surl¡ilÀRY

Recognizing anil anticipating channel instabilitiesis an irnportant part of locating and designing hiqh-ways in river environments. channel instabilitiesincludle oscillations in channel bed elevation, vari-ations in river oríentation and location, and nâjorriver rnÍgrations or neanders. Factors affectíngriver stability have been classlfieil as natural oraccelerated. Natural instabilitles result fromchanges in hy¿lrometeorology rrrhereas acceleratederosion is usually a result of nanrs activitiesrvithin the watershed.

IClentifying channel instabilíties requires anunderstanding of the geonorphic processes occurringwithin the satershed in queslion and an awarenegs ofall activitíes that affect stability. A thoroughanalysis of systern stability should include con-síderation of past system changes and changes inprogress, as r¡ell as a geonorphic analysis to pre-dict future changes.

REFERENCES

1. S.A. Bro$tn. Streanbank Stabilization lleasure forHighway Engineers. Final Report' FHVIA/RD-83,/099.FHWA, U.S. Departrnent of Transportation' 1984.

2. I'.8. Leopold. FIuviaI Processes in Geomorphol-ogy. W.It. Freeman and Cornpany' San FranciscotCalif., and ton¿lon, England' 1964' 522 pp.

3. E.w. Lane. The Inportance of Fluvial I'{orphologyin Hydraulic Engineering. Proc. r AmericanSociety of Civil Engineering, vol. 81' No. 745'1955,17 pp.

163

4. S.A. Schu¡nn. The Fluvial Systen. wiley' Neht

Yorkr 1977.5. D.B. Si¡nons and F. Senturk. Sediment Transport

Technology. lvater Resources Publications' FortCollins, Colo.' 1977, 807 PP.

6. D.G. Decoursey. Stream channel stability, Corn-prehensive Report. U.s. Army CorPs of Engineers'Vicksburg District' Vicksburg, Miss.' 1981.

7. F.M. Henderson. Open Channel Flow. MacmiLlan'Ne\r York' 1966.

8. T.N. Keefer, R.s. !.lcouivey, and D.B. sinons.Stream Channel Degradation and Aggradation:Causes and Consequences to Highways. InterinReport, FHÏvA/RD-80,/038. FHIIÀ, U.S. Departnentof Transportation' 1980.

9. J.C. Brice and J.C. Blodgett. Counterneasuresfor Hydraulic Problens at Bri¿lges, vo1s. 1 & 2.Final Report' FHWA,/RD-78/162. îllwÀ, U.s. Depart-ment of Transportation, 1978.

10. S.A. Brown' T.N. Keefer, an¿l R.S. ÈtcQuívey.Strean channel Degradation and Aggra¿lation:Analysis of Impacts to Highway Crossings. FinalReport, FIIWA,/RD-80,/159. FHWÀ, U.s. Departnentof Transportation' 198I.

lL. w.w. Sayre and ,f.F. Kennedy. Degradation andAggradation of thè lrlissourí River. Report 215.Iowa Institute of Hydraulic Research, ]-978.

Publication of this paper sponsored by Committee on Hydrology, Hydraulicsønd lüater Quality.

piers, is related to stream Èype and can beassessed from aerial photogrâphs.

Hydraulic problems at bridges, although less preva-lent than structural problems, are nevertheless sig-nificant. In the United States the annual damage tobridges and highways irom floods has been esti¡natedat $100 million during years of extreme floods (I).strêam-related danage and maintenance problens alsooccur \dhen there are no floods' but the expense ofsuch damage and problerns is difficult to estinate.A study of hydraulic problems at bridges Ç) has ín-dicated that damage by strearns can be reduced byconsidering channel stability in site selection,bridge design, and counterneasure placernent.

The objective of this paper is to give a briefsunmary of geonorphic nethods used to assess streamchannel stability. These methods are presented ingreater detail in a research report published by theFederal Highway Adrninistration (3), and a checklíst

Assessment of Channel Stability at Bridge Sites

JAMES C. BRICE

ABSTRACT

Assessment of channel stability fron fieldstudy and the conparison of tine-sequentíalaerial photographs provides information thatis needed in site selection, bridge design,and countermeasure placenent. Channel in-stability is indicated by bank erosion, pro-gressive degradation (or aggradation) of thestreambed, or natural scour and fil-l of thestrea¡nbed. Bank erosion rates are relatealto strearn type and are proportional tostrea¡¡ .size. Predictions of future ratesare based on past rates, as measured onti¡ne-sequentíaI photographs or naps, and onthe typical behavior of meander loops. SiS-nifÍcant degradation of the streanbed canusually be detected from inilirect field evi-dence. The sites of greatest potentialscour along a channel can be identified fromchannel confígurâtiôn. Shift of the thal-weg, which is a factor in the alignment of

r64

of geo¡norphic factors that should be consiclered insite selection and bridge design are given in an Ap-pendix at the end of this paper..

