Dynamics of Sediment Transport and Erosion-Deposition Patterns in the Locality of a Detached Low-Crested Breakwater on a Cohesive Coast

Article Dynamics of Sediment Transport and Erosion-Deposition Patterns in the Locality of a Detached Low-Crested Breakwater on a Cohesive Coast

Arniza Fitri 1,*, Roslan Hashim 2,*, Soroush Abolfathi 3 and Khairul Nizam Abdul Maulud 4,5

1 School of Civil and Architectural Engineering, Nanchang Institute of Technology, Nanchang 330099, China 2 Department of Civil Engineering, Faculty of Engineering, Malaya University, Kuala Lumpur 50603, Malaysia 3 School of Engineering, University of Warwick, Coventry CV4 7AL, UK 4 Earth Observation Centre, Institute for Climate Change, Universiti Kebangsaan Malaysia, Bangi 43600, Selangor, Malaysia 5 Smart and Sustainable Township Research Centre, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, Bangi 43600, Selangor, Malaysia * Correspondence: arnizaﬁ[email protected] (A.F.); [email protected] (R.H.)

Received: 8 June 2019; Accepted: 13 August 2019; Published: 19 August 2019

Abstract: Understanding the dynamics of sediment transport and erosion-deposition patterns in the locality of a coastal structure is vital to evaluating the performance of coastal structures and predicting the changes in coastal dynamics caused by a specific structure. The nearshore hydro-morphodynamic responses to coastal structures vary widely, as these responses are complex functions with numerous parameters, including structural design, sediment and wave dynamics, angle of approach, slope of the coast and the materials making up the beach and structures. This study investigated the sediment transport and erosion-deposition patterns in the locality of a detached low-crested breakwater protecting the cohesive shore of Carey Island, Malaysia. The data used for this study were collected from field measurements and secondary sources from 2014 to 2015. Sea-bed elevations were monitored every two months starting from December 2014 to October 2015, in order to quantify the sea-bed changes and investigate the erosion-deposition patterns of the cohesive sediment due to the existence of the breakwater. In addition, numerical modelling was also performed to understand the impacts of the breakwater on the nearshore hydrodynamics and investigate the dynamics of fine sediment transport around the breakwater structure. A coupled two-dimensional hydrodynamics-sediment transport model based on Reynolds averaged Navier-Stokes (RANS) equations and cell-centered finite volume method with flexible meshing approach was adopted for this study. Analysis of the results showed that the detached breakwater reduced both current speed and wave height behind the structure by an average of 0.12 m/s and 0.1 m, respectively. Also, the breakwater made it possible for trapped suspended sediment to settle in a sheltered area by approximately 8 cm in height near to the first main segment of the breakwater, from 1 year after its construction. The numerical results were in line with the field measurements, where sediment accumulations were concentrated in the landward area behind the breakwater. In particular, sediment accumulations were concentrated along the main segments of the breakwater structure during the Northeast (NE) season, while concentration near the first main segment of the breakwater were recorded during the Southwest (SW) season. The assessment illustrated that the depositional patterns were influenced strongly by the variations in seasonal hydrodynamic conditions, sediment type, sediment supply and the structural design. Detached breakwaters are rarely considered for cohesive shores; hence, this study provides new, significant benefits for engineers, scientists and coastal management authorities with regard to seasonal dynamic changes affected by a detached breakwater and its performance on a cohesive coast.

Water 2019, 11, 1721; doi:10.3390/w11081721 www.mdpi.com/journal/water Water 2019, 11, 1721 2 of 28

Keywords: coastal dynamics; coastal resilience; cohesive sediment; detached low-crested breakwater; erosion-deposition pattern; artiﬁtial reef

1. Introduction Detached breakwaters are parallel barrier structures placed in the shallow nearshore water column, to protect any landform area behind them from the direct impacts of wave attack, currents, tides and storms surges [1,2]. Detached shore-parallel breakwaters are also known as artificial reefs. Low-crest breakwaters have small relative freeboard and are commonly built of quarry materials of homogeneous size [3]. Detached breakwaters are being used increasingly worldwide in the intertidal zones of non-cohesive (sand) coasts, to provide protection measures and mitigate erosion problems [4–7]. However, such coastal structures cause local changes to nearshore flow hydrodynamics and sediment dynamics in the coastal zone [7–9]; these are site-specific and affected by parameters such as sediment characteristics, climate conditions, structural configurations (design, shape, dimension, and material) and nearshore hydro-morphodynamics [2,5,10]. The complexity of morphodynamic changes due to the presence of coastal structures is higher when coastal structures are located on intertidal areas of cohesive shores, where significant temporal and spatial variation of the water depths exist [11]. Sediment transport in coastal areas is mainly influenced by dynamic nearshore processes and site-specific environmental conditions, including sediment characteristics, wind, currents, waves, tides and the exchange processes between estuaries and nearshore regions [12–16]. Coastal protection structures, such as a breakwater, can change flow patterns, hydrodynamics and sediment transport, which could then impact erosion-deposition patterns in the coastal zone [17–19]. Breakwaters can reduce the incident wave energy and impact on the sediment transport capacity, allowing sediment deposition on the shoreward sides of the structure [20,21]. An accurate description of the sediment transport around a protection structure is vital for predicting the effects of coastal structures on morphological changes in coastal zone [22]. Previous studies performed on shoreline changes due to the presence of detached breakwaters have mainly focused on non-cohesive (sand) coasts [23–28], while detached breakwaters are infrequently used at cohesive coasts due to the lack of understanding of complicated cohesive sediment dynamics impacts on nearshore morphodynamics [21,27–29]. The sloping bottom of the nearshore acts like a natural defence and protects the shore against waves, currents and storms [30,31] by dissipating the wave energy through wave breaking phenomena and depending on bottom friction coefficient [32]. The breaking wave may re-form and break again several times before finally rushing up the foreshore areas in the swash zone. However, in the presence of a breakwater in the nearshore area, the structure will interact with incident waves and dissipate some of the wave energy while the remaining turbulent enegy will be reflected and transmitted through the breakwater [33,34]. For submerged breakwater, the waves may simply transmit over the breakwater’s structure. However, when the crest of the breakwater is above the mean water level, waves generate a secondary flow at the toe of the structure or transmit through breakwater when the structure is permeable [35]. Previous research has reported that cohesive sediments have low settling velocities and therefore could be transported over a long distance before settling through water column [36–38]. However, the cohesive behaviour of fine sediments allow them to agglomerate and form bigger- sized aggregates called flocks, with higher settling velocities compared to single-particle fine sediments. Thus, flocks can settle in areas where fine sediments can never be deposited [39]. Generally, the formation of flocks can occur when suspended sediments in the water column exceeds 0.01 kg/m3 [39]. On a non-cohesive shore, presence of breakwaters lead to sand deposition on the lee side of the structure and formation of a sandbar (tombolo) [35,40]. The tombolo grows from the shore towards the structure. Prior to the sand deposition reaching the structure, the sandform is referred to as Water 2019, 11, 1721 3 of 28 salient [35,40]. If a breakwater is built perpendicular to the shoreline, the longshore sediment transport is cut at the breakwater; thus, significant morphological changes occur near the breakwater [22]. For a cohesive shore, the hydro-morphodynamic behaviour of sediments in presence of a detached breakwater is not yet fully understood and more research is needed. This paper provides valuable new information on the dynamic of cohesive sediment in presence of a detached low-crested breakwater, and improves our understanding of the complex hydro-morphodynamics of cohesive coasts which enables coastal planners, managers and engineers, to conduct accurate environmental assessment and design suitable coastal defence structures for cohesive shores. In the last few decades, the cohesive shore of Carey Island, Malaysia has been experiencing mangrove degradation and erosion problems due to human interventions in the coastal zone. To reduce erosion problems, a rehabilitation project has been carried out by constructing a detached low-crest breakwater near the degraded mangrove area. The impact of the detached breakwater on sediment transport dynamics and erosion-deposition patterns on the cohesive shore had not been fully investigated to this date. Hence, this study aims to understand the sediment transport and erosion-deposition patterns in the locality of a detached low-crested breakwater on the cohesive shore of Carey Island, Malaysia under seasonal variation of hydrodynamic conditions. This study specifically investigates: (i) the characteristics of, and changes in, the hydrodynamics of the cohesive shore of Carey Island due to the existence of the detached breakwater during the NE and SW seasons, (ii) the pattern of suspended sediment concentration (SSC) around the breakwater structure during the NE and SW seasons, and (iii) the variation of sea-bed elevation and the erosion-deposition pattern in the vicinity of the breakwater structure on the cohesive shore of Carey Island.

