Some efforts have been done to cope with coastal erosion and rob in Sayung Coast including mangrove plantation, hybrid engineering (HE), and hard structure protection. However, those efforts are not considered to be the best solution in reducing the impacts of the hazards. This study aimed to determine the sediment accumulation rates based on natural isotopes 210 Pb dating and hydro-oceanographic modeling technique. According to 210 Pb analysis, we calculated the sediment accumulation rates at Surodadi and Timbulsloko of 0.145 cm/year and 0.06 cm/year, respectively. The sediment compositions are dominated by terrestrial sediment typified by clay deposits. The sedimentary rate average of Sayung Coast prior to HE installation (2011-2013) was 0.195 cm/year. The rate of sediment accumulation rose to around 0.4 cm/year in 2016 in accordance with the increase of Mangrove area from around 409 hectares in 2013 to about 455.79 hectares in 2015. The increase of sedimentary rate recent years became an evidence that mitigation efforts to reduce abrasion temporarily succeeded.
Dinamika kawasan pesisir mencakup proses erosi dan sedimentasi merupakan hal penting untuk mengetahui kondisi pantai. Pesisir Kecamatan Brebes merupakan salah satu kawasan di Pantai Utara Pulau Jawa yang memiliki dinimika perubahan pesisir sangat kuat. Studi sebaran jenis sedimen yang dilakukan secara terpadu di perairan Kecamatan Brebes ditunjukkan untuk memberikan gambaran proses transportasi dan pengendapan sedimen berdasarkan distribusi ukuran butir sedimen. Penelitian dilakukan dengan pengambilan sampel sedimen dasar perairan secara acak dan tersebar di sepanjang pesisir dengan metode grab sampler, kemudian dilakukan analisis lebih lanjut dengan metode granulometri. Hasil penelitian menunjukkan bahwa secara umum, pola sebaran sedimen permukaan dasar laut Kec.Brebes didominasi oleh sedimen berukuran lempung-pasir. Jenis sedimen berdasarkan ukuran butirnya yaitu pasir, pasir lanauan, lanau pasiran dan lanau. Kondisi hidro-oseanografi yang fluktuatif memberikan nilai sortasi buruk hingga sedang, dominasi ukuran halus tersebar pada bagian Timur, sedangkan fraksi kasar tersebar dibagian Barat dekat dengan laut lepas. Kondisi tersebut menjadikan salah satu faktor tererosinya bagian pesisir Kecamatan Brebes dengan dinamika hidro-oseanografi yang berubah-ubah.
The dynamics of erosion and accretion in the coastal area of Brebes Subdistrict have many impacts on mangrove destruction and coastal region. The erosion and accretion are influenced by oceanography parameters that can affect the distribution of coastal sediments. The aims of this research were to determine the characteristics of the phenomenon of erosion and accretion in coastal Brebes Subdistrict and shape appropriate mitigation to reduce the erosion. This research is done with the bed sediment sampling using grab sampler for 26 sampling point. Current and tide measurement are conducted wih ADCP deployment for 15 days' measurement. Sediment characteristic analysis done with granulometri and statistic analysis. The average of sediment sorting is 1.21, skewness is 0.088 and the kurtosis is 3.76. Generally, bed surface sediment distribution pattern is dominated by clay to sand, the grand size distribution of sediments are sand, silt, sandy silt and silty sand. Longshore current speed ranged between 0-0.12 m.s -1 at the spring tide condition and ranged between 0-0.08 m.s -1 at the neap tide condition, the domination of current direction towards to the East and Northeast. The mitigation is very needed to rearrange the mangrove areas, build the coastal protection and recover the main river line which is the sourceof the sediment materials, so that can be provide the supply of sediment deposition in the coastal area. ABSTRAK Dinamika kawasan pesisir Kecamatan Brebes berupa bencana abrasi dan akresi yang terjadi memiliki dampak besar terhadap kerusakan kawasan mangrove dan pesisir. Fenomena erosi dan akresi yang terjadi dipengaruhi oleh parameter oseanografi yang dapat mempengaruhi sebaran sedimen di pesisir. Penelitian ini bertujuan untuk mengetahui karakteristik distribusi sedimen dasar perairan sebagai upaya identifikasi bencana erosi yang terjadi di pesisir Kecamatan Brebes.Penelitian dilakukan dengan pengambilan sampel sedimen dasar perairan menggunakan metode grab sampler terhadap 26 titik pengambilan. Pengukuran kecepetan arus di lokasi penelitian dilakukan dengan pemasangan ADCP selama 15 hari. Analisis karakteristik sedimen dilakukan dengan metode analisis granulometri dan statistic sedimen. Nilai rata-rata sortasi 1.21, skewness 0.088 dan kurtosis 3.76. Secara umum, pola sebaran sedimen permukaan dasar laut Kec.Brebes didominasi oleh sedimen berukuran lempung-pasir. Jenis sedimen berdasarkan ukuran butirnya yaitu pasir, pasir lanauan, lanau pasiran dan lanau. Kecepatan arus sepanjang pantai hasil model pada kondisi purnama berkisar 0-0,12 m.s -1 dan pada saat perbani berkisar antara 0-0,08 m.s -1 , dengan dominasi arah arus menuju ke Timur dan Timur laut. Perlu dilakukan penataan ulang terhadap kawasan mangrove dan bangunan pelindung pantai serta melakukan pencodetan terhadap sungaisungai utama yang merupakan sumber pembawa material sedimen sehingga dapat memberikan suplay endapan sedimen di bagian pesisir.
Mandeh Bay is threatened by sedimentation issue caused by the rapid development of marine tourism area which strongly impacts to the environmental degradation. Due to the semi-enclosed area of Mandeh Bay, the tidal current has a significant role in triggering vertical and horizontal transports within the bay. This study aimed to determine the characteristic of tidal current during the southwest monsoon. We developed a hydrodynamic model based on Navier-Stokes equations using a flexible mesh and tidal forecast in which the validation is performed by ADCP data. The simulation results will be used as the basic data to develop a model which depicts the elliptical pattern of tidal current constituents. Offshore rotary tidal currents which are originally semidiurnal reiterate the elliptical pattern every 6 hours and 12 minutes. The strongest semidiurnal current speeds are observed in the bay mouth ranged from 0.1-0.5 m.s-1. The tidal constituent ellipses are oriented more meridionally and in several areas oriented zonally. The current speed of is the highest at all which the speed is averagely one third of magnitude. While, the two main diurnal tidal constituents ( and ) have the maximum speeds approximately one fifth of magnitude. Thus, the domination of semidiurnal constituents may trigger sediment distribution and accumulation within the bay because of its twice tidal oscillations entering the bay.
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