was m a d e of abrasive forces,g . I n a d e q u a t e a l l o w a n c e wa s m a d e for effect of obstructions.

o R i pr a p m a t er i a l h ad improper gradation.o M a t er i a l wa s pl a c e d improperly.o Side slopes were too steep.o No filter b l a n k e t wa s i n s t a l l e d or b l a n k e t wa s i nadequ ate or damaged.o Excess hydrostatic pressure c aused f ailure of base materi al.o C h a n n e l ch anges caused:

a . I m p i n g i n g flow.b. F low to be directed a t en ds of pr ot ec ted reach,c . D ec r e a s e d ch annel c apacity or i ncreased depth,d . Scour of toe of riprap.

o D i f f er e n t i a l settlement occurred duri ng submergence or periods of excessive precipitation.

E s t i m a t es of p a r t i c le s t a b i l i t y serve as the basis for most riprap design procedures, such as HEC-11, HEC-15, a n d E M- 1 601 . This a p p r o a c h seems sou nd bec ausep a r t i c le e r o s i o n is i nvolved in most of the causes of f ailure described above.

E V AL U AT IO N OF R IPR AP D E S IG N PROCEDURES

Most publications on riprap design compare results o b t a i n e d from d i f f e r e n tmethods. For exa m ple, results from several methods rela ti ng stone size toa l l o w a b l e v e l oc i t y a re compared in a p p e n d i x A of HEC-11. The methods a re thoser e c o m m e n d e d by the C a l i f o r n i a D i v i s i o n of Highways, B u r e a u of Public Roa ds ,HEC-11, U.S. Bureau of Reclamation, a n d U.S. Army Corps of Engi neers. Th atcomparison is reproduced here as figure 1 8 . A n d er s o n and others, in N a t i o n a lC ooper a t i v e H i gh w a y R es e a r c h P r o g r a m (NCHRP) report 108 (1970), compare results

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from m e t h o d s th at relate ston e size to shear stress, as used by L a n e a n d C a r ls o n( 1 953) , a n d Shields (1936). T h a t c o m p a r i s o n is reproduced here as figure 1 9 .In his figure C-l, N o r m a n n (HEC-15) compares the results from the methods th atrelate stone size to critic al shear stress. His c o m p a r i s o n is a s i m p l i f i e dv e r s i o n of the one by A n d er s o n and others. The U.S. Army Corps of E n g i n e e r scompares d es i g n procedures in EM-1601. In their report, ston e size i s relatedto velocity, a n d the relationships suggested b y I s b a s h and the USER a re shown.The Corps expands on the concept of r e l a t i n g ston e size to v e l oc i t y by use ofa d d i t i o n a l hydraulic factors, such as depth, ch annel curv ature, a n d eq u i v a l e n troughness. Various relation ships of stone size to critic al tractive force werecompared by Simons a n d S e n t u r k (1977). The relationships given in theirfigure 7.8 r e p r e s e n t the results of l a b o r a t o r y flume tests a n d are limited tosizes of ston e riprap ( D 5 O ) less th an 0.13 ft (0.040 m) dia m eter .

OOL UW(LL UQ _L UL U

OOL UL U

C CL U

15 -

HEC-11,d>10ftstraight channel

Cal - B & SPstraight channel Ca l - B & SPcurved channelHEC-11, d


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F o r this study, the F e d e ra l H i g h w a y A d m i n i s t r a t i o n r e q u e s t e d th at p r o c e du res i n th e i r H y d r a u l i c E n g i n e e r i n g C i r c u l a r s 11 a n d 15 be e v a l u a t e d in r e g a r dto (1) t e c h n i c a l a c c u r a c y , ( 2) a d e q u a c y o f d e s i g n s p e c i f i c a t i o n s o b t a i n e d b yu s i n g the m e t h o d , (3) e a s e of a p p l i c a t i o n , a n d (4) m e t h o d of p r e s e n t a t i o n .M e t h o d s r e c o m m e n d e d in H E C - 1 1 a n d H E C - 1 5 w e r e to be c o m p a r e d to th ose rec o mm e n d e d b y o t h e r g o v e r n m e n t ag e nc i es. A l arg e p a r t of this e v a l u a t i o n i n v o l v e da stu dy of f i e l d s ites w h e r e r ipr a p h a d f a iled. Th e f ai lu res, such a s o n P i n o l eC r e e k a t P i n o l e , C a l i f o r n i a , a re d e s c r i b e d i n p r e v i o u s sec t i o ns of this r epor t.T he size a n d g r a d a t i o n of m a t e r i a l a t th ose s ites a r e c o m p a r e d to th ose th atw o u l d be o b t a i n e d b y a p p l y i n g the v a r i o u s me th ods. T he resu lts of this e v a l u a t i o n a re to be the b a s i s for n e w d e s i g n gu i d e l i ne s.

