SEDIMENTATION ANALYSIS IN CILIWUNG DIVERSION TUNNEL

• Main Author: FRANSISCA RIA SILALAHI, IRA
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/62660
• Institution: Institut Teknologi Bandung

The construction of the Ciliwung Diversion Tunnel–East Flood Canal is carried out in order to maximize the capacity of the main drainage channels such as the East Flood Canal and reduce the flood water level in the Ciliwung River. This diversion is planned to be able to divert some of the flood discharge from the Ciliwung River to the East Flood Canal through the Cipinang River at 60 m3/second. One of the problems faced by this construction is sedimentation which can reduce the effectiveness of the drainage system. Sedimentation that occurs can be in the form of sedimentation on the river section where the inlet is located or sedimentation that occurs in the tunnel outlet pond. Therefore, this study will analyze the morphological changes of the Ciliwung River from the aspect of agradation and degradation in the wet and dry months and evaluate the amount of sediment deposited in the tunnel and diversion system after the river passes through the flood and the ability to drain sediment in the diversion system before and after normalization. The design flood discharge analysis was carried out on the Ciliwung and Cipinang River using the ITB Synthetic Unit Hydrograph method. The data held is in the form of daily rainfall at 7 (seven) rain gauges stations that are spread fairly evenly both inside and outside the Ciliwung watershed with a data series recording length of 11 years from 2008 – 2018. Before conducting sediment modeling, hydrodynamic modeling (unsteady flow) will be carried out to get the rating curve for the Ciliwung River and the Tunnel Inlet with a bankful capacity scenario. Sedimentation potentials in the Ciliwung River and the Ciliwung Sudetan System were analyzed using a calibrated HEC-RAS model. The calibrations carried out are manning calibration and sediment model calibration. The calibration of the sediment model was carried out by comparing the modeling results of the Ciliwung River with the data from the geometry measurement of the Ciliwung River at the 2021 inlet location. From the results of the analysis, it is known that the bankful discharge of the Ciliwung River before normalization is equivalent to the design discharge of Q10 and after normalization is equivalent to the design discharge of Q50. For the calibration of the sediment model, the invert elevation of the Ciliwung River 4 years after normalization was completed in 2017 was + 7.41 or a difference of 0.079 m from the 2021 measurement results. Wet and Dry Months also affect the amount of sediment transport in the Ciliwung watershed. From the analysis conducted in the period 2017 – 2020, it is concluded that in general the sediment transport in the wet months is greater than in the dry months, but specifically depends on the number of months of occurrence. The riverbed of the Ciliwung River has consistently increased from year to year in the period 2017 to the end of 2020. The upstream and downstream areas of the view experience an average sedimentation of 1 m except in the middle part which has a relatively steeper depth so that the sedimentation that occurs is even greater. . The amount of sediment carried into the tunnel before normalization is greater than after normalization in small return period flood discharges such as Q2 and Q5. After normalizing the amount of sediment that enters is approximately 48 tons at the planned flood discharge Q2, it is smaller than the amount of sediment that enters the condition before normalization which is 104 tons. To prevent sedimentation in the tunnel, it is necessary to pump flood from the outlet pool before 33 hours since the flood was diverted. Analysis of the critical shear velocity of sediment grains was carried out using the Hjulström curve and Shields diagram. By comparing the results of the analysis between the Hjulström curve and the Shields diagram, it is concluded that the flood discharge plan for a return period of 1 year using the Hjulström curve, the maximum movable sediment grain diameter is 1 – 5.5 mm, while with the Shields diagram the sediment grain diameter of 10 mm can be flushed. The sediment grain diameter (d50) of Ciliwung River is 10 mm.