Many engineers have evidently relied on engineer-ing judgment, based on prior experience and hy-draulic analysis of flow (3) to assess scour andother aspects of strean behavior. An âssessnent ofchannel ståbility, fro¡n field observations and in-terpretation of time-sequential aerial photographs,provides additional infornation to decision nakerswho select sites and design bridges.

Ideålly a stable channel ís one that does notchange in size, form, or position over time. Allalluvial channels change to sone extent and there-fore have sone degree of instability. For engineer-ing purposes, an unstable channel is one in whi.chthe rate or magnitude of change is so large that itbecomes a sígnificant factor in planning for ormaintaining a bridge, highway, or other structure.Chãnges considered here are (a) lateral bank ero-sion, (b) degradation or aggradation of the stream-bed that continues progressively over a period ofyears, and (c) natural short-term fluctuations ofstreambed elevation that are usually associated withthe passage of a flood (scour and fill).

Applying assessments of channel stability toplanning biidges and countermeasures is well statedby Klingeman (5):

h¡hereas designers often consider suchchanges (in bed configuration and channelflow alignnent) as a function of stage,it nay be ¡nore important to recognize thechanges that might occur with tine .best studíed fron a series of aerial pho-tographs spanning several years.From this assessnent of channel stabilitythe designer can expect to make sounderrècom¡nendations regarding the best loca-tion of the åxis of the bridge, the Loca-tions of piers in the channel . . . andthe likelihood for channel changes andpotential maintenance problems during thelife of the bridge.

PLANFORM PROPERTIES AND TYPES OF STRBAMS

Stream planform properties índicative of channelstabilíty (or instability) are most readity observedin aerial photographs. Unstable streams have wideunvegetated point bars (a and c in Figure I), cutbanks (b in Figure 1), and recent meander culoffs (din FÍgure l). Stab1e streams (shown in Figure 2)have a fairly constant strean width, narrow pointbars, and erell vegetated banks. ttajor stream types(Figure 3) are characterized by these plånforn prop-erties and by stability that lies within a fairlywell-defined range.

Sinuous Canaliform Streams

The point bars of sinuous canaliform streans (FiS-ures 2 and 3a) are typically covered with permanentvegetation, but narrow crescents of bare sedimentmay be visible at nornal stage. Braidíng and lat-eral bars are rare. If rnarkings are visible onpoint bars¡ they tend to be concentric scrolls.Sinuosity tends to be moderate to high, but somereaches are nonsinuous. Banks tend to be well veg-etated and cut banks are rare. Natural ¡neander cut-offs are at the necks of ¡neander loops, leavingcrescentic oxbor,, lakes on the floodplaÍn. this ísthe most laterally stable of all stream types, butmeãnders gradually migrãte. ff ¡nuch vegetation iscleared along the channel, stability may quickly de-terioratei çut banks are an early indication of

Transportation Research Record 950

FIGUßE I Aerial photograph showing typical features of a laterallyunstable stream (West Fork White River near Nervberry, Ind.).

FIGURE 2 Aerial photogr.aph showing typical features of a laterallystable stream (Àpalachicola River near kistol, Fla.).

JU"-"n tëã ,,rrorJo"^;-"tsr'Ã

I(,' \t _ --4\9l%B SINUOUS POINT RAR

FIGURE 3 Major alluvial stream types.

this. The rate of bedload transport is small in re_lation to suspended toad.

Sinuous Point-Bar Streams

Bare point bars, clearly visible because of theirIight tone, tend to be conspicuous on aeriat photo-graphs of sinuous point-bar streams (Figures I and3b). ¡4arkings on the point bars, if visible, tend

C srNuous sRn¡oeo

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to be concentric (location c in Figure l). Thechannel may be locally braided and lateral bars naybe present. Sinuosity tends to be noderate, butsome reaches are nonsinuous. Cut banks are conmonlypresent on the outside of bends. Both neck cutoffsand chute cutoffs occur r but neck cutoffs are ¡noretypical. The rate of bank erosion at bends is po-tentially high, but nonsinuous reaches may remainstable for decades. Sediment load tends to be rnod-erate, and a signifícant part of the total load istransported as bed load, either sand or gravel.I'tost rivers in the United States are of the sinuouspoint-bar type.

sinuous. Braideg streems

Point bars' Iateral bars, and midchannel bars arelikely to be present in the channel of sinuousbraided streams (Figure 3c). t'larkings on point barsare irregular or braided. Braiding may be loca1 orgeneral. In contrast with nonsinuous braidedstreams (Figure 3d), whose thalweg is discontinuous,the thalweg is continuous and likely to be meander-ing. On sone braide¿l point-bar streans' the posi-tion of the thalweg is fairly stablei on others, itshifts drasticatly during floods. The maÍn channel¡in contrast with the thalweg' tends to have a lowsinuosity. Cut banks arê cornmon along the nainchannel, ân¿l natural cutoffs are generally of thechute Èype.

sinuous braided streams hãve a potentially highrate of bank erosion. Rapid shift of the thalwegnay cause alignment problems and bypassíng of â

bridge. scôur depth in the thalweg is potentiallygreat, particularly if the bed material is silt orsand. Sediment load tends to be large and a signif-icant pårt of total load is transported as bed load(sand, gravel, or cobbles).