2. Study Area The study site is located in degraded environment in an intertidal area of Carey Island on the west coast of peninsular Malaysia, within the Strait of Malacca (Figure1). The Carey Island coast is a mangrove forest reserve, with a semi-diurnal tidal system that receives daily tidal inundations of maximum 2.96 m MSL of neap rise and 4.33 m MSL of spring rise. For the most part, the Carey Island coast is covered by cohesive sediment. Protection of the landward against tidal inundation and prevention of salt water intrusion during high tides are accomplished by Earth dyke constructed by the Department of Irrigation and Drainage (DID) of Malaysia, along the coastline of the island. According to the Malaysian Department of Meteorology data, the temperatures at the Carey Island have ﬂuctuated between Tmax = 35 ◦C and Tmin = 26 ◦C, with an average humidity of 92%, since the year 2000 [40]. Based on the rainfall data collected by the Sime Darby Plantation Berhad in 2015, the total annual rainfall at the study site was 1718 mm and the minimum and maximum monthly rainfalls were 63 mm and 281 mm, respectively (2015). The Langat River is situated near the study site (degraded mangrove area), and the estuary of the river is located approximately 8 km from the study site (Figure1a). The suspended sediments are carried by Langat River to the Malacca Strait as well as the area of the study site. Water 2019, 11, 1721 4 of 28 Water 2019, 11, x FOR PEER REVIEW 4 of 28

(a)

Earth dyke

5 m Earth dyke

CG1 S1 Earth dyke Circulation Gap CG2 S2 Main body

CG3 S3

(without L-block)

(b)

Figure 1. View of the study area: (a) location of Carey Island, Malaysia; (b) study site. Figure 1. View of the study area: (a) location of Carey Island, Malaysia; (b) study site. The problems of erosion and mangrove degradation have plagued the intertidal area of Carey The problems of erosion and mangrove degradation have plagued the intertidal area of Carey Island since 1995. To tackle this problem, a mangrove rehabilitation project was carried in the Island since 1995. To tackle this problem, a mangrove rehabilitation project was carried in the intertidal area in early 2009 which included constructing an 85 m-long stretch of low-crested, detached intertidal area in early 2009 which included constructing an 85 m-long stretch of low-crested, breakwaters consisting of three main segments (main body; S1, S2, S3) and three circulation gaps detached breakwaters consisting of three main segments (main body; S1, S2, S3) and three circulation gaps (CG1, CG2, CG3) (Figure 1b) [22]. The distance between the dyke structure and the northern Water 2019, 11, 1721 5 of 28

Water(CG1, 2019 CG2,, 11, CG3)x FOR PEER (Figure REVIEW1b) [ 22 ]. The distance between the dyke structure and the northern section5 of 28 of the breakwater is approximiately 5 m. The lengths of the three main segments of breakwater section of the breakwater is approximiately 5 m. The lengths of the three main segments of structure (S1, S2, S3) are 20, 30 and 20 m, respectively, with a height of 1.4 m and a width of 2.5 m. breakwater structure (S1, S2, S3) are 20, 30 and 20 m, respectively, with a height of 1.4 m and a width The circulation gaps (CG1, CG2, CG3) are each 5 m long, with a height of 0.8 m, and a width of of 2.5 m. The circulation gaps (CG1, CG2, CG3) are each 5 m long, with a height of 0.8 m, and a width 2.0 m (Figure2)[ 22]. The breakwater was constructed at longitude 101 20 23.5” to 101 20 24.5” E, of 2.0 m (Figure 2) [22]. The breakwater was constructed at longitude 101°20◦ 0′23.5” to 101°20◦ ′024.5” E, and latitude 02 49 27” to 02 49 28.5” N, with the aim reducing current speeds and wave actions and and latitude 02°49◦ ′027” to 02°49◦ ′028.5” N, with the aim reducing current speeds and wave actions and increasing sediment accumulation and sea-bed elevation in the protected mangrove degradation areas increasing sediment accumulation and sea-bed elevation in the protected mangrove degradation behind the breakwater in order to create a suitable tidal regime for restoring the lost mangroves. areas behind the breakwater in order to create a suitable tidal regime for restoring the lost mangroves. The detached breakwater is a rubble mound armoured by L-blocks (Figure2). In addition, there are The detached breakwater is a rubble mound armoured by L-blocks (Figure 2). In addition, there are stones (without L-blocks) with an average diameter of 15 to 20 cm ﬁtted neatly to the right side of the stones (without L-blocks) with an average diameter of 15 to 20 cm fitted neatly to the right side of the breakwater in mounds approximately 0.8 m high, 2.5 m wide and 20 m long. The stones are arranged breakwater in mounds approximately 0.8 m high, 2.5 m wide and 20 m long. The stones are arranged to trap more sediment within the study site [22]. The breakwater is submerged during the spring tide to trap more sediment within the study site [22]. The breakwater is submerged during the spring tide and emerges during the neap tide. and emerges during the neap tide.

Figure 2. Schematic of detached breakwater structure. Figure 2. Schematic of detached breakwater structure. 3. Methods 3. Methods 3.1. Data Collection 3.1. DataIn this Collection study, field-measured and secondary data were collected and analysed to understand the hydrodynamic and sediment transport patterns. Field-measured data include sediment samplings In this study, field-measured and secondary data were collected and analysed to understand the along the coastline of the Carey Island; fine resolution bathymetry data survey along Langat River and hydrodynamic and sediment transport patterns. Field-measured data include sediment samplings Carey Island coastline; measurement of currents, waves, water level, suspended sediment concentration along the coastline of the Carey Island; fine resolution bathymetry data survey along Langat River (SSC) characteristics near the study site area; water samplings at the mouth of Langat River and and Carey Island coastline; measurement of currents, waves, water level, suspended sediment concentration (SSC) characteristics near the study site area; water samplings at the mouth of Langat River and near the study site; and monitoring of coastal sea-bed profiles in the vicinity of the breakwaters. The secondary data included the weather data consist of wind and wave data (2014– Water 2019, 11, 1721 6 of 28 Water 2019, 11, x FOR PEER REVIEW 6 of 28 near2015); the rainfall study site;data and(2014–2015); monitoring tide of at coastal Lumut sea-bedstation, profilesBelawan in station, the vicinity Tanjung of Keling the breakwaters. station and TheDumai secondary station data (2014–2015); included and the weatherbathymetry data data consist for offshore of wind zones and wave (approximately data (2014–2015); 45 × 160 rainfall km). data (2014–2015);The typical tideweather at Lumut on Carey station, Island, Belawan Malaysia station, is primarily Tanjung Kelinginfluenced station by the and Northeast Dumai station season (2014–2015); and bathymetry data for offshore zones (approximately 45 160 km). (NE) from November to March and the Southwest season (SW) from× May to September. April and OctoberThe typical represent weather transition on Carey periods Island, between Malaysia the is primarilytwo seasons. influenced The weather by the Northeast data, consisting season (NE)of the fromdaily November wind and to wave March characteristics and the Southwest were collected season (SW) at lat. from 2°–3° May N, long. to September. 101°–102° April E by andthe Malaysian October representDepartment transition of Meteorology. periods between The the3-hour two seasons.wave climate The weather conditions data, were consisting also ofobtained the daily from wind the andEuropean wave characteristics Centre for Weather were collected Forecasts at lat. of 2Medium◦–3◦ N, long. Range 101 (ECMWF),◦–102◦ E by in the grid Malaysian format Departmentfor longitude of99.75° Meteorology. E–101.45° The E 3-hourand latitude wave 2° climate N–3.25° conditions N. Hourly were rainfall also obtained data for fromthe Klang the European region was Centre obtained for Weatherfrom the Forecasts West Estate of Medium Office, RangeSime Darby (ECMWF), Plantation in grid Berhad, format Carey for longitude Island, and 99.75 hourly◦ E–101.45 tidal ◦dataE and was latitudeobtained 2◦ N–3.25from the◦ N.Malaysian Hourly rainfallSurvey and data Mapping for the Klang Department region was(JUPEM). obtained Figure from 3 presents the West the Estate wind Offirosesce, Simeduring Darby the NE Plantation and SW Berhad, seasons Carey based Island, on the and analysis hourly of tidal weather data wasdata obtainedin 2015. fromThe figure the Malaysianillustrates Survey the dominant and Mapping wind speed Department and direction (JUPEM). du Figurering both3 presents NE and the SW wind seasons. roses The during weather the NE data andwere SW used seasons for setting based onup the the analysis hydrodynamic of weather conditions data in 2015.in the Thenumerical figure illustratesmodelling the study dominant (Section wind3.3). speed and direction during both NE and SW seasons. The weather data were used for setting up the hydrodynamic conditions in the numerical modelling study (Section 3.3).

(b)

(b) (a)

Figure 3. Wind roses in 2015: (a) NE season; (b) SW season (sources of data: Malaysian Department Figure 3. Wind roses in 2015: (a) NE season; (b) SW season (sources of data: Malaysian Department of Meteorology). of Meteorology). During the field measurement campaign, twenty sediment samples were collected along the Carey During the field measurement campaign, twenty sediment samples were collected along the Island coastline and the area surrounding the breakwater, at depths of 0–100 cm to determine the size Carey Island coastline and the area surrounding the breakwater, at depths of 0–100 cm to determine distribution of the sediment particles. the size distribution of the sediment particles. Water samples and flow velocity were collected from the mouth of the Langat River (during the Water samples and flow velocity were collected from the mouth of the Langat River (during the period of 23 December 2014 to 7 January 2015) to determine the total suspended solids (TSS1) values period of 23 December 2014 to 7 January 2015) to determine the total suspended solids (TSS1) values and the discharge values of the river. For each TSS test, 100 mL water of each sample was filtered and the discharge values of the river. For each TSS test, 100 mL water of each sample was filtered through a pre-weighed glass fibre filter. The filters were then dried at 105 ◦C in an oven and weighed. through a pre-weighed glass fibre filter. The filters were then dried at 105 °C in an oven and weighed. The mass differences between the pre and post-filtration of the filter were measured to determine the The mass differences between the pre and post-filtration of the filter were measured to determine the total suspended sediment in 100 mL of water. The TSS analyses were performed in accordance with the total suspended sediment in 100 mL of water. The TSS analyses were performed in accordance with APHA 2540D standard method. Furthermore, in order to obtain information about the average amount the APHA 2540D standard method. Furthermore, in order to obtain information about the average of TSS in the Strait of Malacca (TSS 2), which can affect the sediment transport around the study site, amount of TSS in the Strait of Malacca (TSS 2), which can affect the sediment transport around the ten water samples were collected monthly from a site located at latitude 2◦49’38.38” N and longitude study site, ten water samples were collected monthly from a site located at latitude 2°49’38.38” N and Water 2019, 11, 1721 7 of 28

101◦19’55.39” E, which is approximately 500 m seaward of the breakwater. Table1 summarises the monthly climate conditions at the study site in 2015.

