The Formation and Future of the
A Geolo{Jist Auswers
Ouestious about Saud.
Storms & Ul'iug bu the Sea
John B. Anderson
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Coastal Processes
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\ -ir I\' IIf I lit' brae/!
du r i ng lull' tidf'.
What hoppells to the 5(1/,,1 that erodes ji"Ol1l
tvave MoHon and Sand
Ollr
beaches?
;\1O\-('II1(,'l1t
As waves approach the shore. they begin 10 drag on the seafloor.
which cau ses th em to become steeper and break . A breaking wave is litera lly su rface water that has overrun water ncar the bottom. The location
where waves break is called the brea ker zone. Its location varies a s wave
height varies. La ndward of the breaker zone is the surf zone. whe re extreme tu rbulence fro m breaking waves lifts sa nd off the seabed. The area
where waves swash back an d forth onto the beach is called the swash
zone (fig. 1.1).
.1
CH .\I' T lK
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1
Crest
~---,..--.....,.-
C )
~
/ '-'
VA
~
/'"
-Tr o ug h
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- -------
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I~
Breaker Zone
Surf
(
Zon e
~
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-
->
' -- - .......... Sw ash
Zon e
Sa nd
Sand and mud
--------- -=-=
M ud
fir/ urI'
1 .1 . tr/l('11 IVill'I' S
IlppmncJl O,e beach (hey
" f COI ll e ste eper (IS the y
tlm!T011 til l'
bottom .
l::1'l'lIfu lIlI J/l!lr SII r! ace
water Il l'c r r ll/lS fil e
bouom water. fmd die
WllI'" brto ks••b dlt'
IIr iglll oj ill t lI'are ( HJ
IlIIII
III ,. 1I'II.f1llr of llle
\\'lll'I" ,
1\'(11 '"
LJ ; IICn'IlSes . Ilu'
brrak.'i1 {a n lle r
[mmshorr.
Land wa rd of th e swash zone is the ma in part of the beach. Coastal gcologlsts refer to this as the storm beach beca use it becomes a swash zone
duri ng storms and extremely high tides. Sa nd that is moved la ndwa rd
by swash an d wt nd piles up at the la ndward side of the beach to create a
beach ridge. which in cludes du nes. This is the nat ural ha rrier th at protects the coast from storm wash-over; As the bea ch retreats landwa rd. the
du ne line moves wit h it. Unfort u na tely. construction on the beach has too
often prevented th is na tu ral movement of the dune line. so it is becom ing
a ra rity a long our coa st.
The motion ohvutcr wit hi n a wave is orbita l. or circula r. and t he upward component of wave motion lifts sa nd gra ins off the seabed eac h ti me
a wuvc passes. This is why waves arc so effi cient in transport ing sand . The
depth of wave orbita l motion increases with wave height and length (the
dista nce between wave crests). mean ing th at la rger. longer waves. in essence. dig deeper. transportin g sa nd from greater depths tha n do smalle r
waves. As waves approa ch the beach they lift sand grai ns off the bottom
and toward shore.Hyou stand in the breake r zone you ca n feel this motion
of sa nd from the breaking wave. The sa nd th at is tra nsport ed shorewa rd
is deposited as a sa ndbar. As the bar grows. waves brea k over it. so the
brea ker zone becomes more con fi ned. The location ofthe brea ker zone depends upon the wave length an d height on any given day. When on shore
wind s (those th at move from water toward shore) arc strong, large waves
eOAs 'rAt
PROCESSES
IJ
n., M ndil'S flrmllld Sail
tuh
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1"'1'1/ Irr' I.rlliml 1111 IIIf'
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IIII'
West of San I.uis Pass.•don g Follcts Isla nd an d arou nd Su r fside Beach .
tilt' coast has it low profile: Su r fside has an avera ge elevation of -I feet. The
moo-t sign ifica n t loss uf sa nd from the beach occurs du ring sto r ms when
...utd is wa sh ed across th e beac h into the wet lands a nd hark-harrier bays.
Eventually, th is sa nd w ill be: reclaimed and moved ba rk into t he lon g.Jxlrl' tra ns port sys tem as the sho re line adva nces landwa rd. ;\ Il'a nwh ile.
.he and helps to ma int ai n wetlands by providing a fra mework on wh ich
a r h vegetat ion grows .
