Idfi, Gilang (2025) Model Kolam Olak Tipe Negatif Bertangga (Negative Step) untuk Meningkatkan Performa Hidrolis dan Efektifitas Peredaman Energi. Doctoral thesis, Institut Teknologi Sepuluh Nopember.
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Abstract
Peredam energi merupakan salah satu bagian terpenting pada pelimpah bendungan. Bangunan ini mempunyai fungsi untuk meredam energy aliran superkritis dari saluran peluncur (chuteway) menjadi aliran subkritis untuk dikembalikan lagi ke sungai. Di dalam proses peredaman tersebut terdapat suatu fenomena loncatan hidraulik (hydraulic jump) yang merupakan indikator terjadinya proses peredaman tersebut. Permasalahan di lapangan yang sering terjadi adalah peredam energi tidak bekerja secara optimal, sehingga seringkali mengakibatkan permasalahan gerusan pada bagian geometri sungainya yaitu dasar dan tebing sungai. Berdasarkan permasalahan tersebut, maka dilakukan penelitian peningkatan kinerja peredam energi berdasarkan parameter hidraulik. Model kolam olak yang banyak digunakan adalah model dari United States Bureau of Reclamation (USBR), dimana model ini menggunakan lantai kerja kolam olak yang datar dan dilengkapi dengan bangunan pemecah energi di bagian hulu, tengah dan hilir kolam olak ( Chute Block, Baffle Block dan End Sill). Berdasarkan beberapa kondisi eksisting dan penelitian terdahulu kinerja kolam olak ini kurang optimal dikarenakan kondisi aliran saat keluar dari kolam olak adalah kondisi superkritis dengan kecepatan yang tinggi, sehingga nilai redaman kurang efektif.
Merujuk pada analisis masalah yang ada, penelitian ini bertujuan untuk menutup gap penelitian terdahulu untuk menemukan bentuk kolam olak yang efektif agar nilai redaman energi aliran dapat optimal, sehingga tidak terjadi aliran super kritis yang menyebabkan gerusan pada hilir kolam olak. Upaya tersebut dapat dilakukan dengan merubah bentuk lantai kerja kolam olak dengan merubah dalam bentuk kemiringan negative (semakin ke hilir semakin menanjak) atau sering disebut Negative Steep. Model yang akan dicoba kolam olak bertangga atau Negative Step. Selain model tersebut, maka akan dicoba juga dengan model Negative Step yang disertai penurunan elevasi kolam olak. Penelitian ini dilakukan di Laboratorium Hidrolika dan Teknik Pantai, Departemen Teknik Sipil ITS, menggunakan model fisik yang dibuat di flume dengan kemiringan saluran peluncur yang digunakan kemiringan 1:2 dengan lebar 20 cm. Pada penelitian ini juga digunakan 10 variasi model debit dan dialirkan untuk masing-masing model kolam olak yang dibuat yaitu kolam olak model USBR, Negative Step dan Negative Step yang disertai penurunan elevasi kolam olak. Kondisi batasan hilir (downstream boundary condition) digunakan pelimpah skot balk untuk membentuk tail water (Tw). Pengamatan parameter hidrolik yang dilakukan pada uji experimental ini adalah elevasi muka air (y1, y2 dan yc), panjang loncatan air (Lj), disipasi energi (E1 dan E2), rasio disipasi energi (ΔE) dan faktor gesekan (f).
Hasil penelitian pengujian menunjukkan model experimental kolam olak USBR Type III tanpa Tw menghasilkan rasio disipasi energi (ΔE/E1) sebesar 76.26% dan efisiensi (E2/E1) sebesar 23.74%, sedangkan pengujian dengan Tw menghasilkan rasio disipasi energi (ΔE/E1) sebesar 78.99% dan efisiensi (E2/E1) sebesar 21.01%. Pengujian kolam olak negatif bertangga (Negative Step) tanpa Tw menghasilkan rasio disipasi energi (ΔE/E1) sebesar 77.53% dan efisiensi (E2/E1) sebesar 22.26%, sedangkan pengujian dengan Tw menghasilkan rasio disipasi energi (ΔE/E1) sebesar 81.39% dan efisiensi (E2/E1) sebesar 18.61%. Model experimental kolam olak negatif bertangga (Negative Step) yang disertai penurunan elevasi kolam olak tanpa Tw menghasilkan rasio disipasi energi (ΔE/E1) sebesar 78.41% dan efisiensi (E2/E1) sebesar 21.21%. Sedangkan pengujian dengan Tw menghasilkan rasio disipasi energi (ΔE/E1) sebesar 84.96% dan efisiensi (E2/E1) sebesar 15.04%. Dari hasil pengujian untuk ketiga model tersebut menunjukkan model Negative Step yang disertai penurunan elevasi kolam mempunyai kinerja peredaman energi yang terbaik.
