Received 11 November 2005, revised 13 February 2006, accepted 14 February 2006.

Transcription

1 Papers Experiments on wave motion and suspended sediment concentration atnanghai,cangio mangrove forest, Southern Vietnam OCEANOLOGIA, 48(1), 26. pp C 26,byInstituteof Oceanology PAS. KEYWORDS Wave motion Suspended sediment concentration Can Gio mangrove forest Southern Vietnam Vo Luong Hong Phuoc Stanisław R. Massel Institute of Oceanology, Polish Academy of Sciences, Powstańców Warszawy 55, PL Sopot, Poland; vlhphuoc@iopan.gda.pl Received 11 November 25, revised 13 February 26, accepted 14 February 26. Abstract Biogeochemical and trophodynamic processes as well as hydrodynamic factors play a major role in the structure and function of mangrove ecosystems. This study outlines field experiments on wave motion and suspended sediment concentration carried out at Nang Hai, Can Gio mangrove forest, Southern Vietnam. Pressure sensors were used to measure sea surface elevation, and Optical Backscatter Sensors (OBS) were applied to detect infrared(ir) radiation scattered from suspended particles in order to measure turbidity and suspended sediment concentrations. The experimental results indicate that most of the energy is dissipated inside the mangrove forest as a result of wave-trunk interactions and wave breaking. The suspended sediment concentration depends on wave intensity and tidal current velocity. Wave action is one of the main factors forcing sediment transport and coastalerosionatthestudysite;eventhewavefieldatthestudysiteisnotso strong. The establishment of mangrove vegetation can encourage the deposition of sediment, or at least the retention of the flood-tide sediment influx. The complete text of the paper is available at

2 24 V.L.HongPhuoc,S.R.Massel 1. Introduction Mangroves are very important ecosystems at the boundary between terrestrial and marine environments in tropical and subtropical zones. Coveringabout181km 2 ofthecoastalareasinthetropics,mangroves are considered highly productive yet vulnerable to the impacts of nature and human beings. Mangroves have a beneficial effect on the surrounding environment in that they are an important food supply for humans, protect the land from erosion, facilitate alluvium deposition, and mitigate the effects of typhoons and floods. Mangroves are also important as breeding and nursery grounds for prawns, fish and other marine and terrestrial organisms. In addition, they are a source of timber for firewood, construction, etc. (Snedaker& Snedaker 1984, Tuan et al. 22). Biogeochemical and trophodynamic processes in mangrove forests, and the structure of such forests and their growth are intimately linked to water movements due to tides and waves(massel et al. 1999). Numerous studies of the physical processes within mangrove forests have been made in the past. They have generally focused on riverine-type mangrove forests and tidally periodic processes(wolanski et al. 1992, Furukawa& Wolanski 1996, Furukawa et al. 1997). However, studies of wave processes within the mangrove forests themselves are few. The significant attenuation of surface waves over relatively short distances was studied by Mazda et al.(1997a) and Massel et al.(1999), and a few experimental studies in mangrove forests have been conducted. In particular, field measurements were undertaken on the muddycoast of Thai Binh province, in the Tong King Delta, Vietnam(Mazdaetal.1997a).Itwasfoundthattherateofwavereduction varieswithwaterdepthandtheageofthetrees. Waveattenuationwas greater where the trees were older. Moreover, the efficiency of mangroves in protecting the coast from wave-induced erosion depends on the mangrove species(mazda et al. 1997b). However, these authors did not describe the mechanism of wave energy dissipation. These problems have recently been addressed by Massel et al.(1999), who developed a theoretical model for predicting the attenuation of wind-induced random surface waves in a mangrove forest of constant depth. Examples of numerical calculations as well as preliminary experimental results of wave attenuation through mangrove forests at Townsville(Australia) and Iriomote Island(Japan) were given. Both theoretical analysis and field experiments indicated that for shallow water depths, the combination of bottom friction with the effects of drag caused by mangrove roots and trunks produces a significant amount of attenuation over a relatively short distance. This paper reports on the field experiments on wave motion and wave-induced suspended sediments recently carried out in the Can Gio

