Collecton of Marne Research Works,, X: 1-33 FEATURES OF WAVE FIELD, ITS EFFECT TO EROSION DEPOSITION PROCESSES IN THE CUADAI (HOIAN), CAI RIVER MOUTH (NHATRANG) AND PHANTHIET BAY IN TYPICAL LOCAL WIND CONDITIONS Bu Hong Long, Le Dnh Mau Insttute of Oceanography ABSTRACT Based on the hydrodynac odel and Shore Protecton Manual (CERC - USA) we have calculated wave feld characterstcs n the typcal wnd condtons (wnd velocty equal to 13/s n the hgh frequency drecton of the wnd rege). Coparson between easured and calculated wave paraeters was presented and these results were corresponded to each other. The followng an wave characterstcs were calculated: -Pattern of the refracton wave feld. -Average wave heght feld. -Longshore current velocty feld n surf zone. Fro dstrbuton features of wave feld characterstcs n research areas, t could be suarzed as followng: - The foraton of wave felds n the research areas was unequal because of ther local dfference of hydroeteorologcal condtons, rver dscharge, botto relef - At Cuada (Da outh, Hoan) area n the N drecton of ncdent wave feld, wave has caused serous varaton of the coastlne. The coastlne n the whole regon, especally, at the south of the outh was eroded and the foreland n the north of the outh was deposted. - At Ca rver outh (Nhatrang) area n the E drecton of ncdent wave feld, wave has effected strongly and drectly to the nshore and channel structure. - At Phanthet bay area n the SW drecton of ncdent wave feld, wave has effected strongly to the whole shorelne fro Da pont to Ne pont and caused serous eroson. CAÙC ÑAËC ÑIEÅM CUÛA TRÖÔØNG SOÙNG, AÛNH HÖÔÛNG CUÛA CHUÙNG TÔÙI CAÙC QUAÙ TRÌNH XOÙI LÔ Û- BOÀI TUÏ TAÏI CÖÛA ÑAÏI (HOÄI AN), CÖÛA SOÂNG CAÙI (NHA TRANG), VÒNH PHAN THIEÁT TRONG CAÙC ÑIEÀU KIEÄN GIOÙ ÑÒA PHÖÔNG ÑIEÅN HÌNH Buø Hoàng Long, Leâ Ñình Maàøøu Veän Haû Döông Hoïc TOÙM TAÉT Baèng caùc oâ hình soá trò vaø qu phaï SPM (CERC-Myõ), chuùng toâ tính toaùn caùc ñaëc tröng soùng ven bôø öùng vôù tröôøng goù uøa aïnh, oån ñònh (goù caáp 6), cho caùc höôùng taïo soùng coù taàn suaát cao. Keát quaû tính toaùn ñaõ ñöôïc keå chöùng baèng soá leäu ño ñaïc töø naê 1997 trong khuoân khoå caùc ñeà taø: caáp Cô sô û(1997, 1998, 1999); caáp Trung taâ (1998, 1999); caáp Nhaø nöôùc -ñeà taø KHCN68 (1997, 1998,1999, ) cuûa Veän Haû Döông Hoïc [1, 4, 5, 6]. Caùc ñaëc tröng cuûa tröôøng soùng ven bôø ñöôïc tính toaùn laø: -Tröôøng ta soùng khuùc xaï. -Tröôøng ñoä cao soùng trung bình. -Tröôøng toác ñoä doøng chaûy doïc bôø do soùng ñoå nhaøo gaây ra. Töø söï phaân boá caùc ñaëc tröng cuûa tröôøng soùng ven bôø chuùng toâ böôùc ñaàu ñaùnh gaù taùc ñoäng cuûa chuùng ñeán quaù trình beán ñoäng ñöôøng bôø. Ñeå noå baät laø ñeàu keän taïo soùng taï oã khu vöïc ngheân cöùu coù söï khaùc beät do söï khaùc nhau veà cheá ñoä khí töôïng thuûy vaên, löu löôïng nöôùc soâng vaø ñaëc ñeå ñòa hình Taï vuøng beån cöûa Ñaï (Hoä An) soùng höôùng baéc 1
gaây ra söï beán ñoäng ñöôøng bôø aïnh nhaát: xoù lôû toaøn daû ven bôø nhaát laø bôø phía na cöûa Ñaï, boà tuï aïnh taï uõ bôø phía baéc cöûa Ñaï. Taï vuøng beån cöûa soâng Caù (Nha Trang) soùng höôùng ñoâng taùc ñoäng aïnh nhaát gaây neân söï beán ñoå ñöôøng bôø vaø caùc baõ ngaà beân ngoaø cöûa soâng. Taï vuøng beån vònh Phan Theát soùng höôùng taây na gaây ra söï xoù lôû treân toaøn daû ven bôø töø uõ Ñaù uõ Neù. INTRODUCTION In the coastal zone of Central Vetna (fro Thuathen - Hue to Bnhthuan provnces), the varant processes of shorelne have been happenng very serously, whch nduced the eroson - deposton processes of the shorelne and especally the oveent of the rver outh. These processes have effected drectly to the lfe of