Nonsinuous Braided Streams

A typicåI nonsÍnuous braided stream (Figure 3d) hasa channel bordered by tiistinct banklines; withinthese banklines, the channel is divided by unvege-tated bars or sma1l vegetated islands. The bank-lines tend to be irregularly scalloped, with cutbanks at indentations. Channel width may changedrastically from place to place, but in most placesthe channel Ís wide and shallow, reguiring a J-ong

165

bridge unless confined by suitable countermeasures.AlthÕugh the channel is unstable in Èhe sense thâtbraÍds shift rapidl-y during floods' the bank erosionrates tend to be l"ow or noderate. Bank erosion oc-curs where braids shift randonly against the bank-line, and hence the point of erosion is unpredict-able. Because the banks tend to be erotlible, bankprotection neasures are requíred in the vicinity ofabutments. Braids shift at each high flow¡ and un-expected dèpths of scour may occur where braíds jointo for¡n a deep channel. Much of the load is trans-ported as bed load, either sanil, gravel, or cobblès.

Anabranching

Any of the four major stream types may be anâ-branched (Figure 3). Anabranching differs frombraiding in that the flovr is divided by islands, orsometimes bars' that are relatively pernanent andare large in relation to channel wiclth. The ana-branches, or individual channels' are more widelyand distinctly separâted and more fixed in positionthan are the braids of a braided strean. A longbridge may be requireil unless the strean ís crosseclat a local point where it is not anabranched. ff -there are two or nore anabranches at a crossingsite, suitable countermeasures will pernit a shorterbridge. If two bridges are usedr percentage oftotal flood flow at each bridge rnay be difficult topredict. The stability of anabranches differsgreaÈty on different streamsr and the stability ofeach anabranch needs to be assessed as though it$¿ere a separate strean.

LATERAL STABILITY

Lateral instabil-ity at a bridge site may involveerosion of one or both banks, but commonly only onebank is eroded as the channel migrates låterally andchanges its position relative to the bridge (Figure41. Some degree of låteral migration is to be ex-pected for bridge sites at bends ín the channel' butbends may develop at sites where the channel wasoriginally straight. One objective of stabilityassessment is to anticipate the nigration of bends,or the developnent of new bends. The most co¡nmonproblems associated with lateral migration ãre theundermining of aL¡ut¡nents and the exposure of pilebents that were originally placed on the flood plain.

FIGURIì 4 l.ateral stream erosion and related hydraulic problems at bridges.

166

PRoCESS ttV0LVEDIT HYDRAUIIC PROBLIM

LATERAL EROSION BY STREAM

GENERAL SCOUR

LOCAL SCOUR

CHANN€L DEGRADATION

i{UM8Eß OT SITES

N106

ñs\ñÑ\Ñ\s\s¡ñ\l 55

ñ\s\\ñlN 47

ñ\\ñ\s\\l 34

ACCUMULATION OF OEBRIS ÑÑÑ\ 26

FIGURE 5 Relative importance of different processes inhydraulic problems at bridges.

Lateral erosion is probably nore frequently in-volved in hydraulic problems at bridges than anyother stream process. In a study of 224 bridgesites in the United States (2) r hydraulic proble¡Rswere attributed nainly to lateral strean erosion at106 sites. Other stream processes (general scour,local scour, channel degradation, and accu¡nulationof debris) were less common contributors to problens(Figure 5).

The lateral stability of a channel is measuredfro¡n records of its position at two or nore dif-ferent tines, and the available records are usuallynaps or aerial photographs. Surveyed cross sec-tions, although useful, are rarely available. Formost agricultural regions of the UniÈed States,aerial photographs are availabl-e for about the last40 years. Information on the acquisition of tírne-seguential aerial photographs was given by Brice(3). Maps have the advantage of a longer tine span,but time-sequential naps of suitable accuracy areunavailable for large areas of the United States.Reference Points

Measurement of bank erosion on two time-seguentialaerial photographs (or rnaps) requires the identifi-cation of reference points that äre conmon to both.Discernible reference points are either cultural ornatural features, which can be identífied with ¡nuchgreater confidence by stereoviewing than by examina-tion, of a single photograph. ff a stereopair is notavailable, a nagnifying lens wilI assist in í<ienti-fication on a single photograph. fn most regions,and particularly on floodplains, cultural featuresare more 1ikely to naintain recognizable identityover a period of several decades than are naturalfeatures.

Cultural features useful as reference points in-clude road and fence corners, buildíngs, irrigationcanals, and bridges. In Figure 6, point I is a roadcorner, point 2 is a fence corner, point 3 is theend of a brídge, and point 4 is a farrn building.Points close to the strean have been selected. Be-cause of possible scale variation across the photo-graph, related to canera tilt, the usefulness of areference point decreases with increasing distancefrom the channel. Anong the natural features thatmaintain recognizable identity are rock outcrops andsharp bends in snaIl incised channels. Isolatedtrees are sometimes useful, as are drainage featureson floodplains and lakes of distinctive shape. Onsorne wide, densely forested floodplains, no reliablereference points rnay be discernible in the vicinítyof the channel, and bank erosion distances can onlybe estimated.