Table 1. Monthly climate conditions in 2015.

Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec No. of rainfall 4 5 4 5 9 4 4 12 7 9 11 14 events Rainfall 63 64 118 60 221 128 65 281 254 139 192 133 (mm/month) Signiﬁcant wave 1.2 0.75 1 1 0.75 0.75 0.5 0.75 1 1.5 1.2 1 height (m) Mean wave 545433345554 period (s) Dominant wind 9.1 7.5 8.5 7.3 6.2 5.3 5.5 6.5 5.0 6.5 7.5 8.5 speed (m/s) Dominant wind 300 330 320 270 120 130 140 130 120 290 310 300 direction (degree) Sources of data: Rainfall data were obtained from Sime Darby Plantation Berhad, Carey Island; Wind and wave data were obtained from Malaysian Meteorology Department and ECMWF.

Based on weather data obtained from Malaysian Meteorology Department (Figure3), the wind during the NE season predominantly blew from the Northwest (approximately 300◦–330◦) at a dominant speed range of 7.5–10 m/s, while the wind during the SW season predominantly blew from a Southeastern direction (approximately 120◦–150◦) at a dominant speed range of 5–7.5 m/s. Monthly climate monitoring (Table1), indicates the significant wave heights and intensity of wind speed were higher during the Northeast season compared to the Southwest season. In addition to the climatic data mentioned above, two Acoustic Wave and Current Profiler (AWAC) units and four optical backscatter sensors (OBS-3A) were installed at the study site (see Figure4) to measure the suspended sediment concentration (SSC), water level fluctuations, currents and waves characteristics. Field measurements using AWAC and OBS-3A were carried out from 23 December 2014 to 7 January 2015, covering both the neap and spring tides. The data collected during the field study campaign, were used for the model setup, calibration and validation of the numerical model (Section 3.3). Table2 shows the coordinates and water depth for the AWACs, OBS-3As, water samples and water discharge measurements.

Table 2. Coordinates of ﬁeld measurements.

Station Longitude (x) Latitude (y) Water Depth (m)

Station 1 (AWAC 1 and OBS 1) 101◦20011.18” E 02◦48040.02” N 10.324 Station 2 (AWAC 2 and OBS 2) 101◦18058.14” E 02◦49026” N 12.557 Station 3 (OBS 3) 101◦26010.06” E 02◦4007.910 N 15.221 Station 4 (OBS 4) 101◦06044.34” E 03◦8036.81” N 10.483 Water samples (TSS 1) and water 101 24 6.24” E 2 48 2.72” N 6.242 discharges from LR ◦ 0 ◦ 0 WaterWater2019 2019, 11, 172111, x FOR PEER REVIEW 8 of8 28of 28

3.25°

3.00°

(a)

2.50°

2.00°

100.00° E 101° E 102° E

W1 (b)

Figure 4. (a) Locations of AWAC and OBS-3A units within the study site, (b) location of water and bed samplesFigure in 4. the (a) study Locations site (S of= AWACsoil sample, and OBS-3A W = water units sample within and the discharge). study site, (b) location of water and bed samples in the study site (S = soil sample, W = water sample and discharge). A ﬁne resolution bathymetry survey was conducted around the coastline of Carey Island and the Langat River, covering an area of 17.5 5 km with lines spaced at 20–500 m intervals. The survey was A fine resolution bathymetry ×survey was conducted around the coastline of Carey Island and carriedthe Langat out using River, a boat covering equipped an area with of an 17.5 echo-sounder × 5 km with and lines a DGPSspaced during at 20–500 the m spring intervals. tide on The 8 tosurvey 12 Decemberwas carried 2014. out Bathymetry using a boat data equipped were processed with an echo and-sounder prepared and for settinga DGPS up during the numerical the spring model tide on using8 to ﬂexible 12 December meshing 2014. technique. Bathymetry data were processed and prepared for setting up the numerical model using flexible meshing technique.

3.2. Monitoring of Sea-Bed Elevation Water 2019, 11, 1721 9 of 28

Water 2019, 11, x FOR PEER REVIEW 9 of 28 3.2. Monitoring of Sea-Bed Elevation The sea-bed elevations in the mangrove degraded area of the Carey Island intertidal region were monitoredThe sea-bed using elevationsa Total Station in the (theodolite) mangrove degradedand bed-profiler area of the for Carey a two-month Island intertidal period, regionduring were the periodmonitored from using December a Total Station 2014 (theodolite)to October and2015. bed-profiler The sea-bed for a two-monthelevation monitoring period, during was the mainly period concentratedfrom December at the 2014 breakwater to October region, 2015. especially The sea-bed the elevation mangrove monitoring degraded wasarea mainlyat the landward concentrated of the at breakwater,the breakwater as more region, significant especially se thea-bed mangrove level changes degraded was area expected at the in landward that area. of A the total breakwater, of twenty- as fourmore profile significant lines (CS1 sea-bed to CS24), level changes perpendicular was expected to the shoreline, in that area. were A surveyed total of twenty-four and monitored profile during lines the(CS1 study to CS24), period perpendicular (Figure 5). Monitoring to the shoreline, sea-bed were levels surveyed on a cohesive and monitored shore is a during difficult the task study since period any movement(Figure5). Monitoringon the cohesive sea-bed sea-bed levels could on a cohesive disturb the shore sediment is a di ffi surfacescult task significantly. since any movement In this study, on the woodencohesive poles sea-bed were could pushed disturb into thethe sedimentcohesive sediments surfaces significantly. around the Inmonitoring this study, area wooden at 5 m poles distances were alongpushed each into profile the cohesive line to sediments obtain accurate around thebed monitoring surface da areata. The at 5 mbed distances profile alongmeasurements each profile were line conductedto obtain accurate during low bed tide surface exposure, data. Theaccording bed profile to the measurements datum provided were by JUPEM conducted near during the study low site. tide Theexposure, profiling according outcomes to thewere datum used providedto analyse by the JUPEM erosion-deposition near the study patterns site. The in profilingthe locality outcomes of the breakwater’swere used to analysestructure the by erosion-deposition determining the patterns elevation in the differences locality of thebetween breakwater’s the bed structure profiling by measurementsdetermining the obtained elevation and diff theerences initial between sea-bed the elevations bed profiling in December measurements 2014. obtained and the initial sea-bed elevations in December 2014.

Figure 5. Bed profiling method in the study site (24 profiles). Figure 5. Bed profiling method in the study site (24 profiles). 3.3. Numerical Modelling 3.3. NumericalAccurate Modelling prediction of hydrodynamic and sediment transport characteristics under the influence of wave-currentAccurate prediction interactions of inhydrodynamic coastal regions and is ase challengingdiment transport task due characteristics to multifaceted under nearshore the influencedynamic processesof wave-current which areinteractions varying on in both coastal temporal regions and is spatiala challenging scales [19 task,41 ,42due]. Previousto multifaceted studies nearshorehave shown dynamic that modelling processes nearshore which are problems varying based on both on the temporal shallow and water spatial equation scales provides [19,41,42]. high Previousaccuracy andstudies efficiency have [11shown,43,44 ].that In thismodelling study, the ne MIKEarshore 21 Flowproblems Model based FM including on the Hydrodynamic,shallow water equationSpectra Wave provides and high Mud accuracy Transport and modules efficiency was [11, adopted43,44]. for In this simulating study, the the MIKE flow hydrodynamics 21 Flow Model FM and includingcohesive sedimentHydrodynamic, transport Spectra for the Wave case studyand Mud described Transport in Section modules2. MIKE was adopted 21 Flow modelfor simulating FM is a thecomplete flow hydrodynamics coastal modelling and suite, cohesive which sediment is capable transport of designing for the the case data study assessment described for in coastal Section and 2. MIKEoffshore 21 structures;Flow model and FM environmental is a complete impact coastal assessment modelling of suite, marine which infrastructures is capable of based designing on flexible the datamesh assessment approach. for This coastal model and was offshore established structures by;the and Danish environmental Hydraulic impact Institute assessment (DHI), of Denmark. marine infrastructuresMIKE 21 Hydrodynamic based on isflexible the basic mesh module approa of thech. MIKE This 21model Flow Modelwas established FM system by for the free Danish surface Hydraulicflows which Institute simulates (DHI), water Denmark. level fluctuations MIKE 21 andHydrodynamic flows in response is the tobasic a variety module of forcingof the MIKE functions 21 Flow Model FM system for free surface flows which simulates water level fluctuations and flows in Water 2019, 11, 1721 10 of 28 in lakes, estuaries, bays and coastal areas. The details of the numerical model used for this study are as follows: The modelling approach adopted for this study is based on the numerical solution of the two dimensional incompressible Reynolds averaged Navier-Stokes equations with the assumption of Boussinesq and hydrostatic pressure. The governing equations used for the two-dimesional numerical model in Cartesian coordinates are continuity (Equation (1)) and momentum (Equations (2) and (3)):