:s
IH
C H :\P T I:: R
1
th at th e delta wa s a prominent lobe. marked by shallow depths. offsh ore
of t he river mout h. With in a few decades after th e river was diverted. the
lobe was gone. h avin g been eroded by stor m waves. and a new delta lobe
wa s formed to the southwest. offshore of th e new Brazos River mou th .
This is one of several examples of the fact that sediment th at occurs offshore in water depth s of up to 30 feet is eventually exhum ed as the shoreline advances. Much of the sand that is exhumed is delivered back into th e
lon gshore tran sport system. In this cascothe delta was removed in about
two decades. An oth er exa mple is the erosion oft he old Bolivar Roads ebb
tida l delta that occurred after the con struction of the ship chan nel and
jetties. wh ich blocked sand supply to the tidal delta (fig. 1.5). The lesson
here is that it doesn't take long to sec the impact of h uma n tamperi ng
with th e coas tal system.
NO\\' let's get hack to th e quest ion of where the sand eroded from area
bea ches goes. Most of the sa nd that erodes from Bolivar Pen insula is
trapped on the beach eas t of th e North Jetty. The sa nd eroded from Galveston Island ends up in the San Lui s Pass tidal delta . Sand eroded from Follets Island was hes over this na rrow ba rrier into wetla nds a nd bays. A
similar fate awaits sands removed fro m bea ches betw een th e Freeport
jetties and the Brazos Delta . During major storms . sa nd can be removed
fro m the beach a nd transported far offsho re and deposited as storm beds.
However. sedime nt cores ta ken offsh ore have ra rely sam pled stor m beds.
so t his mechanism of sand remova l from the coast docs not appear to be
significant.
As for what happens to the area imm ediately offshore of the beach as
the shoreline moves la ndward. the answer is pretty well kn own. As the
sho reline retr eats landward. the shoreface retre ats wlth it. leaving litt le
rema ining shoreface deposits on the inner contine ntal shelf. Sand is confined to t he area closes t to shore. generally within a kilometer (0.6 miles)
of shore and wlthln the steepest. most dyna mic portion of th e shoreface .
The ra te of sho reface movemen t is more or less equal to changes that occur on the beach. If sand is taken from the shoreface for beach nou rish ment. an unstable offshore profile is created and sand will be delivered
from the no urished beach back to the sh oreface to reestab lish the equili brium profile.
C ll .\ ST.\1.
I' R tl C I: S S E S
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rx frl' /III' 1r1~/I" IiII' ,II,1'
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C ll :\ P T F: R 1
't\\'o sal/dlmrs, 0/11" nrnr
JIll' IJI'Ilcfl lind IlII mil a
ba r 1I'111'r/' t Ill'
brt'f/ld llff.
\\'tlI'/' .~
are
break fart her offshore to form sandba rs farth er offshore. Surf fis herm en
are well aware th at there is more than one sandbar orr the beach , each
separated by depressions th at run parallel to the coast. The depressions.
or ru nn els. are where the fi sh tend to congregate.
Sandbars form far from shore durin g strong winds, when waves arc
larger. These offshore bars tend to endure, because when winds diminish. the waves arc too small to have much inlluence on the bars. On an y
given day there may be as many as three sandbars and associated breake r
zones. Over periods of weeks. as the weat her and wave conditions va ry.
sandbars migrate landward and old bars are remolded into new ones. If
you were to measu re the seafloo r depth from the beach to <I lew hu ndred
yar ds offshore on a regula r basis. you would observe constant change in
COA S T A l. P R O C E S S E S
Figure 1.2. Sed imr nr
{"Ores ev l ln /r d of'fstio re
of G(I1\'F.~I(Jl ( h /rrllr/
Sture Park shnw til('
dl(lll gr [nun .WII" / co
mud iI/lUI olb lw re direrrtou. Hll'!lllhfl ~ "(I\\,
fl W llllild Ii " IIrt1 11ye }
new buri t s sllIId II~' II
rt sult vf rOIl,~ l/I l rnrm t.
Til t plwro.'lmp" ~ (III ,lit
the location of runnels and bars. This high ly dynamic near-shore zone
is referred to as th e upper shoreface. This part of the coast is u nder the
constant inllucnce of waves. and only sand is deposited here. It is also the
steepest portion of the shoreface and. along the upper Texas coast. occurs
between the sho reline and about 15 to 18 feet 15 to b meters) water depth .
