Fenomena gerusan lokal dan degradasi dasar sungai di hilir Sub Dam Pada Sabo Dam tipe terbuka dan tertutup :: Uji model fisik
MAKWAN, Dr.Ir. Bambang Yulistiyanto
2006 | Tesis | S2 Teknik Sipil (MPBA)Aliran sedimen dalam jumlah besar dapat menimbulkan bencana bagi manusia. Bencana alam sedimen dikendalikan dengan Sabo Dam. Sabo Dam dibangun pada dasar sungai yang terbentuk oleh endapan aliran sedimen, sehingga sifatnya tidak stabil. Beberapa bangunan Sabo Dam ditemukan dalam kondisi rusak setelah terjadi banjir, diduga disebabkan oleh fenomena gerusan lokal dan degradasi dasar sungai. Untuk mengetahui karakteristik fenomena gerusan lokal dan degradasi dasar sungai di hilir sub dam diteliti dengan uji model fisik. Penelitian uji model fisik ini dilaksanakan dengan cara memodifikasi model fisik skala 1:25 dari prototipe sungai Boyong dan BOD-5 yang sudah ada di laboratorium dengan menambah panjang 10,6 m di hilir sub dam. Bangunan Sabo Dam tipe terbuka dimodifikasi untuk tipe tertutup. Uji model dilakukan terhadap Sabo Dam tipe terbuka dan tipe tertutup. Debit puncak diperoleh dari debit desain Q50 187,74 m3/det. Debit dan waktu pengaliran di model skala 1:25 didasarkan pada hidrograf banjir AWLR stasiun Pulowatu pada kejadian banjir tgl 14 Desember 1994 yaitu Q1=11,2 ltr/det, waktu 10 menit; Q2=59,84 ltr/det, waktu 14 menit (sebagai debit puncak); Q3=36 ltr/det waktu 20 menit dan Q4=16 ltr/det selama 16 menit. Hasil penelitian menunjukkan bahwa fenomena gerusan lokal dan degradasi dasar sungai di hilir sub dam dipengaruhi oleh debit aliran dan tipe Sabo dam. Semakin besar debit semakin besar semakin besar gerusan lokal dan degradasi yang ditimbulkannya. Tipe Sabo Dam tertutup (hulu bangunan belum terisi sedimen) dengan aliran puncak (Q2) memberikan dampak yang paling besar terhadap fenomena gerusan lokal dan degradasi dasar sungai daripada tipe yang lain. Gerusan lokal maksimum 0,236 m, total angkutan sedimen dasar sungai sebesar 2,29 m3 berdampak pada degradasi dasar sungai sebesar 0,128 m dan diikuti turunnya kemiringan dasar sungai dari kemiringan awal 6% menjadi 2,6%. Sub dam aman terhadap degradasi tetapi tidak aman dari gerusan lokal.Gerusan lokal dan degradasi dasar sungai yang terjadi di hilir sub dam terisi oleh sedimen yang terangkut aliran Q3 dan Q4.
A huge sediment flow can result in disaster for human life. Sediment disaster is controlled by Sabo Dam. Sabo Dam is constructed in the base of the river which is built by sediment flow so that it has unstable characteristics. Several Sabo constructions are found in damaged condition after flood, and are assumed triggered by local scouring phenomenon and river base degradation. Physical model test research is conducted to understand local scouring phenomenon and river base degradation in down part of sub dam. Physical model test research has done by modifying physical model at 1:25 scale from Boyong river and BOD-5 prototype which is already exist in laboratory by adding 10.6 m its length in down part of sub dam. Open type Sabo Dam is modified for close type. Model test has done toward open and close types of Sabo Dam. Peak discharge is resulted from Q50 =187.74 m3/sec discharge design. Discharge and time flow at 1:25 scale model, is based on AWLR flood hydrograph at Pulowatu station when it was flooding on December 14th 1994, Q1=11.2 liter/sec, 10 minutes; Q2= 59.84 liter/sec, 14 minutes (as peak discharge); Q3=36 liter/sec, 20 minute and Q4=16 liter/sec during 16 minutes. Research result shows that local scouring phenomenon and river base degradation in down part of sub dam is influenced by flow discharge and type of Sabo Dam. The increasing discharge flow resulted on huge local scouring and degradation effect. Close type Sabo Dam which upstream structure has not been filled by sediment, result in greatest effect toward local scouring phenomenon and river base degradation compare to other type at peak flow (Q2) condition. Maximum local scouring of 0.236 m and total river base sediment transport of 2.29 m3, result on 0.128 m river base degradation. It is followed by decrease of river base gradient from 6 % down to 2.6 %. Sub Dam is safe toward degradation. However, it is unsafe from local scouring. Local scouring and river base degradation at down part of sub dam are filled with sediment transport by Q3 and Q4 flows.
Kata Kunci : Aliran Sedimen,SABO Dam,Uji Model Fisik, physical model test, local scouring, degradation