∂u ∂v ∂w + + = S (1) ∂x ∂y ∂z

2 Z η ! ∂u ∂u ∂vu ∂wu ∂η 1 ∂pa g ∂ρ ∂ ∂u + + + = f v g dz + Fu + νt + usS (2) ∂t ∂x ∂y ∂z − ∂x − ρ0 ∂x − ρ0 z ∂x ∂z ∂z 2 Z η ! ∂v ∂v ∂vu ∂wu ∂η 1 ∂pa g ∂ρ ∂ ∂v + + + = f u g dz + Fv + νt + usS (3) ∂t ∂y ∂x ∂z − − ∂y − ρ0 ∂y − ρ0 z ∂y ∂z ∂z where t denotes time and u, v, w are velocity components in x, y, z Cartesian coordinates, respectively. d denotes the still water depth and h is the total water depth (= η + d), g is the gravitational acceleration, pa is atmospheric pressure, ρ is density, ρ0 is reference density of water, νt is the vertical diﬀusivity, f is Coriolis parameter and S is point-source discharge magnitude. Fu and Fv are gradient-stress relations described is Equations (4) and (5), respectively. !! ∂ ∂u ∂ ∂u ∂v Fu = (2A ) + A + (4) ∂x ∂x ∂y ∂y ∂x !! ! ∂ ∂u ∂v ∂ ∂v Fv = A + + 2A (5) ∂x ∂y ∂x ∂y ∂y where A is the horizontal eddy viscosity. The total water depth, h is determined based on the kinematic boundary condition at the surface by use of robust vertical integration of local continuity equation (Equation (6)).

∂h ∂hu ∂hv + + = hS + Pˆ Eˆ (6) ∂t ∂x ∂y − where Pˆ and Eˆ are the precipitation and evaporation rates, respectively and u and v are the depth-averaged velocities in the two-dimensional domain. The model does not consider compressibility of water. A standard k ε model is used for turbulence modelling and the vertical eddy diffusivity is − derived based on Equation (7). k2 νt = c (7) µ ε where k is the turbulent kinetic energy per unit mass, cµ is an empirical constant and ε is the dissipation rate of turbulent kinetic energy. The discretisation of the numerical domain is performed with use of unstructured grid approach to achieve an optimal degree of flexibility in the representation of geometry of the detached breakwater, enabling a smooth representation of offshore and onshore boundaries. The flexible meshing approach used for this study enabled depth-adaptive and boundary-fitted mesh and provided adequate resolution of the bathymetry of study site, as well as high accuracy wave and current generation. MIKE 21 Spectral Wave (SW), a spectral wind-wave model, is used to simulate the growth, decay and transformation of wind-generated waves and swells in the numerical domain for the described case-study. The model solves the spectral wave action balance equation formulated by Komen et al. [45] and Young [46]. The hydrodynamic module was coupled with a sediment transport module capable of simulating fine sediments, to investigate the impact of detached breakwater on the coastal waters of Carey Island Water 2019, 11, 1721 11 of 28 and the sediment deposition-erosion patterns landwards of the breakwater. MIKE 21 Mud Transport module solves an advection-dispersion equation, based on Mehta [47], and the impact of waves and currents are introduced with bed shear stress. The cohesive sediment transport is described by Equation (8):

      i i i i i i  ν i  i ∂c ∂uc ∂vc ∂wc ∂wsc ∂  νTx ∂c  ∂  Ty ∂c  ∂  νTz ∂c  i + + + =   +   +   + S (8) ∂t ∂x ∂y ∂z − ∂z ∂x i ∂x  ∂y i ∂y  ∂z i ∂z  σTx σTy σTz

i th i where c is the i component of the mass concentration, νTx is the anistropic eddy viscosity, S is the i source term, σTx is turbulent Schmitt number, and ws is the settling velocity. The model is capable of considering ﬂocculation as a function of suspended sediment concentration (Equation (9)).

c γ ws = k ( ) (9) × ρsediment where k is a constant, ρsediment is sediment density, and γ is the settling index coeﬃcient. The deposition is described in the model by Equation (10):

SD = wscbpD (10) where ws denotes settling velocity of suspended sediment, pD describes the probability of deposition (= 1 τ /τ ) and c is suspended sediment concentration near the bed. The sediment transport model − b cd b is soved by spatial discretisation of the primitive equations with use of cell-centred finite volume method. An unstructured gird approach is used for the horizontal plane, while in the vertical domain structured mesh is used. The numerical model described above has been adopted successfully in several studies. Jose and Stone [48] and Jose et al. [49] investigated the characteristics of wave transformation with MIKE 21 Spectra Wave model for south-central Louisiana, USA [49]. Patra et al. [44] investigated and validated the characteristics of offshore waves in the Bay of Bengal, India using MIKE 21 Spectra Wave FM. In addition, a study of suspended sediment transport was carried out sucessfully by Sravanthi et al. [10] along the cohesive shore of Central Kerala, India using MIKE 21 Mud Transport FM. Previous studies have shown that MIKE 21 is capable of simulating hydrodynamic processes with over 85% accuracy when compared to field data [11,44,48–50].

3.3.1. Model Setup The numerical model described in Section 3.3 is used to investigate the impact of breakwater on the hydrodynamics and sediment transport patterns in the locality of the breakwater. In this study, the hydrodynamic characteristics, including currents and waves, were simulated in presence of the breakwater and without the breakwater for both seasons (northeast season and southwest season) to quantify the impact of the breakwater on the nearshore processes. Separate models were developed to simulate the conditions for the neap and spring tides. The suspended sediment concentrations (SSCs) were also simulated in the vicinity of the breakwater for both seasons and tidal conditions. The numerical results were compared to the data obtained from ﬁeld measurements, for calibration and validation purposes.

3.3.2. Model Input Model input data consisted of bathymetry data; climate data, including wind characteristics, water level and wave characteristics; sediment data, including sediment characteristics, SSC and TSS; and water discharge from the Langat River. The bathymetry data for the Strait of Malacca at coarse resolution were obtained from the Malaysian National Hydrography Centre, based on navigation survey from 2012. These data were further integrated with bathymetry data provided by DHI in Water 2019, 11, 1721 12 of 28

Water 2019, 11, x FOR PEER REVIEW 12 of 28 C-MAP (2014). Bathymetry data for the Carey Island coast and Langat River areas were measured at fineﬁne resolution resolution during during the the fieldwork ﬁeldwork (8 (8 to to 12 12 December December 2014). 2014). Following Following successful successful model model calibration calibration andand validation, validation, the the NE NE season season simulations simulations were were set set up up by by using using the the dominant dominant wind wind (8.5 (8.5 m/s m/ sof of speed speed andand 310° 310 ◦ofof direction) direction) and and wave wave characteristics characteristics (1 (1 m m height height and and 5 5s speriod) period) during during the the NE NE season season in in 20152015 and and the averageaverage SSC.SSC. For For the the SW SW season season simulations, simulations, the modelthe model was set was up set by usingup by the using dominant the dominantwind (6.5 wind m/s of(6.5 speed m/s andof speed 140◦ ofand direction) 140° of direct and waveion) and characteristics wave characteristics (0.75 m height (0.75 andm height 3 s period) and 3 durings period) SW during season SW in 2015season and in the 2015 average and the SSC. aver Bothage modelsSSC. Both used models SSC data used collected SSC data from collected 23 December from 232014 December to 7 January 2014 to 2015. 7 January 2015.

3.3.3.3.3.3. Computational Computational Domain Domain TheThe computational computational domain domain was was developed developed with with the the use use of of bathymetry bathymetry data data and and by by adopting adopting a a flexibleﬂexible mesh mesh technique. technique. Figure Figure 66 depicts depicts the the co computationalmputational domain domain developed developed for for hydrodynamic hydrodynamic andand sediment sediment transport transport modelling. modelling. This This study study set set the the computational computational domains domains beyond beyond the the study study area, area, accordingaccording to to the the guideline guideline for coastal hydraulichydraulic studiesstudies published published by by the the Department Department of Irrigationof Irrigation and andDrainage Drainage (DID), (DID), Malaysia Malaysia [51]. Therefore,[51]. Therefore, the computational the computational domain domain developed developed for the simulation for the simulation(MIKE 21 Hydrodynamic(MIKE 21 Hydrodynamic and MIKE 21 and Spectra MIKE Wave) 21 Spectra was expanded Wave) towas include expanded the Tanjung to include Keling the and TanjungLumut areasKeling (Figure and Lumut6a). To areas improve (Figure the cost 6a). eToﬃ ciencyimprove of the modelling,cost efficiency smaller of the computational modelling, smaller domain computationalwere generated domain for the were sediment generated transport for the modelsedime (Figurent transport6b). Figuremodel6 (Figure depicts 6b). the Figure computational 6 depicts thedomain computational developed domain for hydrodynamic, developed for wave hydrodynam and sedimentic, wave transport and sediment modelling. transport modelling.