3 Experiments on wave motion and suspended sediment concentration mangrove forest, Southern Vietnam. The field observations provide data on wave parameters and suspended sediment concentration, as well as on bottom current velocity, topography and mangrove characteristics. The measurements and data analysis make it clear that wave motion is one of the main factors driving sediment transport and accumulation/erosion processes in the mangrove environment. 2. Experimental equipment and procedures 2.1. Study site Can Gio mangrove forest, adjacent to Ho Chi Minh city(saigon), South Vietnam,isthefirstBiosphereReserveinVietnam,andhasatotalareaof 7574ha.(Fig.1).TheCanGioareahasacomplex,convolutednetworkof rivers and creeks, where the Vam Co, Saigon and Dong Nai Rivers discharge intothesea.cangioliesinarecentlyformed,soft,siltydeltawithan irregular, semi-diurnal tidal regime. Tidal amplitudes range from about 2 m atmeantideto4mduringspringtides.maximumtidalamplitudes,inthe rangeof4. 4.2m,arethehighestrecordedinVietnam.TheclimateofCan Gio is influenced mostly by the equatorial monsoons a rainy season(from May to October) with prevailing south-westerly winds, and a dry season (from November to April) with prevailing north-easterlies, creating waves upto2. 2.3minheight(Tuanetal.22). ThestudysiteisatthemouthoftheDongTranhestuary(Fig.1). TheDongTranhareaislessaffectedbystrongwind-inducedwaves,asit isshelteredbycapeslynhonandlonghoaeithersideoftheestuary. This situation is conducive to the formation and growth of mangroves. As aresult,onlywindsfromsetosswcancreatedirectwavespropagating todongtranh.thesitechosenforthewavemeasurementswasthenang Hai mangrove forest, located between the Rach Trung and Khe Ca creeks. NangHaiisonlyabout2.5kmdistantfromtheDongTranhestuaryandhas amoredirectinfluenceonwavemotionthantheothersites.nanghaiforest consists of mixed mangroves of Avicenia sp. and Rhizophora sp. in the first 1 meters; beyond this Rhizophora sp. are largely dominant. The field measurements were carried out for 16 days, during January and February25. Theinstrumentationtransect( N, E N, E)waschosenparalleltothedirectionofwave propagation towards the forest(fig. 1b). The topography of the study site was measured at 2 m intervals along the 88 m long instrumentation transect with a YA-28X theodolite. The zero reference level was chosen as the bottom level at Station 2(ST2).

4 22 Lao Cai Fan Si Pan Hanoi Hong Gai China Haiphong b latitude o N Laos Thailand Dao Phu Quoc Gulf of Thailand Vinh Long Xuyen Can Tho Con Dao Gulf of Tonkin Cambodia Hue Da Nang Ho Chi Minh City Qui Nhon Nha Trang Cam Ranh East Sea Khe Ðôi Creek ST5 ST3 ST4 ST1 ST2 a Binh Khanh longitude o E Dong Nai Prov. Dong Tranh River Nang Hai site Trung Creek Khe Cá Creek R C ô C ò Nha Be An Thai Dong Tam Thon Hiep Thanh An Ðông Hò a River Sông Hà Thành Cây Da Creek Tien Giang Prov. Ly Nhon Soai Rap Estuary Cangio Forestry Park Nga Bay River Can Thanh Long Hoa cap Âp Ðông Hoa 1 2 km Dong Tranh Dong Tranh River Long Hoa East Sea Nang Hai study site c 2 topography level [cm] ST2 OBS-Mk9, MK-111 ST3 OBS-3A, MK-111 ST4 MWR-I ST5 CTD ST1 WG-73W distance [m] Fig. 1. Location of the experimental area. Can Gio mangrove Biosphere Reserve, HoChiMinh,Vietnam(a);NangHaistudysiteandtransectofstations(b); Instrumentationlayouton3January25attheNangHaisite(c)

