NUMERICAL MODELLING OF HYDRODYNAMICS AND EVIDENCE OF CAY EROSION IN SERIBU ISLANDS. by Gabriella Alodia

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NUMERICAL MODELLING OF HYDRODYNAMICS AND EVIDENCE OF CAY EROSION IN SERIBU ISLANDS UNDERGRADUATE THESIS Academic Writing as one of the requirements to obtain the degree of SARJANA TEKNIK in Geodetic

1 NUMERICAL MODELLING OF HYDRODYNAMICS AND EVIDENCE OF CAY EROSION IN SERIBU ISLANDS UNDERGRADUATE THESIS Academic Writing as one of the requirements to obtain the degree of SARJANA TEKNIK in Geodetic and Geomatics Engineering Undergraduate Study Program by Gabriella Alodia GEODETIC AND GEOMATICS ENGINEERING STUDY PROGRAM FACULTY OF EARTH SCIENCES AND TECHNOLOGY INSTITUTE OF TECHNOLOGY, BANDUNG 2012

AUTHORIZATION This Undergraduate Thesis entitled NUMERICAL MODELLING OF HYDRODYNAMICS AND EVIDENCE OF CAY EROSION IN SERIBU ISLANDS is actually made by myself and has not been previously written and

2 AUTHORIZATION This Undergraduate Thesis entitled NUMERICAL MODELLING OF HYDRODYNAMICS AND EVIDENCE OF CAY EROSION IN SERIBU ISLANDS is actually made by myself and has not been previously written and submitted either in part or whole, either by me or other people, both in ITB and other educational institutions. Bandung, September 2012 Gabriella Alodia NIM Bandung, September 2012 Supervisor I Supervisor II Dr.rer.nat Poerbandono NIP Dr. Eng. Totok Suprijo NIP Head of Undergraduate Study Program of Geodesy and Geomatics Engineering Dr. Kosasih Prijatna NIP ii

3 ABSTRACT Seribu (Thousand) Islands is a patch reef complexes off Jakarta Bay that have experienced severe erosions. A recent study advises that monsoonal wind-induced wave surf actions are thought to contribute to this by causing seasonal change of beach line positions. This research is intended to understand the pattern of the sand movement transport around a coral island s beach line. A two-dimensional numerical simulation is applied in order to understand the near shore current under different wind scenario, i.e. NW and SE monsoons. It is found that the net neap-spring current vectors resulted from the simulation show directional trend, which fits well the hypothetical directional pattern of seasonal sand movement. The result of this study has helped to complement recent findings on changes of beach line and development of sand spit on the focal point of which are based on the interpretation of remote sensing imagery in different epochs. Keywords: remote sensing, hydrodynamics simulation, along shore currents iii

4 ABSTRAK Kepulauan Seribu merupakan gugusan pulau-pulau karang yang terletak di utara Jakarta yang telah mengalami erosi. Sebuah penelitian terbaru menyatakan bahwa gelombang yang terjadi akibat angin musiman diduga memberikan kontribusi terhadap fenomena erosi tersebut, terbukti dengan adanya perubahan posisi garis pantai yang terjadi secara musiman. Penelitian ini bertujuan untuk mengetahui pola pergerakan pasir di sekitar garis pantai sebuah pulau karang. Dilakukan simulasi numerik dua-dimensi untuk mengetahui arus yang terjadi di dekat garis pantai dalam dua skenario berbeda, yaitu musim barat (NW) dan musim timur (SE). Ditemukan bahwa vektor resultan arus saat purnama dan setengah purnama. yang dihasilkan dari simulasi tersebut menunjukkan kecenderungan arah yang sesuai dengan hipotesis pola pergerakan pasir musiman. Hasil dari penelitian ini telah melengkapi penemuan-penemuan terbaru yang berkaitan dengan perubahan garis pantai serta penemuan lidah pasir berdasarkan interpretasi citra hasil penginderaan jauh dalam epok yang berbeda. Kata kunci: penginderaan jauh, simulasi hidrodinamika, arus dekat pantai iv