Cornparison of Aerial Photographs or Maps

Àssessnent of lateral stâbility, and the behavior of

Transportation Research Record 950

FIGURE 6 Examples of reference points on time-sequential aerialphotographs. (A) Points, indicated by numbers, on l9ó9 photographof Cedar River, Iowa. (B) fürresponding points on 1937photograph. (U.S. Dept. of A$iculture photographs).

meanders is greatly facilitated by a drawing whichshows the changes with tine of banklines and otherfeatures of interest (Figure 7). To prepare such adrawing, the aerial photographs (or maps) arematched ín scale and the pairs of fixed referencepoints are placed in register. This involves either(a) bringing the aerial photographs to the samescale by photographic enlargement, or (b) natchingscales by projecting the image of one photographonto another (or a tracing of the other). Projec-tion can be done with a vertÍcal reflecting projec-tor, a graphical data transfer instrument, or an or-dinary 35-mm slide projector (3).

Bank Erosion Rates

Bank erosion rates tend to increase with an increasein stream size, as expressed by channel width. InFígure 8, channel i{idth refers tö width as neasuredon aerial photographs, at straight reaches and theinflection poínts of bends. Median erosion rate wasmeasured on time-sequential bankline diagrans for 36streams in the United States (3). The dashed curvein Figure I is drawn arbitrarily to have a slope ofI and a position (intercept) to separate most sinu-ous canaliforn strea¡ns fron most sinuous point-barand sinuous braided streans. For a given channelwidth, sinuous canaliform streams tend to have thelowest erosion rates, and sinuous braided streatns,the highest. Bank erosion could not be dÍscernedfor some sinuous canaliform streams, ând an arbi-trary rate of 0.01 meter per year was assigned to

Br ice

EOAP

........ Bonkline, 1937

- Bonkllne, 1969

////// E( o sioî, 1937- 69---- Reference linss

0 500 Meters

FIGURE 7 Time-sequential bank-lines for Cedar River, Iowa.

o 0riõ 50 r0 50 lo0CHANNEL WIDTH, IN METERS

FIGURE B Bank erosion in relation to channel size and type.

these. Nonsinuous braided strèams' not shown ínFigure Ir plot wetl below the arbitrary curve be-cause their channels are wide relative to their dis-charges. If nonsinuous braided streams and sinuousbraided streams (both unconnon in rnost parts of theUniteil States) are excluded, the dashed curve inFígure I provides a preliminary estinate of erosionrates that nay be encountered at a particular site.

The lateral stability of different strea¡n reachescan be conpared by means of a dimensionLess erosioninilex Q). The erosion index of a reach is theproduct of its ¡nedian bank erosion rate expressed inchannel widths per yearr multiplied by the percentof reach l-ength along ethich erosion occurred, multi-plíed by 1r000. Erosion indexes for 41 strea¡nreaches in the Uníted States are plotte¿l against

767

500

sinuosity in Figure 9. The length of nost of thesereaches is 25 to 100 channel wialths. The highesterosion index values are for reaches whose sinuosityis 1.2 to 2 and whose type is either sinuous braidedor sinuous point bar. An erosion index value of 5

(horÍzontal dotted l-ine in Figure 9), which sepa-rates these types from nost sinuous canalifortnstreamsr is suggested as a boundary between stableand unstable reaches. Reaches having erosion indexvalues less than 5 are unlikely to cause lateralerosion problems at bridges. As an exanple of theuse of the erosion index for conparative purposes'the reach of the Vlhíte River in Figure 2 has anerosion index. of about 15' and the reach of theApalachicola River in Figure 3 has an erosion indexof about 4,

lr1-------v-.tliirlilllr

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z_

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t.0

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STREAM TYPE

SINUOUS CANALIFORM

SINUOUS POINT BAR

SINUOUS BRAIDED

IL¡

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168

FIGURE 9

For all strea¡n types, erosion rates are higher atbends than along straight reaches. On the otherhand, strean types characterized by the highest sin-uositíes tend to be the most stable, because somedegree of stability is necessary for high sinuosi-ties to be maintained. An unstable strean will notrenain highly sinuous for very long, because thesinuosity ¡'¡i11 be reduced by frequent neanaler cut-o ffs.