the local people, so the study on laws and predcton of erosondeposton processes n the coastal zone and rver outh was a great nterest of local governent and scentst. In ths paper the authors only present ther researched and calculated results for wave feld characterstcs n the coastal zone and prelnary estaton of pacts of wave feld upon the eroson-deposton processes at Cuada (Hoan), Ca rver outh (Nhatrang) and Phanthet bay (Fg. 1). These areas can represent the coastal zone of Central Vetna by coplexty n the foraton of coastal wave feld and features of eroson - deposton processes. In addton, n recent years (1997- ) at the above areas, the Insttute of Oceanography has carred out any nvestgatons to deterne the prncpal causes, whch control the varaton of the shorelne and rver outh. Data have been collected, theren data of wave feld n the coastal zone were especally nterested. + Cuada (Hoan) area - Botto topography of Quangna area was taken fro the ap wth scale of 1/. ssued n 1977 by Vetnaese Navy and botto topography of Cuada (Hoan) area was taken fro the ap of KHCN68 Natonal Project wth scale of 1/5. ade n 1998. - Other necessary and related data were collected fro surveys n Septeber 1997, May 1998, August 1999, January of the Natonal Project KHCN68 [5]. + Ca rver outh (Nhatrang) -Botto topography of Nhatrang bay was taken fro the ap wth scale of 1/5. ssued n 1964 by Vetnaese Navy and botto topography of Ca rver outh (Nhatrang) was taken fro the ap of CS98.1 Project wth scale of 1/5. ade n 1998. - Other necessary and related data were collected fro surveys n July 1996, Aprl 1997, Aprl 1998, Noveber 1998 of Projects CS97.3 and CS98.1 [4, 6]. + Phanthet bay - Botto topography of Phanthet bay was taken fro the ap wth scale of 1/. ssued n 1976 by Vetnaese Navy. - Other necessary and related data were collected fro surveys n August 1997, June 1998, October 1998 of Phanthet bay Project [1]. BASICS AND METHODS FOR CALCULATION OF WAVE CHARACTERISTICS 1. Calculaton of wave eleent n deep sea - Wave paraeters were calculated based on power spectra ethod. Basc equaton s: V, r cos E h h (1) Where: h - average wave heght; V - wnd velocty; r( ) - wnd fetch length n the drecton ; - angle of ncdent wnd drecton, that angulates wth the noral of shorelne; E : energy coponent of waves wthn the angle of - / to + / (fro total energy of - / to / ), s defned fro E = E( 1 ) - E( ). Wth:
/ cos d 1. sn E cos d 1 () 18 -Wave characterstcs (heght, perod) were calculated n the coplex condtons: wnd feld s varant n space and te and shorelne shape s coplex varaton. We used the progra of the Russan Natonal Insttute of Oceanography, whch was foratted by power spectra ethod. Theren coponents of total wave heght ( h ) and average wave perod were deterned fro followng relatons [3]: gh g. X 1/ 3.417( ) (3a) V V gt g. X ) 4 / 5 17.3( (3b) V V gh gt 5 /1.13( ) (3c) V V g g. h 18.7( V V ) 3 / 5 (3d). Calculaton of wave feld n the shallow water Modfed equaton (1) wll be: h V, r cos K s E h tb (4) Where: K s = K R x K d x K T (5) - If t s supposed that the wave perod s a constant, the celerty of a wave anly depends on the local water depth. Refracton coeffcent (K R ) was solved fro wave rays of dfferental equaton syste, refracton wave zone s fro H=.65 o to breaker zone. - Transforaton coeffcent was calculated ch KH by: K T = (6) KH + sh KH - Dffracton coeffcent (Kd) was calculated accordng to SPM []. 