Hinh Thai Bo Bien

7/24/2019 Hinh Thai Bo Bien

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TRNGI HC THY LIDn Nng cao Nng lc o to

Ngnh KThut BBin ti Trng HTL

ThS. Trn Thanh Tngi hc Thy li, H Ni, Vit Nam

TS. Jan van de Graaffi hc kthut Delft, TU Delft, H Lan

H ni 4-2006

HHHNNNHHHTTTHHHIII BBBBBBIII NNN


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LI NIU

Vic bin son tp bi ging Hnh thi bbin l mt hot ng nm trong khun

khDn Nng cao nng lc o to ngnh Kthut BBin ti Trng i hcThy li do Chnh phH Lan ti tr, vi mc tiu cung cp nhng kin thc cbnvhnh thi bbin v kthut bbin cho sinh vin theo hc ngnh Kthut bbinti Trng i hc Thy li.

Tp bi ging gm 7 chng c chia lm 2 phn, phn 1 cung cp cho ngi hcnhng kin thc csvhnh thi bbin nhcc khi nim, thut ngsdng tronghnh thi bbin, cc hin tng din bin bbin do tc ng ca sng, mc nc,dng chy vng ven bin, cc kin thc cbn lin quan ti vn chuyn bn ct

bbinv.v... Phn 2 strnh by su hn vshnh thnh dng chy vng ven b,cc tnh ton vn chuyn bn ct v din bin bbin, cch m hnh ha bi bin vng bcng nhcc phng php o c mt ct ngang bi bin. Mt snguynnhn gy xi lbbin v cc gii php n nh bbin cng sc gii thiu

phn 2.Tp bi ging c xy dng trn cstham kho tcun Kthut bbin ca

Van der Velden l tp bi ging hin ang sdng ti Trng Kthut Delft, H Lan,v tSTay Kthut BBin (Coastal Engineering Manual, 2002) ca Thp Kthut thuc Qun i M.

Tc gixin chn thnh cm n Gio sMarcel Stive, Gio s. Kee dAngremond c v cho cc kin ng gp qu bu cho tp bi ging ny. Tc gicng xin

by tlng bit n ti Tin s. Jan van de Graaff, ngi khuyn khch, hng dnv tho lun tng ni dung trong tp bi ging ny cng tc gi.

Sau cng, tc gixin trn trng cm n cc cn bPhng Hp tc quc t, i hccng nghDelft (CICAT), vn phng Dn H Lan ti Trng i hc Thy li c nhng gip qu bu v hiu qugip tc gihon thnh tp bi ging ny.

Delft, thng 4 nm 2006

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MC LC

LI NIU i

MC LC ii

CHNG 1 GII THIU CHUNG

1.1 MC TIU, I TNG CA MN HC.......................................................................................1

1.2 CC THUT NGLIN QUAN TI HNH THI BBIN .........................................................2

1.3 KTHUT BBIN, HNH THI BBIN V QU TRNH DIN BIN BBIN..............4

QUTRNHDINBINBBIN......... ................................................................ .................................6

1.4 LCH SV XU THPHT TRIN..................................................................................................8

LCHSPHTTRINCANGNHKTHUTBBIN.......................................... ......................8XUTHPHTTRINTRONGTNGLAI ......................................................... ................................9

1.5 MT SKHI NIM CBN VVNG VEN BIN ......................................................... .........10

1.6 C IM A HNH BBIN VIT NAM..................................................................................13

CIMAHNHVNGBBINMINBC............................................................................14 CIMAHNHVNGBBINMINTRUNG............................................................ ..........15CIMAHNHVNGBBINMINNAM........................................................................ ...16

CHNG 2 SNG, MC NC V DNG CHY

2.1 GII THIU..........................................................................................................................................18

2.2 L THUYT SNG TUYN TNH CA AIRY...............................................................................18CCGIITHITCBNVCTRNGSNG...........................................................................19 NNGLNGSNG............................................................................................................................23 PHMVIPDNGCALTHUYTSNG....................................................................................25

..............................................................262.3 HIN TNG TRUYN SNG V BIN DNG SNG

HINTNGTRUYNSNGVNGNCSU ................................................................ ........26SBINDNGSNGGNB .......................................................................................................29HINTNGKHCXSNGVNHIUXSNG......................................................................31

2.5 THY TRIU V SDAO NG MC NC ........................................................ ....................36

A-THYTRIUVDNGTRIU.......................................................................................................37

NGUN GC THY TRIU...............................................................................................................37PHN TCH V DBO THY TRIU............................................................................................40 DNG TRIU......................................................................................................................................41

B-CHTRIUBBINVITNAM............................................................. ..............................45C-NCDNGBBINVITNAM ................................................................ ..............................46

NC DNG DO GI MA..............................................................................................................47NC DNG DO BO.......................................................................................................................47CIM NC DNG DC BBIN VIT NAM ............................................................ ..........49

D-SBINIMCNCTRONGTHIONDI...................................................................52 SDAONG MC NC DOIU KIN THI TIT ..............................................................52SBINI MC NC BIN........................................................... ............................................53SBINI KH HU TON CU ......................................................... ........................................53

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CHNG 3 NG LC HC BBIN

3.1 SNG VV CC C TRNG SNG V..................................................................................56

CCDNGSNGV ...........................................................................................................................56 ...............................................................................59 GIIHNSNGVVCHIUCAOSNGV

3.2 DNG CHY HNH THNH DO SNG VNG GN B.........................................................62

DNGTIUVDNGTUNHON..................................................................................................64 CCHHNH THNH DNG TIU................................................................................................65

DNG CHY DC B HNH THNH DO SNG TC DNG THEO HNG XIN GC VINGB .............................................................................................................................................66

TNH TON VN TC DNG CHY DC B.................................................................................67PHN BVN TC DNG CHY DC B....................................................................................71 CC GITHIT CA LONGUET-HIGGINS............................................. ........................................73SO SNH PHN BLU TC DC BL THUYT V THCO.............................................74 PHN BLU TC DC BCA MT PHSNG ......................Error! Bookmark not defined.

PHN BLU TC DC BTRN MT CT Y=(x)2/3

....................Error! Bookmark not defined.DNGCHYDOSNGKTHPVIDAONGMCNCDCB ....................................79CCNHHNGCAGIVTHYTRIUTIDNGCHYVENB ..................................81

CHNG 4 VN CHUYN BN CT BBIN

4.1 MT SKHI NIM CBN VVN CHUYN BN CT ....................................................83

KHINIMCBNVVNCHUYNBNCT...........................................................................83 CCHNHTHCVNCHUYNBNCT.......................................................................................87

4.2 CC C TRNG CA BN CT BBIN ............................................................... ..................88

GIITHIUCHUNG..............................................................................................................................88

THNHPHN

BN

CT.......................................................................................................................88 NGKNHHTBNCT..............................................................................................................89

BININGKNHHTTHEOKHNGGIANVTHIGIAN .............................................92HNHDNG.................................................................. ............................................................... ...........94RNG ............................................................ ................................................................ ....................95THTHYLCCABNCT ....................................................... ............................................96

4.3 TC KHI NG /NG SUT TIP TI HN ............................................................ .........98

4.4 TNG QUAN VVN CHUYN BN CT VEN B....ERROR! BOOKMARK NOT DEFINED.

4.5 VN CHUYN BN CT DC B.................................................................................................103

GIITHIUCHUNG............................................................................................................................103 TNHTONVNCHUYNBNCTDCB...............................................................................105

CNG THC CERC (1966) 105CNG THC BIJKER (1967) 108CNG THC KAMPHUIS (1991) 109

VNCHUYNBNCT"TNH"V"TNGCNG"DCB......................................................112SUTCHUYNBNCTDCBTHCT .................................................................................114

4.6 VN CHUYN BN CT THEO PHNG NGANG .....ERROR! BOOKMARK NOT DEFINED.

GIITHIUCHUNG........................................................................ ERROR!BOOKMARK NOT DEFINED.TRNGTHITHYNGLCHCBNNGOIVNGSNGV .....ERROR!BOOKMARK NOTDEFINED.

DNG CHY TI LP BIN Error! Bookmark not defined.17CC SNG PHI TUYN Error! Bookmark not defined.DNG TRI DO SNG TO NN Error! Bookmark not defined.

DNG CHY RI TRUNG BNH Error! Bookmark not defined.

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VNCHUYNBNCTTHEOPHNGNGANGBNNGOIVNGSNGVERROR!BOOKMARKNOT DEFINED.VNCHUYNBNCTTHEOPHNGNGANGTRONGVNGSNGVERROR!BOOKMARK NOTDEFINED.SHNHTHNHVPHTTRINMTCTNGANGBIBINERROR!BOOKMARK NOT DEFINED.

MHNHVNCHUYNBNCTTHEOPHNGNGANGNGINERROR!BOOKMARK NOTDEFINED.6

CHNG 5 DIN BIN BBIN

5.1 HNH DNG MT CT NGANG BBIN V VN CHUYN BN CT THEO PHNGNGANG......................................................................................... ERROR! BOOKMARK NOT DEFINED.

GIITHIU....................................................................................... ERROR!BOOKMARK NOT DEFINED.CTRNGVTNGQUANHNHDNGBIBIN...........ERROR!BOOKMARK NOT DEFINED.TNGQUANGIADCBIBINVINGKNHHTCT.ERROR!BOOKMARK NOTDEFINED.TNGQUANGIADCBIBINVINNGLNGSNGERROR!BOOKMARK NOT DEFINED.

5.2 MT CT NGANG BI BIN TRNG THI CN BNGERROR! BOOKMARK NOT DEFINED.GIITHIUCHUNG........................................................................ ERROR!BOOKMARK NOT DEFINED.CCPHNGPHPXCNHMTCTNGANGBIBINCNBNGERROR!BOOKMARK NOTDEFINED.CCLCTCDNGVNGVENB.......................................ERROR!BOOKMARK NOT DEFINED.

CC LC GY PH HOI BI BIN ..................................................Error! Bookmark not defined.CC LC C TC DNG THNH TO BI BIN ............................Error! Bookmark not defined.

MHNHMTCTCNBNGXYDNGTRNQUANIMCNBNGLC......... ERROR!BOOKMARK NOT DEFINED.

LC DO TIU TN NNG LNG SNG TRONG 1N VTHTCHError! Bookmark not defined.LC DO STIU TN NNG LNG SNG TRN 1N VDIN TCH.Error! Bookmark notdefined.

LC SINH RA DO NG SUT TIPNG NHT Y................Error! Bookmark not defined.MHNHMTCTCNBNGXYDNGTRNQUANIMVNCHUYNBNCTERROR!BOOKMARK NOT DEFINED.CCDNGMTCTNGANGCNBNGKHC.....................ERROR!BOOKMARK NOT DEFINED.

5.3 STNG TC GIA BI BIN VI SNG V MC NC.....ERROR! BOOKMARK NOTDEFINED.

MHNHTNGTCGIABIBINVIBINIMCNC.......ERROR!BOOKMARK NOTDEFINED.

a) SGIA TNG MC NC BIN V TNG TC CA BI BINError! Bookmark not defined.B) QUY TC BRUUN............................................................................Error! Bookmark not defined.C) PHNG PHP EDELMAN ...........................................................Error! Bookmark not defined.

BINIHNHDNGMTCTNGANGTHEOMA.............ERROR!BOOKMARK NOT DEFINED.

BINIHNHDNGMTCTCNBNGDITCDNGCABO ..............................159BINIHNHDNGMTCTDITCDNGCATHYTRIU ....................................161

CHNG 6 M HNH HA BI BIN VNG B

6.1 GII THIU CHUNG........................................................................................................................164

6.2 M HNH HA BBIN BNG M HNH VT L .......................................................... .......166

GIITHIUTNGQUTVMHNHVTL .......................................................... ..................166BTOSNGVMNGTOSNG ....................................................... .......................................168MHNHLNGCNG..... ................................................................ ..................................................169

6.3 M HNH HA NG BBNG PHNG PHP GII TCH ...........................................171

MHNHHNHTHINGBDNGNGN ........................................................ .......172

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PHNG TRNH LIN TC............................................................................................................173 PHNG TRNH CHUYNNG .......................................................... .......................................174GII HPHNG TRNH LIN TC V PHNG TRNH CHUYNNG...................... ......176P DNG M PHNG HIN TNG BI LNG TRCP PH SNG..........................176NH GI CC HN CHCA L THUYTNGN........................................................181

6.4 M HNH TON DIN BIN NG B....................................................................................183 6.5 O C MT CT NGANG BI BIN ............................................................ .............................189

A.KHOSTBNG PHNGPHPTRUYNTHNG .......................................................... .......189B.KHOSTBNGPHNGPHPHINI ........................................................... ..................191

B-1 CRAB......................................................................................... ..................................................191B-2 SEA SLED............................................................ ............................................................... ........192B-3 THIT BO SU THEO NGUYN L P LC THY TNH.................................................192TM TT...........................................................................................................................................193

CHNG 7 SBINING BVCC GII PHP NNHNG B

7.1 GII THIU........................................................................................................................................194

7.2 NGUYN NHN GY XI LNG B.................................................................................195

KHIQUTCHUNGVXILBBIN .......................................................... .............................195SSUYGIMNGUNBNCTTSNGRABIN.............................................................197SSUYGIMNGUNCUNGCPBNCTTCCNCTGNB ................................199DOKHAITHCTRMTCHVKHONGSNBBIN ....ERROR!BOOKMARK NOT DEFINED.SGIATNGNNGLNGSNGDOTHMBIBHTHPERROR!BOOKMARK NOT DEFINED.DOGINONVNCHUYNBNCTDCB ...................ERROR!BOOKMARK NOT DEFINED.DOSTHAYICAGCSNGTISOVINGB ....ERROR!BOOKMARK NOT DEFINED.SGIATNGGCSNGTCDNGTINGB ............ERROR!BOOKMARK NOT DEFINED.DOSGIATNGLNGBNCTBTNTHTTRNBICAO.......ERROR!BOOKMARK NOTDEFINED.DOSGIATNGBOBIN .........................................................ERROR!BOOKMARK NOT DEFINED.SGIATNGXILDOCCSNGPHNX ........................ERROR!BOOKMARK NOT DEFINED.SSUYGIMLNTHYTRIU........................................... ERROR!BOOKMARK NOT DEFINED.