Seaward of the wave-dominated upper shoreface is a zone that is inOucnced by waves only duri ng stor ms. Du rin g storms. sand is delivered
to this zone. the lower shoreface. from onshore. Durin g fair weather conditions. the seafloo r is more quiescent and mud is deposited. With time.
sand mixes 'with mud. Along the upper Texas coast. the lower shoreface
tends seaward of the upper shoreface to water depths of about 24 to
30 feet IS to 10 meters) (fi g. 1.2). The depth s of these zones and their sediment types differ largely according to differences in th e bottom profile
Uxtl:! the coast. Seaward of the lower shoreface is the continen ta l shelf.
For those of us concern ed about coast al erosion and beach nou rishL it is importan t to know where within the coas tal setti ng sand is
g transported by waves and coastal curr ents. The term closure deptll
com mon ly used to describe the maximu m depth of sand transport. One
. 1O estimate closure depth is to use such factors as wave length and
ght. However. in my opinion. the best way to delermine the closur e
is to use the actu al distribut ion of sand in the shoreface and to obthe migration of sand bar s over time.
r ighr a rt !"Xllmp ll's of
sr di mr lll corrs Il .~ f"d 10
construct il lis pm/He.
Thl' li gh fer w ries u ri"
.~m J(f. 1I'Ilidl i.~ \'rrllfill('
g m i lled (!lId Illl.\'ll i/ubk
for bellel! nourishment.
The dllrk er 11l!JI' fJi ur r
",l licli fu rrl lr r
1i",l fs ,Ill' 111/l/Ury of rill'
srdi mtlll fo r IWfI('ll nour-
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is/lIIlrllt . \ /,," ifit tl f rom
Rodri!/llt : .l'lIss rll. (!lId
.4 I1dtr.w II 2 fJll J.
6
'fl it' f O(l SW.l \ \ 'Il U'rs oj
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U II/I III be IIIudtln III l1c l l
oftli,. t i lllt bt'Cflll.~ f'
tar!l"" Wa\' t S rt's ll ·~ IJrll d
"IUd 1I1l11 rests 0 11 , b,.
scaJlollr ill sf offs ho re.
Th l .~ sl'lli mrlll Is IIII'll
IlJOI't'l 1ulml .r/s lw f l' bv
coast fl l f"lI r rrllts.
<.- H ,\ PTER
1
Lon gsh or e Cu rrents
The consta nt movement of waves toward t he beach ca uses water to
pile up at t he coast. and that water ultimately has to move back offshore.
Hut before it docs. the water may How para llel to the shore as a longshore
cur rent. This happens when wa ves app roach the beach from an an gle.
Longs hore cu rrents can be qu ite stro ng if the winds arc stro ng and the
wave s app roach from a sha rp a ngle. These longshore currents accelerate
us more water is continuo usly added to the coastal current.
\ Vc all have experie nce d longshore currents. Recall the time you were
d rifting on your float and suddenly realized that you had dri fted down
the coa st some distance fro m where you entered the 'w ater. You 'were
taken there by longshore cu rrents. The experience could have been mu ch
worse . beca use ultimately longs hore currents veer olTshore as waves move
more wat er ons hore. This olTsho re flow occurs either as discrete currents
kn own as rip currents or as a more disper sed now com monly referred to
as und er tow. These offshore-di rected currents pose a danger to swim-
CO A S T ,' !.
I' RO C f. S S~: S
I. j . I r ll l'l'! IIl11' Ilpp n J/ld l
"" 1111' fl/ ret'
\I ' llt r r 1"'~ 'lfJ rr 11111' 1/ Im' fI.\ lw rl'
I'l!lUU
I lit' CUlI~ t f rtJlII till
fU " ru dll er It/u rl-d lUrr r /lrn'/II .\ ,
Beac h
- -
Lc...lt~"'~ l "ulTt'nl
~.---
Rip l"ulTt'll l
mers. who often panic when th ey feel th em selves being pulled offsho re.
The Irick is to rema in ca lm. because the current will dlsslpate when it
flows a shor t dista nce offshore a nd its water is displaced .