5 Experiments on wave motion and suspended sediment concentration Data collection and methods Instrument layout To determine the surface wave propagation within the mangrove forest and the induced sediment transport, wave instruments and the instruments measuring suspended sediment concentrations(ssc) were deployed along atransectfromtheedgeofthemangrovesforadistanceofabout8m into the forest(fig. 1c). All the instruments were cleaned daily to prevent errors due to mud adhering to the sensors. Additionally, other factors such as bottom current velocity, grain size distribution, bottom topography, and the characteristics of the mangrove trees(circumferences of trunks and roots, density of the forest) were determined Wave measurement Pressure sensors were used to measure sea surface elevation, and time serieswereanalyzedoverperiodsfrom2to3minuteswithasampling frequencyof2 5Hz.Pressuresensorsweremountedunderwateratafixed position to measure the variations in the height of the water column passing above them. Varioustypesofinstrumentswereusedtomeasurethewaves: awg- 73W wave gauge(valeport Co.), an MWR-I wave gauge(sanyo Sokki Co.),anOBS-3A(D&ACo.) andactd-66(valeportco.),asshown infig.1c.atstationst1,5minfrontofthemangroves,thewg-73w sensorwasmounted.1mabovetheseabed.thesamplingfrequencywas 4Hz,and248waveburstsampleswererecordedat3minuteintervals.The other instruments were spaced out along the transect through the mangroves atst3,st4andst5: themwr-iwasinstalledwithaburstof5hz and 6 samples every 3 minutes, the CTD pressure sensor(ctd-66, ValeportCo.)wasinstalledatasamplingfrequencyof4Hz,andtheOBS- 3A optical backscattering sensor was deployed to record the pressure with afrequencyof2hz.thepositionsofthesepressuresensorsinthemangroves were changed in response to water levels and wave heights during the field measurements. Fig. 1c shows the instrumentation layout on 3 January Measurement of suspended sediment concentration(ssc) As Optical Backscattering Sensors(OBS) are able to detect infrared (IR) radiation scattered from suspended particles, they are used to measure turbidity and suspended sediment concentrations. In this experiment, two OBS were installed in the mangrove forest(fig. 1c): an OBS-3A(D&A Co.) with.5-second sampling intervals 2 m from the mangrove edge

6 28 V.L.HongPhuoc,S.R.Massel (ST3), and a self-cleaning OBS-Mk9(James Cook University, Australia) 8mfromthemangroveedge(ST2)recordingeveryminutewitha3-min wiper time. Both sensors were calibrated in the standard laboratory way usingsedimentssampledatthestudysite(riddetal. 21).TwoMK (General Oceanics Inc.) current meters were positioned adjacent to the OBS sensors to measure the current velocities every half-hour. At 5 stations along the transect sediment samples were taken to analyze the grain size distribution(fig. 2). Soft and silty sediments with a size distributionfromveryfinesandtofineclayarecommonatthenanghai site. Bottom sediments contain between 2% and 52% of very fine silt andclay(<5 µm).underhighshearstress,siltsandclayswillremain in suspension and will tend not to be transported as bedload. Therefore, measurement of the suspended sediment concentration(ssc) is required to calculate the induced sediment transport in mangroves(rijn 1989, Mehta &Li23). 6 5 class weight [%] ST1 [%] ST2 [%] ST3 [%] ST4 [%] ST5 [%] < grain size [mm] Fig.2.GrainsizedistributionattheNangHaisite 3. Results and discussion 3.1. Wave data analysis Beforeanalysis,thewavedatawereeditedtoremovethespikesdueto instrument malfunctioning. Because of the tidal motion, spurious trends,

