Concave-hank henches oii the Squamish Kiver, British Colunibia, Canada
19c~prrr-tl~c~lîf
c ? f ' ( l c ~ o g ~ . ~ rSp ih~y ,~ i oFi-~lser
n
I ~ n i i ~ r ~ . B~ iIt Iy .~ I IB.C.,
L ~ ~( ' (~~ I,I ( I ( I ( ~1'5A ISh
Keceived July 18, 1978
Kevision uccepted 4 ~ i g ~ i17,
\ t 1978
'H'tie fosrn, sediinenth, anai possible origin of two ccpnc;ive-bank benches on the Squamish Kiver,
Bi-itish C:ol~imbin, are bi-iefiy described. They coritain the oiily fine-gruiried within-channe1
tleposits in this gruvel-bed stre:inn :armd are genei-nlly siniilai. to features on the Bai-won River in
Aresti-alia.
One coric:ive-t9;ink bencli on the Sq~i:in~ish
Rivei- is :i ni;at~ir-efeotui-e and the othei- is cui-1-ently
being foi-nied. Evidence fi-c~rnthe lattes exuriiple indicates that the feat~ires,like those oni the
B:ii.won Rivei-, 211-edcposited in the sepnriition zones developed at the concave bank of vei-y sh;ii-g
bends. Sedinlents appeai-to be supplied frorn s~isgensionsin dissipziting vortices ndvected into the
sep;ii.:ition zone. Two processes of vortex ~idvection;ire briefly discussed.
On cieci-it hr.ikvenierit Iii forme, les sidiments et l'origine possible cte cieux t7;irics de sive
concave sui- ILL rivière Squaniish eri Colon187ie-IIi-it~inr1iq~ie.
[Iscontiennent les seuls rmi:itériaux fins
dans les dépôts de chcriut de ce cours d ' e a ~ iii l i t de gravier et ont gén6r:ileinent des cai.acté.i-isticlues serriblables ii celles de la rivière R:irwon en Austraiie.
Un des bancs de rive cc1nc:ive sui. ln s-ii1iki.eSquarnisk est gnrvcnu ii rnntiii-iti aloi-s que I'liiiti-e
est en voie d e fc~rm:ition.I.'ohsei-vation d ~ dei-nier
i
exemple indique (lue ces bancs. corlinle ceux
d e l:i rivièi-e H:irwon, se déposent dans les zones de sepai-ation qui se développent sur I;i i-ive
coiicave cic nik:indi.es trks prononcés. Les siciinients semblent &se ;ipportés pal- cles s~aspeiisions
dans des von-tex c8e dissipnticm transportés dans In zone d e séparation. On dici-it br-i;tven~ent(jeux
pi-ocessus d'udvëctior~d e vortex.
[FI'r;iduit pal- le journnll
Ciin. J . Eai-th Sci., 16,200-207 (1979)
Introduction
The termn concave-bank hpench wlis introduced
by Woodyer (1975) to identify fine-gréiined fluvial
material depo\ited at the concave bank of a tightly
cur-ved bend un the Warwon River- in New South
Wales, Au\tralia (\imilar deposits were described
by C'ai-ey (1969) on the Mi\sissippi Miver). He
\peculatecl that these benches were developed in
flow separations in Wood-scoured bank indentations during anore modernte flows. The sediments
foi-nîing concave-bank benches there, are unusually fine gn-ained éind are thought to retlect the low
enei-go/ depositic~nalenvii-onment of the original
\eparation zone (see Taylor clf al. 1971).
The plirpcpse of the prescnt note is to present
additional evidence of the character and oi-igin of
cc~ncave-barikbcnches Bi-oral the Squamish Kiver in
British Columhpica. The two examples to be dexribed :ire particular%y significant for several
relisons. Fir\tly, unlike the cases on the Barwon
Rivei-, these ex;implrs occui' in ii p v e l - b e d channel and constit~atethe onl y wit hin-channe%finegrrained depmits. Secondly, the benchec; are at two
differ-ent stages of development: one is ri mature
feature which is cair-rentIybeing eroded; the othel- is
a normally subaqueous ferat~irewhich is pl-esently
hpeing hi-med.
Field Observations
The developing concave-ban k bench (the ~ i p strcam bench) is located beneath a large cànd wel%
developed separat ion zone lit the concave bank of a
ver-y tightiy crirvecl channe%bend (see Fig. 1). 'l'he
boundary of the bench coincides exactly with that
of the flow-separation envelope. The bealch is
about 400m in length and éivecages 50m in width:
the fo1.n-aof the channe1 cross section suggests that
it represerîts about 4-5 rn of within-channel vertka%
sediment accaimulatiori. 'The surface of the feat~ire
is horizontal and lies 2.5 m benecith the bankf~ill
water s ~ i r h c c .