Alon g th e up per Texa s coas t. prevai ling win ds a rc from th e southeast. Thi s creates waves that app roach th e coast from th e southea st and
lon gshore currents that flow fro m ea st to west. These longshore currents
tr an sport sa nd. a nd th e consta nt movem ent of sa nd a long the coa st is
ca lled longsh ore tran sport. The qua nt ity of sa nd moved wit hin th e
longsh ore tra nsport system is evident where ver lon gsh ore currents a re
blocked by ma n-made struct ures, For exa mple. at the west end of Bolivar
Peninsul a. sa nd t ran sport ed to t he west by prevailin g longsho re currents
is trapped by th e Nort h Jett y. whi ch extends 4.) miles offsho re. It is estimated that the [etty has trapped lX million cubk ya rds of sa nd since it
wa s constructed . Anot her way to visualize the volume of sa nd movin g
with in th e lon gshore tra n spo rt system is to imagine dump t ru cks truvdi ng west a long Galveston Isla nd filled with sa nd. A new load of sa nd
wou ld pass every tw ent y minutes.
From November th rough Ma rch . low-pressure weat her systems
rfronts! frequently move across th e upper Texas coast. When th ey do. we
exper ience st rong offshore wi nds (those t hat blow from th e shore out to
th e seal. These winds da mpen waves. and longshor e curre nt s arc wea k.
/
s
Ctl .\P TER
I
nyurt' I A . .H oIlY
Bol im r PI'IIimll lll.l1l t'
1lIIl!I~lw rl.' Ilrifl S!j Sfl' lII
drlil'as S(lIId fOlI 'llnl 80 -
ti var R l,"d_~ . ,III' l Ja B fll'1\\'1'1'11 (;a"' t'sfO lI ls/lllld
(Ill d fill' P(,II jll ,~ lI ll/ . R ill
IIt'f orr " II' ,m il d " acllt's
I III' inlrt i/ is MOI'kl'lI by
lilt' ,\ 'or d l /1'1111. If !11l1l
IIfi\'1' W til l' urea ju sl
l"aSI of tltl" it'1l.IJ. !JOII can
SCi' f i ll' rx/t'II_~iI'I' !J('lldJ
111lI1 /1l1 .~fllrlllt'dfrOIll
.~ a/lll
truPI,cd on tue
ofdl t'
iet t 1/. ,\ 01 1' r!ll' lor lll/Oll
of lil t' J S in shoreline
(dll,~ / II' d /fur} IJl'f on' t Ill'
jn/ y uns rOIl .~ / rllclt'd.
No/I' also th l' ttneo rfen ~uIJ_drifr8 .~ il/t'
IlIrt'.~ 11/111 t'xlt' llI l 1I10ng
l lll' p e n i ' I.~ 1l 111.
These
1>t'ilrl, ridgt's•.~ t'pll rfl ud
hy ilel'res.dolls n r S\\'l'lls ,
[ornird " ' I l t'll dl t' peninsuirt II'IIS UfO wi tI.IJ 1111111-
rally . J'lw w f rolll rllt'
{}"li , Geologiral SlIrI'eli.
Sh ifling nort herly and southeasterly winds enh an ce tides. and beaches
expand and contract accordingly.
On Galveston Island, as fronts move across the coast and to the east.
winds blow from th e west. and longshore cu rrents 110\\' toward the east
Although th is is not the prevailing longshore cur rent direction. sand is
trapped behind the jetties on the east end of the island. where it remains
because the jetties prevent waves and longshore curr ents from removing
this sand. This is why East Beach is so expansive.
The Bolivar jett ies were constructed to protect ships entering the sh ip
chan nel and to prevent sediment from filling the channel. Before the jet·
ties were construc ted. sand was transported through the Bolivar Roads
in let into Galveston Bay or offshore. where it accu mulated as an exten sive
ba r off the mouth of the inlet. The sand delivered into th e bay accumulated in a sand body that is referred to as a flood tidal delta. because the
5
C O A S T A L Pi!.OCFSS·
. . RS
HYIIU I .1·Alu
ra
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j rldes
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CHA PTER
1
Rork flroills d Ull tXf('lld
offs hort oflIu' Gf/ f\,(..ston
S('f/lI'a ll
ra,'llIrt Sf/ li d
Ihat ",O\'('S 11'11111" lil t
IOllgsllOrt tran sporl
susu",.
:
.-
-: ...
...