(a)

Figure 6. Cont. WaterWater 20192019, 11, 11, x, FOR 1721 PEER REVIEW 13 13of of28 28

Study Area

(b)

Figure 6. Computational domain: (a) hydrodynamic and wave models; (b) mud transport model. Figure 6. Computational domain: (a) hydrodynamic and wave models; (b) mud transport model. 3.3.4. Model Calibration and Validation 3.3.4. Model Calibration and Validation To check the accuracy of the simulation results from the models and to provide confidence in the simulationsTo check ofthe the accuracy current, wavesof the andsimulation patterns results of suspended from the sediment models concentration and to provide in the confidence locality of in the theexisting simulations detached of breakwater,the current, the waves simulation and pattern resultss obtainedof suspended from the sediment models wereconcentration calibrated in initially the locality of the existing detached breakwater, the simulation results obtained from the models were against measured conditions on 23 December 2014 to 7 January 2015 at station 1 (lat: 02◦48040.02” N, calibrated initially against measured conditions on 23 December 2014 to 7 January 2015 at station 1 long: 101◦20011.18” E). The models were further validated against field measurements taken between (lat: 02°48′40.02” N, long: 101°20′11.18” E). The models were further validated against field 23 December 2014 and 7 January 2015 at station 2 (lat: 02◦49026” N, long: 101◦18058.14” E. measurementsThe calibration taken between of the hydrodynamic 23 December model2014 and was 7 carriedJanuary out 2015 by at adjusting station the2 (lat: values 02°49 of′26” the N, bed long:roughness 101°18/′Manning58.14” E. number over the whole computational domain [52]. The calibration of the spectra waveThe model calibration was carried of the outhydrodynamic by adjusting model the values was ofca therried wave out breakingby adjusting parameters, the values bottom of the friction bed roughness/Manningparameters and white-capping number over (deep the waterwhole wavecomput breaking)ational parametersdomain [52]. [53 The]. The calibration calibration of ofthe the spectramud transportwave model model was wascarried carried out by out adjusting by adjusting the values the valuesof the wave of the breaking erosion parameters, coefficient, powerbottom of frictionerosion, parameters settling velocity and white-capping and critical shear (deep stress water for wave deposition breaking) and erosionparameters [39]. [53]. To check The calibration the accuracy ofof the the mud simulation transport results, model the was Theil’s carried inequality out by adjust coeffiingcients, the values R Squared of the (R er2),osion and Rootcoefficient, Mean Squaredpower ofError erosion, (RMSE) settling were velocity calculated and [ 44critical,48,54 ].shear stress for deposition and erosion [39]. To check the 2 accuracyFigure of the7 comparessimulation theresults, simulated the Theil’s and inequality measured coefficients, water level, R Squared current characteristics,(R ), and Root Mean wave Squaredcharacteristics Error (RMSE) and SSC were at calculated station 1. [44,48,54]. Figure8 depicts the comparison between the simulated and measuredFigure water7 compares level, current the simulated characteristics, and measured wave characteristics water level, and SSCcurrent at station characteristics, 2. Table3 presents wave characteristicsthe values of and Thiel’s SSC coe atffi stationcient, R 1.2 andFigure RMSE 8 depicts obtained the during comparison the model between calibration the simulated and validation. and measuredBased on water the standard level, current error allowedcharacteristics, by the DIDwave (2013) characteristics for hydrodynamic and SSC and at station sediment 2. transportTable 3 2 presentsmodelling, the values the values of Thiel’s proved coefficient, that the models R and were RMSE calibrated obtained and during validated the model well. calibration and validation. Based on the standard error allowed by the DID (2013) for hydrodynamic and sediment transport modelling, the values proved that the models were calibrated and validated well. Water 2019, 11, 1721 14 of 28 Water 2019, 11, x FOR PEER REVIEW 14 of 28

0.8 Measured 0.6 Predicted 0.4 0.2 (m/s) 0.0 21-Dec-14 25-Dec-14 29-Dec-14 2-Jan-15 6-Jan-15 10-Jan-15 Current Speed Current Date

(a)

360 Measured 280 Predicted 200 120 40 (degree) 21-Dec-14 25-Dec-14 29-Dec-14 2-Jan-15 6-Jan-15 10-Jan-15

Current Direction Date

(b)

3.0 Measured 1.0 Predicted -1.0 (m) -3.0

Water Level Water 21-Dec-14 25-Dec-14 29-Dec-14 2-Jan-15 6-Jan-15 10-Jan-15 Date

(c)

Measured 0.8 Predicted 0.6 0.4 0.2 Height (m) Height 0.0

Significant Wave Significant 21-Dec-14 25-Dec-14 29-Dec-14 2-Jan-15 6-Jan-15 10-Jan-15 Data

(d)

Measured 360 Predicted 280 200 120 40 Mean Wave Wave Mean 21-Dec-14 25-Dec-14 29-Dec-14 2-Jan-15 6-Jan-15 10-Jan-15 Direction (degree) Direction

Date

(e)

Figure 7. Cont. Water 2019, 11, x FOR PEER REVIEW 15 of 28

WaterWater2019 2019, 11, 11, 1721, x FOR PEER REVIEW 1515 of of 28 28 0.18 Measured 0.15 Predicted ) 3 0.180.12 Measured 0.150.09 Predicted ) 3 0.120.06 0.090.03 0.00

SSC (kg/m SSC 0.06 0.0321-Dec-14 25-Dec-14 29-Dec-14 2-Jan-15 6-Jan-15 10-Jan-15 0.00 SSC (kg/m SSC Date 21-Dec-14 25-Dec-14 29-Dec-14 2-Jan-15 6-Jan-15 10-Jan-15 Date (f)

Figure 7. Comparison between simulation results and(f) field measurements on 23 December 2014 to 7 FigureJanuary 7. Comparison 2015 at station between 1 (long: simulation 101°20′11.18” results E, andlat: ﬁeld02°48 measurements′40.02” N), (a) oncurrent 23 December speed; (b 2014) current to Figure 7. Comparison between simulation results and field measurements on 23 December 2014 to 7 7 Januarydirection; 2015 (c) water at station level; 1 (long:(d) significant 101 20 11.18” wave height; E, lat: 02 (e)48 mean40.02” wave N), direction; (a) current (f) speed; SSC. (b) current January 2015 at station 1 (long: 101°20◦ 0′11.18” E, lat: 02°48◦ 0′40.02” N), (a) current speed; (b) current direction; (c) water level; (d) signiﬁcant wave height; (e) mean wave direction; (f) SSC. direction; (c) water level; (d) significant wave height; (e) mean wave direction; (f) SSC. 0.6 Measured Predicted 0.60.4 Measured 0.40.2 Predicted (m/s) 0.20.0 (m/s) Current Speed Current 21-Dec-14 25-Dec-14 29-Dec-14 2-Jan-15 6-Jan-15 10-Jan-15 0.0 Date

Current Speed Current 21-Dec-14 25-Dec-14 29-Dec-14 2-Jan-15 6-Jan-15 10-Jan-15 Date (a)

(a) 360 Measured 280 Predicted 360200 Measured 280 120 Predicted (degree) 20040 12021-Dec-14 25-Dec-14 29-Dec-14 2-Jan-15 6-Jan-15 10-Jan-15 (degree) Current Direction Current 40 Date 21-Dec-14 25-Dec-14 29-Dec-14 2-Jan-15 6-Jan-15 10-Jan-15 Current Direction Current Date (b)

(b) 3.0 Measured Predicted 3.01.0 Measured

(m) -1.01.0 Predicted -3.0 Water Level Water

(m) -1.0 21-Dec-14 25-Dec-14 29-Dec-14 2-Jan-15 6-Jan-15 10-Jan-15 -3.0

Water Level Water Date 21-Dec-14 25-Dec-14 29-Dec-14 2-Jan-15 6-Jan-15 10-Jan-15 Date (c)

Figure(c) 8. Cont. WaterWater2019 2019, 11, ,11 1721, x FOR PEER REVIEW 1616 of of 28 28

0.8 Measured 0.6 Predicted 0.4 0.2

Height (m) 0.0

Significant Wave Significant 21-Dec-14 25-Dec-14 29-Dec-14 2-Jan-15 6-Jan-15 10-Jan-15 Date

(d)

360 Measured 280 Predicted 200 120

Mean Wave Mean 40 Direction (degree) Direction 21-Dec-14 25-Dec-14 29-Dec-14 2-Jan-15 6-Jan-15 10-Jan-15 Date

(e)

0.18 Measured 0.15 Predicted ) 3 0.12 0.09 0.06 0.03 SSC (kg/m SSC 0.00 21-Dec-14 25-Dec-14 29-Dec-14 2-Jan-15 6-Jan-15 10-Jan-15 Date

(f)

Figure 8. Comparison between simulation results and field measurements on 23 December 2014 to Figure 8. Comparison between simulation results and field measurements on 23 December 2014 to 7 7 January 2015 at station 2 (long: 101 18 58.14” E, lat: 02 49 26” N), (a) current speed; (b) current January 2015 at station 2 (long: 101°18◦ 0 ′58.14” E, lat: 02°49◦ 0 ′26” N), (a) current speed; (b) current direction; (c) water level; (d) significant wave height; (e) mean wave direction; (f) SSC. direction; (c) water level; (d) significant wave height; (e) mean wave direction; (f) SSC. Table 3. Statistical results for the performance of the hydrodynamic, wave and mud transport models. Table 3. Statistical results for the performance of the hydrodynamic, wave and mud transport models. Calibration Validation Parameter R SquaredCalibrationThiel’s Inequality R Squared ValidationThiel’s Inequality RMSE RMSE (R2) CoefficientThiel’s (R2) CoefficientThiel’s Parameter R Squared R Squared Current RMSE inequality RMSE inequality 0.07 m/s 0.92(R2) 0.08 0.08 m/s 0.91(R2) 0.08 Speeds coefficient coefficient Current Current 15 0.94 0.05 17 0.93 0.06 Directions 0.07◦ m/s 0.92 0.08 ◦ 0.08 m/s 0.91 0.08 WaterSpeeds Levels 0.05 m 0.95 0.04 0.06 m 0.94 0.05 SignificantCurrent 0.04 m15° 0.850.94 0.14 0.05 0.05 m 17° 0.830.93 0.16 0.06 waveDirections heights Mean Wave Water Levels 18 0.05 m 0.81 0.95 0.18 0.04 19 0.06 m 0.80 0.94 0.19 0.05 Directions ◦ ◦ Significant 0.004 0.005 SSC 0.04 m 0.83 0.85 0.16 0.14 0.05 m 0.82 0.83 0.17 0.16 wave heights kg/m3 kg/m3 Mean Wave 18° 0.81 0.18 19° 0.80 0.19 Directions Water 2019, 11, x FOR PEER REVIEW 17 of 28