It was nut possible tc~systeiilatic:illy sarnple
sedin-aent fi-om the upstream bench, but an iareaintegrated surface sedin-aent sample collected at
points in the riear-batik zone yielded the grain-si7e
distribiition shown in Fig. 2. 'The silty sands of the
concave-bank bench markedly contrast with the
material of the srnall point bar on the opposite side
8008-4077/79/0 lOêOB)-04[r;O1.
(01979 National Rewarch Council of ë;inadn/C'onseil national de 1-echerches du Canada
bankfull
n
Fib. 1 . 1,ocntiori 'ind pl:inforrii of two concave-txink henches of the Squamish Rivei-, I3.C'. (NT$ 1 : 50000 ni,ip I efeeerice
C.Bmecik,imai\ River 92 G / 14 east: grid sqiinie XI 19).
of the channel. Sediments thei-e consist of ;i basal
gravcl layes of pcbblci with soiiie cobblcs ovei-lain
by a thin ( I l c m ) eleneer of mai-kecily gsaded
medium \rind. The \and of the point bai-, pi-obably
depositeci fi-om suspension, is nioi-e than three
tirnes as coai-se as the sedirmient tlepositcd on the
concave-bank benc h (see Fig. 2 1.
'I'lie rmlode of bench foi-mation is ccleai-ly visible ad
neai- bankfull flow5. At thc flow hepai-ation botind;il-y the highly shear-ed fjow genei-ates a trail of
voietices whiêh may attain dianieter\ of alimiost
lOm. Many of the\e vol-tices, containing watciclrawn fi-om the mai-gins of the main channcl whei-e
there is a relatively high s~ispeiidcdsedirnent concentration, are advectcd into the sepai-ation tone
by the genei-al fltrw cil-culation at an avc.1-agei-ate of
about 0.3 m/s. Most of the vol-tices entes the \ep~irationzone ilcar-the dowiiçti-eam simit of this large
flow structiii.e and here crcate a high encrgy enviroiiinent in which local a-adiiil flow velocity often
exceed5 l .O ari/s. Once the ewrticcs :ire ti-apped in
the \ep:ir;ition zone, however, they i-apidly diccipate and deposit miich of theii- siispended load on
the concave-bank bcnch. Thei-e is a tendency foithe sepaicition boundary to shift sti-eri~rmwas-d
a i discharge incr-eases, but it i\ sensibly stable ove]- the
obsei.ved range of flows (350-930 1rm3/\).
The general pattei-n of secondai-y cil-c~ilationi-esponsible for the advection of vortices into the cepkiration Lone appears to t->eof two basic type\: helical flow and the flow diviiion proces\ desci-iked by
Carey (19699). Although the helical pattern of secondary flow in the bend is lindoubtedly sti-ong
enotigh (see Hiëkin 1978) to induce soxrme vortices in
the main channcl to penetr-ate the shcar plane and
enter the separation r.one, the phciscs of inost in-
103
C'AN. 9. EARTII SCI. VOL,. 16. 1979
SlLT
;
SAND -t
Grain diameter,mm
Fic;. 2 . Cri-iiin-4ze disti-ibkitions of sedinient on the conc;ive-bank bencheh ;ind point
tense peneti-ation appear to be 1-elatedto flow division at tIie dowiîstream cornei- of the beiîd. Carey
(1969) likened this pi-ocess in the R~li\sissippiKiver
to a jet impinging on a wall and dividing; part of the
flow cntei-s a counter circulation away fi-oin the
main flow. cal-rying some of the shear-line vol-tices
with it.
Advection of vortices into the sepai-ation zone by
thi\ latter process is markedlÿ pei-iodic in the
Sqliatni\h River. Intense advection occurs during
pei-iods of about 30 s in length. Dur-ingthis time the
riegative water surface gradient fi-om the separation
zone to the point of maximum superelevation declines and subsequently incseiises over the next
several minutes uiitil another b~irstof acivection
activity commences. Advection of vortices by the
liielicaI pattern of bend flow appeai-s to dominate
during the inter-vening periods of relative quiescciîce. The cause of the periodic behaviour of the
secondary flow systein in the Squamish Kiver has
not been identified.
The downstream concave-bank bench (see Figs.
1, 3) is about 300 m long and was oi-iginally about
75 m wide at its maximum extent. Recent erosion
t x i i - 4 at
the two sui-vey sites.
hiis i-emoved some 2000niZ of sedinient and red~icedthe maximum width of the bench to about
50 m. 'I'he horizontal s~ii-face
of the feature stands :it
about the bankfull stage and approximately I m
above the noi-mal suiîîmer water s~irface (the
Squarnish Kiver is a pi-oglacial streaim with a relatively high and sustained summer flow).