. Prediction of l¿le_ander Loop titigration

Most Lateral erosion problens ât bridges are associ-ated grith Èhe migration of meander loops or with thegrovrth of nerd loops. Prediction of the rate andrnode of loop migration may therefore be needed forplanning purposes. Sone progress is beÍng ¡nade onnu¡nerical prediction of loop deformation and migra-tion, applying Èo time intervals significant for en-gineering purposes (!). At present. however¡ thebest available estimates are bâsed on pâst rates ofIateral migration at a particular reach and on thetlpical migration behavior of loops. As dernon-strated by Nanson et al. 0), erosion rates at aparticular loop may fluctuate substantially (and un-predictably) from one period of years to the next.Even so, a rãtional estimate of erosion at a neanderloop, based on the probåb1e distribution and raÈe oferosion, is better than none.

Erosion at loops involves extension (Figure 10a),translation (Figure lob), or conversion to a coÍl-pound loop (Figure loc). TlFical-ly, a loop nigratesmainly by translatíon, with some component of exten-sion. Meanders tend to becone conpound because theyhave developed, by growth¡ a path length that isIong in reLation to the typicat spacing of pools andriffles (or alternate bars) in the channel. Neckcutoffs (Figure lod) are typical of canalífor¡o ândsinuous point-bar streams. As the degree of braid-ing increases, chute cuÈoffs (Figure 10e) are rnoreprobable. The cutoff of a neander 1oop, whether bynatural or artificial means, tends to increase thebank erosion rate at adjoining l-oops. However, Ioopcutoffs are conmon in nature, and drastic conse-quences have rareÌy been observed.

Transportation Research Record 950

STREAM TYPE

SINUOUS CANALIFORMSINUOUS POINT 8AR

SINUOUS BRAIDED

NONSINUOUS BRAIDED

a-----r

FIGURE l0 Modes of meandel loop developmentand cutoff.

CHANNET. DEGRÀDATION

Progressive vertícåI changes in bed elevation (deg-radâtion or aggradation) are a conmon cause of hy-draulic problems at bridges in sone regions of theUníted States, ând a potential cause in any regionwhere channels are underlain by erodible materials.Degraclation occurs nore frequently then aggradation,and its consequences are nore serious. Of the totalnumber of gradation sites reported by Keefer et aI.(8), sites having degradation problems were aboutthree times more numerous than sites having aggrada-tion problems.

Annual rates of degradation that are averagedover a period of time following some rnan-inducedevent, such as the closure of a darn or the straight-ening of a channel, are not a good basis for esti-mâting future rates. Àccoraling to Sínons andSenturk (9), tne rate of degradation dogrnstream fr.gna dam ís rapid initially and decreases gradually âsa nei{ stâble profile evolves. Sinilarly, availableeviclence suggests that the rate of degradation fol-lowing channel straightening is likety to be rapidat first and to decrease gradually (I9.). From aconprehensive study of the effects of channelstraightening on streans in western Tennessee,Robbins and Simon (fl) were able to describe thechannel degradation by an exponential decay func-tion. They concluded that if there were no furtherdisturbance the degradation or aggradation follor¡eda predictable pattern and rate.Fiêld Àssessment of Degradation

ff a chânnel- has been recently degra¿líng at a rate

a

¡

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2.A2.6?.4

,{ l:\.// \\,æ

l'I

l.? r.4 r.6 r.8 2.o 2.2SINUOSITY

Erosion index in relation to sinuosity.

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that would significantly affect planning of engi-neering norks, there is likely to be some observableevidence for this åIong the channel, as seen in thefiêld or by stereoviewing of aerial photographs.Indicators of degradation are listed below in ap-proximate order of reliability:

l. Channel scarps (headcuts, knickpoints). A

rnigrating scarp ín the long profile of a channel isunequivocal evidence of degradation' ånC the rate ofdegradation is related to the height and migrationrate of the scarp. ChanneL scarps are easy to ob-serve in ephemeral streans or snall perennialsÈreams, but are rarely observed in large perennialstreams.

2. cullying of ninor side tributaries. As a

stream degrades, its tríbutaries also degrade' andscarps may be present along their profiles.

3. High, steep, unvegetated banks. sone chan-nels have higher banks than others of about the same

wÍdth. Arnong the factors that determine bank heightare degree of incision and erosional resistance ofthe banks. Where hígh banks are also raw and un-graded, recent degradation is suggested.

Other Metho¿ls of Assessment

other nethods of assessing channel degraclation havebeen described by Keefer et al. (8). Changes in theelevation of the vrater surface are determined over aperiod of yearsr in relation to a fixed datum.¡4ethods include (a) periodic neasurernents frombridge deck to streambed, where allowance can benade for local or general scour at the bridget (b)plotting change in the stage-discharge relation atgauging stationst and (c) rePetitive ¡neasure¡nent ofthe longituilinal or cross profile of the streamchannel.

NATURAL SCOUR AND FILL

Naturat scour and fill has been neglected as a fac-tor in bridge site location, probably because of itsconplexity and the lack of useful infornation on

it. Bed elevation is clifficult to measure duríngfloods, and a reliable analysis of scour and fillreguires neasurement of several cross sections atabout the same time. A continuous longitudinal bedprofile along the thal¡.teg is al-so highLy desirable.In a particular cross section, the amount of scourand fill is unevenly distributed' and both the ref-erence bed and the scoured bed are likely to be ofirregular shape.