3. Calculaton of wave characterstcs n breaker zone Accordng to SPM [] wave characterstc was calculated as below: -At the surf zone below crteron s used: h b =.78H (7) -At the breaker zone used crteron of Weggel (197) : bh h b = (8) 1 ( ah / g ) Where: a=43.75/1-e (-19) /; b=1.56/1+e (-19.5) / -Wave nduced alongshore current velocty accordng to Longuet-Hggns (197) V b =M 1 (gh b ) 1/ sn b (c/s) (9) Where: b angle between ncdent wave ray wth orthogonal contour n the shallow water ( o ). M1 coeffcent dependng on frctonal coeffcent and perturb coeffcent, by experent of Galvn and Eagleson, 1965 and t s supposed M 1 =.7. CALCULATION OF WAVE FIELD CHARACTERISTICS 1. Features of studed areas We have calculated wave characterstcs n the typcal wnd condton: wnd velocty s equal to 13/s n the hgh frequency drecton. General wave feld characterstcs n the deep sea are calculated, obtaned result s h o =1.7, =6.7s. 1.1. Cuada (Hoan) area (Fgs. 1b,, 3, 4) -Statstcal wnd data fro 1977 to 1997 [5] showed that the ajor drectons forng wave feld were N, NE, E. Ths s an open sea, but n the offshore area, Cha sland (about 8k wde) whch s located about 1k n the NE drecton far fro Cuada s barrer holdng back the NE wave. Therefore, Cuada s fallen down nto shadow zone of wave n the NE onsoon. Relef slope s about.4-.5. Here, the bars syste s fored very strongly n front of the outh; nteracton between rver and sea s very strong. -Refracton and dffracton coeffcents are calculated n the whole Hoan area n the ncdent wave fro N, NE, E drectons by grd syste (space nterval: X= Y=1k). -Wave characterstcs n Cuada were calculated by grd syste (space nterval: X= Y=). I.. Ca rver outh (Nhatrang) area (Fgs. 1c, 6, 7) 3
-Statstcal wnd data fro 1983 to 1994 [1] showed that the ajor drectons forng wave feld were NE, E, SE. Ths s a relatvely close sea area, because Nhatrang bay s bounded by forelands n the South, the North and Tre sland n the offshore. Therefore, n the SE onsoon t s very strongly effected by dffracton stress; n the NE onsoon t s fallen down nto shadow zone of ncdent wave. Relef slope s relatvely strong (about.7-.), so wave energy fro deep sea s effected strongly and drectly toward the shorelne. Here, the nteracton between rver and sea s average. -Refracton and dffracton coeffcents are calculated for the whole Nhatrang bay fro NE, E, SE drectons by grd syste (space nterval: X= Y=). -Wave characterstcs n Ca rver outh are calculated by grd syste (space nterval: X= Y=5). I.3. Phanthet bay area (Fgs. 1d, 8, 9) - Statstcal wnd data fro Hydroeteorology Staton at Phuquy sland showed that the ajor drectons forng wave feld were NE, SW [1]. Phanthet bay s a relatvely open sea area blocked by foreland n the South and North. Therefore, n the NE and SW onsoons, t s fallen down nto shadow zone of ncdent wave feld. Relef slope s relatvely sall (about.-.3), so when wave fro deep-sea travels to the coastal zone, ts energy wll be reduced. Here, nteracton between rver and sea s weak. -Refracton, dffracton coeffcents and wave characterstcs are calculated n the whole Phanthet bay n the ncdent wave fro NE, SW drectons by grd syste (space nterval: X= Y=1k).. Coparsons between the calculated and the observed data The observed wave data by GM61 (Russa) and AWH16-M1 (Japan) autoatc recorders have been analyzed and processed. Stes of wave observng statons are shown n Fgs. 1b, 1c, 1d. Durng the feld surveys, the data observaton s conducted spontaneously for soe perods on both the wave data and wnd data. The wave heght and the drecton n the deep waters can be deterned by the prevously entoned way. The tdal level at each staton at any te s deterned by the dfference between the ean sea level (obtaned fro wave recorder). The calculated wave heght has been copared wth the observed ones at soe statons as below: Table 1: Coparsons between observed and calculated wave paraeters at Nhatrang bay (Locaton: =1 o 15.46N, =9 o 11.94E; Depth: H ; Date: 5-6/4/1998) Date and tes 15h/5/4 17h/5/4 19h/5/4 h/5/4 Wnd features NE (6/s) NE (5/s) NE (6/s) ENE (9/s) Wave features G/L G/L G/L G Measured average wave heght () Calculated average wave heght () Error (%) Measured average wave perod (s) Calculated average wave perod (s) Error (%) Measured wave drecton( o ) Calculated wave drecton( o ) Error ( ).4.3 5 6.7 3.8 43 65 5 15.4.3 5 7. 