0.004 0.005 Water 2019SSC, 11, 1721 0.83 0.16 0.82 0.1717 of 28 kg/m3 kg/m3

4. Results and Discussion 4. Results and Discussion 4.1. Sediment and Water Samples Analyses

4.1. SedimentAccording and toWater the resultsSamples of Analyses sediment particle analyses, the median grain diameter (D50) of the cohesive sediments in the intertidal area of Carey Island, at depth of 0–40 cm, was determined to According to the results of sediment particle analyses, the median grain diameter (D50) of the cohesivebe 0.015–0.022 sediments mm, in which the intertidal was consisted area of Carey of 10% Island, clay, 71%at depth silt andof 0–40 19% cm, ﬁne was sand. determined Furthermore, to be 0.015–0.022the sediment mm, fraction which at was depths consisted of 40–100 of 10% cm consistedclay, 71% ofsilt sti andﬀ clay. 19% Basedfine sand. on water Furthermore, samples andthe sedimentvelocity measurements fraction at depths at the of mouth40–100 ofcm the consisted Langat River,of stiff TSSclay. 1 Ba showedsed on that water the samples Langat Riverand velocity carries 3 measurements180–261 mg/L ofat suspendedthe mouth of sediments the Langat to theRiver, Malacca TSS 1 Straitshowed with that 698–1130 the Langat m /Rivers of water carries discharges 180–261 3 mg/Lduring of ebbsuspended tide and sediments 121–479 m to/s th duringe Malacca spring Strait tide. with Table 698–11304 present m TSS3/s of 2 water analyses discharges from the during water ebbsamples tide and collected 121–479 at 500m3/s m during seaward spring of the tide. breakwater. Table 4 present TSS 2 analyses from the water samples collected at 500 m seaward of the breakwater. Table 4. Monthly amount of TSS 2 in 2015.

Table 4. Monthly amountMonth of TSS 2 in 2015. Jan Feb Mar Apr May JunMonth Jul Aug Sep Oct Nov Dec TSS 2 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 0.047 0.043 0.058 0.049 0.079 0.071 0.06 0.081 0.076 0.058 0.061 0.056 (kg/TSSm3) 2 0.047 0.043 0.058 0.049 0.079 0.071 0.06 0.081 0.076 0.058 0.061 0.056 (kg/m3) Table4 shows that the amounts of TSS2 were higher during the Southwest season, when the monthlyTable rainfall 4 shows intensities that the were amounts also higherof TSS2 (see were Table higher1). High during concentration the Southwest of suspended season, when sediment the monthlyfrom the rainfall Langat intensities River into were the Strait also higher of Malacca (see Table is expected 1). High during concentra the Southwesttion of suspended season, duesediment to the fromhigher the rainfall Langat intensities. River into Thethe windStrait fromof Malacca the Southeast is expected direction during transports the Southwest the suspended season, due sediment to the higherfrom the rainfall mouth intensities. of the Langat The Riverwind from to the the study Sou site.theast direction transports the suspended sediment from the mouth of the Langat River to the study site. 4.2. Hydrodynamic Changes in the Locality of the Breakwater Structure 4.2. Hydrodynamic Changes in the Locality of the Breakwater Structure Figure9 illustrates the simulation results for the currents and waves characteristics with and withoutFigure the 9 breakwater illustrates duringthe simulation the NE season, results for for the the maximum currents tidaland waves level of characteristics the neap tide and with spring and withouttide conditions. the breakwater Figure during10 presents the NE the season, simulation for th resultse maximum for the tidal currents level of and the waves neap tide characteristics and spring tidewith conditions. and without Figure the breakwater 10 presents during the simulation SW season results for the for maximum the currents tidal and level waves of the characteristics neap tide and spring tide conditions.

(a) (b)

Figure 9. Cont. Water 2019, 11, 1721 18 of 28 Water 2019, 11, x FOR PEER REVIEW 18 of 28

(e) (f)

(g) (h)

FigureFigure 9. 9.Simulated Simulated currentcurrent and wave wave characteri characteristicsstics during during the the Northeast Northeast season: season: (a) CS (a during) CS during NT NT(WT); (WT); (b ()b CS) CS during during NT NT (W); (W); (c) CS (c) during CS during ST (WT); ST (WT); (d) CS ( dduring) CS during ST (W); ST (e) (W); SWH (e during) SWH NT during (WT); NT (WT);(f) SWH (f) SWH during during NT NT(W); (W); (g) (SWHg) SWH during during ST ST(WT); (WT); (h) ( hSWH) SWH during during ST ST (W). (W). Notes: Notes: W W= =withwith breakwater,breakwater, WT WT= without= without breakwater, breakwater, CS CS= =current current speed,speed, SWH = significantsigniﬁcant wave wave height, height, NT NT = neap= neap tide, ST = spring tide. tide, ST = spring tide. Water 2019, 11, 1721 19 of 28

(a) Changes in Current and Wave Characteristics during the Northeast Season (NE) The simulation results shown in Figure9 demonstrate that during the NE season and without the existence of the breakwater structure, the currents flow directly to the study site (degraded mangrove area) from a Northwest direction at approximately 300◦ to 330◦ (magnetic) towards the Southeast direction with velocity between 0.12 and 0.22 m/s. At the same time, incident waves impose force on the mangrove degradation areas from a Southwest direction at approximately 230◦ to 250◦ (magnetic) to the Northeast direction, with the significant wave heights between 0.10 and 0.21 m. In addition, the simulation results also show that the current velocity and wave heights were reduced in the nearshore area, before reaching the study site, due to the resistance forces of the cohesive sediment. Previous studies have reported that cohesive bed slopes have higher friction coefficients compared to sandy beach slopes [30,31]. Therefore, current and wave turbulent energies are reduced on cohesive coastal bed slopes. Due to the presence of the breakwater at the study site, the current and wave characteristics changed dynamically around the degraded mangrove area. For the maximum tidal level during neap tide conditions, the water levels were lower than the main segment’s (S1, S2, S3) crest levels and higher than circulation gaps’ (CG1, CG2, CG3) crest levels (the breakwater was not submerged). Currents and waves entered the mangrove degradation area via overtopping the circulation gap’s (CG1, CG2, CG3) of the breakwater and through the available gaps between the breakwater and earth dyke or between the breakwater and the stone (without L-blocks). At this moment, the current speeds on the landward side of the breakwater decreased from 0.18 to 0.10 m/s (approximately 20 m from the breakwater structure to the landward side area) and from 0.18 to 0.04 m/s (closest to the breakwater structure). The wave heights also decreased on the landward side of the breakwater from approximately 0.165 to 0.06 m. In addition, the current speeds slightly increased around the circulation gaps in the breakwater (CG1, CG2, CG3), above the stone (without L-blocks) and available gaps between the breakwater and earth dyke from 0.20 to 0.24 m/s due to the turbulent flows in these areas as the result of return flow occurrences, while the wave heights also increased slightly at the toe of the breakwater’s main segment (S1, S2, S3) from 0.15 m to 0.18 m due to reflection incidences [41]. At the maximum tidal level during the spring tide, the water levels were higher than the breakwater’s crest (the breakwater was submerged); consequently, the current and wave energies could pass through the study site via overtopping the structure. The overall current speed and wave heights on the landward side of the breakwater (mangrove degradation area) were reduced slightly from approximately 0.18 to 0.16 m/s and from 0.18 to 0.15 m, respectively. However, the current speeds on the landward side of the first main segment (S1) of the breakwater were found to be reduced by a greater amount, i.e., from 0.18 to 0.06 m/s. Additionally, current speeds and wave heights at the toe of the main body (direction the currents/waves come from) increased from 0.20 to 0.28 m/s and from 0.18 to 0.24 m, respectively. These increases might be due to refraction occurrences (due to the differences in water depths) combining with reflection incidences when the current and waves pass through the breakwater at an angle to underwater contours and through shallower depths [41]. Water 2019, 11, 1721 20 of 28 Water 2019, 11, x FOR PEER REVIEW 20 of 28

(a) (b)

(e) (f)

Figure 10. Cont. Water 2019, 11, 1721 21 of 28 Water 2019, 11, x FOR PEER REVIEW 21 of 28

(g) (h)

Figure 10. SimulatedSimulated current current and and wave wave characteristics characteristics during during the the Southwest Southwest season: season: (a) CS (a) during CS during NT NT(WT); (WT); (b) CS (b) during CS during NT (W); NT (W);(c) CS (c )during CS during ST (WT); ST (WT); (d) CS (d during) CS during ST (W); ST (e (W);) SWH (e) SWHduring during NT (WT); NT (WT);(f) SWH (f) SWHduring during NT (W); NT (W);(g) SWH (g) SWH during during ST ST(WT); (WT); (h) ( hSWH) SWH during during ST ST (W). (W). Notes: Notes: W W = with breakwater, WTWT= = without breakwater, CSCS= = current speed,speed, SWHSWH == signiﬁcantsignificant wave height,height, NTNT == neap tide, ST = springspring tide. tide.