7 Experiments on wave motion and suspended sediment concentration or low frequency components with a wavelength longer than the recorded length, had to be removed. Moreover, as this study concentrated on the highest-energy part of the spectrum, high-frequency components with negligible energy had first to be filtered out(bendat& Piersol 1986, Hughes 1993, Massel 1996). Time domain statistics were extracted from the time series records, and individual waves were determined by zero-downcrossing. Waveheightwastakentobethetotalverticaldistancebetweenthewave crest(highest elevation of the wave) and the wave trough(lowest elevation of the wave). Figs3a,b,cshowthesignificantwaveheightsatstationsalongthe transect. The ranges of wave heights give the maximum, mean and minimum valuesfromtheobserveddata.therecordingtimeofthewavemotionwas selectedtoprovideachosenwaterdepth.thewaterdepthatst1varied from1.9m(fig.3a),2.1m(fig.3b)to2.5m(fig.3c)asaresultoftidal motion.thewaterdepthof2.5matst1waspracticallythehighestwater levelinthenanghaimangrovesandwasrecordedonlyduringthehigh spring tides in January 25. Therefore, wave height data for a water depth of2.5mweresparse:only4dataserieswerecollectedatwaterdepth h ST1 =2.5m,but1dataseriesfor h ST1 =2.1mand11for h ST1 =1.9m. Ingeneral,thewaveheight H ST1 atst1wasabout.35.4m.thewave height decreased very quickly over relatively small distances, especially in shallower waters. About 5% 7% of the wave energy was dissipated within only2minthemangroveforestwhenthewateratst1was1.9mor 2.1mdeep,whilethewaveheightfellbyabout5%overadistanceof4m whenthewaterwas2.5mdeep.afterthisabruptfall,waveheightthen continued to decrease only slightly. Theoretical models indicate that wave breaking and interactions between the waves and the mangrove vegetation are the dominant mechanisms of energy dissipation as waves propagate into the mangroves. After most of the waves have broken, the further slight decrease in wave height is due mainly to wave-trunk interaction(hong Phuoc in preparation). The wave heights at particular stations are random variables that follow certain probability laws. As the water depth strongly influenced the wave motion, Glukhovskiy s distribution for wave height for a finite water depth was applied(massel 1996). Fig. 4 illustrates a comparison of experimental data with the Glukhovskiy distribution for wave records at the edge of the mangroves(fig.4a)and2mfromthemangroveedge(fig.4b).ingeneral, the distribution of wave heights does follow a Glukhovskiy distribution. If the water depth decreases, the distribution becomes narrow and more symmetrical, because high waves disappear as a result of wave breaking, and

8 3 V.L.HongPhuoc,S.R.Massel a significant wave height Hs [m] distance x [m] water depth [m] b significant wave height Hs [m] distance x [m] water depth [m] Fig. 3. Experimental wave height attenuation with varying water depth at the NangHaimangrovesite: h = 1.9m(a), h = 2.1m(b), h = 2.5m(c). The filled circles denote mean values and the horizontal bars indicate the range of wave height changes

9 Experiments on wave motion and suspended sediment concentration c significant wave height Hs [m] distance x [m] water depth [m] Fig. 3.(continued) small waves attenuate because of their interaction with roots and trunks. Moreover, the most probable wave height shifts towards lower values. The wave spectrum presents the distribution of wave energy as a function of frequency. The Blackman-Turkey method was used to estimate frequency spectra.fig.5givesthespectralenergydensityatst1andst3(4m inside the mangroves). These spectra were obtained at high tide, when h ST1 =2.6m.Theyindicatethatenergyisstronglydissipatedwithinthe mangroveforestandintheshallowerwaterzone,wherewaterdepth h ST3 =1.35m;thespectrahave2peaks,oneinthelowerfrequencypartand other in the higher frequency part. This can be explained by the nonlinear interaction mechanisms in shallow waters Suspended sediment concentration(ssc) analysis Fig. 6 shows suspended sediment concentrations for 6 days, correspondingtotidesmeasuredattheoffshorestationinthedongtranhestuary, about1.6kmfromthestudysite(fig.6a).atst2,thehighestwater levelwasabout.65m.thehalf-hourlymaximumvelocitiesatst2inthe mangrovesareshowninfig.6b.itcanbeseenthatthesscchangesin accordance with current velocities(fig. 6c).