7.3 BO VBBIN ....................................................... ..........ERROR! BOOKMARK NOT DEFINED.

KHNGLMGDIDIVDCHCHUYNTINIANTONERROR!BOOKMARK NOT DEFINED.GIIPHPBOV"MM"............................................................ ERROR!BOOKMARK NOT DEFINED.CCCNGTRNHNNHBGIIPHPCNG.............ERROR!BOOKMARK NOT DEFINED.P MHN ........................................................... ............................Error! Bookmark not defined.K BO VB.....................................................................................Error! Bookmark not defined.TNG BIN............................................................. ...........................Error! Bookmark not defined.XI LSAU KHI XY DNG TNG BIN............................... .......Error! Bookmark not defined.P PH SNG NGOI KHI........................................... .................Error! Bookmark not defined.

TI LIU THAM KHO

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CHNG 1GII THIU CHUNG

1.1 MC TIU, I TNG CA MN HC

Mn hc Hnh thi b bin" c thit k nhm cung cp cc kin thc cbnphc vphn tch, tnh ton din bin hnh thi trong lnh vc kthut bbin. Ccni dung chnh ca gio trnh bao gm cc kin thc cbn c lin quan ti hnh thi

bbin v din bin bbin nh cc kin thc vc trng sng, dng chy, mcnc; cc kin thc cbn vvn chuyn bn ct, cchvn chuyn bn ct ti b

bin; dng ven bv vn chuyn bn ct ven b, vn chuyn bn ct ngang bv dinbin bbin, m hnh ha bbin v tnh ton din bin ng b; cc gii php bovbbin chng st l.

Mc tiu ca mn hc l trang bcho sinh vin nhng hiu bit cbn nht vmtbn cht vt l cc qu trnh din bin bbin, v cch vn dng cc nguyn tc kthut gii quyt cc vn c lin quan ti k thut bbin nhxi lbbin,ngp lt bbin, bi lng cng bin, vv.

Gio trnh ny c thdng l ti liu tham kho cho cc ksmt sngnh xydng cng trnh hoc cc lnh vc nghin cu c lin quan ti kthut bbin.

Ni dung ca gio trnh bao gm vic gii thiu cc hiu bit chung v cc qu

trnh xy ra di ven b, cch tnh ton vn chuyn bn ct, tnh ton din bin bbin v a ra cc bin php kthut nhm gii quyt hoc hn chnhng bt li dodin bin b bin gy ra. Sau khi hon thnh mn hc, cc kin thc sinh vin thunhn c sc chia thnh 2 mc : mc thnht: sinh vin phi nhn bitv gii thch c cc din bin bbin theo quan im hnh thi bbin. mc th2: sinh vin bit vn dng cc cng cv kin thc c hc nhcc cng thc,cc l gii vmt chuyn mn, cc chng trnh tnh ton my tnh gii thch vtnh ton cc c ch din bin bt li ca b bin v sinh vin c th xut cc

phng n thch hp gii quyt cc vn ny.Gio trnh bao gm 7 chng sln lt cp ti nhng ni dung cbn vhnh

thi bbin nh nu trn. Chng 1 gii thiu mn hc v cc khi nim, thutnglin quan ti mn hc hnh thi bbin. Chng 2 scp ti cc iu kin binquan trng trong tnh ton din bin bbin, l sng, mc nc v dng chy. Docc kin thc v sng, mc nc v dng chy c gii thiu cc mn hctrc, nn chng 2 chgii thiu li nhng khi nim cbn nht vsng, mc ncv dng chy c lin quan ti din bin bbin.

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Chng 3 scp ti cc vn ng lc hnh thi bbin. Cthl cc hintng sng v shnh thnh cc dng chy vng ven b.

Chng 4 trnh by cc kin thc cbn vvn chuyn bn ct bbin, c bit

nhn mnh ti cc csl thuyt cbn sc ng dng trc tip trong tnh tonvn chuyn bn ct ven b.

Cc qu trnh din bin bbin, mt ct ngang bi bin trng thi cn bng v sbin i hnh dng mt ct ngang bi bin di tc dng ca sng v dng chy sc trnh by trong chng 5.

binBn cnh cc phng php tnh ton lng vn chuyn bn ct b thngthng, chng 6 trong gio trnh cn gii thiu mt scng ctnh ton hin i, rtcn thit cho cc ksngnh kthut bbin. Cc cng cny cho php tnh ton,m phng v dbo cc qu trnh din bin bbin.

Trong chng 7, cc hnh thc din bin bbin bt li sc xem xt, m tvl gii. ng ph vi cc din bin bt li ny, mt sgii php sc cp, baogm cc gii php cng trnh v phi cng trnh, snh hng ca cc cng trnh nyti din bin bbin cng sc xt ti trong chng 7.

1.2 CC THUT NGLIN QUAN TI HNH THI BBIN

Dean (2002), khi bn vcc thut ngc sdng trong ngnh kthut bbin

nhn mnh rng, trong thc tcc bi bin mi ni trn khp thgii u c ktcu v hnh dng rt ging nhau. Hnh dng mt ct ngang bi bin, c nh nghal hnh dng ca mt ct ngang ly theo phng vung gc vi ng b, ni chung sc kt cu gm bn phn l: phn ngoi khi, phn gn b, phn bi v phn b

bin, nhhnh vminh ha (1-1)Nhtrn hnh v, c ththy ng bthng c nh ngha l ng ranh gii,

ni c s tip gip gia bin v t lin, tng ng vi s giao ng mc nc dohin tng thy triu m ranh gii ny cng sdch chuyn su vo trong t lin hocxa ra pha bin. Trong mt thi on nht nh, ng b l ng mp nc trung

bnh ca thy triu.Vtr chnh xc ca ng bsphthuc trc tip vo trng thica thy triu, iu kin sng ti v dc ca bi bin. Bi bin (hay bbin) lvng c xc nh nm gia ranh ri ca mc nc triu thp v gii hn tc dngca sng vpha t lin; thng thng, l im chn cc mm hoc ng ranhri xut hin thc vt tn ti trong thi gian di trn bbin. Bi bin c chia thnh2 phn, phn bi trc v phn bi sau. Bi trc l phn bi nm trn ranh ri giamc nc cao v mc nc thp khi sng dn ln bbin v khi sng rt ra khi b.Bi sau c gii hn tphn nc cao n gii hn trn cng vpha t lin ca

sng.

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Hnh 1-1.Mt ct ngang ng b(theo Shore Protection Manual, 1984)

Theo Silvester (1997), thut ngbbin (coast), thng hay bhiu nhm l bibin (beach), c nh ngha l di t c chiu rng khng xc nh (c thln tivi km) c ko di tvtr ca ng btrn t lin ti im u tin c sbin

i ln vc im a hnh vpha bin. Theo nh ngha ca Silvester, mt thutngthng c nhc ti ca vng ven bin c thbao gm cc thnh phn nhb

bin, bi bin v vng gn b(l vng c chiu rng ko di tmc nc thp ti giihn sng v). Mt ng bbin c thc hiu theo 2 cch khc nhau. Vmtkthut m ni th ng bbin l ng hnh thnh ranh gii bbin v bi bin,nhng n cng thng c coi nhl ng hnh thnh ranh gii gia t v nc.Theo Bird (1984) th tng chiu di ng bbin trn thgii c tnh vo khong500,000 km, trong chc 20% l bbin c cu to ct.

Mt ct ngang bbin c to nn tct c o gt li khi sng chuyn ng tvng nc su vo b; khi vo ti vng nc nng, sng sbvkhi n gp cc dict ngm. Bi trc (foreshore), hay cn gi l vng sng v, l vng m mt ct bithng xuyn trng thi kh, t mt cch lun phin nhau khi sng x vo phnmi dc trn bi. Phn bi bin kh ro, khng bngp nc c thc mt hoc nhiuthm bi, cc thm bi l nhng phn bi nm ngang c dc rt nh; ni tip vicc thm bi l cc vch ng, l phn bi bct gn nh thng ng do tc dngca sng vo thi im mc nc dng cao (c thdo xy ra khi xut hin bo). Tiptheo phn vch ng vpha trong t lin c thc cc cn ct, n ct c hnh

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thnh khi gi dch chuyn ct tbi vo pha bn trong (qu trnh to nn cc cn ctthng kt hp vi sxut hin ca thc vt c mt trn bbin nhc, chng c tcdng by v chn ct thi tbin vo t lin). C ni cc n ct, cn ct c thay

thbng cc vch hoc cc dc ct dng ng (c bit l phn bbin bxi lphn pha trn)

Trn mt bng (trn nh chp my bay phn bbin), ng bc thc mt snt c trng ng quan tm. Ti hnh (1-2.A), bbin c n ngbi mt o chnvi cc lch triu ct ngang qua o tnhiu pha. Hnh thbbin ny ny xut hinrt nhiu ni trn thgii. Cc lch triu l ni trao i dng chy tbin vo hthng m, ph pha bn trong cc o chn v ngc li. Thng thng, qu trnhlng ng bn ct sko theo hin tng ng cc lch triu c ththy xut hin nhiu ni.

Hnh 1-2.Hnh dng mt bng ng bbin dng o chn

Mt o chn ti ca vnh c thxut hin khi cc di ct ngm hnh thnh phttrin tmt mi t (head land) nhhnh (1-2.B).

1.3 KTHUBBIN, HNH THI BBIN V QU TRNH DINBIN BBIN

Theo Velden (2001), ni chung thut ngkthut bbin c lin quan ti ttccc vn kthut din ra vng bbin. Do kthut bbin bao hm cc vn

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qu rng, nn n c chia thnh 3 lnh vc chnh tng ng vi 3 vn ln vmt k thut c trong thc t. Ba lnh vc chyu l: Cng bin, Hnh thi v

Nghin cu vng ngoi khi. S l ch quan nu ni rng Hnh thi l lnh vc

quan trng nht, nhng cc nghin cu trong hai lnh vc cn li l Cng bin vVng ngoi khi u c lin quan cht chti nghin cu Hnh thi. Cc cng binc thbnh hng ca hin tng bi lng trong cng hay lung tu vo ra cnh,trong khi cc cng trnh ngoi khi nh cc ng ng dn du, hoc ng cpngm qua bin c thbnh hng ca hin tng xi l. Gio trnh schcp ticc khi nim c lin quan ti Hnh thi bbin.

Hnh thi bbinc hiu l cc hnh dng vt l v cu trc ca bbin. Hayni cch khc: hnh thi bbin l khoa hc nghin cu vstng tc gia cc yutng lc nhsng, dng chy, vv ti bbin m cc tng tc ny c thgy nn

sdch chuyn ca bn ct vng ven bv dn ti sbin i hnh dng ca bbin.

Bbin trong tnhin c thchia lm 3 loi chnh cn ctheo cu to ca cc vtliu thnh to v c mt trn bbin, l bbin c cu to , bbin ct v b

bin bn. Bbin c cu to hu nhc cnh (hoc c y v ng bkhngthay i) trong khong thi gian tng i di. Do nhng bin i ti bbin c cuto chxy ra sau mt thi gian rt di (c khi hng vi trm nm) nn n thngc xem xt trong lnh vc a cht hc hn l trong lnh vc kthut bbin. B

bin ct hay bi bin ct, ngc li, rt dbbin dng, di tc dng ca sng vdng chy. V tr ng b bin ca mt bi bin ct thng xuyn dch chuyn,hoc tin ra pha bin hoc li vo trong t lin, v l i tng tc ng ca casng, dng chy ... Hu ht cc bbin trn thgii u c cu to l ct, chc mtst l c cu to v bbin bn. Bbin bn c cu to l cc ht st, bn, ctmn v l cc vt cht c tnh dnh kt, hon ton khc vmt bn cht vt l v hahc vi cc ht vt cht khng kt dnh nhct, cui, si. Cc qu trnh ca cc htvt cht c tnh kt dnh ny bchi phi mnh bi cc tng tc vt l v ha hc

gia cc ht n lv mt nhm ht bn ct (v dnhqu trnh keo kt, ngng t,lng ng), cng skhng c cp ti trong gio trnh ny

c thtnh ton v dbo c din bin hnh thi bbin th trc tin phihiu v nm bt c cc quy lut chuyn ng ca nc, cc tc ng ca cc yut tnhin ln bbin. Do vy, trc khi tip cn vi mn hc hnh thi bbin,ngi hc cn c cc cc kin thc cbn vchc cht lng, thy lc, sng gi,thy triu, a l ..vv.

Cho ti nay, nh hng ca sng do gi v cc dng chy ln bi bin vn cn cha

c hiu r mt cch hon ton. Vn chuyn bn ct vng ven bl mt ni dung

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quan trng trong nghin cu ca hnh thi bbin. Cc kt qunghin cu ny vang c sdng nhm nng cao chnh xc cng nh tnh thc tin ca cc mhnh ton dng m phng hnh thi bbin, trong c m phng, dbo bin bin

ng b.Khi khng phi tt ccc bin i ca bbin trong tnhin u ng theo mong

mun ca con ngi th cc cng trnh bo vbbin l gii php cn thit ng phvi nhng bin i bt li trn. Cc cng trnh bo vbthng c sdng lmchm cc tc ng bt li do cc qu trnh tnhin xy ra bbin, i khi l v hiuha cc nh hng ca cc din bin ny. V dvic xy dng cc p mhn vunggc vi b, hay cc p ph sng ngoi khi lm chm qu trnh xi lbbin. Xydng bin v hthng k, hay tng bin bo vmi pha ngoi bin cng l mtgii php thng c s dng chng li tc ng ca nc dng, sng, v triucng i vi nhng vng trng pha trong t lin. Ngy nay, bn cnh cc gii phpcng trnh, cc nc pht trin ngi ta bt u hng ti nhng gii php mang tnh"phi cng trnh", thn thin vi mi trng v p ng c yu cu pht trin bnvng, hi ha vi thin nhin.