(b)(b) ChangesChanges inin CurrentCurrent andand WaveWave CharacteristicsCharacteristics duringduring thethe SouthwestSouthwest SeasonSeason (SW) (SW) The simulation results shown in Figure 10 illustrateillustrate that without the coastal defence structure at the site, the currents flowflow toto thethe degradeddegraded mangrove area from aa SoutheastSoutheast direction approximately 120120°–150°◦–150◦ (magnetic)(magnetic) towards towards the the Northwest Northwest direction direction with with speeds speeds between between 0.09 and 0.09 0.18 and m/s, 0.18 while m/s, whilewaves waves force forcethe degraded the degraded mangrove mangrove area area from from the the south south between between approximately approximately 170° 170◦ andand 185185°◦ (magnetic)(magnetic) withwith significantsignificant heightsheights ofof waveswaves betweenbetween 0.10.1 andand 0.2250.225 mm duringduring thethe SouthwestSouthwest season.season. After constructionconstruction ofof the the breakwater breakwater in in the the intertidal intertidal area area of Careyof Carey Island, Island, the currentsthe currents which which flow fromflow from Southeast Southeast directions directions reach reach the degraded the degraded mangrove mangrove areas areas first, first, and and then then they they flow flow seaward seaward by overtoppingby overtopping the the circulation circulation gaps’ gaps’ of the of breakwaterthe breakwater (CG1, (CG1, CG2, CG2, CG3) CG3) and throughand through the available the available gaps betweengaps between the breakwater the breakwater and earth and dykeearth or dyke between or between the breakwater the breakwater and stone and (without stone (without L-blocks) L-blocks) during theduring neap the tide. neap In this tide. case, In this currents case, were currents reduced were on reduced the landward on the side landward of the breakwater side of the structure breakwater from approximatelystructure from 0.13approximately to 0.06 m/s, and0.13 they to 0.06 increased m/s, and above they the circulationincreased above gaps’ structuresthe circulation and around gaps’ thestructures gaps between and around the breakwater the gaps between and the stonethe breakw (withoutater L-blocks) and the stone/earth (witho dyke fromut L-blocks)/earth approximately dyke 0.13 tofrom 0.18 approximately m/s. During the 0.13 spring to 0.18 tide, m/s. the During current speedsthe spring in the tide, degraded the current mangrove speeds area in werethe degraded reduced frommangrove 0.17 to area 0.09 mwere/s, whilereduced the currentfrom 0.17 speeds to slowed0.09 m/s, quite while significantly the current on the speeds seaward slowed side ofquite the firstsignificantly main segment on the of seaward the breakwater side of (S1), thei.e., first by main up to segment approximately of the0.04 breakwater m/s. In addition, (S1), i.e., the by current up to speedsapproximately increased 0.04 at them/s. toe In of addition, the breakwater the curren (directiont speeds the currentincreased comes at the from) toe from of the approximately breakwater 0.18(direction to 0.21 the m /currents. comes from) from approximately 0.18 to 0.21 m/s. In termsterms ofof thethe wavewave characteristics,characteristics, the wave heights in thethe degradeddegraded mangrove area werewere reduced andand concentrated concentrated on on the the landward landward side side of the of breakwater the breakwater structure structure (from the (from middle the structure middle tostructure the earth to dyke).the earth Also, dyke). wave Also, heights wave increased heights from increased 0.2 to 0.24 from m at0.2 the to toe0.24 of m the at breakwater the toe of andthe stonesbreakwater (without and L-blocks).stones (without Such L-blocks). an increase Such in wavean increase heights in was wave due heights to refraction was due and to refraction reflection incidencesand reflection as indicated incidences by as mechanics indicated ofby wave mechanics motion of in wave shore motion protection in shore manual protection [41]. manual [41].

4.3. Suspended Sediment Transport inin thethe Breakwater’sBreakwater’s SurroundingsSurroundings Figure 1111 presentspresents the the simulations simulations of of suspended suspended sediment sediment concentration concentration during during the the NE NE season season for thefor maximumthe maximum tidal tidal level oflevel the of neap the tide neap and tide spring and tide spring conditions tide conditions in the vicinity in the of thevicinity breakwater. of the Inbreakwater. addition, In Figure addition, 12 presents Figure the12 presents simulations the simulations of suspended of sedimentsuspended concentration sediment concentration during the during the SW season for the maximum tidal level of the neap tide and spring tide conditions around the breakwater structure. Water 2019, 11, 1721 22 of 28

SW season for the maximum tidal level of the neap tide and spring tide conditions around the Water 2019, 11, x FOR PEER REVIEW 22 of 28 Waterbreakwater 2019, 11, x structure. FOR PEER REVIEW 22 of 28

((aa)) ((bb)) Figure 11. Simulation of suspended sediment concentration during the Northeast season: (a) neap FigureFigure 11. Simulation ofof suspendedsuspended sediment sediment concentration concentration during during the the Northeast Northeast season: season: (a) neap(a) neap tide; tide; (b) spring tide. tide;(b) spring (b) spring tide. tide.

((aa)) ((bb))

FigureFigureFigure 12.12. 12. SimulationSimulationSimulation ofof of suspendedsuspended suspended sedimentsediment sediment concenconcen concentrationtrationtration duringduring during thethe the SouthwestSouthwest Southwest season:season: season: ((a (a)a) ) neapneap neap tide;tide;tide; ( (b (bb)) )spring spring spring tide. tide. tide.

The average concentration of suspended sediment brought and transported by current and wave TheThe average average concentration concentration of of suspended suspended sediment sediment brought brought and and transported transported by by current current and and wave wave actions from the seaward side of the breakwater to the mangrove degradation area was approximately actionsactions fromfrom thethe seawardseaward sideside ofof thethe breakwaterbreakwater toto thethe mangrovemangrove degradationdegradation areaarea waswas 0.048 kg/m3, during the NE3 season (Figure 11) and 0.052 kg/m3, during the3 SW season (Figure 12). approximatelyapproximately 0.048 0.048 kg/m kg/m3, ,during during the the NE NE season season (Figure (Figure 11) 11) and and 0.052 0.052 kg/m kg/m3, ,during during the the SW SW season season Hence, there is a possibility that individual particles of the suspended sediments in water column (Figure(Figure 12). 12). Hence, Hence, there there is is a a possibility possibility that that indi individualvidual particles particles of of the the suspended suspended sediments sediments in in water water become attached to each other and form flocks in the areas where the hydrodynamics conditions were columncolumn becomebecome attachedattached toto eacheach otherother andand formform flocksflocks inin thethe areasareas wherewhere thethe hydrodynamicshydrodynamics calm enough. Based on the simulation results, the concentrations of suspended sediments brought conditionsconditions werewere calmcalm enough.enough. BasedBased onon thethe simulationsimulation results,results, thethe concentrationsconcentrations ofof suspendedsuspended from the seaward are slightly reducing in line with the mean wave direction towards the landward area. sedimentssediments broughtbrought fromfrom thethe seawardseaward areare slightlyslightly reducingreducing inin lineline withwith thethe meanmean wavewave directiondirection This result may have been obtained because some suspended sediment settles down on the sea-bed towardstowards thethe landward landward area. area. ThisThis resultresult may may havehave been been obtainedobtained becausebecause somesome suspendedsuspended sedimentsediment settlessettles down down on on the the sea-bed sea-bed due due to to the the reduction reduction of of wave wave and and current current energies energies triggered triggered by by the the high high bottombottom friction friction coefficients coefficients of of the the cohesive cohesive sediments sediments [31,55]. [31,55]. TheThe simulationsimulation resultsresults ofof mudmud transporttransport modulemodule (Figures(Figures 1111 andand 12)12) showshow thatthat therethere areare reductionsreductions in in the the SSCSSC ofof thethe waterwater columncolumn atat thethe ststudyudy sitesite after after thethe constructionconstruction ofof thethe breakwater,breakwater, Water 2019, 11, 1721 23 of 28 due to the reduction of wave and current energies triggered by the high bottom friction coefficients of the cohesive sediments [30,31]. Water The2019,simulation 11, x FOR PEER results REVIEW of mud transport module (Figures 11 and 12) show that there are reductions23 of 28 in the SSC of the water column at the study site after the construction of the breakwater, especially in theespecially landward in the area la ofndward the breakwater area of the during breakwater the NE andduring SW the seasons. NE and In addition, SW seasons. reductions In addition, in the SSCreductions are higher in the around SSC are the higher first main around segment the first of main the breakwater segment of (S1) the (seawardbreakwater and (S1) landward (seaward areas) and duringlandward the areas) SW season. during the SW season.

4.4. Variation of Sea-Bed LevelsLevels andand Erosion-DepositionErosion-Deposition PatternPatternin inthe the Locality Locality of of the the Low-Crested Low-Crested Breakwater Breakwater Figure 13 presents the variation of sea-bed levels and the pattern of erosion-deposition in the localityFigure of the 13 breakwater presents inthe the variation degraded of environment, sea-bed levels i.e., and the cohesivethe pattern intertidal of erosion-deposition area of Carey Island, in measuredthe every two months from December 2014 to October 2015.

