Most studies of the hydrodynamics and sediment transport in the swash zone in recent years have stressed the importance of swash processes in terms of science advancement and practical applications in this field of study. Clearly, the hydrodynamics of the swash zone are complex and not fully understood. The hydrodynamics of the swash zone characteristics likes swash water depth and velocity will affected the sediment transport mechanisms that mostly control beach face morphology especially under dry and wet periods in east coast region of Peninsular Malaysia. Field monitoring works at Desaru beach for data of rainfall distribution, morphological changes and swash characteristics likes flow depths and velocities have been conducted and analysed for both different periods (dry and wet). From observation, it is believed that rainfall distribution during the seasonal variation of dry and wet seasons in Malaysia affects the swash characteristics mainly by the infiltration processes in beach areas and significantly controlled the morphological changes in the swash zone

Abd. Wahab, Ahmad Khairi

Jamal, Mohamad Hidayat

Othman, Norasman

Samat, Shairul Rohaziawati

Universiti Teknologi Malaysia Institutional Repository

TABLE OF CONTENTSForeword by The Minister of Natural Resources and Environment MalaysiaForeword by The Vice Chancellor, Universiti Teknologi MalaysiaForeword by The Chairman of The Organising CommitteeAbstract Keynote SpeakersAbstracts- Flood Management- Watershed Management- Water Supply and Water Economics- Coastal Engineering and Management- Enviromental Science and Engineering- Mathematics and Computing Applications- Other Related DisciplinesFIELD INVESTIGATION OF SEASONAL VARIATIONS IMPACT ON FLOWDEPTHS AND VELOCITIES IN THE SWASH ZONENorasman Othman1, Ahmad Khairi Abd. Wahab2,3,Mohamad Hidayat Jamal2,3 and Shairul Rohaziawati Samat11Faculty of Civil Engineering and Earth Resources, Universiti Malaysia PahangE-mail: norasman@ump.edu.my, shairul@ump.edu.my2 Faculty of Civil Engineering, Universiti Teknologi Malaysia3Center for Coastal and Ocean Engineering (COEI), Universiti Teknologi MalaysiaE-mail:akhairi@utm.my, mhidayat@utm.myMost studies of the hydrodynamics and sediment transport in the swash zone in recent yearshave stressed the importance of swash processes in terms of science advancement and practicalapplications in this field of study. Clearly, the hydrodynamics of the swash zone are complexand not fully understood. The hydrodynamics of the swash zone characteristics likes swashwater depth and velocity will affected the sediment transport mechanisms that mostly controlbeach face morphology especially under dry and wet periods in east coast region of PeninsularMalaysia. Field monitoring works at Desaru beach for data of rainfall distribution,morphological changes and swash characteristics likes flow depths and velocities have beenconducted and analysed for both different periods (dry and wet). From observation, it isbelieved that rainfall distribution during the seasonal variation of dry and wet seasons inMalaysia affects the swash characteristics mainly by the infiltration processes in beach areasand significantly controlled the morphological changes in the swash zone.Keywords: beach morphology, swash zone characteristics, uprush, backwash, infiltrationIntroductionThe swash zone is commonly defined as the part of the beach area between theminimum limit of backwash and maximum limit of up-rush. A detailed schematicillustration about the swash zone area in coastal area was shown in Figure 1 and moststudies clearly found that the swash zone also recognised as the most energetic area ofbeach sediment movement, characterized by strong and unsteady flows, highturbulence levels, large suspended sediment concentrations, high sediment fluxes andrelatively rapid rates of morphological change (Baldock et al., 2005; Longo et al.,2002; Masselink and Puleo, 2006). Bakhtyar (2009) also reviewed and concluded thatthe swash zone hydrodynamics are poorly understood if compare to the surf zone.This is happen by the complexities of data collecting in intermittent, shallow, bubblyand rapidly varying flows, which makes data analysis and critical comparisonsbetween theory and observations difficult. As a result, there are only limited fieldmeasurements and experimental data on swash zone study. So, there is an urgent needto improve the currently available equipment or tools in order to conduct a highquality study in the swash zone. Swash processes are totally forced by surf zonewaves and are controlled by factors such as the beach morphology, sedimentcharacteristics and beach groundwater (Hughes and Turner, 1999). Due to itscomplexity and difficulty of making field measurements, the swash zone remains oneof the least understood regions of the nearshore (Hughes et al., 1997). Also theunderstanding of swash hydrodynamics likes bed levels, flow depths and velocitiesduring wave uprush and backwash in the swash zone is becoming very importantmechanism in the beach morphological development (Jensen et al., 2010; Baldock,2009).Figure 1: Schematic illustration of swash zone (Elfrink