Simulasi numeris tiga dimensi kantong lumpur Bendung Sapon

NINDITO, Dwi Agung, Dr.Ir. Istiarto, M.Eng

2007 | Tesis | S2 Teknik Sipil

Salah satu keberhasilan sebuah perencanaan kantong lumpur terletak pada efisiensi pengendapan sedimen. Pada kenyataannya, cukup sulit untuk memodelkan secara detail kasus pergerakan sedimen pada sebuah model fisik. Fenomena pengendapan sedimen dapat dirumuskan dengan persamaan matematik secara cukup detail, sehingga simulasi numeris dipilih untuk mengetahui kinerja kantong lumpur. Simulasi dilakukan pada perencanaan model kantong lumpur Bendung Sapon menggunakan model numeris tiga dimensi SSIIMWin 1.1. Efisiensi pengendapan dicek untuk dua keadaan yang berbeda yaitu pada saat kantong lumpur kosong dan penuh. Untuk kantong lumpur kosong, dilakukan pengecekan terhadap kecepatan dan efisiensi pengendapan awal. Untuk kantong lumpur penuh, dicek apakah pengendapan masih efektif. Efisiensi pengendapan tergantung dari karakteristik sedimen (kecepatan jatuh butiran) dan karakteristik aliran air di kantong lumpur (kecepatan aliran). Simulasi aliran ditampilkan melalui vektor kecepatan tiga dimensi. Hasil simulasi distribusi kecepatan menunjukkan bahwa komponen horisontal kecepatan aliran dari hulu ke hilir (bertambahnya nilai x/b) nilainya semakin mengecil. Kecepatan aliran horisontal minimum terjadi di dekat dasar saluran dan bertambah besar terhadap z/h menuju ke permukaan. Semakin mendekati dinding batas, kecepatan aliran horisontal semakin kecil dan maksimum di y/B=0,5. Komponen vertikal kecepatan aliran menunjukkan bahwa semakin mendekati dasar saluran, kecepatan arah vertikalnya semakin besar. Komponen lateral kecepatan aliran menunjukan bahwa terjadi penambahan kecepatan pada saat terjadi pelebaran saluran. Di awal kantong lumpur, timbul pusaran tiga dimensi akibat penurunan elevasi dasar. Pusaran tidak tampak di tengah kantong lumpur. Pusaran kembali muncul di dasar saluran saat mendekati ujung hilir kantong lumpur. Profil distribusi konsentrasi sedimen tampak bahwa konsentrasi sedimen suspensi semakin besar ke arah dasar saluran dan semakin kecil ke arah permukaan. Semakin ke hilir saluran, konsentrasi sedimen suspensi semakin kecil. Sebesar 69,85% dari sedimen yang terendapkan terdiri dari partikel-partikel dengan diameter diatas 0,06mm. Perubahan volume tampungan akibat penambahan volume sedimen yang mengendap menyebabkan efisiensi pengendapan mengalami penurunan. Kantong lumpur sudah tidak efektif mengendapkan sedimen pada saat volume endapan sedimennya sudah mencapai 71,84 % dari seluruh volume tampungan.

The success factor of a settling basin planning is the trap efficiency. In reality, it is difficult to create a detailed physical model of the sedimentation process. In a more detailed way, sedimentation process phenomena can be formulized by using mathematical equation, for this matter numerical simulation is selected to observe the settling basin performance. A three-dimensional numerical model from SSIIMWin 1.1 is used to carry out this settling basin simulation of the Sapon weir planning. The trap efficiency of the sedimentation process is observed in two different situations: at the time when the settling basin is empty and when it is full. During the empty situation, the velocity and the initial trap efficiency process is measured. When the settling basin is full, inspection is performed to check whether the sedimentation trap process remains effective. The trap efficiency depends on the characteristics of the inflowing sediment (fall velocity of the particle) and the water flow characteristic in the settling basin (flow velocity). Flow velocity simulation is demonstrated using a three-dimensional velocity vectors. Velocity distribution simulation indicates that the value of the horizontal flow velocity component from the upstream to the downstream (augmentation of x/b value) is decreased. The minimum horizontal flow velocity occurs in locations close to the bottom of the bed channel and increasing towards z/h to the surface water. Approaching to the wall, the horizontal flow velocity is decreasing and reaches its maximum at y/B=0.5. The vertical flow velocity component indicates that approaching the bed channel, the vertical velocity is increasing. The lateral flow velocity component indicates a velocity increase at the horizontal expansion. A three-dimensional vortex occurs on the inlet of the settling basin, due to the vertical expansion. The vortex does not exist in the middle location of the settling basin. Another vortex occurs at the bottom of the channel approaching the downstream of the settling basin. The profile of sediment concentration distribution indicates an increase from the water surface to the bottom (bed channel). The sediment concentration is decreased in quantity to the downstream of the channel. The ratio of the incoming sediment that is deposited (trapped) and inflow the sediment mass entering a settling basin is 69,85 % for the upper 0.06 mm diameter particles. Volume changes of the pond, due to the deposit sediment volume, have lead to a decline in the trap efficiency process. When the volume of the sediments reaches a percentage of 71,84 % of the total pond volume, the settling basin is considered ineffective in performing sedimentation trap process.

Kata Kunci : Pengendapan Sedimen,Simulasi Numeris Tiga Dimensi, three-dimensional numerical simulation, trap efficiency, settling basin