5 0 i iIiiTTOOLLUJC C .

OC OC CU JQ .COQzDOQ .I?C Oc oU JC CHc oC CICO

CCO

S t o n e s i z e DSQ measured t2 5 f i e l d i n s t a l l a t i o n s

I Stone size D S Q normally available from quarries.Sizes smaller than 0.5 ft. require extra crushingan d screening

10 Upper limit of stone size 050 shown inNCHRP-108 an d HEC-15

Shear stress for natural channels assumingmedian values given in table 2 (N = 7.297)TC= YwdmSw =62.4x10.3 x0.00368=2.4 Ib/ft

Upper limit of stonesize 050 shown inEM-1601 (1970)

T C = 5D50 (HEC-15, 1975, " X s = 165 Ib/fT)Shields (1936) Lane and Carlson (1953)

USBR for canals in coarse~ noncohesive material /(from HE R Rept Hyd '504, 1962) ' *TC = 0.040 ("y s -62.4)050=4.10 050 (EM-1601, 1970, G s =2.64 Ib/ft 3 )0.1 0.5 1

MEDIAN STONE SIZE, D 50 , IN FEET10

FIGURE 1 9. Comparison of procedures fo r estimating stone size on channel bed using critical shearstress (adapted from report by Anderson and others, 1970) .

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Procedures r e c o m m e n d e d by M an-54, C al-B&SP, USBR-EM-25, an d HEC-11 forcurved ch annels give stone sizes th at m a y ha ve b ee n adequ ate to pr e v e n t the riprap f ailures on Pi nole C r ee k a t Pi nole, C aliforni a, and the S a c r a m e n t o R i v er atE - 1 0 n e a r Chico, C a l i f or n i a (table 4). In terms of a p pl i c a b i l i t y a n d a b i l i t y toh a n d l e situ ations such as v a r i a b l e side slopes a n d ch annel curves, the methodsby U S B R - E M - 2 5 a n d H E C - 1 1 for curved ch annels appear to be the most appropri ate.

Three of the five methods compared in figure 19 do n ot c o n s i d e r m e d i a nstone size (D 5O ) larger th an 1.0 ft (0.30 m), but the m e d i a n size a v a i l a b l e fromrock qu arries r a n ges from about 0.5 to 4 ft (0.15 to 1.2 m) . To o b t a i n a D 5Os m a l l e r t h a n 0.5 ft (0.15 m) requires extra crushi ng and screen in g. M a t e r i a l sof this size are u s u a l l y o b t a i n e d from roa d p a v i n g stoc kpiles.

The average shear stress for open cha n n els , based on the m e d i a n slope(0.00368) for 297 sites in table 1 of v o lu m e 1 , is 2. 4 lb/ft 2 (11.7 k g / m 2 ) .This v a lu e of shear w ou ld i ndic ate a D 5O of 0.5 to 0.6 ft (0.15 to 0. 1 8 m) ,w h i c h is n e a r the lower size limit of c o m m er c i a l l y av ailable rock riprap.

The ch annel slope for about 25 p e r c e n t of the sites listed in table 2 issteeper th an 0.010. The shear stress for a slope of 0.010 a n d a flow d e pth of10.3 ft (3.14 m) is 6.43 lb/ ft 2 (31.4 k g / m 2 ) . O n l y E M - 1 6 0 1 p r o v i d e s a ston esize for a shear stress this large.