For most streams the nagnitude of scour is sub-stantialty greâter at so¡lìê places along the channelthan at others. Accordíng to Neill (L?) '

The locaÈion of a bridge rdith respect tothe river channel pattern in plan has animportant bearing on its liability to bedscour. Bends and narrow sections may betiable to scour at high stages' regar¿l-less of the effects of bridge struc-tures . . . . Straight or gently curvedreaches with stabte banks are to be pre-fer red .

If, however, a crossing must be made on a neanderingreach, iilentification of the segrnents of least po-tentiâl scour may be a deciding factor in site 1o-cation.

Scour below prefloocl bed elevation probably oc-curs at most cross sections of an alluvial stream atsome tine during the passage of a floodr althoughnot at the same time nor to the same degree. At

169

some sections, the scour is due to the migration ofbed forms and the nean strearnbed elevation doês notchange significantly. rn a detailed field study ofscour and fill at LI cross sections on the East ForkRiver in Wyorning, Andrerrs Q]) neasured scour thathe considered significant (though less than 0.5 m)

during flood crests at 6 of the 11 cross sectionstand fill at 5 of the sections. At sone ti¡ne orother during Èhe floodr net scour occurred at allexcept 2 of the sections.

Natural scour and fill refers to fluctuations ofstreambed elevatíon about an equilibrium position'which is comnonly taken to be the position at lovtflow. These flucÈuations are associated nainly vtithflooals, anil they occur without artificial constric-tion of the channel and without the presence of ar-tificial obstructions such as bridge piers. I'hescour induced by a britlge is adclitive to naturalscour. A bed elevation that has beên raised or low-ered is likely to return to Íts equilibríun positionduring the falling stages of the flood; however' thereturn rnay require weeks, months, or even years forsorne streams, particularly those having coarse bedmaterial. Natural scour and fill occurs by threedifferent nechanisms, erhich may operate jointly orindependently: (a) bed form migration, (b) conver-gence and divergence of flow, anct (c) Iateral shiftof thâIweg or braíds.

Bed Form Migration

The mígration of dunes ¡nay result in an amount ofscour that is sufficient to warrant consideration inthe design of pier foundations. Allowance for scourdue to the migration of sand waves is more problen-atical and erould have to be determined fron a con-tinuous tong profile of the stream at high stage.The maximum scour induced by the mígration of a dune

is about one-half dune height¡ and dune height isroughly esti¡nated at one-third the nean flow depth.In sand-bed streams' dune migration can be expectedif the guantity of bed load in transPort is suffi-cient for dune formation. strearn type is a reason-ably good indicatíon of the bed-Ioad characteristicsof a stream. Canaliform streams that have very nar-row point bars are likely to be bransPorting ninoramounts of bed load. An increasing transport of bedload is indicated by an increase in degree ofbraid ing.

Most migrating bed for¡ns in gravel-be¿l streanscan be regarded as bårsr the height of which is re-late¿l to flow depth. Migration of a bar through a

briilge waterway is rnainly o{ concern because of itsdeflection and concentration of flow. Bar nigrationtends to be a rando¡n process, and the tendency ofhars in a stream to nigrate is best determined frontime-sêquential aerial photographs.

Convergence of Flow

The flow conclitions associated with changes in mean

bed elevation are summarized by the convergent-divergent flow criterion of Leliavsky (14). Conver-gent currents in a natural stream are associatedwith erosion (scour), and divergent currents are as-sociated with deposition (fi11).

Persistent pools (Figure tI) in the long profíleof an all-uvial channel have the strongest conver-gence of flow and the greatèst Potential for scour.Such .pools are best identified by a continuous bedprofile along the thahdeg' as soun¿led at hÍghstage. On a gravel-becl 'pool-ancl-rifflen streamtthe water-surface profite at lold stage is flattestover the pools and steepest over the interveningriffles. on a sand-bed streamr however, persístentpools rnay fill at 1ow stage, their position nay

170

FIGURE ll Pools, crossovers, and trace of thalweg.

shift to sone degree, and they may be difficult todistinguish fro¡n random irregularities in the longprofíle. Pools, as well as riffles and crossovers,tend to be several channel widths in length, whichis longer than most rando¡¡ irregularíties. As thedegree of braiding of a stream increases, the proba-bility of persistent pools in the long profile de-creases. Scour holes ín braided strearns are mostlyat the confluence of braids.

For streams havíng wíde point bars, crossovers(Figure l-I) can usualLy be identified on aeríaI pho-tographs taken at low flon¡. pools cannot be ob-served directly, but they are typically located¿lownstream from the apexes of bends and opposite thepoint bar. Pools mây also occur in straightreaches, where their position is sornetimes rnarked byan alternate bar. At low stage, water-surface widthtends to be least at pools, greatest at ríffles, andof interrnediate value at transítional sites. Atbankfull stage, the water-surface width tends to begreater at pools than at crossovers or transitionalsections.