QU TRNH DIN BIN BBIN

Mt trong nhng ni dung quan trng trong hnh thi bbin l nghin cu cc qutrnh din bin bbin. Vy qu trnh din bin bbin l g ? N c hiu l cc qu

trnh tnhin c tc ng ti sbin i hnh dng ng bv vng ven bv cxem xt, nghin cu nhiu phm vi khng gian v thi gian khc nhau ty thuc votnh cht v mc pht trin ca cc qu trnh ny. V dnhqu trnh xi lbi bin,ti chn cc n ct do bo thng c xem xt trong thi gian xy ra bo (c thvigi, hoc 1 ngy), nhng qu trnh tkhi phc li bi bin sau c thxy ra trongmt vi thng hoc trong ma ktip.

B bin lun bin i mt cch lin tc di tc dng ca sng v dng chy tinhiu phm vi khng gian v bc thi gian khc nhau. V dnhkhi bbin chu tc

ng ca mt con sng n lm bn ct ven bni llng trong nc v dng chydo sng sinh ra svn chuyn bn ct bni llng ny vpha hlu ca dng chydc b. Qu trnh tc ng ca sng n ny chdin ra trong vng vi giy v c phmvi tc ng trong di sng vm thi. Nhng khi qu trnh ny din ra lin tc trongnhiu ngy hoc nhiu nm, n c thgy ra hin tng xi lbbin ko di trn mtvng rng vi trm mt n hng chc kilmt. Hin tng xi lhoc bi tlin tctrong thi gian nhiu thng, nhiu nm sdn ti ng bbsuy thoi (hoc pht trin)vo trong t lin.

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Li c nhng qu trnh din bin bbin i hi phi c xem xt trn mt phm virng n hng trm kilmt v trong khong thi gian hng trm nm, v dnhnhngqu trnh thnh to v pht trin ca cc ng bng chu thsng Hng hay ng bng

sng Cu Long, hay qu trnh pht trin, suy thoi v thay thca cc ca sng trn mththng cc ca sng ra bin.

Nhng hiu bit vcc din bin bbin trong qu khvi thi on di (khong vitrm nm n hng nghn nm) l rt quan trng v n scho bit c nhng thng tincn thit din gii c cc nhn tchnh tng c tc ng n hnh dng cang bbin trong qu kh; thng qua cc suy on trn cscc lun c c. Cthcc nhn ttc ng ny vn ang tip tc din ra trong hin ti tuy vi mc nhhn so vi qu kh.

Theo Dean (2002), trong vng 50 nm trli y, kthut bbin trthnh mtngnh khoa hc hon chnh vi cc nghin cu chuyn su vi mc tiu l nm bt ccc quy lut ca qu trnh din bin bbin v pht trin cc chin lc ng ph c hiuqui vi hin tng xi lbbin. Vi cc tip cn nghin cu ngy cng tinh vi,su sc vqu trnh din bin bbin, cc ksk thut bbin c th thit kmtcch hiu qucc cng trnh bo vb. Vi p lc ca sgia tng dn ston cu, nhtl khu vc ven bin; v mi e da vsgia tng mc nc bin, sgia tng slngv mc tn ph ca bo bin, th nhu cu v k thut b bin v cc nghin cuchuyn su vdin bin bbin chc chn sngy cng tng.

Cho ti nay, nhng hiu bit tt nht vqu trnh din bin bbin (trong baogm c nhng hiu bit v dng chy ven b, sng v tng tc gia sng vi b

bin)bao gm: nng lc phn tch, tng hp cc qu trnh din ra trong tnhin/ nnglc din gii, gii thch cc hin tng phc tp; i khi l smu thun gia cc cnc, bng chng/ cc kinh nghim t c tcc nghin cu trn mt lot cc bbinv tcc cng vic thc hin ti cc dn c lin quan ti kthut bbin.

Tuy vy, vic m tton hc cc hin tng tnhin din ra bbin, nht l m tcc chuyn ng phc tp ca dng chy v bn ct bbin vn cn hn ch, y

cng l nguyn nhn lm hn chkhnng m phng v dbo cc din bin bbintrong thi on di bng cc m hnh ton hc. Nhiu nghin cu vcc hin tng, qutrnh din bin bbin ang tip tc c thc hin thng qua vic so snh cc ktqu tnh ton vi cc o c thc t ti b bin v cc nghin cu trong phng thnghim. Tuy nhin nhng nghin cu chuyn su vlnh vc ny, nht l vng lc

bbin trong iu kin t nhin l mt trong nhng nghin cu th vv thu ht squan tm ca nhiu nh khoa hc v ccc sinh vin theo hc ngnh kthut bbin.

Ni chung, cc bi ton nghin cu bbin c thchia thnh 2 loi: thnht l cc

bi ton vm, sdng cc nh lut bo ton hoc tm ra cc lun ci n cc gii

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php hp l; v thhai l cc bi ton vi m, loi ny thng bao gm cc nghin cuchi tit bn cht vt l ca cc hin tng din ra bbin. Ngy nay, cc tip cn theohng cc bi ton vm thng hu ch hn cho cc ksngnh kthut bbin; cn

i vi cc nghin cu chi tit vbn cht vt l ca cc qu trnh din bin bbin thvn cn l vn hc ba, hy vng trong tng lai, nhng thnh tu nghin cu theohng tip cn vi m sthay thhoc kt hp vi cc tip cn theo hng vm.

1.4 LCH SV XU THPHT TRINLCH SPHT TRIN CA NGNH KTHUT BBIN

Kthut bbin l mt ngnh khoa hc tr, mi chhnh thnh v pht trin trongnm 50 ca thk20. Trong lch s, vic xy dng cc cng trnh ven bin u c lin

quan ti cc ksdn sv cc ksphc v trong qun i. Ti M, hu ht ccnghin cu vbin v xy dng cc cng trnh ven bin u do cc ks thuc Thp k thut qun sca qun i M thc hin. Thut ng k sbbin cnhc ti v c sdng phbin vo nhng nm 1950, vi Hi tho u tin vkthut bbin ti Long Beach, California. Trong li ni u ca tuyn tp cc nghincu khoa hc ti Hi tho ny, M.P. O'Brien vit, N (ngnh kthut bbin),khng phi l mt ngnh mi hay l mt ngnh ring bit ca k thut m trc htKthut bbin l mt ngnh nm trong Kthut Dn dng, ngnh ny chyu thin

vhi dng hc, chc cht lng, in t, kt cu cng trnh v cc ngnh khc.Bn cnh cn phi k ti a cht, a mo, phn tch thng k, ha hc v khoahc vt liu.

Khi nim ny cho ti nay, vcbn vn ng. Tuy vy, k thut bbin cnhng pht trin vt bc trong vi thp kgn y. Tln hi tho u tin vi tuyntp hi tho c 35 bi bo khoa hc; cho ti nay, ti hi tho ln th30 (tchc nm2006 ti San Diego, M) c hn mt ngn bi bo, bo co tham dhi tho. Ngoira, mi nm hai ln, cc Hi tho Quc t v k thut b bin cho mt s chuynngnh nhcng, no vt, bn ct bbin, di ven bv cng trnh bin, o c v

phn tch sng v kthut bbin v xy dng cng ti cc nc ang pht trin cng c tchc.

Cc lnh vc c lin quan ti ksngnh KTBB c m ttrong danh sch diy Cc nghin cu trin khai (thng qua o c thc tv khi phc tsliu lch s)

c lin quan ti sng vng nc nng, dng chy v cc iu kin mc nc. Thit kcc dng cng trnh bin n nh, bn vng, hiu quv kinh tbao gm:

cc p ph sng, chn sng, p mhn, k bin, tng bin, cu tu, cc kt

cu ngoi khi v cc ng ng di bin.

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Khng chhin tng st lbbin bng cc cng trnh bo vbbin hoc bngcc gii php "mm" mang tnh "phi cng trnh" v thn thin vi mi trng nhnui bi nhn to, trng cy chn sng ven bin, ...

n nh cc ca sng, lch triu, lung tu, phc vgiao thng thy v thot lthng qua vic no vt, xy dng cc cng trnh hoc bng cc cchchuyn ctnhn to

Dbo sbin i ca dng chy, mc nc cc lch triu, ca sng v nh ginh hng ca chng ti sn nh ca lung lch cng nhcht lng nc.

Nghin cu pht trin cc cng trnh bo vvng ven bin c nguy cbngp ltdo hin tng nc dng hoc sng thn

Thit kcng v bn neo u tu thuyn v cc cng trnh ph trbao gm: bncng, tng neo, trneo, cu cng v hthng neo tu vv

Thit kcc cng trnh bo vo ngoi khi Gim st cc dn khc nhau vbbin thng qua cc quan trc a dng ti hin

trng

XU THPHT TRIN TRONG TNG LAI

Cng vi spht trin vt bc ca cng ngh thng tin v my tnh in tmngnh Kthut bbin cng c sbng nvloi v stinh vi ca cc m hnh tontrong phn tch cc hin tng c lin quan ti bbin. Trong hu ht cc lnh vc ca

kthut bbin, nhng khng phi l tt c, cc m hnh ton bsung v thay thdn cho cc m hnh vt l. Trong mt s lnh vc nghin cu nh tnh ton dbonc dng do bo, th chc m hnh ton mi l cng cthch hp v hiu qu. Mtkhc, mt sbi ton nhtnh ton sng leo v sng trn qua nh cc cng trnh b

bin hay tnh ton n nh ca cc vin bo vchu tc dng ca sng chc ththc hin c tt nht trong phng th nghim.

Robert (1997) khi bn v phng hng quan trng ca ngnh k thut b bintrong thc tin nhn mnh rng, ang c xu hng xy dng cc cng trnh bo v

bdng mm v t gy cn trdng chy. V dnhvic xy dng cc cng trnhp ph sng ngoi khi bo vb thng c cao trnh nh ngay di mc ncbin trung bnh, ti cao trnh ny cc p vn c tc dng khng chcc sng, v tgy nh hng bt li ti cnh quan.

Cc thit bo c, quan trc ngoi thc a ngy cng nhiu vslng v nngcao vmt tnh nng ca thit b. V dnhcch y 20 nm, cc thit bo sngthng cho c cc dao ng trn mt nc ti 1 im (do vy m khng thoc cc thnh phn ca sng theo cc hng khc ca phsng). Ngy nay, cc thit

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bo sng c tho c phsng theo nhiu hng ng thi ang c sdng mtcch phbin trong cc nghin cu ngoi thc a.

Tnh tng ca my to sng trong phng th nghim c ci thin mt cch

ng k. Trc nhng nm 60, mi chc thto c cc sng n v u c chu kv chiu cao sng khng i bng my to sng. Trong nhng nm 70, cc my to

phsng mt chiu trnn thng dng. n nay, cc my to sng a phnhhng c mt nhiu phng th nghim.

1.5 MT SKHI NIM CBN VVNG VEN BIN

Nh c nh ngha trn, ng bc xem nhl ranh gii gia t linvi i dng, bin hay h. Vng ven bin l phn din tch ca t v nc bao bc

ly ng bv ko di vchai pha t lin v bin. Vpha t lin vng venbin c ko di ti mt gii hn thch hp, cn v pha bin th n ko di tivng thng xuyn din ra cc qu trnh ng lc quan trng i vi vng b.

Phn loi bbinCc tc ng ti bbin l rt khc nhau, ph thuc vo phm vi ca tc ng

theo khng gian v thi gian; cc thnh to a cht; loi bn ct; cc yu tng lcxut hin vng ven bin cng nhcc tc ng ca con ngi trong khu vc ny.Bn ct ca bi bin c ng knh ht bin i trn mt di rng, t cc ht bn

mn ti ct th, dm, cui si v tng. Da vo thnh phn, c tnh ca cc vtcht c mt trn bi bin m ngi ta c thphn bbin thnh cc loi sau: bbinbn, bbin ct, bbin cui, si v bbin c cu to v cc mi .

Trn cscc c tnh c trng (vmt hnh thi hc v hin tng din bin bbin) m ngi ta c thphn loi bbin theo mt cch khc; bao gm: bbinc cu to l cc o chn, bbin chu th, bbin cn ct; bbin cu to vch, bbin san h, bbin rng ngp mn v bbin m ly, cbin, vv

Da trn lu lng dng chy (bao gm lu lng dng chy v lu lng bnct) tsng ra bin, c trng sng theo ma (biu ththng qua cao sng trung

bnh vng nc nng - k hiu H) v ln tng i ca thy triu (bin triu- k hiu l TR), mt cch phn loi bbin khc c cp ti nhsau: bbinc qu trnh dng chy sng givai tr thng trv bbin m nng lng sng givai tr thng trc tng quan (TR/H = 0,5-1), bbin m nng lng thy triu givai tr thng tr(TR/H >3) v bbin hn hp (TR/H =1 n 3)

Khi tc dng ca sng v dng chy tng i nhv dng sng mang mt lngbi tch ln chy ra bin, th c thdn n shnh thnh mt tam gic chu ko dira pha bin, theo xu thpht trin ca ng b. Ti mt sni, ng bbchia

ct to thnh cc ca sng hoc lch triu n thng vi vnh pha trong t lin.

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Cc ca sng hoc lch triu thng c duy tr nhc tc dng ca dng chy tsng hoc do tc dng ca dng triu. Ngoi ra cn c mt sbbin c vch dcm phn bi pha trc n l cc bi bin nhhoc khng cn bi bin di chn

vch na. Do cc bbin ct thng chim a sv qu trnh ng lc loi bbin ny thng rt mnh v vi nhiu c im l th nn loi ng b nythng nhn c squan tm ln ca cc nh nghin cu.