Hasil penelitian ini juga menghasilkan formula baru untuk perhitungan panjang loncatan hidrolis (hydraulics jump). Formula untuk model kolam olak negatif bertangga (Negative Step) Lj = 6.37 (Y2-Y1), sedangkan kolam olak negatif bertangga (Negative Step) yang disertai penurunan elevasi kolam olak Lj = 6.29 (Y2-Y1). Rata-rata panjang loncatan hidrolis (Lj) pada kolam olak bertangga tanpa penurunan elevasi kolam olak adalah 41.85 cm, sedangkan pada model kolam olak negatif bertangga yang disertai penurunan elevasi kolam olak adalah 40.02 cm. Model bertangga (Negative Step) dapat menghasilkan faktor gesekan sebesar 0.819. Besarnya faktor gesekan secara tidak langsung akan mempengaruhi pula tingkat peredaman energi pada kolam olak. Semakin besar nilai faktor gesekan maka kehilangan energi akan semakin besar, sehingga kemampuan peredaman energi akan semakin baik
Kata Kunci : kolam olak, Negative Step, tail water, superkritis, hydraulic jump
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The stilling basin is one of the most crucial components of a dam's spillway. A stilling basin reduces the supercritical flow energy from the chute way into a subcritical flow, which is then returned to the river. During the dissipation process, a hydraulic jump phenomenon occurs, indicating the completion of the dissipation process. The reality is that the energy dissipator often does not work optimally, resulting in erosion problems in the river geometry, specifically the river bed and river banks. Based on these problems, research was conducted to improve the performance of energy dissipators based on hydraulic parameters. The stilling basin model that is widely used is the model from the United States Bureau of Reclamation (USBR), where this model uses a flat stilling basin working floor and is equipped with energy-breaking structures in the upstream, middle, and downstream parts of the stilling basin (Chute Block, Baffle Block, and End Sill). Based on several existing conditions and previous research, the performance of this stilling basin is less than optimal because the flow conditions when leaving the stilling basin are supercritical, characterized by high speed, resulting in an ineffective energy dissipation ratio.
Building on the analysis of existing problems, this study aims to bridge the gap in previous research by identifying an effective form of a stilling basin that optimizes the flow energy attenuation value, thereby preventing supercritical flow from causing erosion downstream of the stilling basin. This effort can be made by changing the shape of the working floor of the stilling basin by changing it into a negative slope (the further downstream, the steeper it is), often called a Negative Steep. The model that will be tried is a stairwell or a Negative Step. In addition to this model, the Negative Step model will also be tried, which is accompanied by a decrease in the elevation of the stilling basin. This research was conducted at the Laboratory of Hydraulics and Coastal Engineering, Department of Civil Engineering, ITS, using a physical model created in a flume with a slope of 1:2. The chuteway used a slope of 1:2 with a width of 20 cm. In this study, 10 variations of the discharge model were also used and applied for each model of the stilling basin made, namely the USBR model stilling basin, Negative Step, and Negative Step accompanied by a decrease in the elevation of the stilling basin. The downstream boundary condition is utilized by the weir to form tailwater (Tw). The hydraulic parameter observations conducted in this experimental test are water surface elevation (y1, y2, and yc), hydraulic jump (Lj), energy dissipation (E1 and E2), energy dissipation ratio (ΔE), and friction factor (f).
The results of the test research showed that the experimental model of the USBR Type III stilling basin without Tw produced an energy dissipation ratio (ΔE/E1) of 76.26% and an efficiency (E2/E1) of 23.74% while testing with Tw produced an energy dissipation ratio (ΔE/E1) of 78.99% and an efficiency (E2/E1) of 21.01%. Testing of the Negative Step stilling basin without Tw produced an energy dissipation ratio (ΔE/E1) of 77.53% and an efficiency (E2/E1) of 22.26% while testing with Tw produced an energy dissipation ratio (ΔE/E1) of 81.39% and an efficiency (E2/E1) of 18.61%. An experimental model of the negative stilling basin with stairs (Negative Step), accompanied by a decrease in the elevation of the stilling basin without Tw, produces an energy dissipation ratio (ΔE/E1) of 78.41% and an efficiency (E2/E1) of 21.21%. While testing with Tw produces an energy dissipation ratio (ΔE/E1) of 84.96% and an efficiency (E2/E1) of 15.04%. The test results for the three models show that the Negative Step model, accompanied by a decrease in the elevation of the stilling basin, has the best energy dissipation performance.
The results of this study also produced a new formula for calculating the length of the hydraulic jump. The formula for the Negative Step model is Lj = 6.37 (Y2-Y1), while the Negative Step model, accompanied by a decrease in the elevation of the stilling basin, is Lj = 6.29 (Y2-Y1). The average length of hydraulic jumps (Lj) in the Negative Step Stilling Basin without a drop in the stilling basin elevation is 41.85 cm. In contrast, in the Negative Step Stilling Basin model with a drop in the Stilling Basin elevation, it is 40.02 cm. The Negative Step model can produce a friction factor of 0.819. The magnitude of the friction factor will indirectly affect the level of energy dissipation in the stilling basin. The greater the value of the friction factor, the greater the energy loss, resulting in better energy dissipation capability.
Keywords: stilling basin, negative step, tailwater, supercritical, hydraulic jump
| Item Type: | Thesis (Doctoral) |
|---|---|
| Uncontrolled Keywords: | kolam olak, Negative Step, tail water, superkritis, hydraulic jump stilling basin, negative step, tailwater, supercritical, hydraulic jump |
| Subjects: | T Technology > TC Hydraulic engineering. Ocean engineering > TC555 Spillways. Energy dissipation. Hydraulic jump. |
| Divisions: | Faculty of Civil Engineering and Planning > Civil Engineering > 22001-(S3) PhD Thesis |
| Depositing User: | Gilang Idfi |
| Date Deposited: | 25 Jul 2025 02:25 |
| Last Modified: | 25 Jul 2025 02:25 |
| URI: | http://repository.its.ac.id/id/eprint/121761 |
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