As suggested by Neill (lI), field ¡neasurement ofcross sectional area and flow velocity at an íncised(straight) reach near bankfull stage provi¿les a goodbasis for câLculation of scour by extrapolation tothe alesign flood. Furtherrnore, a comparative tnea-surement of this same section at low stage gíves theanount of scour from 1o\r stage to bankfull stage,which is valuable for confirming results obtained byconputational netho¿ls. Klinge¡nan G) recornrnendedthat "The loe¡est undisturbed streambed elevation ator near the bridge crossing (other than a localscour hole) be used as a reference level in settingscour elevations of principal píers at or near themain channel."

Shift of Thalweg

Of the 224 bridge sites studied by Brice andBlodgett (2), hydraulic problens atÈributed to shiftof the tha].weg occurred at 6 sites. One of these(site 164, Fort ceorge River, Florida) is at a sand-bed estuary, subject to strong tidal currents. Site207, Leaf Riverr Mississippi, and site 170, RedRiver, Arkansas, are on sinuous poínt-bar streamshaving sand beds. At both sites, the thalvreg shiftwas related to a slight curvature of the channel andtook place over a period of years rather than duringa single flood.

Site 16, Deer Creek, California, and site 186,White Riverr South Dakota, are at sinuous braidedstreans in r¡hich the thalweg tends to wander. Site226, Boulder Creek¡ Washington, is on a steep non-sinuous braided stream having a cobble-boulder bed.The bridge clearance was greatly reduced by aggrada-tion, and the thah,reg shifted against an abutrnent.Àlthough nonsinuous braided strea¡ns are commonly re-

Transportation Research Record 950

-> ÍHALWEGêoe Irc¡OLxxx cßossoyEF

garded as unstable because of the râpíd and unpre-dictable shift of bars and braidsr they are readilycontrolled v¡ith suitabte countermeasures and are nota particular cause of hydraulic problems.

Instability of the streambed that results fromshift to thalweg (or braiits) is, Iike bânk instabil-ity, relâted to stream t]¡pe ancl can be assessed fromstudy of aerial photographs. On sinuous canalifor¡nstreams, shift of the thalweg during flood is minl-mal. The channel tends to be relativety deep, nar-row, and unifor¡n frorn one place to another. Instraight reaches, the alternate bars that indicatemeandering of the thalg¡eg are rarely present.

A greater shift of the thahreg, both at bends andin nearly straight reaches, can be expected on sinu-ous point-bar streams. Flood flow cuts across theends of the point bars, which are wide and bare atIoe, stage. In straight reaches, alternate bars¡visible on aerial photographs taken at low stage,are comnonly present. These alternate bars are sig-nificant in the planning of bridges and counter¡¡ea-sures, because they indicate the potential for shiftof the thalweg and also for bank erosion where thecurrent is deflected against the bankline.

coNcLusIoNs

1. Study of strean morphology on time-sequentialaerial photographs provides infor¡nation that is ap-plicable to site selection and brídge deeign. Bythis means, information can be obtained on lateralstream stability, degratlation, and natural scour andfÍII. .

2. Lateral stability is related to stream tlrpe.StreaÍìs that have a unifor¡n r¿idth and. narrow pointbars (canaliform streams) tend to be the moststable. Streams that have wide point bars and cutbanks tend to be less stable and, for sinuousstreans, stability tends to ¿lecrease ¡vith the degreeof braiding. For a given strean tl4)e, median bankerosion rates tend to increase in direct proportionto stream size, as expressed by channel width.

3. Channel degradation is deternined by neasure-¡nent of progressive changes with tirne of streambedelevation in reference to a fixed datum. For rnan-induced degradation, the curve of cu¡¡ulative degra-dation versus time is more likely to be aslmptoticthan linear. Equilibriun bed etevation is difficultto predict.

4. Scour by bed for¡n nigration is of consequencenainJ-y in sand channels. crâvel bars tend to mi-grate on braided streams and to remain fixed at rif-fLes on unbraided, pool-and-riffle streams. e mi-grating gravel bar nay concentrate flow at a bridgeand cause lateral bank erosion or local scour âtpiers.

5. Scour by converçtence of fl-ow is relate¿t tochannel configuration and is greatest at persistent

ìì-.i\"j=..---

Br ice

ileeps or pools in the channel-Iong profile, wherethe water velocity cluring floods is likely to begreatesÈ. È1any cross sections along a strean aretransitional between pools and riffles. In general¡the scour iniluceil by a bridge will be greater atpools or pool-like cross sections than at riffles orriffle-like cross sections.

6. Shift of the thalweg with increase in stageis a significant factor in bridge design, not onlyfor estirnation of the Point of maximum bed scour(and bank erosion) but also for alignment of pierswith flood f1ow. Thalweg stability is relateil tochannel stabilíty and to stream typer and can be

assessed fro¡n aerial photographs.