Khi thit k v xy dng cc cng trnh ven bin, cn phi lng trc cc nhhng ca cng trnh gy ra i vi chdng chy, ng lc sng v bin i b

bin ti vtr xy dng cng trnh v khu vc ln cn . iu ny trnn cn thithn nu yu cu t ra khi xy dng cng trnh l n nh v khng gy nhng tcng khng mong mun. Cc hu qukhng mong mun khi xy dng cc cng trnhdc bbin c th l hin tng bi lp hoc xi lbbin ln cn khu vc xydng cng trnh do qu trnh vn chuyn bn ct theo hng dc bhay theo hngvung gc vi ng bbgin on bi cng trnh xy dng.

vng ven bin, sng thng l yu tng lc mnh v c nh hng ln ting b. ng kv dnhn thy nht l cc tc ng do cc sng hnh thnh tgi. Mt loi sng khc cng c tm quan trng khng km l cc sng c ngun gcthin tng thy triu, l cc sng di hnh thnh do tc dng ca lc hp dnv tr tmt trng v mt tri i vi tri t, hay cn gi l sng triu. Bn cnhsng gi v sng triu l cc sng khc c tn sut xut hin nhhn rt nhiu so vi

sng triu hay sng gi. Xt vtng th, th cc sng ny c tm quan trng nhvtn sut xut hin rt thp, nhng n li c nhng nh hng rt ln ti nhng nim n tc ng, cc sng hnh thnh do hin tng a chn di lng i dnghay cn gi l sng thn (tsunamis). Mt loi sng khc cng thng ng cpti trong cc nghin cu khu vc ca sng, ven bin l sng do tu thuyn khichuyn ng to ra.

Sng do gi gy nn nhng tc dng ng knht i vi sbin i ca bi bin.Mt ct ngang ca bi bin c hng vung gc vi ng b thng bin i hnh

dng khi ct bmang ra ngoi khi v quay trli pha btrong mt khong thi gian.nhiu ni, mt lng ln ct cng bvn chuyn dc theo bbin di tc dng casng khi n tc dng theo hng xin gc vi ng b. Cc tc ng ca dng chythng chi phi cc din bin ti ca vo ca cc vnh v cc ca sng, ni dng chyc vn tc ln hnh thnh.i vi cc khoa hc v cc ksang nghin cu hoc lm cc cng vic c lin

quan ti bin th nhng hiu bit vc chhnh thnh ca sng v c khnng dbocc c trng ca sng hnh thnh tgi l iu khng ththiu.

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Ni chung, sng c hnh thnh vng nc su, v sau c truyn ra khpi dng, thm ch l tbin ny ti bin khc, v khi i ti gn bdo nh hng caa hnh y, sng bbin dng lm thay i chiu cao sng v hng sng. Cc sng

ny thng c gi l sng lng, hay sng cn (swell waves). Trong qu trnh lantruyn trn bin, ch c mt phn nhnng lng ca sng b tiu hao. S tiu haonng lng ny chlm gim chiu cao sng chkhng lm thay i chu kca sng.Cc sng lng, c xem nhc tc dng phn phi li nng lng gi (ngun nnglng ny l rt ln) mt cch hiu qunht ti mi bbin trn thgii. Cc sngny (hoc l c ngun gc tbo hoc l do bin ng) trc khi ti c bbin s

phi di chuyn qua thm lc a. Sbin i cc c tnh ca sng khi n di chuynqua vng thm lc a v vng nc nng ln cn gn b, cn phi c hiu r. Cuicng, khi i vo gn b, do nh hng ca ma st y, sng bvv gy nn sbini nng lng rt phc tp trong vng bni n vv i vi cc cng trnh cxy dng trong khu vc ny.

Bi vy, khi nghin cu vcc qu trnh din bin bbin v vn chuyn bn ctti mt on bbin cthno th cc thng tin vloi sng, thi gian xut hinca n trong cc thng, cc ma v trong cnm cn phi c xc nh mt cch cth. Nhng thng tin ny c gi l c trng sng theo ma (wave climate).

Khi xt ti tc ng ca mt sng n ln bi bin, ngi ta thng xc nh mctc ng ln hay nhcn cvo 3 thng schnh, l: chiu cao v chu ksng,

hng sng tc ng ti ng b v tn sut xut hin ca sng trong nm.Hng sng tc ng ti ng bl mt tham squan trng, khng chi vi tnhton thit kcng trnh, m cn i vi ccc nghin cu vn chuyn bn ct ven

b. Khi tnh ton thit kcc cng trnh ven bin, ngi ta thng quan tm ti cc tiliu thng kvsng, c bit l s liu c lin quan ti con sng ln nht tngxut hin ti khu vc xy dng cng trnh. Nu khu vc xy dng cng trnh c y cc ti liu quan trc sng trong qu khth vic tnh ton xc nh cc thng ssngthit k theo tn sut thng khng qu phc tp. Tuy nhin, nu khu vc xy dng

cng trnh khng c ti liu quan trc sng, th cn tnh ton phc hi li sliu sngln nht tcc sliu vcc trn bo ln, xy ra trong qu kh.

Cao trnh mc nc bin cng l mt tham squan trng trong tnh ton thit kcng trnh v tnh ton din bin bbin. Thng y l cn ctnh ton cao trnhnh ca cc cng trnh xy dng ven bin v tnh ton sng leo trn bi bin hocsng leo trn mi cng trnh. Mc nc triu c thdbo c rt chnh xc da vo

bng thy triu xy dng tmc nc thc o trong mt, hai nm.Nc dng do bo l hin tng thng hay xy ra dc bbin, c bit l ti

nhng vng thng xuyn chu nh hng ca bo. Nc dng l hin tng dng cao

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t ngt mc nc bin dc theo bbin trn mt on bbin nht nh no c.Hin tng xy ra khi on bbin chu tc ng ca gi bo, thi lin tc vi vn tcln theo hng nht nh tbin vo t lin, kt hp vi sbin i kh p cc b

(xut hin khi c bo). Hin tng nc dng do bo c thgy ra nhng thit hi rtln trn mt din rng v n gy ngp lt cc vng t thp ven bin v to iu kincho cc sng tc dng vo su trong t lin.

Khi sng vgn b, n c thto vi ng bmt gc xin. Do vy thnh phnca sng theo hng dc bsto thnh dng chy dc b, xut hin trong vng sngv(surf zone). Trong vng sng vc chuyn ng ri ca cc cht im nc, kotheo cc ht bn ct y cng bkhuy ng v ni llng. Do tc dng ca dngven bcc ht bn ct llng sbcun i theo hng dc bv svn chuyn bnct ny c gi l tc dng vn chuyn bn ct dc b. Dng chy dc bc thkthp vi dng tiu (chy vung gc vi b), to nn nhng nh hng ring bit ivi s phn tn bn ct di ven b. Bn trong ng sng (breaker line), daong ca cc cht im nc ti y do sng gy ra cng lm cho bn ct ni llngtrong nc. Cc dao ng ny c gi trln nht ti y, v lm dch chuyn bn ct llng dc bbin, chyu trn mt su ti mt ct ngang bbin

Ct trn bi bin v trn cc n ct, cn ct, ngoi cc tc ng ca sng v dngchy ven b, cn chu tc ng ca gi. Ct bgi cun ln phn thm bi ca cn ctngoi cng pha bin v chng c tc dng cng cphn pha trc ca cn ct. Trong

trng hp sau mt vi nm m chng khng bsng bo lm cho xi lth chng stip tc c pht trin trthnh cn ct ln hn.

Bn cnh vn xi lbbin th hin tng lng ng bn ct ti cc ca sng,lch triu cng l vn thng hay gp vng ven bin. Bn ct khi chuyn ngdc theo b bin ti cc ca sng hay lch triu, do c tng tc vi dng chy tsng i ra hay tng tc vi dng triu xung, s to thnh cc di ngm ngoi ca,i khi l cc doi ct ngm nng dn ln trn mc nc bin. Vtr v kch thc cacc di ct ngm hay doi ct ngm ny thng xuyn thay i do chng chu tc ng

trc tip ca sng v dng chy dc b, v gy cn tr rt ln cho giao thng thy.Ty thuc vo ngun bn ct c vn chuyn ti ca sng, lch triu l nhiu hay tm ca sng sbbi lp hon ton hay mt phn, dn n nhng chi ph no vt cahng nm rt tn km.

1.6 C IM A HNH BBIN VIT NAM

Bbin Vit Nam c chiu di hn 3260km, l mt di bbin rt phc tp. ngvmt tc ng ca cc qu trnh ng lc tbin ln lc a, tlc a (sng ngi

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v.v.) ln vng bbin v stng tc ln nhau gia chng, th c im ca di bbin l mt trong nhng yu tquan trng. Cc c im ny c thksbnhsau:

- c im ca cc ca sng v ca cc hthng sng chy ra bin.

- Cao trnh v cu trc a cht ca ng bbin.- Hng v a hnh y ca cc on b.- Hsinh thi dc ng bv vng ngp mn- Hthng , k bin v.v.Vic phn chia ranh gii cc min Bc, Trung, Nam khng c trng cho sng

nht va hnh v c im ca vng b. Trong mi min, c im a hnh vng brt phc tp.

C IM A HNH VNG BBIN MIN BC

Nhn chung va hnh di bmin Bc l phc tp nhng tng i bng phng,dc nh. l mt trong nhng nguyn nhn to iu kin cho nc dng bo phttrin mnh sut cdi bny.

Mc d ton di bny c thchia thnh hai phn khc bit nhau: phn pha bc lon bkhc khuu, nhiu i ni to thnh vch ng v hng ngn o che chn ;

phn pha nam l on bthuc vng ng bng c nhiu ca sng chia ct. Nhng y c mt c im chung l ton bdi bu c hng chung l ng bc - tynam. a hnh y thoai thoi, dc khng ln, c bi triu rng. cng l c

im thun li cho cc hin tng nhsng, nc dng v.v . pht trin.a) ng btQung Ninh n Hi Phng- Vng b tnh Qung Ninh, t pha lc a, y l vng b ca sng dng hnh

phu. Cc ca sng chnh y l: Ca Lc, Tin Yn, H Ci v Ka Long. Cc casng ny u c lu lng nh, t ph sa. Nhng do vng ny l vng st chm nn cdin tch ngp triu kh rng. Hn na, y l ni c bin triu ln nht Vit Nam(4,5m). Vpha bin, y l vng bc a hnh phc tp nht do c rt nhiu o nhche chn pha ngoi b(Vnh HLong, Bi TLong).

- Di bbin Hi Phng: y l vng ca sng ca hthng sng Thi Bnh. Ccca sng chnh l Lch Tray v Ca Cm. Di bny c hthng lch triu dy c.c im ni bt ca di bny l qu trnh xi lc xu th tng trong nhiu nmqua.

b) ng btHi Phng n Thanh Ha- Di b thuc chu th sng Hng (t Sn - Hi Phng n Lch Trng-

Thanh Ha)c cc ca sng: Ba Lt, Ca y, Vn L. Trvng Vn L trong mtgii hn hp c xi lcc b, cn cdi bny c xu hng bi t. Qu trnh bi tc xu hng mrng ngang- tbra bin c thti 25km, nhca sng y, ca

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Ba Lt (ti Vn L chkhong 2-4km). y, qu trnh ng lc sng chim u thso vi bin.

C IM A HNH VNG BBIN MIN TRUNG

Di bbin min Trung, xt theo c im a hnh c thchia lm 3 on nhsau:a) Di bbc Trung btThanh Ho n H Tnh.Phn pha bc ca on ny nhsk tip ca phn pha nam ca on b thuc

di chu thsng Hng. Phn pha nam c a hnh phc tp hn v bchia ct bi ccca sng nhCa Nhng, Ca Khu...

Ni chung on bny kh bng phng, dc khng ln. ng ng su 20m v50m nm cch bkh xa.

Mc d cng c sng ngi ra, nhng cc sng di bny thng nh, ngn v

dc, nn khng to c nhng ca sng ln nhsng Hng, sng Thi Bnh, do vydi bny khng c nhng bi triu rng nhdi bchu thsng Hng. Hsinhthi (rng ngp mn) y kh ngho.

b) Di btrung Trung btQung Bnh n Qung NgiL on bc sxen kgia ni cao (o Ngang) v cc bi ct lm cho ng b

trnn khc khuu. on bny c c tnh nhon bpha bc ca min Trung,vi nhiu dy ni m ra bin nhmi Chn My, o Hi Vn, bn o Sn Tr, miLay, mi Dc,.. v ng thi di bny tuy ngn nhng c nhiu sng ra : Ca

Tng, Ca Vit, Ca Thun An, Ca i...ng bng ven bin y hp. Nhvy, ni, sng, ng rung nm an xen nhau

lm cho ng bkhc khuu. Gia cc mi vn ra to thnh cc vng vnh ln nh: vng Chn My, vnh Nng.

su di bbin ny ln. ng ng su 50m nc nm cch bkhng xa,c bit ng ng su 20m nc gn nhnm st b, c ni chcch bkhong10km.

c) Di bnam Trung btQung Ngi ti Bnh Thun.

y l on bc t sng ra bin nht. Sng ln ng knht trn on bnyl sng Tr Khc thuc tnh Qung Ngi v sng Rng Tuy Ho, thuc tnh PhYn. on bny c mt smi v bn o nh: mi Ba Lng An, mi i Lnh,mi Dinh, mi K G, bn o Hn Gm. ng thi c mt svnh kh kn, thun licho tu b tr n khi c dng, bo nhvnh Quy Nhn, vnh Vn Phong, vnh CamRanh.

Hthng o ven bvng ny nm tha tht trn mt di bdi nho L Sn(Qung Ngi), C Lao Xanh (Ph Yn), Hn Tre (Khnh Ho).

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Nt ni bt nht ca a hnh y on bny l thm lc a rt hp v dc. Ccng ng su 50m,100m, thm ch cng ng su 1000m i rt st b, nht lon tQuy Nhn n Nha Trang.