D a t a for C o m p a r i s o n of D es i g n Procedures

F or this report, the v arious methods for d e s i g n i n g riprap were c o m p a r e du s i n g a set of hydraulic da ta for Pi nole Creek at Pin ole, C aliforni a. D a t a forthe P i n o l e C r e e k site (fig. 20 a n d table 5) were used b ec a u s e the cause of ripra p f ailure wa s k n o w n a n d a n e x t e n s i v e set of hydraulic da ta was av ailable.P i n o l e Creek is loc ated a p pr ox i m a t e l y 1 7 m i (27.4 km) n o r th e a s t of Sa n Franciscoon I n t e r s t a t e 80. The reach of ch annel studied wa s pr ot ec ted w i th ripr a p by theCorps of E n g i n e e r s in c ooper a t i o n w i th Contra Costa C ou nty in 1966. Severalf ailures occurred over the surveyed reach as a result of high flows from theflood of J a n u a r y 4 , 1982. Two i ndirect m e a s u r e m e n t s of the pe a k flow, high-w a t er profile da ta , a n d 1 7 cross sections were surveyed in M a r c h 1 9 8 2 tod e t e r m i n e the pe a k disch arge a n d ch annel geometry. Origi nal design data wereo b t a i n e d to supplement the field data.

The ch annel of Pi nole Creek wa s d e s i g n e d for a flood disch arge of about2,500 ft 3 /s (70.75 m 3 /s). The ch annel h ad a b otto m w i d th of 20 ft (6.1 m) formost of the reach tha t is over 1,400 ft (426.7 m) long. C h a n n e l b a n k s wered e s i g n e d w i th a 2:1 side slope. R oc k riprap pl a c e d throughout the reach i n 1966wa s s e v e r e l y d a m a g e d d u r i n g the J a n u a r y 4 , 1982, flood ( disch arge 2,250 ft 3 /s)by p a r t i c le erosion . The M an ni ng 's rough ness coefficient n r a n ges from 0. 027 to0 . 0 4 8 b a s e d on v er i f i c a t i o n studies made after the J a n u a r y 1982 flood. A chutestructure, built to reduce the ch annel g r a d i e n t in the v i c i n i t y of cross section0.4 ( D 5 O = 2.3 ft or 0.70 m) , c aused supercritic al flow. Cross section 0. 2 intable 5 is in a straight reach of cha n n el, a n d cross section 3 is just dow nstream from the apex of a 66 curve, as shown on the aeri al ph oto gr a ph of thesite in figure 20. The da ta in table 5 summarize the h y d r a u l i c a n d riprap chara c t e r i s t i c s at cross sections 0. 2 a n d 3 a n d were used as a common base fore v a lu a t i n g the v arious riprap design procedures in subsequent sections of thisreport.

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Table 5 . Hydraulic properties of the J a nu a ry 4 , 1982, flood at crosssections 0.2 and 3 on Pinole Creek at Pinole, California.

[Locations of cross sections are shown in figure 2 0 . Values are based oncross-section geometry surveyed March 2 , 1982]Cross-section

0.2(straight reach)

Cross-section3(curved reach)

Discharge (ft 3 /s)Water-surface elevation ( f t )Water-surface slope ( S ) (ft/ft)Area (ft 2 ) WWidth of w a t e r surface ( f t )Mea n velocity (ft/s)M a xi m um depth ( f t )Average depth ( f t )Hydraulic radius ( f t )D 5 0 ( f t )M a n n i n g 's nRatio D 8 5 /D 5 oSide slope, zAngle of curvature ( )Radius of curvature ( f t )Specific weight of rock ( y ) (lb/ft 3 )S

2,25092.590.0036275548.28.35.14.740.470.0351.741.6:100172

2,25088.290.0054293617.77.74.84.620.600.0301.402:16 6150178

The failure of the riprap at section 0.2 i s related to ( 1 ) the steep bankslope, a nd ( 2 ) the expansion of flow, a nd associated turbulence that resultedwhen flow velocities decreased downstream from the culvert under Interstate 8 0 .Hydraulic data for cross section 0.2 were not used in the comparison of designmethods, but are included for comparison with hydraulic data at cross section 3 .The riprap failure at cross section 3 i s attributed t o excessive shear stressand inadequate size of rock in the vicinity of a chan nel bend.

