On the sediment deposition on Barugbug Situdam Weir

In this paper, we investigate the behavior of total sediment transport in the Barudbug weir, Situdam, Karawang, West Java, Indonesia, which is a crucial aspect of water management in the Citarum river basin. We utilize SESAME instrument observation data, encompassing rainfall, TSS, and depth parameters, gathered over a period of approximately three years. Additionally, we employ a two-dimensional Advection-Diffusion analytic model that takes into account settling velocity. The observation results reveal a positive correlation between TSS and depth, with TSS levels increasing following heavy rainfall. Furthermore, there exists a time delay between rainfall events and the subsequent rise in TSS concentration, suggesting an upstream source of sediment. The model outcomes demonstrate an inverse relationship between TSS concentration and the velocity of river flow, as the river flow acts as a leaching mechanism.


Introduction
The weir is a hydraulic structure that very important for humans life so that it has received serious attention, especially in the aspect of structural durability due to environmental influences.With increasingly widespread land use changes in the upstream section, the input of sediment into the river becomes high, increasing in the sedimentation process in the dam.No exception will occur in Barugbug Weir.The Barugbug Weir or Situdam Jatisari Weir was built in 1949 and is a meeting place for two rivers, namely the Ciherang River, which has its source in Darang and Cikalong, Purwakarta, and the Cilamaya River, which has its source in Cijalu, Subang.In this dam, the water of the two rivers is then regulated to then be forwarded to the Cilamaya river which leads to the Cilamaya market and also the Ciherang river which also empties into the Cilamaya Karawang area.
Knowledge of the distribution of TSS in a dam or river is very important for weir and river management.The distribution of TSS in a weir is not only determined by the nature of the waters and geographical course but also on the structure of the Weir made.They show that Limestone was used instead of gravel as an adsorbent material for the porous weir to improve its efficiency for removing selected contaminants, including lead (Pb), turbidity and total suspended solids (TSS).Another study looked at how the existence of a weir affects the rate of sedimentation in a river.For example, the presence of the Sembayat Dam changed the sediment transport rate to increase under the influence of erosion in the river branches.Sediment transport rates decrease by about 30% during the rainy season.Thus, one of the objectives of building a weir is to save water so that the agricultural water supply is maintained in the dry season, but also to filter suspended sediment so that the water that is channelled remains The sedimentation process of dissolved materials in a weir is important to know.Several methods have been used by researchers such as using environmental fluid dynamics (EFDC).They demonstrated that as rainfall increases, so does the total suspended solids (TSS) concentration.TSS variability was IOP Publishing doi:10.1088/1755-1315/1343/1/012026 2 observed depending on the lid of the weir gate.Other researchers used the HEC-RAS simulation software to simulate the hydraulic behaviour of sand traps for irrigation management of a weir.Recommended rinse the period is 315 minutes, with a flow rate of 2 m3/s.They further suggest the frequency of flushing sand traps is conducted twice a year, and can be done in late March and October.It coincides with the end of the first and third planting seasons of the irrigation scheme in the study area of Kulonprogo district, Indonesia.[5][6] [7] This research aims to understand the process of sedimentation from dissolved sediment material through measurement data of three parameters namely TSS, rainfall and water depth obtained for more than two years in the case of Barugbug weir (See Figure 1).To understand the mechanism of sedimentation, we use a simple advection-diffusion model that takes into account the settling velocity of dissolved sediment material.

The data
Barugbug weir has a door width of 12 x 2 m with a normal elevation of 31.40MSL and a flood elevation of 33.00 MSL (see Figure 1).Water sources come from the Ciherang River and Cilamaya River with a maximum water discharge of 1200 m 3 /s.The data for this study were taken from PJT II's Sesame instrument with the parameter of Total Suspended Sediment, Rainfall and Water Depth in a time series from October 1, 2016 to January, 29 2020.where  is a horizontal river velocity,  is river turbulence diffusion coefficient,  0 is settling velocity and  is the sediment sources.By using Laplace transform, the analytic solution of Eq. (1) with a point source is given by [8] IOP (2

The model
where Q is the initial TSS source, erfc is a complementary error function and   =  (x).

Results and discussion
Time series data from Situdam with TSS parameters, rainfall and water depth in the form of a time series from October 1 2016 to January 29 2020 is depicted in Figure 3.The data has been expressed in normalization (data/max(data)) that we get the form balanced for easy analysis.TSS units in (mg L -1 ), rainfall in (mm) and water depth in (m).
From the Google Earth survey results, the distance between Kalvari Church Lubang Buaya and Halim Perdana Kusuma Meteorological Station is 3.31 kilometers.Previous studies have shown that in general, labyrinth-type fixed weirs have self-cleaning capabilities.From the results of TSS measurements for almost two years in Barugbur weir, it shows that there were periods when TSS had small concentrations, in other words, a self-clearing mechanism occurred.The results (Fig 2 ) of the observations show that in general the concentration of TSS (on the surface) will be directly proportional to the depth, that is when the concentration of TSS is high (the water condition is more turbid), the depth in the water will be shallower and this will increase after rain.This indicates that the source of sediment originates from upstream because there is a time lag between the rain falling and the increase in TSS concentration.
The effect of water discharge or river current velocity on TSS concentrations is depicted in Figure 4.The current velocity acts as a leaching effect where if the speed of the river water is large, the concentration of TSS will decrease.The distribution of TSS concentration to depth is shown in Figure 5.It is clear that river velocity plays a significant role in sediment deposition.In other words, the advection effect is more dominant than the diffusion effect in a river dam.This condition is the opposite for estuaries which are characterized by high tidal ranges.

Conclusions
The presence of a weir is very necessary for the governance of water management in a watershed irrigation system.In this paper, we study the behaviour of total sediment transport in the Barudbug weir, Situdam, Karawang, West Java, Indonesia, which is part of the water management of the Citarum river basin.SESAME instrument observation data with parameters of rainfall, TSS and depth with a time range of 1 October 2016 to 29 January 2020 were used in this study along with a simple two-dimensional model of the Advection-Diffusion equation.The analytical solution in the form of an error function has been obtained.The observation results show that TTS has a positive correlation with depth and rises after heavy rains.There is a time lag between rain events and an increase in TSS concentration which indicates TSS originates from upstream.The model results show that the concentration of TSS is inversely proportional to the velocity of the river flow.
In this paper, we investigate sedimentation processes in weir based on the simple model.The dispersion model of a flume is needed in an effort to relate the rate of sediment transfer by water flow and the sedimentation process in a weir and river.The model tries to correlate quantities water discharge, average velocity, tractive force, or energy dissipation rate, and mixing related to sediment transport.In a mathematical model they are connected through the advection-diffusion equation which is an open equation because it requires another equation (Navier-Stokes for the advection term) so that the equation becomes closed.In short, the study of advection-diffusion problems is still an open book.In this study we study the disposition and transport of sediment in a weir by looking at the steady-state diffusion advection equation in two-dimensional conditions by calculating the settling velocity which describes the settling velocity of dissolved solids material.The advection-diffusion equation describing sediment deposition is given,

Figure 3 .
Figure 3.Time series of low-pass filter of total suspended sedimen (bold red line), rainfall and water depth (bold blue line) at Situdam.The yellow line is the TSS, green line is water depth original time series data, and the black bar is the rainfall.

Figure 4 .
Figure 4. Time series of low-pass filter of total suspended sediment over varying river velocity.

Figure 5 .
Figure 5. Dispersion of total suspended sediment over varying river velocity.
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