Nhtrn ni, bo vng ny t vtn sv nhhn vcng , v thncdng y khng ln. Nhng mt phn quan trng na lm hn chspht trin canc dng y l a hnh y su v dc. Vhsinh thi, nht l rng ngp mn on bny khng ng k.

C IM A HNH VNG BBIN MIN NAM

Cdi bny c nhiu c im khng ng nht vhnh thng bv a hnhvng b. Nhn chung, n c chia thnh 2 on

a) Di bng Nam bgm hai hthng ca sng chnh :

- Vng ca sng ng Nai. Trc khi ra bin, sng ng Nai chia ra nhiunhnh nhv ra vnh Gnh Ri nng v kn- y l vnh ln nht ca min Nam.

- Tip n l vng ca sng Cu Long. y c rt nhiu ca sng chia ct di bny thnh nhiu on. Cc ca sng chnh gm c: ca Soi Rp, Ca Tiu, Ca i,Ca Ba Lai, ca Hm Lung, ca CChin, ca Cung Hu, ca nh An, ca Tranh, ca MThnh v ca B. Cc ca sng ny hng nm ra bin mt lngrt ln nc ngt v ph sa. Chnh v vy m vng bin cc ca sng c bi p,

bi triu rt rng. Sut cdi b l vng nc nng, y thoi, dc nh. ng

ng su 50m cch rt xa b, cng v pha nam cng xa. Phn lnh hi nm trongng ng su 20m c din tch rt ln, ng ng su ny c thcch b50km.

Di bbin ny hu nhkhng c o. Duy nht chc o Hn Khoai nhb, nmcch mi C Mau khong 15-20 km.

Khc vi bbin min Trung, ni c nhiu mi, vng, vnh, cc o ; di bbinny c nt c trng l c rt nhiu ca sng rng chia ct.

b) Di bty Nam bDi bny ko di tmi C Mau ti mi Nai - H Tin, thuc tnh Kin Giang.

Nhn chung on bny t khc khuu, t bchia ct v c rt t sng ra. Chcsng Ci Ln ra vng Rch Gi l ln hn c. y cng c mt smi nh,khng vn ra xa bin nhmi Hn Chng, mi ng Thy vv. Cc vng, vnh ycng t nhvng Rnh Gi, vng C Mau, vng Cy Dng ; trong vng Rch Giln hn nht. a hnh y vng bthoi, nng, dc rt nh. ng ng su 20mnm cch bkhong 50 -60 hi l.

H thng cc o thuc di bny kh phong ph, vi cc o ln nho PhQuc, ThChu (nm xa b). Nm gn bhn l hthng cc o nhnh: Hn Tra,

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Hn Minh Ho, Hn Anh ng, Hn Anh Ty, Hn Thm, Hn Nam Du, Hn Trc,Hn Mau, Hn Du, Hn Trong vv..

Di bny phn ln c hng Nam - Bc v ng - Nam chu nh hng ca gi

ma Ty Nam tng i ln.Di bchung quanh mi C Mau (ng v ty) l ni c rng ngp mn ln nht

nc mc d - nhchng ta bit: nhng nm gn y din tch rng ngp mn bthu hp i rt nhiu do bcon ngi tn ph v do chy rng.

Do vng bthoi v nng nn sut cdi bbin min Nam u c bi triu rng.Vmt a hnh, y l di bthun li nht cho vic pht trin nc dng do bo. Rtmay l khu vc thng rt t khi c bo v cng bo cng yu.

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CHNG 2SNG, MC NC V DNG CHY

2.1 GII THIU

Van der Velden (1998) vit rng, nu bin lc no cng hin ha v phng lngth cc nghin cu v bin s chng cn g th v na. Tht may l trong t nhinkhng phi l th, mc nc bin lun dao ng theo cc thi on di, ngn khcnhau mt cch thng xuyn v theo cc quy lut phc tp khin cho bin ctrnntht b n v cc nghin cu vbin tht th vv ng quan tm.

Sng hnh thnh tgi (gi tt l sng gi) l mt tc nhn quan trng c tc dngtruyn nng lng t gi qua i dng ti b bin. Khi ti vng nc nng, nnglng sng chuyn thnh dng chy dc bv gy nn hin tng vn chuyn bn ctven b. Shnh thnh sng tgi ph thuc vo tc gi v thi gian gi thi v

phm vi khng gian c gi thi. Sng hnh thnh trong vng c bo c gi l sngbo v chng thng rt phc tp. Ti mt thi im, c rt nhiu chiu cao sng khcnhau, sng dng nh xut hin t ngt v bin mt t ngt. S dm sng bo

phc tp nhvy l do bo khng chn gin to nn mt loi sng m l to nnton bphsng vi mt di cc gi trchu kv chiu cao sng khc nhau. Tuy vy,khi sng di chuyn ra khi vng c bo th chng li tr nn u n v pht trin

thnh sng lng, (swell wave), y l cc sng c chiu cao v khong cch gia ccnh sng ng u nhau. trng thi u n ny, mt con sng c thni tip cccon sng n khc trn mt qung ng di ng k khi chng lan truyn qua idng. Sng lng c vai tr truyn nng lng qua i dng ti bbin, ti ccsng bvdo nh hng ca ma st y v gii phng nng lng m n mang theotrong vng sng v.

Cc kin thc cbn v cthvl thuyt sng gi c trnh by trong giotrnh Sng gi c bin son cho sinh vin nm th3, gio trnh ny skhng nhc

li cc kin thc . Chng 2 s chgii thiu mt cch tm tt v l thuyt sngbin nh, tuyn tnh ca Airy, gii thiu cc tnh ton, m tcc thng ssng clin quan trc tip ti hin tng vn chuyn bn ct v din bin bbin. Nhng khinim c bn vmc nc v dng triu c lin quan ti hnh thi b bin cng sc gii thiu trong chng ny

2.2 L THUYT SNG TUYN TNH CA AIRY

hiu c chuyn ng ca sng khi truyn qua i dng sau khi thot ly khi

ni hnh thnh v sbin i cc tham ssng khi n i vo vng nc nng gn b,

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th vic biu din ton hc cc phng trnh tng quan dtnh tc chuyn ng vsthay i chiu cao sng theo su, cc phng trnh biu thnng lng sng l rtcn thit. Trong tnh ton vn chuyn bn ct v din bin ng b, cc thng ssng

cbn v cc phng trnh m tquo chuyn ng, vn tc ca cc cht im nctrong sng l c bit quan trng.

Cc l thuyt sng p dng trong tnh ton c nhiu loi khc nhau, tl thuyt sngtuyn tnh tng i n gin ca Airy ti l thuyt sng phc tp ca Cnoidal. Tuynhin, do tnh cht phc tp ca sng v cchvn chuyn bn ct cng nhmi tngtc gia cc yu tsng, dng chy v bn ct m kh c thbiu din ton hc ccc tng quan ny thng qua cc l thuyt sng phc tp. y snhn mnh ti vicng dng L thuyt Sng Tuyn tnh ca Airy, l thuyt c p dng ti vng ncsu, cc tng quan ton hc tng i n gin, m tchuyn ng ca sng. Dotnh cht phc tp m l thuyt sng Cnoidal t nhn c squan tm v t c ngdng rng ri trong thc tin mc d y l mt l thuyt sng c khnng ng dng rtrng.

CC GII THIT CBN V C TRNG SNG

Hnh dng, vn tc v schuyn ng ca cc cht im nc khi kt hp vi nhauca mt chui sng n l rt phc tp; chng strnn phc tp na khi ngoi thct, tc l trn mt bin, ni c v s cc sng c kch thc, tn sv hng truyn

sng khc nhau cng c mt. Nh nu trn, l thuyt sng n gin nht m tchuyn ng ca sng l l thuyt ca Airy, hay cn c gi l l thuyt sng tuyntnh v l thuyt ny sdng mt sgin thit nhm tuyn tnh ha cc c trngsng. L thuyt ny da vo cc githit nhsau: trong mi trng cht lng khngnn c (githit ny tng i hp l), cht lng khng tham gia chuyn ng quay(gi thit ny nhm m ch rng coi mi trng nc khng c tnh nht, tuy khnghon ton ng trong thc tnhng cc sai sxut pht tgii thit ny l c thchpnhn c), trn mt bmt y khng thm nc ( khng hon ton ng trong thc

t) v xt cc sng c bin nh(tuy y khng phi l githit tt nhng kt qutnhton kh ph hp vi thc ttrong trng hp sng khng qu ln).Cho ti nay, l thuyt sng n gin nht, l thuyt ca Airy, vn c th s dng

c trong nhiu ng dng, cthl trong m tsng lng vng nc su c chiucao sng gn nhu nhau hoc tuyn tnh ha tng cc sng hnh sin to thnh ccsng bo phc tp. Khi trin khai l thuyt sng ca Airy, mt githit rt quan trngc cp ti l coi chiu cao sng l rt nhso vi chiu di sng v su nc ni

c sng. Bng vic ly gn ng ny, phng trnh m tcao trnh mt nc (x,t) scdng nhsau:

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( , ) os (kx- t)2

Hx t c = (2.1)

Trong H l chiu cao sng, x l trc ta theo phng chuyn ng sng, t l

thi gian, k = 2/L l ssng xut hin, trong L l chiu di sng v l = 2 /T tnssng c n vl radian vi T l chu ksng c n vl giy. Cc thng strnv cc thng s c lin quan trong phng trnh (2.1) c trnh by hnh 2-1. Bng2-1tm tt cc phng trnh cbn trong l thuyt sng ca Airy. Hnh dng ca consng trong l thuyt Airy l hnh sin vi khong cch x ti mt thi gian cnh hocvi thi gian t ti vtr cnh (x l hng s).

Hnh 2-1. Cc k hiu sdng trong l thuyt sng Airy.

Gc h tr c ta l im Otrn mt nc tnh vi trc z chng t y ln pha trn mtnc. Ti y bin c z = -h. Bmt nc bin c dng hnh sin,mi cht im nc chuyn ngvi vn tc theo phng ngang uv theo phng ng w theo qu

o elp vi ng knh trc lnl d v ng knh trc nh l s.

C mt quan hcbn v c bit quan trng c khai trin tl thuyt sng caAiry, l phng trnh phn tn:

(2.2)2 tanh( )gk kh =

Nu thay cc i lng bit l = 2 /T v k = 2 /L, vo phng trnh 2.2 sc

2 2tanh2g hL T

L

=

(2.3)

Trong : h l su nc.Phng trnh ny khng thgii c trc tip v c gi trchiu di sng L c2 v

ca phng trnh v gii hn ph hp ca L c xc nh thm hyperbolic tangent vphi ca phng trnh. Hnh (2-2) m tthhm hyperbolic ca gi tr(r). T

thny c ththy rng khi r = kh = 2h/L c gi trln th tanh(2h/L) l, do vyphng trnh (2.1) c rt gn li thnh:

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20 2g

L T

= (2.4)

ChsL0c dng chcc tham ssng ln cn vng nc su, sdnhvy

v ti vng nc su, theo githit th su nc h ln hn nhiu ln so vi chiu disng Lnn tanh(2h/L) l

Bng 2-1 Cc phng trnh c xy dng tl thuyt song tuyn tnh ca Airy

Hnh 2-2 Cc gi trc a hm hyperbolic

Vng nc su thng c giihn t im c su h > L0/2,trong trng hp ny, sai s tnhton snhhn 5% so vi tnh tontrong phng trnh (2.3). Th vo

phng trnh cbn C = L/T, sthuc vn tc pha C0 tng ng vng nc su nhsau:

0 2

gC T

= (2.5)

i vi vng nc nng, khi su h < L/20 (vi sai s l 5% ), cthly gn ng phng trnh (2.1),

trong tanh(2h/L) 2h/L (hnh 2-2), t phng trnh (2.1) c rt gn li cn

sL T gh= (2-6)

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v vn tc pha tng ng Csl

sC g= h (2-7)

Chs s trong cc phng trnh (2-6) v (2-7) c dng chcc tham ssng

c ly gn ng vng nc nng. Tri ngc vi vng nc su, ni c chiu disng v vn tc pha chphthuc vo chu ksng; th vng nc nng, cc tham sny li phthuc chyu vo su nc.

Tm li, phm vi ng dng cc tnh ton gn ng nhsau:

Vng nc su0

12

hL

>

Vng chuyn tip (phng trnh tng qut)0

1 14 2

hL

> > 0 (2.8)

Vng nc nng0

120

hL

>

Hnh 2-3. S bi n i casng theo l thuyt Airy lhm ca t s gi a sunc, h, vi chiu di sng vng nc su L , trong Ll hm ca chu ksng

Ti su chuyn tip, khikhng th ly gn ng cctnh ton nh i vi vngnc su v vng nc nngvi sai s tnh ton nh hn5%, th cc tnh ton phi sdng phng trnh tng qut

(2.3). Cc bng biu c xydng v trin khai t vic gii phng trnh trn c th tham kho ca Wiegel(1954,1964) hay Stay Kthu t Bbi n(Coastal Engineering Manual - CEM, 2002).

Sbin thin ca L/L v C/Ctheo h/Lc thtra thnh (2-3). Ngoi ra c thtnhgn ng phng trnh tng qut (2.3), thay thcho cc bng biu tnh, bng trin khaica Eckart (1952) nhsau:

0,5

00

2tanh

hL L

L

=

(2.9)

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Phng trnh ny c thc dng vng chuyn tip, v c thc gii trc tiptchiu di sng Lv su h, sau khi tnh ton c chiu di sng L0vng ncsu tphng trnh (2.4). Phng trnh 2.9 ca Eckart cho thy r hn rng khi su

nc tng ln lc sng ti gn bth chiu di sng L v vn tc pha tng ng C=L/Tsgim so vi gi trban u ca chng vng nc su.