The actual size riprap (in terms of D 5 0 ) that would be required to adequately protect the b a n k near cross section 3 for a discharge of 2,250 ft 3 / s(63.67 m 3 / s ) i s not precisely known. I t m a y be safely assumed, however, thatthose methods that result in a m edi a n stone size smaller than the size that wasinstalled ( D 5 O =0.47 ft or 0.14 m at section 0.2 a n d 0.60 ft or 0.18 m atsection 3 ) (table 5 ) would not have provided adequate protection of the banks.

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E v a lu a t i o n of H y d r a u l i c E n g i n e er i n g Circular No. 11 (HEC-11)

The procedures in HEC-11 a n d HEC-15 for d e s i g n i n g rock riprap each requiredifferent hydraulic d a t a a n d provide different results for the same designsitu ation, as i n d i c a t e d in table 4 . The procedure in HEC-11 wa s developed onthe basis of slope pr ot ec t i o n methods in use prior to 1948 and on research onthe pr ot ec t i o n of upstream slopes of e a r t h dams, which was current in 1967. Thep r i n c i p a l sources of m a t e r i a l u s e d in pr ep a r a t i o n of the circular include thesubcommittee report "Review of Slope Pr ot ec t i o n Methods" by the A m er i c a n S oc i e t yof Civil E n g i n e e r s ( 1 9 48 ) , a n d procedures developed by the Corps of E n g i n e e r sfor the hydraulic design of rock ripr a p (Campbell, 1966 ) . The circular a lsodiscusses the types of riprap in common use a n d compares methods for d e t e r m i n i n gstone size and design of filter blankets. Al th ou gh n ot stated in the circular,the procedures a re applic able for subcritic al flow conditions only (Frouden u m b e r less th an 1.0).

D es c r i pt i o n

The basic design procedure outli ned in HEC-11 i s illustrated by the flowchart in figure 21. The size of rock (stone) riprap n e e d e d to protect thes t r e a m b a n k a n d bed i s d e t e r m i n e d on a t r i a l - a n d - e r r o r basis u s i n g a n e s t i m a t e dston e size, design flow velocity, a n d depth.

Two graphs in HEC-11 a re used to rela te the v e l oc i t y of flow a g a i n s t thech annel bed to the size of ston e n e e d e d to resist d i s p l a c e m e n t from the banksfor v arious side slopes. Depths of flow that exceed 10 ft (3.05 m) a re multi-"pl i e d by 0.4. F o r i m p i n g i n g flow, the v e l oc i t y i s m u l t i pl i e d by a f actorr a n g i n g from 1 to 2 to o b t a i n a n e s t i m a t e of the flow v e l oc i t y a g a i n s t the rock.

The rock dia m eter , k , is e qu i v a l e n t to the m e d i a n di ameter ( D 5 o ) of theriprap. Procedures outli ned in HEC-11 for d e t e r m i n i n g rock size a re b a s e d onthe u nit weight of ston e equal to 165 lb/ft 3 (2,640 kg/m 3 ) . A procedure i s presented to adjust the rock size required for other specific weights of stone.This procedure, described on page 11-4 in HEC-11, should be shown as:

w w-.

where k = ston e size from figure 2 of HEC-11k = stone size for ston e of w pou nds per cubic footww = u nit weight of proposed stone riprap, in pou nds per cubic footThe basic requirement in ripr a p design i s to rela te the forces of stream-flow to the resisti ng forces of i nerti a a n d friction of the riprap. HEC-11 uses

stream v e l oc i t y as a m e a s u r e of s t r e a m f l o w forces, which are then related to therequired rock size that will resist displacement.

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Depth of flow > 10 feet, subs titute0.4d for d in ratio

Impinging f low, apply factoro f1 to 2 t imes Vs .

Select trial D5 Q . Determine d, Va ,fo r design con ditions.