10 32 V.L.HongPhuoc,S.R.Massel a probability density distribution b experimental data 2 1 normalized wave height ST1 h = 2.1 m x = m Glukhovskiy distribution ST3 h =.65 m x = 2 m probability density distribution experimental data normalized wave height Glukhovskiy distribution Fig. 4. Comparison of the Glukhovskiy s probability density function for wave heightwithexperimentaldataatst1(a)andst3(b)on1february25at19: Fig.7showsthedetailedchangesinSSCatST2inonetidalcyclewhen thewaterlevel h max.65mforhighwaves(fig.7a)andweakwaves (Fig.7b).Whenwaveswereweak(H max ST2 =.1m),thevelocitywas

11 Experiments on wave motion and suspended sediment concentration normalized spectral density ST1 h = 2.6 m x = m ST3 h = 1.35 m x = 4 m normalized frequency Fig.5.WavespectrumatST1andST3on11January25at: low(.12ms 1 )andthesscwasalsolow,about.1kgm 3. For highwaves(h max ST2 =.6m)withahighvelocityupto.45ms 1,the SSCincreasedrapidlyto.3kgm 3.ThechangesinhighSSCsarechaotic and irregular, and independent of tides. Meanwhile, low SSCs change only slightlyinresponsetotidallevels;sscswerehigherduringtheebbandthe flow,butdecreasedatthecrestofthetidewhenthetidalcurrentisalmost zero.thecomparisonshowsthathighwavesplayamoredominantroleon SSC than tidal currents, especially when the water level is low enough. As a wave propagates into the mangrove forest over shallower depths, most of the wave energy will dissipate not only because of trunk-wave interaction but also because of depth-controlled wave breaking. Fig. 8 shows the suspended sediment concentration measured in January 25 during the highspringtide. Forahighwaterlevel,say h max at ST2 =1.1m,when thewavewashighenough(h max.6m),thetidalregimewasmainly responsible for the changes in SSC. Concentration is higher during the ebb andtheflow,anddecreaseswhenthetidecrestappears(fig.8a).the influence of wave motion is demonstrated by the high amplitude of SSC fluctuations and the sudden increase when the water level is lower(fig. 8b). This means that wave action strongly influences SSCs in shallower waters, where waves break and enhance sediment suspension. A mangrove forest can be considered an effective buffer zone against erosion processes. This can be demonstrated by comparing SSC values at two different points(stations ST2 and ST3) in the mangrove forest(fig. 9).

12 34 V.L.HongPhuoc,S.R.Massel a b c water level at the offshore station [m] maximum current -1 velocity [m s ] : 1.2 : 2.2 : 3.2 : 4.2 : 5.2 : 6.2 : time [dd.mm h:mm] 31.1 : 1.2 : 2.2 : 3.2 : 4.2 : 5.2 : 6.2 : time [dd.mm h:mm] suspended sediment -3 concentration [kg m ] : 1.2 : 2.2 : 3.2 : 4.2 : 5.2 : 6.2 : time [dd.mm h:mm] Fig. 6. Suspended sediment concentration during a 6-day experiment. Water level at the offshore station in the Dong Tranh estuary(a); maximum current velocity at ST2(b); suspended sediment concentration at ST2(c) ST2andST3wererespectivelylocated8and2mfromtheedgeofthe forest. Obviously, the stronger the wave action, the higher the SSC. When waveswereweak(h max in ST1.1m),theSSCatbothstationswerelow, about.6.7kgm 3,andtheSSCatST2wasonlyslightlyhigherthanat ST3.Ontheotherhand,whenwaveswerestronger(H max in ST1.6m), thesscwasmuchhigher: about.23kgm 3 atst2and.19kgm 3 at ST3. The lower concentrations at the more distant station demonstrate that wave energy decreases as waves propagate in the mangrove forest and

13 Experiments on wave motion and suspended sediment concentration a water level at ST2 [m] b high wave time [dd.mm hh:mm] : 1.2 6: : : 2.2 : suspended sediment -3 concentration [kg m ] water level at ST2 [m] suspended sediment -3 concentration [kg m ] : 5.2 8: : : : 6.2 4: weak wave time [dd.mm hh:mm] Fig. 7. Comparison of suspended sediment concentration in one tidal cycle in the caseofhighwaves(a)andweakwaves(b) that mangrove vegetation can encourage the retention of sediment influx due towavesorfloodtides.the2-minuterecordshowninfig.9corresponds tothetimewhentheinfluenceofwavemotionandtidalcurrentswere dominant Erosionatthestudysite The north-east monsoons can generate high waves propagating directly towards the Nang Hai site. Nevertheless, these experiments have shown that besides tidal currents, wave action can make a significant contribution to the increase in SSC in mangroves and to accelerate the movement of suspended sediments. Furthermore, when water levels in the mangroves are low, wave