and Baldock, 2002)According to Elfrink and Baldock (2002), during the up-rush or backwashprocesses, water will infiltrate or exfiltrates the beach surface depends ongroundwater elevation, beach material permeability and degree of soil saturation. Thisall conditions will be significantly affected the flow depths and velocities duringup-rush and backwash processes, resulting the changes of beach morphologicalprofiles.In Malaysia especially in east coast region of Peninsular Malaysia, the annualvariations of seasonal period during dry (March until June) and wet (November untilJanuary) period will significantly controlled the condition of the beaches. During thedry period, there are very limited of rainfall events and this situation will besignificantly dropped the water table and moisture content in the beach area which isenhanced the infiltration process and resulting the accretion of the beaches.Meanwhile, during the wet period due to the northeast monsoon season effect, theexposed areas like the east coast of Peninsular Malaysia experience heavy rain spellsand contribute increasing level of water table and moisture content. This uniquecondition will enhanced the erosion rate at the near shore area. At the same time, itbelieved that the seasonal variation (dry and wet season) factor in Malaysia havesignificantly influence on the sediment transport processes especially in the swashzone. This paper aims to investigate the swash zone characteristics likes flow depthsand velocities during two different periods in order to help the understanding ofsediment transport processes impact from both characteristics in beach morphologychanges.Field SiteDesaru beach has 25 km of white and sandy beach plus lush tropical greenerywhich has made it as the well-known beach in Johor. It is approximately about 2°above the Equator, located at the east coast of Peninsular Malaysia, 98 km northeast ofJohor Bahru; capital city of Johor and about 460 km from Kuala Lumpur; federalcapital of Malaysia. Figure 2 shows the location of Desaru beach in the PeninsularMalaysia maps. The beach is composed of fine sand with a median diameter, D50 of0.2 0.4 mm. The beach experiences semidiurnal tide, with tide ranges ofapproximately 1.2 m.Figure 2: Location of Desaru beachMethods and InstrumentationsThere are several of field experiments were conducted during selected dry and wetperiod at Desaru beach in order to monitor and investigate the effect of seasonalvariations on sandy beach profile changes. All data for swash zone characteristics forthis field investigation likes flow depths and velocities were collected during the hightide.Figure 3: Photograph showing the PT and ACM were installed at the swash zoneFor the swash zone characteristics monitoring as shown in Figure 3, two types ofequipment were used for this study. Data on swash flow water depths were obtainedusing a pressure transducer (PT), which is two PTs buried 30 cm below the sand surfacewere deployed during this monitoring in the upper part of the swash zone area at aseparation distance about 5 meter. This method has been used by Baldock et al., (2005)to measure swash flow water depths changes over a time frame of several hours orinter-swash changes. More on that Jensen et al., (2010) concluded that the deploymentof self-logging pressure transducers in the swash zone is an accurate, easy and flexiblemethod to acquire highly detailed data on water levels and bed elevations.For swash velocities monitoring, about two Acoustic Current Meters (ACMs) weredeployed for data of swash velocities during uprush and backwash were recorded at thesame point as for PTs. The ACMs were installed as close to the bed as possible, about 1cm above the bed. Both the PTs and ACMs were synchronized at a same sampling timeat 1 Hz or every 1 sec per data and operated continuously during the experiment wererunning. For this paper, only the data from the upper swash point will be discussed here.Pressure Transducer(PT)Acoustic Current Meter(ACM)From this monitoring station, the data for swash water depths and velocities wereseparated and analyzed for every single uprush and backwash events during high tide.All these experiments also recorded by a video camera as a backup for next dataanalysis processes. This separation processes for every single uprush or backwashactivities were separated and analyzed by the direction of fluid velocities and alsorechecked by using recorded video for every experiment. The position of samplingbased on the beachface point (x = 35 m) and the tidal elevation (MHWS at 1.56 m LSD)at the field work were selected at approximately as the same high tide condition duringthe dry and wet periods. This condition setup is very important for this experiment inorder to get clear view about the effect of difference period in Malaysia at the sameposition on the swash flow water depths and velocities.especially for dry and wet periods,one rain gauge station (Figure 4) was installed close to the study area to record rainfalldata for every 10 min during the observation period. This rainfall data is very importantfor this study in order to relate the contribution of