Although a British Columbia government aerial
photogi-aph (HC 5105-203) taken in 1964 shows the
bank indentation whei-e the berich has been deposited (see Fig. l). it does iîot show the concave-bank bench in its present form. There are,
nevertheless, 5everal snîall bars there indicating
that sediment accumulation was occurring at that
time. These facts suggest that the downstreain
concave-bank bench is a 1-elatively recent feature
which lcirgely has formed during the last 15 yeai-S.
This interpretation is further supported by the pattern of vegetation on the bench; most of the feature
is covered with horsetail (Eqrlisetrrtrl spp.) and is
backed by a bordering grove of alder (Alnus rrrbrcr
Bong.) none of which exceeds 20 years in age.
There is presently no separation zone here over
the observeci range of flows. Local velocities are
NOTE
203
Fic;. 3. The ~ur.fiicrcarid ci-odingf~>i.t.
edge of the Jownsti-eani concave-barak hench.
i-clatively lob, howevea-, and a weak separatia~n
zone OCCLBPB~
j u s t upstream of the feature (Hickits
1978).
'I'ht. ritreizmvvard edge of the beassh, a B rn high
vertical filce, is in places ctir-rently being eroded by
d~impingand gullying (see Fig. 3) :kt less tkan
bankfrrll stages. 'The fore edge of the hench ksehind
the ai-ea of maaimains bench a-etreat, however, appears now to be relcativelq. stiable; horsetail covers
mmt of the f0~f2-edgesedirnents in this sirea. %t
Lippears thak the positiesrà of the hench foa-e edge
fluctuates by lateral erosion and efepositia~nabout
some mean stre;~mw;~rdh$~unCB;~ry.
3'h;it is, it iu
pi-obably a long terln ( 5 yeor-s?')equilibi-i~innfeature
wkish at aisy point in tirne méay be undergninig extension by depositioin o r coiatsactia~nby erosion,
Longer- ter-m stability is cle;ai-iy dependent on the
general pattern of river naigraticrn.
The sedirnents of the dapwnsti-eam concave-hank
kench consist c~fhorizontéilly bedded iipward-fining
silty sands. Inciividraa1 strcata, i-angingfrûm 5 1nm to
4 clsa in thicknesî psobably coi-!-espond with single
flc~od eveimts. Area-integl-ated sediment sramples
from basal scctisi~s4 1 rn belsw the bench siarfiice)
and from surfi,ce locations yieldecl the grain-size
distributions hhcbwn in Fig. 2. Clearly, the texture of
the ~naten-kal
in tkis concave-baa~kbench is about the
saine as that in the iapstream case. Also, like the
eppstrearn concave-bank bench, the downstream
bench consist in of sedimentç which are very much
finer- and better soi-ked thün the cobble aprcsn and
the 3Ocm of ove!-lying ~ i i hrrning
d
tane opposite
point bas QseeFig. 2 for the sand grain-size distributiomtb.
Conêludia~gRemarks
'I'he êowcéave-bank benches on the Squarnish
River are vei-y bimilaa- fo that desci-ibed by
Vvroodq.ei-(1975) e~rnthe Biinva~nRiver. The rn;%jc~=
difference betcveen thc twc~ex;l~nples is thiat the
Squiimish River benaches crcrnsist of fine band and
silt, while the Bai-~vamRiver bench ior co~~kpessed
a~f
silt and clay. 7'his difference is sirnply a respofnse to
sedimea~tstippiy: thei-e is very llttle cIa5 arnd silt
tninspoa-ted by the Sqiaamish River-.
It seemo, likelq. that these types of features wIiI
deveisp in iiny sE.aani~el
wkere the bend crtrvabur-e Be,
such t hrat fiow jeparation occur-s altirng the ccsrmcüve
Esank.
Aeknowledgement
1 wsuld like to gratefully acknowledge thcat tkis
stiady forrns part o f a project ft'sanded by the Natui4aH
Sciences and Engineeria-eg Wesearch Corancil of'
Canada.
C'ARLY, W. C. 1949 Foi-n~~ition
of îlcaodplain land\. Pr-oceedings
of the An~ericarnSociety of C'ivif Engineet\. MY3, 95. pp.
981-994.
Hic k i u , 1:. J. 1978. Mean Aow-~t~.~~cfiire
111 rne~indcr-4
of the
Squami\h Riktei-, Rriti\h b'olunabia. Canadian Jo~ernztlof Eai-tli
Science\, 15, pp. 1833- 1849
I'411 O R . G., C'ROOK, K. 4. W . , nnd W O O I > Y ~
KM
. Il.,
. 1971.
L!p\ti-easn-dipping foi-ewt cr-054-\tr.atification.or-igin a n d implic:itioile\ foi- püleoilope ,tnaly\e\. Jour-nai of Sedimeritary
Peti-ology, 41, pp 578-5XI
W o o ~ w.
u ~K . LZ. 1945. Concrave-kairk henche\ or1 the Barwon
River, n ' e ~S o u t h C%'ale\.Au\tr.alian Gcographer-. 13. pp.
36-40.