DEVELOPMENT OF SEDIMENT TRAP WITH VORTEX SETTLING DESILTING BASIN MODEL IN WEIR FOR HEAD STRUCTURE FOR IRRIGATION SCHEME

• Main Author: Isnaeni, Muhammad
• Format: Dissertations
• Language: Indonesia
• Subjects: Teknik sipil
• Online Access: https://digilib.itb.ac.id/gdl/view/71710
• Institution: Institut Teknologi Bandung

The sediments entering from the river on the weir, which will then enter the irrigation system, will reduce the wet perimeter cross-section of the main canal. This results in a reduced discharge of technical irrigation needs that pass through the cross-section of the canal, and then a sediment trap is required that can desilt non-cohesive type sediments and be quickly flushed. In Indonesia, irrigated areas covering an area of more than 3,000 ha, which is the central government's authority, have 2,871 weir units, of which 64% of which 1,834 weirs units were built during the colonial to the post-independence period (1890 - 1960). At that time, the river flow had not carried much sediment transport, and there had not been much damage in the Watershed. A total of 1,304 units did not have sediment traps yet. In irrigation modernization, sediment traps are an absolute must-have component of the head structure for irrigation and contribute to the performance assessment of irrigation systems, 20% of which are the head structure (consisting of the weir and sediment traps). Now a day, suppose this conventional type of sediment trap will be built. In that case, it is necessary to have sufficient land area to meet the needs of hydraulic parameters so that performance can reach an optimal of 77.50% based on the Design Criteria (DC)-02 Head Structure issued by the Ministry of Public Works and Housing, Directorate of Water Resources in 2015. In addition, the rapid population growth and the addition of road and bridge facilities and infrastructure that intersect with the head structure site have narrowed the available area. As required in DC-02 above, the sediment allowed to enter the irrigation system is a cohesive type of sediment whose diameter is < 0.06 mm because it is very difficult to depose sediments that move with the flow of water. This study aims to develop a modern form of sediment trap in the shape of a Vortex Settling/Desilting Basin (VSDB) proposed to replace conventional sediment traps with rectangular shapes in the same case on this Macan weir. In other countries in mid-east Asia, this sediment trap is built with a desilting function for non-cohesive sediments in some Water Treatment Plant buildings and Mini-Hydro Power to avoid the ingress of particles > 0.30 mm in the turbine drive system. Some authors have previously analyzed sediment desilting processes and not analyzed settling processes and not for head structure in irrigation systems. As a novelty, analyzing the efficiency of settling and desilting VSDB-shaped sediment traps as sediment traps for the head structure of irrigation systems in Indonesia that have never been studied before. As a case study, this study takes the case of one of the weirs that do not yet have a sediment trap and a narrow area if a conventional shape of sediment trap is to be built. The location of this study located on Macan weir in Subang Regency, West Java Province. It serves a technical irrigation scheme of 9,670 ha, with current conditions ±35% of its wet cross section reduced due to sedimentation. The methods began: a) Numerical analysis with Computational Fluid Dynamic (CFD) simulation with Ansys 2021 R2 Fluent Studentsto obtain a geometric shape approach, b) Dimensional analysis with geometric, kinematic, and dynamic similarities, c) Laboratory experiments with 1:40 model scale prototypes to undistorted three-dimensional physical models d) Observation of flow profiles with water level series (h), Q80% of Net Field Requirement (NFR) = 3.21 cm, Q100% of NFR = 5.15 cm, and Q120% of NFR = 5.84 cm, with analysis of hydraulic parameters such as velocity, dimensionless numbers, and shear stress with the help of Acoustic Doppler Velocity (ADV) and High-Speed Probe Current Meter, and e) Analysis of velocity changes in vortex streamlines so that flow ingress on the radial side can be analyzed because this vortex plays an important role in depositing and desilting of sediments. To simulate flow and analyze settling and desilting performance, in a laboratory test, a physical model of 12 scenarios was performed with four series each for a hollow round basin (chamber orifice) with a basin base slope of 1:10, 1:5 and 1:2. As a result of the analysis, the area of this VSDB sediment trap is slimmer at 38.57% than the conventional shape by optimizing the chamber orifice. The kinematic scale similarity of running time in the physical test model in the Laboratory is 11.38 hours of running, equivalent to 72 hours or 3.25 days on a prototype with sediment transport feeding through sediment feeders amounting to 2,468,62 cm3. By comparing the conventional sediment traps with the same hydraulic parameters, the settling efficiency was 69.12%, and the desilting efficiency was 53.33% against this research. As a result, an orifice chamber with a slope of 1:10 settled sediment of 1,811,97 cm3 or 73.40% of the sediment transport rate, and from the desilting fraction, it can be flushed within 0.36 hours in the amount of 1,538,36 cm3 or 76.90%. Chamber orifice 1:5, sediment settling 2,095,86 cm3 with an effectiveness of 84.90%, can flush the sediment fraction that settles 1,842.26 cm3, equal to 87.90%. For a 1:2 chamber bottom slope where the orifice chamber is steeper, no better than other chamber slopes, it can only deposit 1,641.63 cm3 sediment with a settling efficiency of 66.50 % and flush the sediment fraction by 71.46%. The orifice chamber with a base slope of 1:10 is the maximum result of settling and desilting sediment fraction of more than the optimum value required by DC-02.