seasonal variations during the dry andwet periods to the swash zone characteristics. This station was checked monthly by aresearcher in order to make sure this rain gauge operated successfully and the data fromthis station was downloaded at the same time by using a laptop. This data also wasdouble-agency, Department of Drainage & Irrigation (DID) at Bandar Penawar rainfall station,which is about 10 km from the study area.Figure 4: Rain gauge station at Desaru beachData AnalysisIn order to determine the variation of different periods, rainfall data from the studyarea was used to determine the most suitable monitoring time for dry and wet periods atDesaru beach. Based on recorded rainfall data, month of March 2014 was identified assuitable dry period time due to lowest monthly rainfall occurred in February 2014.Meanwhile for the wet period, month of November 2013 was straightly selected basedon the heaviest rainfall distribution on that month due to the significant effect fromNortheast monsoon. For data analysis of swash characteristics, the aim is to compareand analyze the 300 seconds of selected swash characteristics likes flow water depthsand velocities for this field study under different seasonal variations (dry & wetperiods). In order to investigate the effect of local seasonal variations to swash zonecharacteristics, the flow water depth and velocity profiles were selected and analyzedfor the same highest tidal elevation during the dry and wet periods in the study area asshown in Figure 5 and Figure 6 respectively.Figure 5: Profiles of flow water depths and velocities during dry periodIn Figure 5, the profiles data for swash flow depths and velocities taken at the samehigh tide elevation on 16 March 2014 have shown the effect of seasonal variations onswash characteristics. During this month, the study area was experiencing the dryperiod and the end of the Northeast monsoon which has a lower wave attack andenergy. This condition totally affected the swash water depth and velocity levels tobecome lower than during the wet season as shown in Figure 6. For the swash waterdepths comparison, it clearly showed that decreasing of swash energy happened duringthis period with an average depth was 2.1 cm with the maximum and minimum valueswere 12.3 cm and 0 cm respectively. This an important condition can explain why mostberm was occurred at study area during the wet period before did not change due towave attack with the limitation of observed uprush length occurred as far as atlandward, which is shorter if compared with the furthest uprush length during the wetperiod. For the velocity data during this period, the highest values of uprush andbackwash were detected at 2.85 m/s and 1.58 m/s respectively. It clearly found that theswash velocities was decreasing for the uprush and backwash flow but the backwashvelocity was significantly reduced by greater infiltration effect due to believably highvalue of unsaturated beach surface during the dry period.Figure 6: Profiles of flow water depths and velocities during wet periodDuring the wet period in Figure 6, the average depth was 7.1 cm with the maximumand minimum values were 27.0 cm and 0 cm respectively. For the velocity data, thehighest values of uprush and backwash were recorded at 3.76 m/s and 2.13 m/srespectively. The sampling point for data profiles during the wet period, when theuprush flow came, there was always a maximum instantaneous acceleration in uprushvelocity and followed by more steadily decrease until zero velocity at the end of theuprush flow. During this moment, it clearly showed that the maximum water depthalways occurred when zero velocity reading at the end of uprush flow. In contrast, whenthe backwash flow took the action in the swash zone, the velocity increased graduallyuntil it reached its maximum value. After that, the backwash flow depths and velocitiesdecreased to zero until the new uprush flow came. This observation was consistent withother field study work done by Hughes (1997).Table 1: Result Analysis of Swash Water Depth and VelocitySwash ZoneCharacteristicsParameters Dry Period Wet PeriodSwash WaterDepthMean Uprush, humean (cm) 4.6 13.8Mean Backwash, hbmean (cm) 2.2 10.4Ratio humean to hbmean 2.11 1.33Max Uprush, humax(cm) 12.3 27.0Max Backwash, hbmax (cm) 7.2 21.8Swash VelocityMean Uprush, vumean (m/s) 1.30 2.00Mean Backwash, vbmean (m/s) 0.35 1.08Ratio vumean to vbmean 3.71 1.85Max Uprush, vumax(m/s) 2.85 3.76Max Backwash, vbmax (m/s) 1.58 2.13From Table 1 as shown above, it is clearly shown that seasonal seasons at Desarubeach (dry and wet periods) have significantly affected the swash zone characteristicslikes swash water depth and velocity. For every single swash event of water depthanalysis, the ratio of mean water depth for uprush to backwash during dry and wetperiod is 2.11 and 1.33 respectively. These values are very important in order to explainthe seasonal variations effect on beach morphological changes. For example, the ratiovalue during the dry period was higher than the wet period due to the effects of lowervalue of moisture content and groundwater table. This phenomenon havepromoted the