NNG LNG SNG

Nhvy chng ta xt qua cc phng trnh cbn ca sng tuyn tnh Airy biuthchuyn ng trn mt nc. L thuyt ca Airy cng a ra cc phng trnh nnglng sng v phng trnh truyn nng lng sng qua i dng. Mc d khng c schuyn ng thc ca nc khi sng truyn qua, v theo l thuyt ca Airy th cc chtim nc ch dao ng ln xung xung quanh v tr ca n, nhng bn thn chuyn

ng ca sng cng sinh tc dng truyn nng lng trn mt bin. Sdch chuyn mtnc bin khi trng thi mt nc tnh, phng to cho sng mt thn ng. Ti cng thiim , chuyn ng theo quo ca cc cht im nc bn di sng sto nnmt ng nng. Kt hp cthnng v ng nng dc theo ton bchiu di sng sthu c tng nng lng sng Etrn mt n vdin tch (1m2) ca theo l thuyt sngAiry nhsau:

21

8E gH= (2.10)

Ec tng quan trc tip ti chiu cao sng Hv trng lng ring ca nc bin .Tng quan ny biu din mt ca nng lng c tng hp v tch hp trn mtn vdin tch, v l tng nng lng trn mt n vchiu di nh sng. Trong hmt, cc thnguyn v n vca nng lng sng E c biu din bng Newtons/mhay June/m2 (thnguyn l nng lng/chiu di 2).

Khi sng i qua thm lc a v tin su dn vo trong vng nc nng, do nhhng ca a hnh y, chiu cao sng H sthay i, thng l chiu cao sng stngln cho ti khi sng tin ti gn bv v. iu ny cng c ngha l nng lng sng E

cng sbin i khi c hiu ng nc nng lm thay i chiu cao sng, v nng lngsng skhng c bo tn nhthng c xt (khng phi l nng lng n thunm l mt nng lng). Nng lng sng tng cng (E) c thay thbng i lngthng lng nng lng sng (P) - c gi tt l thng lng sng (flux of energy). Pc ly xp xbng hng s.

Thng lng sng c nh ngha l mt n ng lng sng b d ch chuyn dctheo hng truyn sng v c c biu thbng cng thc sau:

P = E C n = E Cg (2.11)

Trong C l vn tc ca mt sng n ( hay cn gi l vn tc pha), v

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( )

1 21

2 sinh 2

khn

kh

= +

(2.12)

Vn tc Cg= C n l vn tc truyn nng lng sng, vn tc ny c gi l vntc nhm, n biu thtc chuyn ng ca mt nhm cc sng. Vn tc ny c

phn bit rt r so vi vn tc ca mt sng n chuyn ng vi tc pha C. thgi trnbin thin theo h/L c m ttrong hnh (2-3). Ti vng nc su (n = ), gitrn stng khi sng chuyn ng dn vo vng chuyn tip v gi trn = 1 khi vo nvng nc nng. iu ny c ngha l, ti vng nc su, cc con sng n l dchchuyn vpha trc vi tc pha C chuyn ng vi tc gp i ca cnhmsng, nhanh hn vn tc truyn ca nng lng sng. Hiu ng ca skhc bit ny cthquan st thy to cc mng sng trong phng th nghim, khi to ra mt shu hn

cc sng vng nc su. C thquan st thy rng, ssuy gim dn chiu cao ca ccsng dn u, thm ch l cc sng ny bbin mt khi n vt qua tc truyn canng lng sng ca nhm, trong khi cc sng mi c hnh thnh cui nhmsng.

i lng thng lng sng trong phng trnh (2.11) c k hiu l P v n ctrng cho nng lng ca sng, hay ni chnh xc hn l nng lng trn mt n vchiu di nh sng. Snyder, Wiegel, and Bermel (1957) thng qua vic xc nh nnglng sng ti mt u ca mng sng trong phng th nghim v nng lng sng

c truyn ti u kia ca mng chng trng cng thc (2.11) l hon ton phhp khi biu din sut chuyn nng lng sng.Cng vi s dch chuyn sng cn c sut chuyn nng lng hoc truyn ng

lng. Longuet Higgins and Stewart (1960,1964) a ra nh ngha vng sut ta(radiation stress)l sgia tng ng lng dng chy do sc mt ca sng. ngsut ta chnh l phn ng lng gia tng thm, trong c xt ti p sut thyng, tng p sut mang gi tr tuyt i tri p sut thy tnh. iu ny khngnh rng, ng nng chc lin quan trc tip ti sxut hin ca sng. Nu trcx c t ngc vi hng truyn sng v trc y ly song song vi nh sng thc 2 thnh phn khc khng ca ng sut ta (radiation stress): thnh phn mchng theo phng x, y ca ng nng -x v ng nng y. ng sut ta hay sbin thinca ng nng chuyn qua mt phng (x = hng sv c h ng song song vi b) theohng dch chuyn sng (hng x) c xc nh bng cng thc sau

2 12

sinh(2 ) 2 2xxkh

S E E nkh

= + =

1 (2.13)

Mc d, thnh phn c quo chuyn ng song song vi nh sng bng khng,nhng mch ng ca m men theo phng y vn c gi trkhc khng do khi c sng,

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p lc mch ng sc hnh thnh. Mch ng m men theo phng y khi chuynqua mt phng y= hng sl

1

sinh(2 ) 2yy

khS E E n

kh

= =

(2.14)

Ti vng nc su, khi n=1/2; Sxx= E/2, v Syy = 0;ti vng nc nng khi n = 1, Sxx = 3E/2, v Syy = E/2.ng sut ta l mt cng cmnh phn tch cc trng thi khc nhau ca cc hin

tng c lin quan ti sng. Trong nhiu trng hp, ng sut ta sc dng dbo sbin i mc nc bin (do hin tng dng, hmc nc) kt hp vi cc sngchuyn ng ti gn b. Thnh phn theo hng dc bca ng sut ta c xem nhl nguyn nhn to nn dng chy theo hng dc b. Mt ng dng khc l xt shnh

thnh ca sng vb(surf beat), tng tc gia sng vi dng chy n nh, v dcca cc sng ngn trng lc trn nh cc sng di hn (Longuet-Higgins andStewart,1964).

PHM VI P DNG CA L THUYT SNG

L thuyt sng ca Airy chl mt trong snhiu l thuyt sng c nghin cu,v l l thuyt sng n gin nht nh cp phn u ca chng 2. Cc l thuytsng ny u c phm vi p dng khc nhau, phthuc vo mc st thc ca chngkhi biu din cc hin tng sng trong tnhin. Phm vi p dng ca cc sng c

cp y c ch ra c th trn hnh (2-4), trong phm vi p dng ca tng loisng slm hm ca tsH/h v h/L. C thnhn ra rng, l thuyt sng ca Airy l cvng p dng rng nht, trong khi l thuyt sng Stoker c vng p dng hp hn nhiu

Hnh 2-4. Phmvi p dng ca cc l

thuyt sng khcnhau, l hm ca tsH/h v h/L

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Gii hn sng , hay cn gi l gii hn v dc sng ca l thuyt sng n

c chra trn hnh (2-4) l b = Hb/hb = 0.78. Gii hn p dng l thuyt sng Airy l

( )2 21

w 0.05

2

u g+ < H (2.15)

trn csgithit n gin ha phng trnh sng ca Euler, trong chra rng,vn tc quo ca cc cht im nc theo phng u v w ti mt nc l nh(nhhn 5% ca tch sgH). Muir Wood (1969) chra rng, gii hn trn tng ngvi

1 2tanh

16

H

L L

h<

(2.16)

Gii hn ny chnh l ng ranh gii gia l thuyt sng Airy v l thuyt sng

Stokes nhtrn hnh (2-4). Sdng ng gii hn2 23

32

3

HL

h

= (2.17)

l ranh gii gia l thuyt sng cnoidal v l thuyt sng Airy, do Keller (1948) vLittman (1957) pht trin.

2.3 HIN TNG TRUYN SNG V BIN DNG SNG

Khi chuyn ng tngoi khi vo gn b, sng tri qua rt nhiu qu trnh bin i

(bin dng) khi lan truyn. Qu trnh bin dng sng ny tng ng vi sbin i caa hnh y bin hoc khi sng gp phi cc chng ngi vt trn ng truyn sngnhcc cng trnh ven b. Nhvy, khi tnh ton cc din bin ca a hnh y binvng gn bhay tnh ton thit kcho cc cng trnh ven b, cc kscn phi nmc cc hin tng ny v bit cch tnh ton cc thng ssng. phn trc, chngta c bit cc phng trnh m tnng lng v schuyn ng sng, thng quacc phng trnh ny c thtnh ton c sbin dng sng khi sng truyn tvngnc su vo vng nc nng trc khi n chuyn ng vo b.

HIN TNG TRUYN SNG VNG NC SUKhi sng di chuyn ra khi ni hnh thnh v khng chu nh hng ca gi th

chng txp xp th t theo sphn r ca sng v trnn u hn. Tc truynnng lng sng v tc dch chuyn ca nhm sng c xc nh bng vn tcnhm sng theo cng thc nhsau:

1.

2 2 2gC g

C C nT

= = = (2.18)

Theo cng thc (2.18) th tc chuyn ng ca nhm sng chphthuc vo chu

ksng, do vy m cc sng c chu kdi sdi chuyn nhanh hn cc sng c chu k

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Hnh 2-5. S bi n i phnng lng sng t n i snghnh thnh (vi phsng r ng)ti ph c a cc sng lng bthu hp li trong qu trnhphn tn v tiu tn nnglng sng, v cui cng tikhu sng vc ph sng h p .

Mi chu k sng trong phc mt nhm cc sng phi hp

vi n, di chuyn ra khi ni sng c hnh thnh, v c ly xp xsb, chuynng c lp vi cc sng c chu kkhc nhau. Cc nhm sng khc nhau vi chu kring bit c tc chuyn ng khng xc nh; do vy mc d cng bt u xut phtri khi ni hnh thnh sng nhng chng snhanh chng tch ri nhau trong qu trnhchuyn ng.

Hnh 2-6. Nhm sng chnh "ch nh t" c cng mtchu k sng trong m t phsng hon chnh khi n chuynng tn i hnh thnh ti imquan trc A trn bbi n.

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Xt mt nhm sng ring bit c chu kT khi n chuyn ng t ni hnh thnhsng ti bbin nhhnh (2-6). Trong iu kin l tng, nhm sng ny schim mtvng hnh chnht c chiu rng bng chiu rng nh hng ca cn bo hnh thnh

sng v chiu di xc nh tchu kbo v tng chiu di gi. Coi bin ca hnh chnht githit ti im x=0 v sng c hnh thnh ti thi im t=0. Nhm sng c

bao bi hnh chnht schuyn ng theo hng gi thi theo chiu dng ca trc xvi vn tc chuyn ng m ttrong phng trnh (2.18). Nu cn bo ko di trongvng 30 gi, th ng bin y ca hnh chnht theo hng gi thi si qua ngx = 0 chm hn sau khong 30 ting khi ng bin mt trc ca hnh chnht vichiu di ca hnh chnht sl Cg 30 ting. Thc t, cc cnh ca hnh chnht skhng duy tr c hnh dng ban u, mt scon sng sbxo trn v lt ra khi hnhchnht i vo vng nc tnh bao quanh n. Mt ssng thp svt ln cnh baongoi ca hnh chnht v nng lng sng sbkhuych tn theo phng ngang, lmcho cc cnh ca hnh chnht skhng cn sc cnh na m chuyn thnh gc t. Cchiu ng ny c thnh, tuy vy cc sng skhng ti c im B trong hnh (2-6) doqu trnh ny. Sau ny, c ththy, tuy cc sng trong thc tc thi ti im B tnihnh thnh bo, nhng iu ny l l do gc khuych tn ca cc sng, v khng phi ttccc sng sau khi hnh thnh u chuyn ng theo hng gi trung bnh. Thm chvi gc khuych tn sng ny cc sng c hng vo mt vng hp t gy nh hngti khu vc ln cn. Hin tng hnh thnh sng tmt trn bo cng tng tnhhin

tng to sng khi nm mt hn xung mt ao, khi cc sng c hnh trn v khuychtn i tt c cc hng; nng lng trn mt n v chiu di nh sng s gim rtnhanh. Do slan ta ca sng trong i dng l hu hn nn nhm sng svn duy trhu nhton bnng lng khi n chuyn ng ti bbn kia.

Tm li, ngay khi sng ri khi ni vng hnh thnh sng, do hiu ng phn tn,chng sc xp xp li thnh cc sng c chu kdi hn chuyn ng nhanh hntheo phng truyn sng. Kt qul, ti cc im quan trc cch xa nhau, cc sng cchu kdi hn sdi chuyn ti trc, tip theo sau l cc sng c chu kngn hn.

Hiu ng phn tn v tnh nht ca nc bin cng lm cho phsng thu hp li vbin i trng thi sng phc tp ni hnh thnh thnh cc sng lng u n. Nnglng sng cng chu nh hng ca hin tng sng v v hin tng tng tc,

phn tn gia cc sng. Mt khi cc sng pht trin hon ton thnh cc sng lngu, chng c thdi chuyn hng nghn km qua i dng m chtiu hao mt lngnng lng nh. Cc sng hnh thnh do bo Thi Bnh Dng c thquan trc thyti bbin Vit Nam, Indonexia mt cch tng ng.

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SBIN DNG SNG GN B

Ti vng nc su, cc sng lng c dng hnh sin vi nh sng thp v trn. Ngaykhi sng i vo vng nc nng, chng s tri qua mt qu trnh bin dng c h

thng, nhc miu thnh (2-7).

Hnh 2-7. M t s bi ndng sng khi chng dichuyn dn vo trong vngnc nng v tip cn tibbi n.