3.5 5 65 47 18.4.3 5 6.1 4. 34 65 5 15 Notes: G/L - Wnd wave cobne wth Swell; G - Wnd wave; L - Swell.5.6 5.9 4.5 3 9 79 11 4
Table : Coparsons between observed and calculated wave paraeters at Phanthet bay (Locaton: = o 55.9N, =8 o 1.5E; Depth: H ; Date: 13//1998) Date and tes 15h/13/6/98 1h/13//98 14h/13//98 16h/13//98 Wnd features S (5/s) SW (5/s) S (7/s) S (4/s) Wave features G/L G/L G G/L Measured average wave heght () Calculated average wave heght () Error (%) Measured average wave perod (s) Calculated average wave perod (s) Error (%).1.15 5 6.5 3.8 41.15. 3 7. 4. 4.45.4 5.3 4.5 15 Notes: G/L - Wnd wave cobne wth Swell; G - Wnd wave; L - Swell.4.3 5 5. 4. 16 At the Cuada (Hoan), we have copared the calculated and observed wave felds, whch were obtaned fro 167 seral observaton statons and 4 long-te statons n August, 1999 (KHCN68 Natonal Project). Coparatve results are shown n Fg. 5. In Tables 1, and Fg. 5 the coparatve results between easured and calculated wave paraeters were presented and these results were relatvely satsfed to practce. However, calculated results are good whle swell feld s weak or equal to zero; durng strong swell feld there ust be have two statons to easure wave paraeter at soete, one staton s located at the deep sea zone, the other staton s located at the nearshore zone. FEATURES OF WAVE FIELD IN THE NEARSHORE ZONE AND ITS EFFECTS TO EROSION DEPOSITION PROCESSES Accordng to LEO data [], the longshore transport rate was calculated by: I L =.39 P LS () P LS = ghsbwv 5 V ( )( ) V LEO LH C f (11) Where: I L Longshore transport rate, P LS longshore energy flux factor, - flud densty, W wdth of surf zone, V LEO average longshore current velocty due to breakng wave, C f frcton factor, X dstance to dye patch fro shorelne, (V/V ) LH the densonless longshore current. The equatons () and (11) showed that relaton of longshore transport rate (I L ) and average longshore current velocty due to breakng wave (V LEO ) was strong. Therefore, we can coent upon eroson deposton processes by dstrbuton of longshore current velocty feld. 1. Cuada (Hoan) area - In the N drecton of ncdent wave (Fg. ): ost of studed area was strongly effected by ncdent wave feld, especally at the bars syste n the north of the outh. The wave heght n the northern part of the outh was h tb., n the southern part was h tb 1.. The wave nduced alongshore current alost oved southward, especally at the bars syste n the southern part of the outh and the hghest velocty was recorded at Anbang, Phuoctrach wth values of V L (ean).-.3/s, V L (ax).8-1./s. Generally, ost of coastlne was eroded, especally the bar syste and rver bank n the southern part of the outh were ost eroded. Foreland and bars n the northern part of the outh were strongly deposted. -In the NE drecton of ncdent wave (Fg. 3): ost of studed area wasn t affected by wave, because the whole area was n the wave shadow regon of Cha sland. The wave heght n front of the outh and at two banks was h tb.5-1., at the southern part (Dongson) was h tb 1., the hghest value was recorded at Anbang h tb..the wave nduced alongshore current alost oved 5