COASTAl. PROC r.SSI:::S
11
A u illI l' jell' oJ•lie
S IlII
is delivered there du ring th e rising or 1I00d tide . Pelican Isla nd was
ztnally pan of th e flood tidal della . The offshore sa nd deposit is referred
.. the ebb tidal delta becau se the offs hore-directed or ebb tide delivers
sand to these a reas. After the jett ies were con structed. th e ebb t ida l
was eroded. which is evident in th e d iffe rences in offsho re bath ym water depth ) between the two charts in figu re 1.5. The flood tida l
was mostly bu ried in mud.
JOg Galves ton Island. sa nd is most ly transported to th e west. But th e
~e dr ift ha s been alt ered by rock groin s that extend offshore of the
1,lIis Pass lit/al
dr/lll. PllOw! m m
GIn/II:' XfJlorr r.
11
C I, ,~P T ER
1
Til,. bt'tlc" es ('fU f o!Sm,
I.u h Pt'SS IUII'e tx ptrl,./lr ,.d sif/,lij i rn lll gro\1'II,
(II1f1 rt t UtU III rt'Cf llf
lI fn rs. III fl u' rrcr nt
past IIlt ulul\'t t rodt d
rnpi dill. ThIs nrtn is
pnrt o!tllt tida l ddfa
alld. as such. is sub/rcf
to cons tant d,mlf/t .
seawall. The gro in s were bu ilt to tr ap sa nd moving with in the longshore
drift system an d to slow the rat e of beach erosion. They have a lso slowed
the fat e of sa nd delivery to beaches west of th e Galveston Seawall. Actually. so little sa nd is cu rre ntly moving in th e longshore transport system
th at removing the rock groi ns would ha ve litt le effect on coastal ero sion.
Most of the sand eroded from Galveston beaches is tr a nsported westward a nd ultim ately end s up in th e Sa n Luis Pa ss tida l delta . You can see
t hese sand accumulations from th e bridge at San Luis Pass . Or. if you are
a boater, you may have run aground on one of th ese ba rs . With tim e, th e
tidal inlet and delta migra te towar d the west, the direct ion of longs hore
tra nspor t. Sediment cores collected to th e cast of th e modern tida l delta
within West Bay h ave sam pled sa nd from the former delta that is now
buried beneath bay mud.
San Luis Pass is one of the few remai ning natu ral tidal inlet s on the
Texas coast. As such, it ha s a history of west wa rd migration that occurs
a s more a nd more sa nd is delivered there by th e westward longshore
transport system. The beach es adjacent to the ebb tida l delta have a histor y of con stant fluctuation a s the tida l delta a lternately shifts la ndwa rd
a nd seawa rd, As the deep tida l in let m igrates towa rd th e west. it undermines hou ses.
eOAs 'rAt
PROCESSES
IJ
n., M ndil'S flrmllld Sail
tuh
"'''~ I
err am ollg 11,1'
rtlpid l" dlarlgillg
rfl U
TI'.fflS I.,.tld,I' S " I'CI/ll ~1'
~f1 "tJ
Is cml.\ / t11 I/ II1 ~ " ifl­
ill!1f m m IIII' I.,..,dl i lllo
"J!.\horl' l>tln .md
l"ic't' \·r nll. llrrl' . lw lt~r.~
/II T n 'll.m r l' hlllml,lrr
lI, rrn I/!, .\ 1/1 illll ~I'II\l 'fl n l
u( ,II I' \II 1'1' : 1>111'. Iwl'l'lfl
1"'1'1/ Irr' I.rlliml 1111 IIIf'
rver -.\1111/ JII!! I'M. / Mil f
.Ir/, II .
IIII'
West of San I.uis Pass.•don g Follcts Isla nd an d arou nd Su r fside Beach .
tilt' coast has it low profile: Su r fside has an avera ge elevation of -I feet. The
moo-t sign ifica n t loss uf sa nd from the beach occurs du ring sto r ms when
...utd is wa sh ed across th e beac h into the wet lands a nd hark-harrier bays.
Eventually, th is sa nd w ill be: reclaimed and moved ba rk into t he lon g.Jxlrl' tra ns port sys tem as the sho re line adva nces landwa rd. ;\ Il'a nwh ile.
.he and helps to ma int ai n wetlands by providi ng a fra mework on wh ich
a r h vegetat ion grows .
·,
.\ er illl I'iell' west f~r
lilt' Bm:os Dr/lfillflcr
Tropical SWrm Fnlllf "I' .\,
,\'0 11' /Wll' salld lUIs /11'1'11
wlls/led across 1"1' II/' firll
lire !nHomls ami
What happells to the area immediately offshore
of the beach as the shoreline moves landward?
illl/)
\\'I'llr",ds .