T>OO

Fig. 1 - Velocity of stone on channel bottom.Determine Vs , V s = (Va, D 50 /d).

C Oz =

D50Fig. 2 - Size of stone that will resist displacementfor various velocities an d side slopes.

Determine D50 , D50 = (V s , z) .

Continue until D50 selected in step 1 is in reasonable agreementwith size from Fig. 2 in step 3.

FIGURE 21 . Flow chart of riprap design procedure for HEC-11 . (The factor k in figure 1 of HEC-11equals the median particle size, 050).

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Hydraulic Factors

Ch annels designed a n d built as trapezoids develop a pr i m a r y flow p a t hw i th i n the trapezoid tha t g e n e r a l l y results in local velocities tha t a re highert h a n a n t i c i p a t e d in the design. E s t i m a t e s of a v e r a g e v e l oc i t y for m a i n ch annelflow of strea ms with o v e r b a n k flow m a y be low if large areas of o v e r b a n k floware i ncluded in the c alculations. In a ddition , the accuracy of the e s t i m a t e da v e r a g e v e l oc i t y i s d e p e n d e n t on the reli ability of the computed water-surfacee l e v a t i o n a n d flow area. For exa m ple, in a trapezoidal ch annel of widthsn o r m a l l y e n c o u n t e r e d in the field, with side slopes of 3:1 a n d depths of 5 to10 ft (1.5 to 3.05 m) , a 20 p e r c e n t error in water-surface e l e v a t i o n ca n resultin a c r o s s - s e c t i o n a l a r ea tha t i s as much as 40 p e r c e n t in error.

Curves of v e l oc i t y a g a i n s t stone, V s , given in figure 2 of HEC-11 a reextended to velocities greater th an those g e n e r a l l y encou ntered. In a straightreach of subcritic al flow, m a x i m u m poi nt velocities ( at a n y loc ation in thecross section ) exceedi ng 16 ft/s (4.9 m/s) a n d mean velocities exceedi n g about8 ft/s (2.4 m/s) a re u nusu al (table 2 ) for a typic al gravel a n d cobble bedcha n n el. H i gh er velocities may occur in s a n d bed streams.

A relationship of v e l oc i t y versus ston e weight, similar to figure 2 ofHEC-11, is pr es e n t e d in EM-1601. The relationship labeled Isbash in EM-1601,plate 29, is similar to the curve in figure 2 of HEC-11 for a ch annel with 2: 1side slope. The da ta used to develop the relationships shown in plate 29 ofE M - 1 6 0 1 a n d figure 2 of HEC-11 are from hydraulic model experiments a n d observ ations of speci al events such as dam closures or stilli ng basin a n a lyses . Theserelationships, therefore, m a y n ot be realistic i ndic ators of the i n t e r a c t i o nb etw ee n v e l oc i t y a n d ston e weight (size) for typic al streamflow conditions."

In HEC-1 1 , figure 1 relates D 50 /dm to V s /Va a n d figure 2 rela tes V s to ston esize. Figure 1 is reproduced here in m o d i f i e d form as figure 22. The curve fora 2:1 side slope from figure 2 is reproduced here as figure 23. For straightcha n n els , no h y dr a u l i c a l l y reasonable v alues of d a n d V s ca n be selected tha tresult in a D 50 larger th an about 0.5 ft (0.15 m). This size i s too sm a ll toresist d i s p l a c e m e n t duri ng most floodflows .

The factor of 1 to 2 for a d j u s t i n g v e l oc i t y a g a i n s t ston e for i m p i n g i n gflow at bends i s a r b i t r a r y a n d no guideli nes for a p p l y i n g this f actor a re given.The o n l y j u s ti f i c a t i o n for e x t e n d i n g the o r d i n a t e in figure 2 of HEC-11 above10 ft/s (3.05 m/s) i s to accommodate velocities that may occur near energy dis-sipaters or velocities tha t ha ve been derived to accou nt for ch annel curvature.

Appl i c a t i o n of the ripr a p design procedure requires a n i niti al estimate ofD 50 . The tr i a l - a n d - er r or proce

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EVALUATION OF RIPRAP DESIGN PROCEDURES - USGS report

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