14 36 V.L.HongPhuoc,S.R.Massel a 1.4 water level at ST2 [m] suspended sediment concentration [kg m ] 7.1 : 8.1 : 9.1 : 1.1 : 11.1 : time [dd.mm h:mm] b water level at ST2 [m] : 1.114:1.119: 11.1: 1.14: 11.19: time [dd.mm h:mm] suspended sediment concentration [kg m ] Fig. 8. Suspended sediment concentration for the high wave case: high spring tideinjanuary25(a),onetidalcycle(b) actionismorelikelytoaffectsscthantides.generally,waterlevelsatnang Hai are rather low. Fig. 1 illustrates the predicted maximum water level in25atthecangioopen-seastation.theannualwaterlevelmaximum occursinjanuaryandalmostalltidalpeaksarelowerthan4.m.when thewaterlevelattheopen-seastationreachesabout3.25m,therewillbe hardly any water in the Nang Hai mangroves. Similarly, when there are 4.mofwaterattheopen-seastation,thewaterlevelatST2intheNang Haimangrovesisabout.75m.Theoryshowsthatwhenthewaterlevelat ST2isabout.75mandifthereisahigherwave,sayofabout.4m,then

15 Experiments on wave motion and suspended sediment concentration suspended sediment concentration [kg m ] : 19:5 19:11 19:17 weak wave time: 19:, [hh:mm] 5: 5:5 5:11 5:17 high wave time: 5:, [hh:mm] SSC at ST2 for high waves SSC at ST2 for weak waves SSC at ST3 for high waves SSC at ST3 for weak waves Fig. 9. Comparison of suspended sediment concentrations at ST2 and ST3 in the caseofweakandhighwaveaction predicted maximum water level [m] date in 25 [dd.mm] Fig. 1. Predicted maximum open-sea water level off Can Gio in 25. (Source from Southern Regional Hydro-Meteorological Center, Vietnam)

16 38 V.L.HongPhuoc,S.R.Massel almost all the waves will dissipate just beyond the mangrove edge(hong Phuoc in prep.). Therefore, wave energy dissipation can cause movement of suspended sediment at the mangrove boundary itself. Furthermore,waveswillnotbestrongintherainyseason(fromMayto October),sincethemonsoonisfromtheSW.Fig.1alsoshowsthatannual waterlevelsarelowestfrommaytooctober,whichisequivalenttovery shallowwaterornowateratallatthenanghaisite. Fig.11showshowthetopographyatNangHaichangesduringthe year. Obviously, the rate of erosion changed rapidly, especially at the mangrove edge. Only in one year, the mangrove boundary shifted inwards about3mandthisisthetrend.withinthemangroveforest,erosionand accumulation along the transect were probably due mainly to current flows from neighbouring creeks. These results agree with the assessment of Mazda etal.(22)ofthecoastalerosioninlonghoavillage,wherethelandhas beenerodedatarateofapproximately5myear 1 sincetheearly2th century topography level [m] topography on 18 April 24 topography on 8 January 25 topography on 23 February 25 distance [m] boundary on 28 April 24 boundary on 23 February 25 Fig.11.ChangesoftopographyattheNangHaisite,CanGiomangroves,in 1year 4. Summary and conclusions TheCanGioareahasacomplexconvolutednetworkofriversand creeks, with a irregular, semi-diurnal tidal regime, and is influenced by