Vn tc v chiu di sng sgim dn khi chiu cao sng tng dn, chc chu ksng l khng thay i. Ti su nc gn vng sng v, chui sng bao gm mtdy cc sng c nh nhn c ngn cch bi cc bng sng tng i phng. Khinh sng trnn qu dc, v mt n nh, lc ny cc sng sbv. Chiu cao sngvvcbn c th ln hn chiu cao sng nc su. Sbin dng sng dnhnthy nht i vi nhng sng lng c chu kdi, vi hnh dng ban u ca sng gn

vi hnh sin, khi cc sng ny c sma st vi y trong qu trnh di chuyn ti stb. Cc sng c chu kngn hnh thnh mang tnh cc b, thng c c dc sng lnngay khi n hnh thnh, vng nc su, do m qu trnh bin dng sng thngkh nhn thy hn.

Hin tng thay i cao c a sng do c sthay i su khi sng bi n truyntngoi kh i vo ven b c gi l hiu ng nc nng

Mc d, li gii vchuyn ng sng trn mt mi dc c (Stoker, 1957), nhngchng rt phc tp v him khi c sdng. Cch tip cn n gin nht c dngvi trng thi ca sng c ni ti trn l githit trong iu kin a hnh y

bin l bng phng (Rayleigh,1911). p dng l thuyt sng bin nhca Airy, rtra phng trnh sau:

12

0 0 0

2 2tanh tanh

L C h h

L C L L

= =

(2.19)

Phng trnh (2.19) a ra c sbin thin chiu di sng L v vn tc pha C khichiu su nc h thay i. Sbin thin ca cc yu tny cng c m thnh(2.3), c th thy c chiu di sng v vn tc pha gim mt cch c h thng khi

chiu su nc gim dn, khi sng chuyn ng ti gn b. Tng tnhtrn, gi tr

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Sbin thin chiu cao sng khi gp hiu ng nc nng c thc tnh tontnghin cu vthng lng sng. Nu coi tn tht nng lng do ma st y v

phn xsng l khng ng k, githit ny tng i hp l khi xt cc sng bn ngoi vng ven b, th thng lng sng c coi l hng s v thu cphng trnh sau:

P = ECn = (ECn)0= hng s (2.20)Trong (ECn)0 l thng lng sng xc nh ti vng nc su. Nu khng xt ti

tn tht nng lng, th thng lng sng khi sng gp hiu ng nc nng sbng vigi trban u ca thng lng sng ti vng nc su. p dng biu thc tng quan

nng lng sng theo l thuyt sng bin nhca Airy, th : 21

8E gH=

v tsgia chiu cao sng Hti im c su nc hvi chiu cao sng ti vngnc su H0sbng:

12

0

0

1

2

CH

H n C

=

(2.21)

C n v t s C/C0 u l hm s ca su nc (theo phng trnh 2.12) v(2.19)), tng ng nhvy, ts H/H

0 cng sph thuc vo su. Kt qu tnh

ton tng cong H/H0 thhin trn hnh (2-3) cho thy chiu cao sng stngkhi sng chuyn ng vo vng nc nng. Xem xt chi tit hn ng cong ny, cth thy rng, ng cong dtnh ban u sgim ngay khi sng vo ti vng ncnng, lc ny chiu cao sng snhhn mt cht so vi chiu cao sng vng ncsu (H/H0 < 1). Ssuy gim tm thi chiu cao sng slm tng vn tc ca nhmsng Cg= C n. Do tch E.Cnlun duy tr l hng snn khi vn tc nhm sng Cg tng,m vi vn tc ny nng lng sng sc dch chuyn v pha trc , th mt nng lng E sgim khi chiu cao sng gim. Cc hin tng bin i chiu cao sng

v tng vn tc nhm sng c quan trc trong phng th nghim v trn cc mngsng, v i khi c ththy c trong tnhin khi gp phi cc sng lng rt u chochu kca cc con sng xp xnhau.

dc sng H/L cng bin i khi sng gp hiu ng nc nng. dc sng gimtm thi bn di gi trdc sng vng nc su khi sng truyn qua vng ncc su trung bnh v sau t ngt tng khi chiu cao sng H tng v chiu disng L gim. dc sng tng t ngt tng ng vi sgia tng chiu cao sng l cim ni bt ca hiu ng sng nc nng. dc sng tip tc tng nhanh ti im

khi sng mt n nh v bv.

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Cc phn tch trn vsbin dng sng ngay khi chng i vo vng nc nnghon ton c xy dng trn csl thuyt sng tuyn tnh ca Airy, m trong thc tth l thuyt sng ny khng p dng c i vi vng nc nng. Bng vic n gin

ha cc phng trnh, l thuyt ca Airy a ra cc phn tch r rng, dhiu v dbo ng xu th chung sbin i ca vn tc pha, chiu cao sng, vv ca hiu ngsng nc nng. Tuy nhin cng phi nhn mnh rng, trong thc tth mc bin icc c trng sng skhc vi kt qudon bng l thuyt Airy, lm tng thm saiskhi sng chuyn ng ti vng sng v.

Tm li, khi cc sng lng chuyn ng tvng nc su vo vng nc nng vkhi sng tip cn b bin, chng tri qua mt qu trnh bin dng c h thng, nhnchung qu trnh bin dng ny lm tng chiu cao sng v gim chiu di sng cng nhgia tc sng, dn ti lm tng dc sng cho ti khi dc sng vt qu im giihn, lc ny sng mt n nh v v. Khi ng dng cc l thuyt m tcc chuynca sng nu trn th l thuyt sng tuyn tnh ca Airy, m tcc sng c hnh sinv dc sng nh, nhn chung c phm vi ng dng tng i hn ch, l thuyt nychp dng c trong iu kin nc nng, trong khi l thuyt sng ca Stokes hayca Cnoidal c yu cu p dng ti vng chuyn tip v vng nc nng, khi dcsng tng.

HIN TNG KHC XSNG V NHIU XSNG

Khi cc sng chuyn ng ti gn b theo hng xin gc vi cc ng ngmc y bin, vi gi thit l cc ng ng mc l sng song vi ng b,chng sbun gn nhvung gc vi ng bti thi im sng vv chiu caosng cng s thay i mt cch tng ng. Hin tng ny c gi l khc xsng. Sbi n i ca vn tc truyn sng gy ra do s thay i su s lm chosng thay i l n v hng truyn sng gi l qu trnh khc x sng.

Hnh 2-8. Hin tng

khc x sng. T i thi imt1, nh sng ti im B vng nc su s chuy nng nhanh hn nh sng tiim A nm trong vng ncnng lm cho ng nhsng b u n theo phngsong song vi ng b

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Khi i t vng chuyn tip vo vng nc nng, sng s chu tc ng ca hintng khc xlm thay i hng truyn sng khi su nc gim dn v ng nhsng c xu thtrnn song song vi cc ng ng mc y. i vi ng bbin

thng c cc ng ng mc song song vi nhau, do hin tng khc x nn ccng nh sng s c hng song song vi nhau v song song vi ng b. Hintng khc xsng c xem nhl kt quca sphthuc ca vn tc pha vo su nc, khi sng di chuyn t vng chuyn tip vo vng nc nng, su nccng nhth tc truyn sng cng nhtng ng. Nhm ttrong hnh (2-8), iuny dn ti hin tng quay ca ng nh sng tng ng vi ng ng mc y. Trong biu ny, phn nh sng ti im B c su nc ln hn im A, vdo vy n chuyn ng nhanh hn, ti im B, biu din sdch chuyn ln hn so vikhong cch tim A n im A, ni vn tc truyn sng chm hn. Xu thny tiptc xy ra cho n khi ng nh sng xoay dn v tr nn gn nhsong song ving b.

Hin tng khc xsng cng tng tnhhin tng un cong ca cc tia sng, vsbin i hng do hin tng khc xgy ra c lin quan ti sbin i vn tc phaca sng cng tun theo nh lut ca Snell's:

1 2

1 2

sin sinonstantc

C C

= = (2.22)

Khi hai tia sng truyn ti ng thng c sthay i t ngt su, sau khi vt

qua ng ny hng truyn sng sthay i t1sang 2, trong 1v 2l ccgc gia cc ng nh sng v ng ng mc tng ng ti y bin, C1v C2lvn tc pha ca sng ti hai su tng ng. Trong trng hp n gin, khi ng

bbin thng c cc ng ng mc song song, gc truyn sng ti su bt ks

c tng quan ti gc truyn sng ti vng nc su 0nhsau:

00

sin sinC

C = (2.23)

Khi m vn tc pha C gim so vi gi trca n vng nc su khi sng n gn

b, gc truyn sng s gim theo tng ng so vi gi tr ca gc truyn sng tivng nc su.

Trong trng hp y bin khng ng u, hin tng khc xsng c thgy raphn khoc hi tnng lng sng. Hiu ng ny c ththy c rt r khi quan stcc tia sng, l cc ng vung gc vi ng nh sng c c hng dng theochiu chuyn ng ca sng hay chiu truyn nng lng sng nhhnh (2-9). Trn b

bin thng c cc ng ng mc song song vi nhau, hin tng khc xsng slmcho cc tia sng b khuych tn, iu ny cng c ngha l nng lng sng s b

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khuych tn v do vy m chiu cao sng sgim i so vi trng hp sng khng bkhc x

Nng lng sng bgim do hin tng khc xsng c thc gii thch nhsau:

do sut chuyn nng lng sng hay nng lng nm trong khong 2 tia sng k tipnhau l hng s(bqua ma st y) v skhuych tn sng do hin tng khc xsngcn c nng lng tng ng mrng trn chiu di nh sng ln hn. iu ngcli cng ng i vi cc tia sng hi t.

Nu gi s0l khong cch gia cc tia sng vng nc su v s l khong cch cacc tia sng trong vng nc nng (nhhnh 2-9), th

P = ECns= (ECns)0= hng s (2-24)

Hnh 2-9 Sb o ton sut chuynnng lng sng ECnsgi a hai tiasng trn ng nh sng

Trin khai phng trnh (2.24) ging nhphng trnh hiu ng nc nng (2.21)nhng khng xt ti hin tng khc xsng, thu c

1 12 2

0 0

0

1

2

C sH

H n C s

=

(2.25)Skhc xsng c xt ti trong phng trnh thng qua tss0/s, tsny hon

ton khng c xt ti trong (2.21). Trong thc t, cc bng tra hin tng bin dngsng cho bit tsca H/H0 , skhng xt ti hin tng khc xsng, y cng ltsc biu ha trong hnh (2-3). Tsny c gi l hskhc xsng

12

0r

sK

s

=

(2.26)

i vi bbin thng c cc ng ng mc song song, c a hnh y n gin

v ng nht th tsc thly bng:

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0 0os

os

s c

s c

= (2.27)

Trong trng hp ny h s khc x sng K c tnh ton kh n gin. Trong

trng hp bbin c a hnh y phc tp v c hin tng khc xsng th cn xydng mt biu khc xsng v xc nh trc tip sbin thin ca s dc theo b

bin so vi khong cch ngoi khi. Sau khi xc nh c s, c thdng gi trnytnh hs K, dc theo bbin v dng phng trnh (2.25) xc nh sbin thinca chiu cao sng.

Trng hp a hnh y bin khng u c thlm sng bkhc xrt phc tpv dn n sbin thin ng kca chiu cao v nng lng sng dc theo bbin.Sng sbkhc xv phn kti nhng ni c vc bin su hoc nhng ni y bin b

st ln, do vy sng trn bi bin phn pha trong bca cc vc bin su sgim chiucao trong khi pha hai cnh ca vc su c hin tng hi tsng lm chiu cao sngti cc im ny tng ln (nhhnh 2-10)

Hnh 2-10. Sh i t v phn kc a cc tia sng trn mt vc bin su v ti ccmi t nh ra bin gy nn hin tng khc xsng

Chai trng cc tia sng v trng nh sng c bao gm trong biu , nhng

cc tia sng khng c khong cch bng nhau vng nc su. Cc tia sng tin ngn cc vc su ban u c khong cch gn nhau sau nng lng ca chng bphnkv lan ta rt ln khi i vo phn vc su lm gim chiu cao sng ti mt khut cami vc su. Ngc li, hai bn cnh ca vc su hoc ti vng nc nng chuyntip nm gia hai vc su ktip nhau theo chiu ngang li c shi tca cc tia sngdo nh hng ca a hnh.

Hin tng ny dn ti sgia tng chiu cao sng v nng lng sng dc theo dibbin. Sng cng bkhc xv un cong vpha u ca cc mi t hng ra bin

do phn nc nng pha ngoi khi kt hp vi mi t (nhhnh 2-10). Nng lng

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sng do vy shi tti phn mi t hng ra bin, lm cho chiu cao sng vcbnc thln hn cc vng ln cn, ni cc tia sng phn klm gim nng lng sng.

C th thy trong mt s trng hp cc tia sng c th giao nhau, y l vn

thng hay gp khi tnh ton khc xsng bng phng php biu vi iu kin ahnh y bin phc tp. i vi trng hp ny, khong cch gia hai tia sng ktip sgim ti gi tr0, v theo phng trnh (2.25) th chiu cao sng tnh ton stng ti gitrv cng ln ti im hai tia sng giao nhau. Ty nhin trong thc tkhng xy raiu ny. Thay v chiu cao sng stng ln gi trv cng ln th khi chiu cao sngtng, dc sng tng gn im giao nhau gia hai tia sng, nng lng struyn sang

bn dc theo ng nh sng v c thto thnh con sng th2.

Hnh 2-11.Hin tng nhiu xsng khi sng ti gp vt cn lp ph sng xa b

Lin quan ti hin tng khcx sng l hin tng nhiu xsng (hnh 2-11). Khi sng bingp phi mt chng ngi vt vd nh mt chn sng trn

ng lan truyn ca mnh, vtgy cn mt phn nh sng slm tiu tn nng lng sng hoc

gy phn xnng lng sng, phn cn li ca con sng struyn qua vt cn v bun cong vo vng khut sng pha sau. Khi cc sng truyn qua vt cn giao thoavi cc sng ti sto thnh hin tng nhiu xsng.