5 ACKNOWLEDGEMENT I give thanks to The Lord who gave me the strength to pull through my limit in this undergraduate education processes. The One who made me believe that everything will be beautiful in His time (Ecclesiastes 3:11). This Undergraduate Thesis titled Numerical Modelling of Hydrodynamics and Evidence of Cay Erosion in Seribu Islands would not be completed without the supports from: 1. Dr.rer.nat. Poerbandono as the 1 st Supervisor of this Undergraduate Thesis, who have given his full support on facilities and ideas on this research. 2. Dr. Eng. Totok Suprijo as the 2 nd Supervisor of this Undergraduate Thesis, who have given me the chance to learn things beyond my major study. 3. Dr. Kosasih Prijatna as one of the examiners of this Undergraduate Thesis, also as the Head of Geodetic and Geomatics Engineering Study Program. 4. Dr.rer.nat. Wiwin Windupranata as one of the examiners of this Undergraduate Thesis. 5. Dr. Irwan Meilano as my Faculty Trustee for the whole undergraduate year. 6. Mr. Moch. Id Fitrayana, Ms. Sripardi Jamelina, Ms. Leslie Bissey, Mr. Waskito, Mr. Untung, Mr. Salim, Mr. Sarwan, Budi Dipa Prabowo, and Rendy Hermawan for the support on the field experiments. 7. Umar Abdul Aziz for the discussions and references. 8. Muammar Kamadewa and Satya Mujahid for the MIKE tutorials. 9. Hydrographic room colleagues, for the discussions and accompanies. I would like to dedicate this Undergraduate Thesis to my parents, Djohansjah Adiputra and Dr. Wita Anggraini, for the full support of letting me continue my study in Bandung, to my brother, Dick Perthino Sebastian, for all the things we have had since we were only kids, to my whole big family, for the love I will never have in another, and to my greatest comrades, Ikatan Mahasiswa Geodesi. This Undergraduate Thesis is part of a research that was funded by ASAHI Glass Foundation. I hope the results could be used for the next researches. Bandung, September 2012 Gabriella Alodia v

6 TABLE OF CONTENT AUTHORIZATION... ii ABSTRACT... iii ABSTRAK... iv ACKNOWLEDGEMENT... v TABLE OF CONTENT... vi LIST OF FIGURES... viii LIST OF TABLES... ix 1 INTRODUCTION Background Problem Statement Objectives Study Area Research Method Structure and Contents MATERIALS AND METHODS Sand Movement on Coral Cays Monsoonal Wind Nodal Point Theory Evidence of Erosion Characteristic of Hydrodynamics in Study Area Field Observation Analysis of Observation Data Numerical Simulation Setup Mesh Generation Sensitivity Analysis Scenario of Bed Resistance Scenario of Wind Forcing Calibration and Validation Verification of Tidal Data Verification of Current Velocity Data Hydrodynamics Simulation Terms and Definition vi

7 Governing Equations Parameters of Simulation RESULTS AND ANALYSIS Terms and Definition Bathymetric Modelling Hydrodynamics Simulation Typical Flow Field Typical Total, Tidal, and Non-Tidal Current Along Shore Currents The Relation of Hydrodynamics and Sand Movement CONCLUSIONS AND SUGESTIONS Conclusions Suggestions REFERENCES vii

8 LIST OF FIGURES Figure 1.1 Study area... 3 Figure 1.2 Research flowchart... 4 Figure 2.1 Period mean monthly wind energy resultant vector, (Poerbandono, 2012)... 7 Figure 2.2 Nodal point accretion at cay's leeward under predominant windinduced waves (Flood, 1986)... 8 Figure 2.3 Semak Daun cay in two monsoonal seasons (Alodia et al., 2012)... 8 Figure 2.4 Nodal point model proposed for ESE waves during SE monsoon in Semak Daun cay (Poerbandono, 2012)... 9 Figure 2.5 Erosion evidences found in Semak Daun cay (Lumingkewas, 2009)... 9 Figure 2.6 Model domain and observation points Figure 2.7 Domain wire frame Figure 2.8 Sensitivity of tide simulation due to bed roughness Figure 2.9 Sensitivity of the current simulation due to bed rougness Figure 2.10 Sensitivity of tide due to wind speed Figure 2.11 Sensitivity of current due to wind speed Figure 2.12 Time series comparison between observed water level and simulated tide Figure 2.13 Equality line of observed and simulated water level Figure 2.14 Comparison of observed and simulated velocity Figure 3.1 Typical flow field in SE season; (a) flood; (b) ebb Figure 3.2 Typical flow field in NW season; flood (a); ebb (b) Figure 3.3 Current velocity hodographs Figure 3.4 Typical time series of total, tidal, and non-tidal current in SE monsoon Figure 3.5 Along shore currents in SE and NW monsoon viii

9 LIST OF TABLES Table 2.1 Mean monthly wind direction, in degrees (Poerbandono, 2012)... 6 Table 2.2 Mean monthly wind magnitude, in ms -1 (Poerbandono, 2012)... 6 ix

2 MATERIALS AND METHODS

2 MATERIALS AND METHODS 2.1. Sand Movement on Coral Cays Sand movement in Semak Daun cay was recognized by the monsoonal morphological change of the beach line. It is found that certain beach lines advanced