17 Experiments on wave motion and suspended sediment concentration the equatorial monsoons. Therefore, river flows, rainfalls, tidal currents and wave action are the main factors forcing the movement of suspended sediments. Field measurements carried out at the Nang Hai site indicate that most of the energy dissipated within the mangrove forest is due mainly to wave-trunk interactions and wave breaking. Suspended sediment concentrations depend on the wave intensity and tidal current velocity. The results show that wave action is one of the main factors inducing sediment transportanderosionprocessesatnanghai;eventhewavefieldinthedong Tranh estuary is not so strong. Furthermore, the results also demonstrate that mangrove vegetation can encourage the deposition of sediment, or at least the retention of the flood-tide sediment influx. A comparison of the theoretical model with the experimental data as well as the determination of wave and tide-induced sediment transport in the mangrove forest will be presented in a separate paper, which is in preparation. Acknowledgements This experimental study was partially supported by the Centre of Excellence for Shelf Sea Science(CeSSS) at the Institute of Oceanology, Poland. Sincere thanks are due to the SEDYMAN project, which provided logistical support. The authors would like to express their gratitude to Prof. LaT.Cang,NguyenC.ThanhM.Sc.,Mr. DangT.AnoftheHo Chi Minh University of Natural Sciences, and Drs. K. Schwarzer and K. Ricklefs of Kiel University for their invaluable advice and assistance with thefieldobservations,andfinally,tolet.x.lanm.sc. ofthesouthern Regional Hydro-Meteorological Center(SRHMC) for the weather and tide forecasts in Can Gio. References Bendat J. S., Piersol A. G., 1986, Random data. Analysis and measurement procedures, 2nd edn., Wiley-Intersci., New York, 566 pp. Furukawa K., Wolanski E., 1996, Sedimentation in mangrove forests, Mang. Salt Marsh.,1(1),3 1. Furukawa K., Wolanski E., Mueller H., 1997, Currents and sediment transport in mangrove forests, Estuar. Coast. Shelf Sci., 44(3), Hong Phuoc V. L., Energy dissipation in non-uniform mangrove forests of arbitrary water depth,(in preparation). Hughes S. A., 1993, Physical models and laboratory techniques in coastal engineering, World Scientific Publ., Singapore, 568 pp. Massel S. R., 1996, Ocean surface waves: their physics and prediction, World Scientific Publ., Singapore, 491 pp.

18 4 V.L.HongPhuoc,S.R.Massel Massel S. R., Furukawa K., Brinkman R. M., 1999, Surface wave propagation in mangrove forests, Fluid Dyn. Res., 24(4), MazdaY.,MagiM.,KogoM.,HongP.N.,1997a,Mangrovesasacoastalprotection fromwavesinthetongkingdelta,vietnam,mang.saltmarsh.,1(2), MazdaY.,MagiM.,NanaoH.,KogoM.,MiyagiT.,KanazawaN.,KobashiD., 22, Coastal erosion due to long-term human impact on mangrove forests, Wetlands Ecol. Manage., 1(1), 1 9. MazdaY.,WolanskiE.,KingB.,SaseA.,OhtsukaD.,MagiM.,1997b,Dragforce due to vegetation in mangrove swamps, Mang. Salt Marsh., 1(3), Mehta A., Li Y., 23, Principles and process modelling of cohessive sediment transport, Univ. Florida, Gainesville, 86 pp. RiddP.,DayG.,ThomasS.,HarradenceJ.,FoxD.,BuntJ.,RenagiO.,JagoC., 21, Measurement of sediment deposition rates using an optical backscatter sensor, Estuar. Coast. Shelf Sci., 52(2), Rijn,L.C.van,1989,Sedimenttransportbycurrentsandwaves,Vols.1,2,Delft Hydraulics, Delft. Snedaker S. C., Snedaker J. G., 1984, The mangrove ecosystem: research methods, UNESCO/SCOR, Paris, 251 pp. TuanL.D.,OanhK.T.T.,ThanhC.V.,QuyN.D.,22,CanGiomangrove biosphere reserve, Agric. Publ., Hanoi, 311 pp. Wolanski E., Mazda Y., Ridd P., 1992, Mangrove hydrodynamics,[in:] Tropical mangrove ecosystems, A. I. Robertson& D. M. Alongi(eds.), Am. Geophys. Union, Washington,