ÀPPENDIX: CHEeKLIST OF GEOùIORPHIC FACIIORS

ceotnorphic factors relevant to site selection andbridge design are listed below ín the form of ques-tions. Exact answers to these questions can rarelybe obtainecl' but even probable answers are worthconsider ing.

selection of crossíng site

site on a Nonsinuous Reach

1. rs site at a pool, riffle (crossover)' ortransition section?

2. Are alternate bars visible at low streamstage?

3. If midchannel bars are present' ldhât would be

the effect of their nigration through the bridgewaterway?

4. Is cutoff imninent at a¿ljacent meanders?

site at a Meander

1. what has been the rate and mocle of migrationof the rneander?

2. whaÈ is its probable future behavior' asbased on the past?

3. Is site at pool, riffle (crossover), or tran-sition section?

4. Is cutoff of the neander, or of adjacent me-anders' probable during the life span of the bridge?

Design of Bridge

Piers on Flood Plain or A¿ljacent to Channel

1. Is the channel nigration rate sufficient toovertake piers ¿luring the 1ífe span of the bridge?

Piers ín Channel

1. For pier orientation, what is probable posi-tion of thalweg at design flood?

2. For scour estination, Idhat is probable bedforn height at ilesign flood?

3. For scour estimationr what is natural neanbed scour at design flood?

4. For scour estítnation, what is lortest undis-turbed streambed elevation at or near the crossings ite?

5. Does the strea¡n have an unstable thalvreg thathas shifteil vtith tine?

6. rs there evidence of recent channel degraila-tion?

7. Are any works of man in prospect that areIikety to induce degradatlon or bank erosion?

I71

ACKNOWÍ,EDGMENT

This paper was prepared under an interagency agree-ment between the Federal Highway ÀdmÍnistration andthe U.S. Geological Survey. Iquch of the informationrelating to hydraulic problerns at specific sites hasbeen obtaíned through the generous cooperation ofengineers in state highway agencies.

REFERENCES

1. F.M. Chang. A Statistical Summary Õf the Causeand cost of Briclge Failures. Final RePort.FHWA' U.S. Departrnent of Transportation, 1973.

2. J.c. Brice and J.c. Blodgett. Counterrneasuresfor Hydraulic Problems at Bridges. vo1. 1,Analysis and Assessment. Report FIIWA-RD-78-162; Vol.2, Case Histories for Sites 1-283,Report FIIWA-RD-78-163. FIIWA, U.S. DePartmentof Transportationr 1978.

3. J.C. Brice. Stream Channel Stability Assess-ment. Report FHWA/RD-82/021. FHWA' U.S. De-partnent of Transportationr 1982.

4. Scour at Bridge waterways. NCHRP Synthesis ofHighway Practice 5, HRBr National ResearchCouncil, washington. D.C., 1970.

5. P.C. Klingeman. Hydrologic Evaluations inBri¿lge Pier Scour Design. Journal of the Hy-clraulics Division, vol. 99' Anerican Society ofCivil Engineers, 1973' pp. 2175-2184.

6. G. Parker. Stability of the Channel of theMinnesota River near State Bridge No. 93' Min-nesota. Project Report 205, St. Anthony Fa1lsHydraulíc Laboratoryr !4inneapo1is, Èlinn., 1982.

7. c.c. Nanson and E.J. Hickin. Channel Migrationan¿l Incision on the Beatton River. Journal ofHydrautic Engineering' voI. 109r Atnerican So-ciety of Civil Engineers' 1983, pp. 327-337.

8. T.N. Keefer, R.S. McQuivey, ancl D.B. Sinons.fnterím Report on Strean Channet Degradationand Aggradationr Causes and Consequences toHighlrays. Report FHWA/RD-8o/038. FHWA' U.S.Departnent of Transportation, 1980.

9. D.B. Si¡nons and F. Senturk. Sediment TransportTechnology. water Resources Publications' Fortcollins' Co1o., L977' 807 PP.

10. L.w. Yearke. River Erosion due to channel Re-location. Civil Engineering, vol. 41, AmericanSociety of Civil Engineers, 1971, pp. 39-40.

11. C.H. Robbins and A. Sinon. Iqan-rnduced channelAdjustment in Tennessee Streams. wâter Re-source Investigation 82-4098. U.S. GeologícalSurvey' 1982, 129 PP.

f2. C.R. NeiII. River-Bêd Scourr A Review f.orBriilge Engineers. Technical Publication 23,Canadían Good Roads Association' Ottawar can-a¿la, 1964' 37 PP.

13. E.D. Andrews. Scour ancl FilI in a Stream chan-nel, East Fork River, western wyoning. Profes-sional Paper I117. u.S. Geological Survey,1979, 49 pp.

14. S. Leliavsky. Àn Introcluction to Fluvial Hy-draulics. Constable and Conpany, London, 1955,257 PP.

15. C.R. Neill. Guicte to Bridge Hydraulics. uni-versíty of Toronto Press' Toronto' Canadât1973, 191 PP.

Pubtícat¡on of this pøper sponsoted by Committee on Hydtology, Hydraulics

ønd lløter Qualíty.