The fact is, the beach is not the only part of the coast that is re treating
landw ard. The shoreface is also retreating. and at the same rate as the
beach. This is because nature works to maintain a constant beach and
shoreface profile as the coast retreats landward. Coastal geologists refer to
this as the equilibrium profi le.
Figure 1.6 shows two of the several geological models for hO\',7 the
shoreface and beach move landward. Note th at a consta nt shoreface pro Hle is maintained. The only exception occurs when the rate of sea level rise
ch an ges or when th e amou nt of material being eroded by waves changes
C UA STAL
I'KO CESSES
15
III us natural sune. file beac"
\\'i1l maillt aill a COI/ Sh t/ ll
proji le as rr rrt rems lalld ward.
H Oll'el 'a , iftl,e bead l ls /l ot allowed to m inral( landll'ard, If
nroll's narrower al/d ns pro]lte
groll's stuprr. That is tl,e case
here , where a bu lk head \l'a ,~
cOllstrucud to stop coasta l
retreat . H u result is /1 morr
ullstable beach , alld 'lie I/Om rs
Oil tl,e waellare hl.q/llil susceprible to SIOrltl llllderClllt l" ".
The srairs II' " d s pl/Ologra ph
" a1'e bU/l Im derCU l by e rosloll,
leal,j" g them s"spn lded one
foor abol'e rhe beach ,
TIl ls lJ/lOtograpl , il lustrates rile degree ro
11'111("/1 rll(' head l proji le
Is low ered witl! rime.
111 Oils cus e, tI,C mctal
Iw lk/wnd is fl p]JroxlIIIfl lf'! y -I f ed liig ll. so
til{' IWlld,proji le lias
bc(',, /owerl'd tliat uiucti.
lVitl' li me, or du ri llil a
IUlljor storlll. ti,e hul kIWll d wI/IIII' d(,stroyed,
/lml lll{' 1lI11l/wlllelu'II
Jlroji ll' will h(' rees·lab·
lislll'd, l/Il(ll'rcllf l i" g
tI lt' l lO lISI'Sl,('/,I " d d, e
'IIII"" clld.
Ih
C H :U' TE R
I
nfl llrl' 1.6 a end b. T11t..sr two IIImirls
A.
auempt to caplU rr 11011' IiiI' .\ /lOrl'jrlC/' flllfl lll'a d l lll iu rm l' lalli/ ward jll
rt'.'po rl't' 10 rrIl1Ill '" st'll ltl'rJ r ls r . S OIf"
dln l ill lIIodrl .-l, II consmm .d lOrl'! flCl'
Ilroji /1' is 11I1/i IItil i rleil rtu ri llfl reI rear .
SllIld dUll Is l'rodl"djrorll ri ll' slw rl'jflf"t
Is l' il ll a mO\'l"d IlIIlflwlI ffl o r r"/IIl/illS ill
Jlw 10ByS/lOre t ralls port s!Jstell/. :\ Iollf'l
B i llul>l ra U l> lilt' manner by wlliell 1111"
......
---
....
"),'_.Sou..,. ___
\\ b
,"' leal _ ln.
•
.•.•".
'o,~ .o,. ~ ' ~
01_ __ _ "-
.... '"
•.•.
I
Bonom _
'*
- . .01
Inl"," bottono
- - - - - j- - - - - :-•.
b ·· b '
~:l~~.:,
~=
.nd
_ _~
m. ..n"
sl w r l'! f1Cf" I!lfll't!l la lldward tllld c uts 1I
11 1'11'
prufik . Tltl' .• Ilal/(' ofr/I is proJill'
I s CO lll ro llfl l 1m the ra f, of sea Jr \'1'/
rtse fllltl by .• , d l",,.m slI pp ly . During
n [ast ri se, I l r
/I
sf[frlljk nlJr red UCfioll
1/1 sedllll,./ll s" PI, ly . sll"p.~ tire cr r flud
I" lilt pmJH,. II.(filt' .d w rt'f acl" sllifu
rnp id l .IJ l andwflrd. CIHlllf/f'S ill rill'
olJ. lwff" profilr 1Ii so (It'l 'ur w h erl' 1I'lt\'I' S
,. lJfOW IUr d iff t'tl'n l su ll straUs rlw f a re
rill"' r rlls /a or hnrde r 1Il rrnde, ,\ lot! I-
B.
