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

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## Abstract

**:**

## 1. Introduction

^{3}[39].

## 2. Study Area

_{max}= 35 °C and T

_{min}= 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 (Figure 1a). The suspended sediments are carried by Langat River to the Malacca Strait as well as the area of the study site.

## 3. Methods

#### 3.1. Data Collection

#### 3.2. Monitoring of Sea-Bed Elevation

#### 3.3. Numerical Modelling

_{a}is atmospheric pressure, $\rho $ is density, ${\rho}_{0}$ is reference density of water, ${\nu}_{t}$ is the vertical diffusivity, f is Coriolis parameter and S is point-source discharge magnitude. F

_{u}and F

_{v}are gradient-stress relations described is Equations (4) and (5), respectively.

^{th}component of the mass concentration, ${\nu}_{Tx}$ is the anistropic eddy viscosity, ${S}^{i}$ is the source term, ${\sigma}_{Tx}^{i}$ is turbulent Schmitt number, and w

_{s}is the settling velocity. The model is capable of considering flocculation as a function of suspended sediment concentration (Equation (9)).

_{s}denotes settling velocity of suspended sediment, p

_{D}describes the probability of deposition ($=1-{\tau}_{b}/{\tau}_{cd}$) and c

_{b}is suspended sediment concentration near the bed. The sediment transport model 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.

#### 3.3.1. Model Setup

#### 3.3.2. Model Input

#### 3.3.3. Computational Domain

#### 3.3.4. Model Calibration and Validation

^{2}), and Root Mean Squared Error (RMSE) were calculated [44,48,54].

^{2}and RMSE obtained during the model 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.

## 4. Results and Discussion

#### 4.1. Sediment and Water Samples Analyses

_{50}) of the cohesive sediments in the intertidal area of Carey Island, at depth of 0–40 cm, was determined to be 0.015–0.022 mm, which was consisted of 10% clay, 71% silt and 19% fine sand. Furthermore, the sediment fraction at depths of 40–100 cm consisted of stiff clay. Based on water samples and velocity measurements at the mouth of the Langat River, TSS 1 showed that the Langat River carries 180–261 mg/L of suspended sediments to the Malacca Strait with 698–1130 m

^{3}/s of water discharges during ebb tide and 121–479 m

^{3}/s during spring tide. Table 4 present TSS 2 analyses from the water samples collected at 500 m seaward of the breakwater.

#### 4.2. Hydrodynamic Changes in the Locality of the Breakwater Structure

#### 4.3. Suspended Sediment Transport in the Breakwater’s Surroundings

^{3}, during the NE season (Figure 11) and 0.052 kg/m

^{3}, during the SW season (Figure 12). Hence, there is a possibility that individual particles of the suspended sediments in water column become attached to each other and form flocks in the areas where the hydrodynamics conditions were calm enough. Based on the simulation results, the concentrations of suspended sediments brought from the seaward are slightly reducing in line with the mean wave direction towards the landward area. This result may have been obtained because some suspended sediment settles down on the sea-bed due to the reduction of wave and current energies triggered by the high bottom friction coefficients of the cohesive sediments [30,31].

#### 4.4. Variation of Sea-Bed Levels and Erosion-Deposition Pattern in the Locality of the Low-Crested Breakwater

#### 4.5. Discussion

#### 4.5.1. Sediment Transport and Erosion-Deposition Pattern during the Northeast Season

#### 4.5.2. Sediment Transport and Erosion-Deposition Pattern during the Southwest Season

## 5. Conclusions

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 3.**Wind roses in 2015: (

**a**) NE season; (

**b**) SW season (sources of data: Malaysian Department of Meteorology).

**Figure 4.**(

**a**) Locations of AWAC and OBS-3A units within the study site, (

**b**) location of water and bed samples in the study site (S = soil sample, W = water sample and discharge).

**Figure 7.**Comparison between simulation results and field measurements on 23 December 2014 to 7 January 2015 at station 1 (long: 101°20′11.18” E, lat: 02°48′40.02” N), (

**a**) current speed; (

**b**) current direction; (

**c**) water level; (

**d**) significant wave height; (

**e**) mean wave direction; (

**f**) SSC.

**Figure 8.**Comparison between simulation results and field measurements on 23 December 2014 to 7 January 2015 at station 2 (long: 101°18′58.14” E, lat: 02°49′26” N), (

**a**) current speed; (

**b**) current direction; (

**c**) water level; (

**d**) significant wave height; (

**e**) mean wave direction; (

**f**) SSC.

**Figure 9.**Simulated current and wave characteristics during the Northeast season: (

**a**) CS during NT (WT); (

**b**) CS during NT (W); (

**c**) CS during ST (WT); (

**d**) CS during ST (W); (

**e**) SWH during NT (WT); (

**f**) SWH during NT (W); (

**g**) SWH during ST (WT); (

**h**) SWH during ST (W). Notes: W = with breakwater, WT = without breakwater, CS = current speed, SWH = significant wave height, NT = neap tide, ST = spring tide.