toward the south (n the southern part of the outh) and oved toward the north (n the northern part of the outh). Generally, the velocty of the alongshore current was slow V L (ean).1-./s. So, the sedent oveent dd not occur n ost of coastal area of the outh. -In the E drecton of ncdent wave (Fg. 4): ost of the outh regon was strongly affected by wave. Generally, the wave heght was h tb 1.5-., especally at the bar syste n the southern part of the outh. The wave nduced alongshore current alost oved toward the north, especally at the northern bars and shore of the outh, here current velocty reached to V L (ax).8-1./s. In the southern shore of the outh ths value was V L (ean)./s. Therefore, ost of the southern shore of the outh was deposted, especally at the southern foreland the deposton happened at the hghest rate. The northern shore of the outh was eroded, whle the southern foreland of the outh was deposted.. Ca rver outh (Nhatrang) -In the NE drecton of ncdent wave (Fg. 6): the wave heght n ost of the northern regon of the studed area was h tb.5, because t was n the shadow regon of ncdent wave. Around the War Meoral ths value was h tb..the wave nduced alongshore current alost oved toward the south, generally, V L (ean).3-.4/s, V L (ax) 1.-1./s, ost of the shore fro Lau Ong Tu to the War Meoral was ost strongly effected by the wave, so the eroson happened ost serously. -In the E drecton of ncdent wave (Fg. 7): ost of the coastal regon especally at Lau Ong Tu the value of wave heght was h tb.. The wave nduced alongshore current alost oved toward the south, but at several shorelne sectons, the drecton of alongshore current was rregular, generally, V L (ean).3/s; at the War Meoral and Lau Ong Tu V L (ax) 1.-1./s. Most of the coastal zone of the studed area was strongly effected by wave and eroson deposton processes. 3. Phanthet bay area -In the NE drecton of ncdent wave (Fg. 8): ost of the studed area wasn t strongly affected by ncdent wave, the wave heght was h tb.5-.7, because t was n the wave shadow regon of Ne foreland. In the regon fro Da to Ne forelands ths value was h tb <.5; the regon around Ne and Checa forelands was ost strongly effectedby wave, h tb 1.-1.5. -In the SW drecton of ncdent wave (Fg. 9): ost of the regon fro Checa foreland to Caty rver outh wasn t strongly affected by wave, h tb <.5, because t was n the wave shadow regon of Checa foreland. The regon fro Da to Ne forelands was strongly effected by wave, h tb 1.-1., ths regon has been strongly eroded, especally the regon fro Thenlong to Ne foreland was ost strongly effected, h tb >1.5. CONCLUSION - In the coastal zone of Vetna, especally the central regon (fro Thuathen- Hue to Bnhthuan provnces), varant processes of coastlne, channel, bars systes n front of the rver outh have been happenng very serously. However, these processes were not unfed at the above areas. These areas were dfferent n hydroeteorologcal condtons, botto relef features, rver dscharge The nteracton level between rver and sea caused varant processes of coastlne, the strongest levels were recorded at Cuada (Hoan), Ca rver outh (Nhatrang) and Phanthet bay. The pact level of wave feld to the coastal regon was strongest at Cuada (Hoan), then at Ca rver outh (Nhatrang) and Phanthet bay. - At Cuada (Hoan) the varant process of the shorelne was a consequence of nteracton process between rver and sea, n whch Cha sland played an portant role n stoppng the wave. The underwater bar syste n Cuada was ether the cause and the consequence of 6