.~~~~
Slow
R l se ------.~_ ,
"" ......._~ ~
\
\
/ ~~
Fast Rise
-- -- -- -- - - - -~
Slow Rise
ji l,tl.fr(l'" I:Irlll '" 196 2 alit! Sll'lfl 19 7:;,
rn pt'Clh'l' l.lt.
significantly. Sand that is eroded from the beach a nd upper shoreface
moves landward or rema ins in the longshore tran sport system, Th is is
why it makes litt le sense to dredge sand from the shoreface to nourish a
beach. Doing so increases the slope oft he shoreface. creati ng ..I ll un stable
profil e that must be compensated for by the offshore movement of sediment un til the sta ble prolile is reestablished.
As the shoreface migrates landward, an erosion surface is left behind
on th e shelf. Compare the models in figur e 1.6 with the actual profile
shown in figure 1.2. Note that at a distan ce of4 kilometers from the beach,
the sandy sediments tha t composed the beach an d inner shoreface have
been entirely eroded, almost as though a hu ge bulldozer had plowed its
way landward, removing anything above the base of the shoreface. That
is exactly what storm waves do to the shoreface and beach. This process
is called shoreface ravlnement. and it produces a su rface where marine
mud rests directly a ll deposits th at were laid down many thousands of
COASTAl. l'R O C F. S SE S
:;....
Ii
l'i llurt' 1, 7. T/u' st'
clw rls s uo w du' 8ro:os
1)1'1/0 b~forr mill flfrrr
..
IIII' 8ra:os Ri1'~r
ll'O S
tli l'l'rCt'd i ll 192 9. SoCt'
l ilt/Ill prmnlnr nt IIr,Ita
(.~ I/(/ llo w
--
an'a) /'xi sted
oJ]slllJre oli/If' rh'l' r brfort' 1111'IIS dh·uIt'd. Sou
ulse dl~ \\'~sl "'tlrd s/df!
ofdl ~ dr lt tl IIf lr r till'
rtver diversion. w/lic ll
i",lirtllr s d WI IIII' stm d
/ iwi was rrod/'rl [rom til l.'
ol d IIr,Ita l\'n.~ r/'t1/'jJ(Jsitr rl
ill tlJ/' new ddw .
--,_.. ....
•
...
•
'0
~
--------
wars ago in th e Pleistocene. Th is surface is called the sh ore face ravinet surface. OIT the west Flor ida a nd Ala ba m a coa st s. sa nd is so plentlthat the shoreface profile does not erode below the level of' these sa nds.
lis why Floridia ns have been a ble to nourish their beaches with sa n d
the area directly offshore of t he shoreface.
Based on the models shown in llgu re 1.6. any part of t he coa stline th at
. ua ted above the shore face ravinem en t su rface is desti ne d to complete
ruction a s th e sh orelin e advances lan dwa rd. Sed iment cor es fro m offcon fir m these m odels (fi g. 1.1 ). 1-10st of Ga lveston Isla nd and Bolivar
Pm.insu la are rest ing a bove th is surface an d will be destroyed in com in g
uries. Th is is a lso w hy there is litt le in the way of an offshore re cord
old ba rrier islands that existed on the shelf prior to 5.50 0 years ago .
.. we re removed by sho reface ruvl nem ent . Sa nd banks are an cxcepc those arc discu ssed in cha pter 2 .
One of the most im press ive ch aracteris tics of th e n ort h Texa s coast is
rapi d rate at which sho reface ruvin ement occurs. For exam ple. fig1,7 shows old bathymetric m a ps for th e a rea offshore of'Sur fslde Beach.
the Brazos Delta wa s loc ated before the rtver diverslon of 1929. Note
44
HY lIrrl.U•. T"i s sf'l of
block d if/!J m", .~ S" 111 mari:rs d lr r l'(lllIfifJ/l of
tl,r Ilppr r Tnm s coast.
S ou d lfil r lrl'fII ;o rlS e rr
II;gll ly r :Cfl!1!Jrra frd.
Cou rUS~1
of K. ,\I i1/ikrn.
CH A P T ER
1
rn
t
F.VOLtTTtON OF TIlE .\10 0 ER N CO /\S T /\ !. SYST F.:\l
45