**Figure 10.**Simulated current and wave characteristics during the Southwest season: (

**a**) CS during NT (WT); (

**b**) CS during NT (W); (

**c**) CS during ST (WT); (

**d**) CS during ST (W); (

**e**) SWH during NT (WT); (

**f**) SWH during NT (W); (

**g**) SWH during ST (WT); (

**h**) SWH during ST (W). Notes: W = with breakwater, WT = without breakwater, CS = current speed, SWH = significant wave height, NT = neap tide, ST = spring tide.

**Figure 11.**Simulation of suspended sediment concentration during the Northeast season: (

**a**) neap tide; (

**b**) spring tide.

**Figure 12.**Simulation of suspended sediment concentration during the Southwest season: (

**a**) neap tide; (

**b**) spring tide.

**Figure 13.**Variation of the sea-bed levels and erosion-deposition pattern in the locality of the breakwater structure over two-month periods from December 2014 to October 2015. Notes: + represents the deposition in cm, (

**a**,

**b**) NE season, (

**c**,

**d**,

**e**) SW season.

Month | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|

Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |

No. of rainfall events | 4 | 5 | 4 | 5 | 9 | 4 | 4 | 12 | 7 | 9 | 11 | 14 |

Rainfall (mm/month) | 63 | 64 | 118 | 60 | 221 | 128 | 65 | 281 | 254 | 139 | 192 | 133 |

Significant wave height (m) | 1.2 | 0.75 | 1 | 1 | 0.75 | 0.75 | 0.5 | 0.75 | 1 | 1.5 | 1.2 | 1 |

Mean wave period (s) | 5 | 4 | 5 | 4 | 3 | 3 | 3 | 4 | 5 | 5 | 5 | 4 |

Dominant wind speed (m/s) | 9.1 | 7.5 | 8.5 | 7.3 | 6.2 | 5.3 | 5.5 | 6.5 | 5.0 | 6.5 | 7.5 | 8.5 |

Dominant wind direction (degree) | 300 | 330 | 320 | 270 | 120 | 130 | 140 | 130 | 120 | 290 | 310 | 300 |

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

Station 1 (AWAC 1 and OBS 1) | 101°20′11.18” E | 02°48′40.02” N | 10.324 |

Station 2 (AWAC 2 and OBS 2) | 101°18′58.14” E | 02°49′26” N | 12.557 |

Station 3 (OBS 3) | 101°26′10.06” E | 02°40′7.91′ N | 15.221 |

Station 4 (OBS 4) | 101°06′44.34” E | 03°8′36.81” N | 10.483 |

Water samples (TSS 1) and water discharges from LR | 101°24′6.24” E | 2°48′2.72” N | 6.242 |

**Table 3.**Statistical results for the performance of the hydrodynamic, wave and mud transport models.

Parameter | Calibration | Validation | ||||
---|---|---|---|---|---|---|

RMSE | R Squared (R^{2}) | Thiel’s Inequality Coefficient | RMSE | R Squared (R^{2}) | Thiel’s Inequality Coefficient | |

Current Speeds | 0.07 m/s | 0.92 | 0.08 | 0.08 m/s | 0.91 | 0.08 |

Current Directions | 15° | 0.94 | 0.05 | 17° | 0.93 | 0.06 |

Water Levels | 0.05 m | 0.95 | 0.04 | 0.06 m | 0.94 | 0.05 |

Significant wave heights | 0.04 m | 0.85 | 0.14 | 0.05 m | 0.83 | 0.16 |

Mean Wave Directions | 18° | 0.81 | 0.18 | 19° | 0.80 | 0.19 |

SSC | 0.004 kg/m^{3} | 0.83 | 0.16 | 0.005 kg/m^{3} | 0.82 | 0.17 |

Month | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|

Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |

TSS 2 (kg/m^{3}) | 0.047 | 0.043 | 0.058 | 0.049 | 0.079 | 0.071 | 0.06 | 0.081 | 0.076 | 0.058 | 0.061 | 0.056 |

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**MDPI and ACS Style**

Fitri, A.; Hashim, R.; Abolfathi, S.; Abdul Maulud, K.N.
Dynamics of Sediment Transport and Erosion-Deposition Patterns in the Locality of a Detached Low-Crested Breakwater on a Cohesive Coast. *Water* **2019**, *11*, 1721.
https://doi.org/10.3390/w11081721

**AMA Style**

Fitri A, Hashim R, Abolfathi S, Abdul Maulud KN.
Dynamics of Sediment Transport and Erosion-Deposition Patterns in the Locality of a Detached Low-Crested Breakwater on a Cohesive Coast. *Water*. 2019; 11(8):1721.
https://doi.org/10.3390/w11081721

**Chicago/Turabian Style**

Fitri, Arniza, Roslan Hashim, Soroush Abolfathi, and Khairul Nizam Abdul Maulud.
2019. "Dynamics of Sediment Transport and Erosion-Deposition Patterns in the Locality of a Detached Low-Crested Breakwater on a Cohesive Coast" *Water* 11, no. 8: 1721.
https://doi.org/10.3390/w11081721