(a) (b)

Figure 13. Cont. Water 2019, 11, 1721 24 of 28 Water 2019, 11, x FOR PEER REVIEW 24 of 28

(e)

Figure 13. Variation of the sea-bedFigure levels 13. andVariation erosion-deposition of the sea-bed pattern levels in theand locality erosion- of thedeposition breakwater pattern in the locality of the structure over two-month periodsbreakwater from structure December over 2014 two-month to October periods 2015. Notes: from +Decemberrepresents 2014 the to October 2015. Notes: + deposition in cm, (a,b) NE season,represents (c,d, ethe) SW deposition season. in cm, (a, b) NE season, (c, d, e) SW season.

The erosion-deposition patternsThe erosion-deposition for the cohesive patterns sediment for in the cohesive intertidal se areadiment of Carey in the Island intertidal area of Carey Island caused by the constructioncaused of the by 85-m the long,construction low-crested of the breakwater 85-m long, are low-cr evident.ested Unlike breakwater sand coasts,are evident. Unlike sand coasts, deposition patterns on cohesivedeposition coasts patterns are more on cohesive dynamic. coasts Depositions are more dyna aremic. found Depositions to be higher are atfound to be higher at lower lower sea-bed elevations insea-bed protected elevations areas with in protected calm hydrodynamic areas with calm conditions. hydrodynamic Based on conditions. Figure 13, Based on Figure 13, it is it is evident that there isevident an increment that there of sea-bed is an elevationsincrement inof thesea-bed degraded elevations mangrove in the area degraded after mangrove area after construction of the structure.construction During of the the NE structure. season, theDuring sediment the NE accumulation season, the sediment rates were accumulation higher rates were higher on on the landward side of mainthe landward segments side of the of breakwatermain segments (S1, S2,of the S3), breakw while theyater were(S1, S2, lower S3), on while the they were lower on the landward side of the circulationlandward gaps’ side in of the the breakwater circulation (CG1, gaps’ CG2, in the CG3). breakwater During (CG1, the SW CG2, season, CG3). the During the SW season, the sediment accumulations aresediment much higheraccumulations surrounding are much the ﬁrst higher main surrounding segment of the the first breakwater main segment (S1), of the breakwater (S1), while lower amounts accumulatewhile lower around amounts the circulation accumulate gaps’ around in the the breakwater circulation and gaps’ the in gap the between breakwater and the gap between the breakwater and the stonethe breakwater (without L-blocks). and the stone (without L-blocks). Overall, the sea-bed elevationsOverall, in thethe degraded sea-bed environmentelevations in inthe the degraded intertidal environment area of Carey Islandin the intertidal area of Carey increased by approximatelyIsland 8 cm increased near to the by ﬁrst approximately main segment 8 of cm the near breakwater to the first (S1), main while segment other areas of the breakwater (S1), while on the landward side of theother breakwater areas on the increased landward approximately side of the breakwater 4 cm, on average, increased from approximately December 4 cm, on average, from 2014 to October 2015. December 2014 to October 2015.

4.5. Discussion 4.5. Discussion The hydrodynamic characteristics,The hydrodynamic cohesive characteristics, behaviour of thecohesive sediments, behaviour concentration of the sediments, of concentration of suspended sediment andsuspended existence of sediment a low-crest and detached existence breakwater of a low-crest at the deta studyched site breakwater have resulted at the study site have resulted in speciﬁc patterns of sedimentin specific transport patterns and of erosion-depositionsediment transport in and the erosion-deposition cohesive intertidal in zone the ofcohesive intertidal zone of Carey Island during bothCarey the SW Island and NEduring seasons. both the SW and NE seasons.

4.5.1. Sediment Transport and Erosion-Deposition Pattern during the Northeast Season 4.5.1. Sediment Transport and Erosion-Deposition Pattern during the Northeast Season Water flows from the nearshore area bring suspended sediments to the study site. Water flows from the nearshore area bring suspended sediments to the study site. Figures 9, 11 Figures9, 11 and 13 show the flow patterns, sediment transport and erosion-deposition in the locality and 13 show the flow patterns, sediment transport and erosion-deposition in the locality of the of the detached breakwater during NE season. During the neap tide, the detached breakwater obstructs detached breakwater during NE season. During the neap tide, the detached breakwater obstructs the the transportation of some suspended sediments from the seaward side to the mangrove degradation transportation of some suspended sediments from the seaward side to the mangrove degradation area. It was found that the concentrations of suspended sediments are higher on the seaward side than area. It was found that the concentrations of suspended sediments are higher on the seaward side the landward side of the breakwater. Some suspended sediments can enter the degraded mangrove than the landward side of the breakwater. Some suspended sediments can enter the degraded mangrove area through the gap between the breakwater and earth dyke or stone (without L-blocks) and also, by overtopping the circulation gaps’ structure. The concentrations of suspended sediments Water 2019, 11, 1721 25 of 28 area through the gap between the breakwater and earth dyke or stone (without L-blocks) and also, by overtopping the circulation gaps’ structure. The concentrations of suspended sediments in the water column were reduced further on the landward side of the breakwater because the sediment settles in this area due to the calm hydrodynamic conditions created by the breakwater and sediments are trapped [8,9]. During the spring tide, the suspended sediments from the seaward direction entered the degraded mangrove area via overtopping the breakwater structure. Some suspended sediments in the water column formed flocks and settle down in regions with lower hydrodynamic energies within the degraded mangrove area, while others were transported back seawards, all depending on turbulent properties resultant of the wave breaking and wave-current interactions. Suspended sediments containing flocks have higher settling velocities compared to individual particles; hence, they can settle and be deposited faster in regions with lower turbulent energies. The observed results were in line with the findings of previous studies conducted by Pejrup and Zhu et al. [37,38]. As current speeds and wave heights were reduced, the amount of sediment deposited increases in the landward area behind the breakwater, thus, the sediment accumulations were also higher during the NE season. The results were found to be in line with the field measurement (Figure 13a,b) which indicated that sediment accumulations were concentrated in the landward area behind the breakwater.

4.5.2. Sediment Transport and Erosion-Deposition Pattern during the Southwest Season During the neap tide and spring tide in the SW season, the detached breakwater trapped suspended sediments before they flowed seaward from the degraded mangrove area. The suspended sediments settled in areas with lower hydrodynamic conditions, especially in the vicinity of the first main segment of the breakwater (S1). According to Figure 12, a lower amount of suspended sediment concentrations appeared around the first main segment of the breakwater (S1) due to the calmer hydrodynamic conditions (with current speeds of less than 0.07 m/s) and higher settling rate. The field measurement confirmed that sediment accumulations were concentrated around the first main segment of the breakwater structure and in the landward area behind the breakwater (Figure 13c–e). Sediment accumulations were found to be slightly higher during the SW season compared to the NE season. This higher amount could be attributed to the calmer hydrodynamic conditions during the SW season. In addition, a higher amount of the suspended sediments were transported from the seaward to the study site during SW season. Calm hydrodynamic conditions and high suspended sediment concentrations could have accelerated the sediment deposition rate [4]. Finally, minor sea-bed elevation changes, with an average increment of 4 cm, were recorded in the landward area behind the breakwater after a year, which could have been influenced by the consolidation of the deposited cohesive sediment as indicated by several researchers as Kirby, Edmonds, & Ranasinghe [27,36,55,56].

5. Conclusions The numerical modelling study showed that without having the breakwater in the degraded environment of the intertidal area, waves and curernts directly reach to the bare area at the study site with higher turbulent energy which means that in absence of the breakwater more intense degredation will occur in the coastal mangrove reserve area. The numerical model found that the pattern of suspended sediment around the breakwater during both seasons is strongly aﬀected by the local hydrodynamic conditions. It was shown that less energetic wave-current conditions eﬀectively reduced the amount of SSC in the water column, as the suspended sediments were deposited in the water column under calm hydrodynamic conditions. In addition, the patterns of erosion-deposition in the locality of a low-crested breakwater on a cohesive coast were more dynamically evolving compared to a sandy coasts. Also, it was found that the deposition rate and sea-bed elevations increased more prominent under calm nearshore hydrodynamics. Water 2019, 11, 1721 26 of 28

Both ﬁeld measurements and numerical modelling indicated that sediment accumulation increased in the degraded environment of the intertidal area of Carey Island with the presence of the detached breakwater. Field study showed increase in the sea-bed elevations, particularly near the ﬁrst segment of breakwater structure. A smaller increased in bed elevations occured in the landward area behind the breakwaters. The results showed that the presence of the low-crested detached breakwater lead into sediment accumulations in the degraded environment in the intertidal area of Carey Island and reduce the erosion problem at the site. Sediment deposition during the calm seasons could create a suitable tidal regime for mangrove survival. The overall results indicated that conduction of the breakwater structure is vital to reduce the erosion problems in cohesive coasts of Carey Island and play a key role in success of mangrove rehabilitation projects in the region.

Author Contributions: A.F. and R.H. conceived and designed the study; A.F. performed data analyses; A.F. and S.A. wrote the manuscript; all authors (A.F., S.A., K.M. and R.H.) revised the manuscript for publication. Funding: This work was funded by the High Impact Research, University of Malaya [Grant Number UM.C/HIR/MOHE/ENG/47] and the Ministry of Higher Education, Malaysia for TRGS/1/2015/UKM/02/5/1 & TRGS/1/2015/UKM/02/5/3. Acknowledgments: The authors would like to thank the Department of Survey and Mapping Malaysia (JUPEM), the Department of Meteorology, Malaysia and the National Hydrography Centre, Malaysia for sharing the important data that enabled us to undertake this study. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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