higher infiltration rate of swash flow into the sand beach and it is alsoexplained why the water depth of backwash have significantly reduced during dryperiod if compare during wet period. This unique phenomenon also explains why theratio of swash velocity for uprush to backwash flows during dry period (3.71) higherthan wet period (1.85). Still use the same mechanism such as swash water depthanalysis, the backwash flow velocity during dry period was significantly reduced about73.1% by greater infiltration effect and this phenomenon also have promoted theaccretion processes in the swash zone area due to deposition of sediment transportespecially during backwash activities. In contrast during wet period, the backwash flowvelocity reduced only about 46.0% and this value contributed by the high degree ofmoisture content and created slightly decreased the flow volume and velocity in theswash zone area especially at the upper part.ConclusionsThe patterns of swash water depths and velocities for every single uprush andbackwash during the dry period have shown why the accretion processes significantlyoccurred in the swash zone due to high infiltration effect during dry period. In thiscondition, the sediment or sand carried by swash water during the uprush and backwashflow has settled down easily on the beach face due to high infiltration process and thissituation put the beach in the accretion mode. However, in order to understand swashzone processes clearly and how these affect beach profile response, Horn (2006) stressesthe importance of direct measurements of key parameters such as hydraulicconductivity, moisture content and infiltration rates which must be carried out in thefield.Field measurements or monitoring have been relatively few in numbers andsurprisingly, in Malaysia, there has previously been no interest in such study orresearch of swash zone hydrodynamics if compare to the surf zone. This complicatednature of flow within the swash zone include intermittent, shallow, bubbly and rapidlyvarying flows, making data collecting and analysis comparison between theory andexperiments difficult to achieve. As a result, there are only limited experimental dataon swash zone study. There is an obvious need to attract academicians and researchersin our country especially from universities, government bodies and local agencies tostudy the swash zone sediment transport processes. The seasonal climatic factors thataffect the coastal groundwater table levels experienced here also would present anideal opportunity for ideal field experiments or site observations to be conducted here.Thus, the outcomes or findings of these studies at least would contribute to theknowledge of swash zone sediment transport and would be highly anticipated by thecoastal engineering scientific community around the world.AcknowledgementsThe authors would like to acknowledge the financial support provided by theUniversiti Teknologi Malaysia through Research University Grant No. 02H58 and07J10. We also would like to thank the Desaru Development One Holding Sdn. Bhd.for the permission to use the beach as a study area and also for the support in the fieldespecially during monitoring works. Norasman Othman is grateful to the Ministry ofHigher Education Malaysia and Universiti Malaysia Pahang for the sponsorship of hisresearch studies at the Universiti Teknologi Malaysia.ReferencesBakhtyar, R., Barry, D.A., Li, L., Jeng, D.S., Yeganeh-Bakhtiary, A.Y.,2009.Modeling sediment transport in the swash zone: A review. OceanEngineering 36, 767 783.Baldock, T.E., Barnes, M.P., Hughes, M.G., 2005. Field observation of instantaneouscross-shore free surface profiles and flow depths in the swash zone. Proc. CoastalDynamics 2005, Barcelona, ASCE.-by-wave bed-levels in the(2008) 1237-1242]. Coastal Engineering 56, 380-381.Elfrink, B., Baldock, T.E., 2002. Hydrodynamics and sediment transport in the swashzone: A review and perspectives. Coastal Engineering 45, 149 167.Horn, D.P., 2006. Measurements and modelling of beach groundwater flow in theswash-zone: a review. Continental Shelf Research 26, 622 652.Hughes, M.G., Turner, I., 1999. The beachface. In: Short, A.D. (Ed.), Handbook ofBeach and Shoreface Morphodynamics. John Wiley and Sons, pp. 119 144.Hughes, M.G., Masselink, G., Brander, R.W., 1997. Flow velocity and sedimenttransport in the swash zone of a steep beach. Mar. Geol. 138 (1 2), 91 103.Jensen, S.G., Aagaard, T., Baldock, T.E., 2010. Swash zone bed level changes andsediment entrainment at the surf-swash boundary. Proceedings of 32ndInternational Conference on Coastal Engineering (ICCE2010), Shanghai, China.Longo, S., Petti, M., Losada, I.J., 2002. Turbulence in the surf and swash zone: areview. Coastal Engineering 45, 129-147.Masselink, G., Puleo, J.A., 2006. Swash-zone morphodynamics. Continental ShelfResearch 26, 661-680.

Field investigation of seasonal variations impact on flow depths and velocities in the swash zone

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Field investigation of seasonal variations impact on flow depths and velocities in the swash zone

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