Cc cng trnh ny phn chia mt phn nng lng sng, to thnh mt vng khutpha sau cng trnh, thng l ni neo u tu thuyn. Qu trnh nhiu xsng c gy

nn hin tng thot nng lng sng vo trong vng khut sau cng trnh, lc ny nsto nn nhng iu kin sng khng mong mun bn trong cng. Hin tng nhiuxsng c thcng l mt yu trt quan trng c nh hng ti hnh dng ng b

bin, phn ln hnh dng chnh xc ca cc bi bin hnh ti (pocket beach), bi bin chc duy nht mt phn nhtip gip vi bin , v tng ti vi cc bi bin c hnhcong, ni bchn bi cc cng trnh ph sng hoc cc p chn sng xa b.

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2.5 THY TRIU V SDAO NG MC NC

Trong tnh ton thit k cc cng trnh b bin, bn cnh tnh ton sng, th tnhton mc nc l mt ni dung tnh ton thit kquan trng. Gi thit c mt cng

trnh n gin, c t gn bv chu tc dng ca sng. Khi mc nc tng ln,cng trnh schu tc dng trc tip ca cc sng ln hn so vi khi mc nc trngthi trung bnh. l do khi mc nc tng ln, n to iu kin cho cc sng c xmnhp su hn vo trong b, v quan trng hn l khi mc nc tng ln th nh hngca ma st y i vi cc sng sgim i, nng lng sng khi vstp trung vo vtr st cng trnh. iu ny dn ti cc lc tc dng ln cng trnh cng tng ln vsng trn qua nh cng trnh sgy hhi cho cng trnh v vng bn trong bccng trnh bo v. Ngc li, khi mc nc hthp, cng trnh skhng cn chu tc

dng trc tip ca sng v do m cc hhi ca cng trnh schyu xy ra tithi im mc nc cao.

Tng tnhvy, hin tng dng cao mc nc bin gy nn ssuy thoi ca bc cu to ct. Tng tnh gii thch trn, khi mc nc bin dng cao stoiu kin cho cc sng ln hn xm nhp vo gn b. Cc sng ny sgy xi lnct v phn pha trn ca bi bin v a bn ct lng ng pha ngoi khi. Sdngcao mc nc trong thi on di sdn ti hin tng mt bn ct vnh vin. Bbin

bao gm cc mi , vch c cu to bng cc vt liu dbxi lthnh to trong

kbng h hoc l tr, mm slin tc bxi ldo tc dng ca sng. Mc nccao, do vy, scho php cc sng ln hn tc dng trc tip vo cc vch gy nnhin tng xi lnhanh v lin tc bbin trong thi on di.

Sdao ng mc nc c thphn thnh mt sloi v chng c chia thnh cchnh thc khc nhau ty thuc vo tn sut lp li ca cc hin tng :

Chu krt ngn- Sng gi v sng bin (sng lng)

Chu kngn- Thy triu-

Nc dng do bo v do sbin i kh p- Sdao ng mc nc do bin i kh p

Thi on ma, hoc thi on di- Sdao ng mc nc mang tnh ma- Sgia tng ca mc nc bin- Snng, hca lc a- Sbin i kh hu ton cu

Trong phn tip theo strnh by cc ni dung lin quan ti sdao ng mc nc

do tc dng ca thy triu, hin tng nc dng do bo (sdao ng mc nc trong

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thi on ngn) v sdng ln ca mc nc bin (sbin i mc nc trong thion di)

A-THY TRIU V DNG TRIU

NGUN GC THY TRIU

Triu thin vn thng c hn ch trong schuyn ng ca nc vng venbin. Chng gy nn snng hmc nc v to thnh cc trng dng chy c quym ln, i khi xut hin vi vn tc dng chy tng i ln. Thy triu c nhhng trc tip hnh thi bbin, vn ti thy, thy sn, v sctr ca cc loi vcc hot ng gii tr, du lch. Do tnh cht quan trng ca cc vn ny nn trong

phn ny sgii thiu kvthy triu v chthy triu Vit Nam.Nm 1687, Niutn khm ph ra b mt thng ngn nm nay vhin tng thy

triu. Thy triu ln xung hng ngy trn tri t, khi yu, khi mnh, xy ra mt cchtun hon, u n l do nguyn nhn no ? Khoa hc trli rng nguyn nhn sinhra thy triu l do lc hp dn vtrvtr, chyu tMt trng v Mt tri. Tronghnh (2-12), thy triu c gi l triu thin vn v n l kt quca githit lc thytriu tc dng ln nc bin trong thi gian di l cn bng vi lc ht thin thv lcthy triu.

Thy triu l kt quca skt hp ca cc lc tc dng ln mt cht im ncring l, chng bao gm:

- Lc ht hp dn tc dng ln qut,- Lc ly tm pht sinh do chuyn ng quay ca thp qut v mt trng,- Lc ht ca mt trng cho cc cht im nc trn qut,- Lc ht ca mt tri ln qut.Equilibrium Tide (Mt trng)

Hnh 2-12Triu thin vn

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Trc tin, githit rng bqua lc ca mt tri v ton btri t c bao phbi nc. Lc cn tc dng ln cc cht im nc l mt thnh phn lc nh theophng nm ngang. Lc ny c tc dng lm chuyn ng cht im nc A trong

hnh (2-12) v hng mt trng v cht im nc B ra khi mt trng, dn ti haivng li nc cao. Khi tri t chuyn ng quay vi tc gc E, quanh trc camnh ti CE theo hng ca mi tn, iu ny lm cho khi cu nc bao bcquanh tri t b bin dng v to thnh hai ln nc cao v hai ln nc thptrong 1 ngy m. Kt qul thy triu sc chu k12gi, tuy nhin, do hthng

mt trng - mt tri cng chuyn ng quay quanh CMEvi vn tc ME theo cng1 hng quay vi tri t. Hai vng nc li si theo vtr ca mt trng v vthchu kca thy triu l 12gi42 pht.

Equilibrium Tide (Mt tri v Mt trng)

Hnh 2-13Triu thin vn v lc tng tc ca Mt tri v Mt trng

Lc hp dn ca mt tri to thnh mt vng nc tri thhai, nhhn vng ncli u tin c hng v pha mt tri v theo hng ngc li. Chu k triu hnhthnh do lc hp dn mt tri s l 12 gi, tng ng vi thi gian tri t chuyn

ng quay quanh mt tri.Chai chtriu cn bng ny u xy ra ti cng 1 thi im v chng sc

tnh gp li khi mt trng, mt tri nm trn mt ng thng (vo ngy 1 v 15 mlch tng ng vi thi im trng non v trng trn). Ti cc thi im ny, thy triusc bin cao hn mc trung bnh. Ti mt trng, lc hp dn ca mt trng v

mt tri lch pha nhau 90v lc thy triu strit tiu nhau ti cc thi im ny, lcny thy triu c bin nhhn mc trung bnh. Thi im thy triu ln nht gi lnc ln (spring tides) v thy triu nhnht gi l nc rng (neap tides).

Sln xung khng u ca Thy triu

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Trong trng hp l tng, nu trc ln ca hnh bu dc ca mng nc bin lunlun trng vi mt phng xch o v chuyn ng ca mt tri rt u n theong trn th triu dng v triu rt cc ni trn tri t c cng vsht sc

u n. Nhvy sthy hai ln nc ln cao nhnhau, hai ln nc rng cng tngtnhau v thi gian triu dng cng nhtriu rt u bng nhau. Nhng trong thc tta li thy chng chnh lch nhau.

Trc ht l do li ca xch v, tc nghing ca mt trng. Khi mt trng nmtrong mt phng xch o, ta sc xch vcc tiu bng khng. Trong cc trnghp khc, quo ca Mt trng ct mt phng xch o mt gc nhn m gi trcc

i ca xch vl 27-28(vpha Bc hay vpha Nam).GisMt trng vtr c xch vcc i nhhnh 2-14. Theo l thuyt tnh

hc vthy triu, mng nc bin c hnh bu dc hng vMt trng nhtrong hnhv. Schnh lch khng u ca thy triu ngy cng nhnu xch vcng nhvkhi mt trng nm trong mt phng xch o, theo l thuyt, schnh lch ny sbinmt. Thy triu ng vi trng hp Mt trng c xch v ln nht l thy triutropic hay thy triu ch tuyn vi schnh lch triu hng ngy t gi trcc i.

Thy triu vo thi kMt trng c xch vbng khng l thy triu phn imhay xch o.

Tng tnhvy ta sc schnh lch thy triu hng thng do mt trng quayquanh tri t khng phi theo hnh trn m l hnh elp vi cn im v vin im,

ty trng hp mt trng gn hay xa nht.

Hnh 2-14nh hng ca xch v

Vo thi k trng trn hoc trng non (gia hoc u v cui thng) ta c hin

tng giao hi hoc xung i : Mt trng, mt tri v Tri t vtr thng hng vi

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nhau ta nhmt xu to. Mng nc bao quanh tri t bchai lc hp dn vtrcng ko vmt hng nn hnh bu dc trnn nhn hn. l thi ktriu cng,ln y v xung kit hn cc ngy khc trong chu kna thng.

Tri li trong nhng ngy trng thng huyn hoc hhuyn, Mt trng, Mt tri vTri t th vung gc vi nhau khin cho lc hp dn v tr b phn tn v haihng vung gc. Trng hp ny gi l trc th, dng bu dc ca mng nc kmnhn, trc ln v trc nhca bu dc t chnh lch nhau, do triu ln xung yu. l thi knc km. Nhvy, n nhp vi cc tun trng trong vng na thng(chnh xc l 14.77 ngy) triu bin thin tcng sang km ri tkm ln dn thnhnc cng.

Cc nh hng khcNhtrn chng ta bit rng c tnh ca thy triu c xy dng trn cs4

yu tchnh l lc hp dn thin thca mt trng, mt tri v gc xch vca mttrng v mt tri. Ngoi ra cn c rt nhiu yu t nh hng khc nh qu ochuyn ng ca mt trng v mt tri vi tri t, v trong thc tcc quo chuynng ny l hnh elp chkhng phi l hnh trn. Mi mt nh hng do cc sngtriu thcp gy ra u c ln, chu kv gc pha ring bit so vi cc thnh phnkhc. Do vy m thy triu thc t s phc tp hn nhiu so vi tnh ton trn ccsng triu chnh to bi nh hng ca mt trng v mt tri

PHN TCH V DBO THY TRIUphn tch thy triu ngi ta chia mt con triu thc o thnh nhiu thnh phn

thy triu khc nhau v chng c gi l cc sng triu, cc sng triu ny hon tonc thxc nh c khi chng ta c sliu o c thy triu tng i di. lnthy triu c githit l bng tng ca cc hm iu ha theo cng thc sau:

( )i i1

( ) os t +I

T i

i

t a c =

= (2-28)

trong T(t) l mc nc triu ti thi im t, aiv il bin v gc pha ca

cc sng triu; il tn sgc ca chng. V dsng bn nht triu mt trng, ck hiu l M2, c chu k l 12.42 gi v do vy m M2 = 2/(3600 l2.42) =1.405x10-4giy-1.

Nhvy cc phn tch thy triu thc cht l vic i xc nh gi trca cc thnh

phn aiv i. Nhng thnh phn quan trng nht to nn thy triu trn tri t baogm cc sng bn nht triu mt trng (M2) v mt tri (S2) c chu k12.42 giv 12gi, v cc sng nht triu hay bn nht triu khng u c chu kl 24.48 giv 24gi. Ngoi ra cn c rt nhiu sng khc cng cn c xt ti trong tnh ton phn

tch thy triu nhcc sng nht triu O1,K1,P1,Q1v cc sng do nh hng ca nc

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nng hay nh hng lch quo chuyn ng ca mt trng, mt tri. Mi mtsng triu c c trng bi cc hm iu ha khc nhau. Sliu o c thy triutrong mt nm hon ton c thdng tnh ton phn tch bin v gc pha ca

hn 60 loi sng triu khc nhau. Mt sng triu quan trng c chu kl 18.6 nm. Nkhng thtnh ton c tcc sliu o c vi di chui sliu o c hp lv do vy m sc tnh ton theo cng thc. Cc thng sc tnh ton iu chnh

aiv itheo hm thi gian tng ng vi chu k18.6 nm ni trn. Vi sliu o cngy cng di hn, chng ta cng c thphn tch tmvi nhiu sng hp thnh hntrong tnh ton phn tch thy triu. V dnh, sau khi phn tch hm iu ha thytriu, ta bit rng ti cng Hn du, sng nht triu c ln ti 74cm (sng O1),72cm (sng K1) trong khi cc sng bn nht triu li nhhn rt nhiu: 5 cm (sng

M2) v 4 cm (sng S2). Tcc sliu gc l cc hng siu ha vbin v gcpha, ta chcn a thm cc iu kin thin vn tng ng vi thi gian cn tnh l cthtnh ton ra c cao thy triu vo ngy cn tm.

Qu trnh tnh ton hng siu ha kh cng phu, phc tp. Cc php tnh trongdtnh thy triu vi hng chc sng hp thnh cng l cng vic rt cng phu. Nuch tnh bng tay vi my tnh c nhn cng phi c hai ngi tnh tho trong nathng c thd tnh thy triu ca mt thng. n nhng nm sau ny, khi mytnh in t trnn phbin v ngy cng pht trin mnh vi hng triu php tnhmt giy th vic tnh ton thy triu vi kt quchnh xc ti nhiu ni, hu ht l cc

cng bin trn thgii trnn phbin. Nhiu phn mm v website vtnh tonthy triu c xy dng v phbin mt cch rng ri trn internet. Nhng i vinhiu dn xy dng cc cng trnh ven bin th cc thng tin v thy triu v trxy dng cng trnh nhiu khi li khng c v do vy m cc phn tch tnh ton v oc thy triu ti v tr dn l rt cn thit. Thng thng ngi ta thng o cthy triu trong thi gian khong 1 thng v t tnh ton ra cc hng siu hachnh nhsng bn nht triu mt trng, mt tri, v nht triu khng u. Tt nhin lcc kt qutnh ton trn chmang tnh sb, cc phn tch tnh ton chi tit lun c

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