above hydro-ltho-dynac process. As the wnd feld n the N drecton s strongest and has hghest frequency, Cuada s contnung to ove Southward. - At the Ca rver outh (Nhatrang) the relef slope s hgh (.), therefore ncdent wave cong fro deep sea alost drectly pacted to the nearshore regon. However, ths s a relatvely close bay, only the NE drecton wave, especally E drecton one strongly effected to nearshore regon. Generally, the shore fro Lau Ong Tu to the War Meoral was pacted by the wave feld, so ths regon was eroded very strongly. The bar syste and foreland n the south of the outh were deposted n the SE onsoon. At present, perhaps Ca rver outh would be oved northward, but there s a headstone at the northern bank of the outh and now ts ste s fxed. - Phanthet bay s a relatvely open arne area, the an drectons of ncdent wave are NE and SW, however, ost of the area s prevented fro the wave by Ne and Kega forelands. The regon around Ne foreland and nearshore regon fro Da to Ne forelands were pacted by wave. The shore fro Da to Ne forelands was eroded very strongly n the SW onsoon. Because the relef slope s sall, the wave energy fro deep sea propagated nto the nearshore regon and decreased.,. Table 3: Sybols used n the paper Seres nuber Sybol Physcal eanng Unts 1 3 4 5 6 7 8 9 11 1 13 14 15 h h tb h b V L V T H K C, C g X Average wave heght n deep water Average wave heght n shallow water Average breaker heght Average perod of wave Average wave length n shallow water Average wave length n deep water Longshore current velocty nduced by wave Wnd speed Duraton of wnd actvty Botto depth Relef slope Wave nuber Wave celerty n deep and shallow waters Acceleraton of gravty Wave fetch length s /s /s h 1/s /s k REFERENCES 1. Bu Hong Long (Edtor), 1999. Actual stuaton and prncpal orentaton of anageent and reasonable developent of the Phanthet bay. Scentfc Report. Insttute of Oceanography, Nhatrang, 155 pp. (In Vetnaese). Departent of the Ary. US Ary corps of Engneers, 1984. Shore protecton anual. Washngton. DC. Volue I, 514 pp. 3. Krölov. IU. M, 1969. Spectru ethod for calculatng arne wave and applyng to constructon n waters. Transp. Publ. House Moskva Press, 133 pp. (In Russan) 4. Le Dnh Mau (Edtor), 1997. Copleton of the prograe for calculaton of eleents of wnd wave n the Nhatrang bay. Scentfc Report of Project 97.3. Insttute of Oceanography, Nhatrang, 93 pp. 5. Le Phuoc Trnh (Edtor), 1998. Study on 7
laws and predcton of eroson-deposton habtude n the coastal zone and rver ouths of Vetna. Scentfc Report of Natonal Project- KHCN68. Insttute of Oceanography, Nhatrang, 3 pp. (In Vetnaese) 6. Nguyen Ba Xuan (Edtor), 1998. Studyng and estatng the pacts of Ca rver water to the hydrologcal and hydro-lthodynacal condtons n the Ca rver estuary. Scentfc Report of Project- 98.1. Insttute of Oceanography, Nhatrang, 15 pp. (In Vetnaese) 7. The Board of Hydro-eteorology, 1991. Manual of coastal hydrologcal observaton. Hano, 17 pp. (In Vetnaese) 8
(a) o o o o o o o 4 6 8 1 11 4 4 o (b) 8.37 8.38 8.39 8.4 8.41 8.4 o o o Haphong Hanan sland o 15.89 Caan Phuoctrach HA4 15.89 o 18 o 18 15.88 B 15.88 o 16 Hoan o 16 o 14 o 1 o Nhatrang Phanthet : Studed areas o 14 o 1 o o o 8 o o o o o 8 o 4 6 8 1 11 15.87 B4 HA5 Duyha 15.86 : Eroson regon ( 1967-1998) : Deposton regon ( 1967-1998) 15.85 Dongson 8.37 8.38 8.39 8.4 8.41 8.4 15.87 15.86 15.85 6 5 (c) Honchong 5 15 5 3 35 3 Thenlong Longson Halong Phanthet Da headland 5 (d) Ca rver 3 5-55'N 4 Lau Ong Tu 3 ( x 1k) 15 Thanhbnh 15-45'N 5 5 War Meoral 5 15 5 3 35 4 Legend: - Eroson regon. Ke Ga pont 5 : Contour of depth n (n LSL) 5 15 5 ( x 1k) 3 35 8-'E 8-'E 8-'E - Locaton of easurng statons Fgure 1: Locaton and features of researched areas: Da outh (Hoan Fg. b); Ca rver outh (Nhatrang Fg. c); Phanthet bay (Fg. d) 9
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