SEDIMENT TRANSPORT MODELLING WITH SHEN AND HUNG MODELS FOR ANALYSIS PREDICTION OF SEDIMENT INCREASE IN UPPER SERAYU BARRAGE

Serayu Barrage is one of the weirs built in the river segment with a gentle slope of the riverbed. Serayu barrage, which was built in 1993, is used to meet the needs of irrigation water for irrigation areas Gambarsari - Pesangrahan with an area of ± 20,795 ha of irrigated land and to meet the needs of raw water in Cilacap Regency. Because this weir is located in a sloping river segment, with these characteristics the weir will easily increase sediment upstream of the weir due to the low flow velocity of the upstream weir. This study is intended to analyze the increase in sedimentation in the upstream weir based on Shen and Hung's model of sediment transport analysis. An analysis of the increase in sediment upstream of the weir was carried out for the study year from 2007-2036. The results of this study indicate that the increase in sedimentation upstream of the Barrage of Serayu's motion continued throughout the years 2007-2036. Then with using predictive analysis of the second order polynomial model, until 2036, occurs increased sedimentation in the upstream Serayu Barrage of 15,392 tons/day.


INTRODUCTION
Serayu Barrage is one of the weirs in Banyumas Regency. This weir is used to meet water needs in the Irrigation Area D.I Gambarsari -Pesangrahan and serves the fulfillment of water needs in the Banyumas and Cilacap areas as well as in Kebumen with a total irrigation area of 20,795 ha. In addition to irrigation services, this weirs also functions to supply raw water for the domestic municipal and industrial cities of Cilacap. Kroya and Maos amounted to 5.26 m3 / sec [1] [2] [5] [7] [13]  The Serayu Barrage is located in the Serayu River segment which has a gentle slope. One of the characteristics of a weir with these characteristics is that it is easy to increase the sediment upstream of the weir due to its sloping contours and causes sediment deposits to tend to be high. Besides indicated by the turbidity of river water, many deltas have been formed upstream of the weir. This indicates that the surface area of the river has begun to decrease due to sediment or sediment that forms the delta. The development of sediment in the upstream is felt to have continued to increase since previous years since the weir was functioning. By narrowing the riverbed, or silting due to sedimentation, will result in a reduction in discharge capacity in the mobile weir [1] [2] [5] [7].  [1] . Serayu Barrage has a very important role in providing water to irrigate rice fields in Banyumas Regency, Cilacap and partly in Kebumen Regency and raw water supply in parts of Cilacap Regency. Increased sedimentation or narrowing of the river's face upstream of the weir, will result in reduced capacity of the river and weir. This will impact on water supply both irrigation and raw water [ From the simulation results obtained by the dam base elevation for each formula used. The formula that gives the best results, Ackers-White, the combination of Toffaletti with Meyer-Peter and Müller gives results that are close to the dam bottom measurement data [14]. For Vanoni's formula, and the combination of Toffaletti with Schoklitsh gives greater deposition results from the measurement data. Whereas other sediment transport formulas on average provide too little depositional value. The measurement results show the sediment content in the mouth of the river Serayu Total Suspended Solid sediment concentration of 98-126 mg / l.
Meanwhile, from T. Marhendi's research, 2008, it was explained that the increase in the peak of the Serayu river flooding each year experienced changes in the factors of rainfall intensity, duration of rainfall, soil moisture, and the destruction of rain retention areas in the watershed [6]. The effect of rainfall intensity on surface runoff depends on the infiltration capacity. If the intensity of rainfall exceeds the capacity of infiltration, the amount of surface runoff will increase in accordance with the increase in rainfall intensity and this will increase sedimentation entering the river. Andry Rustanto, Dhruba P. Shrestha, Victor G. Jetten, 2010, through his research explained that based on the analysis of Landsat image classification, the dominance of land cover in the Serayu River watershed was dominated by forests and dryland agriculture in the period 1989 to 1999 [8 ] While in the next period up to 2003, the dominance of land cover in this area was dry land agriculture and plantations, while up to 2009 it was dominated by plantations and dry land agriculture. Changes in land cover in the Serayu Hulu watershed during the period 1989 to 1994 were dominated by the conversion of forests, plantations, shrubs and rice fields into dry land agriculture. Meanwhile, during the period 1994 to 1999 the dominance of changes in land cover was from forests, dry land agriculture, shrubs and rice fields to plantations. Changes in rice fields, forests, dryland agriculture, plantations and grasslands into shrubs dominated the change in land cover between 1999 and 2003 in this area. While the conversion of paddy fields, dry land agriculture, forests and shrubs into plantations dominated the change in land cover in the Serayu Hulu River watershed between 2003 and 2009 [3].
To overcome the high sedimentation in the River Basin, what needs to be done is to reduce the rate of land erosion that occurs in the reservoir catchment. Efforts to reduce the rate of erosion can be considered as an effort to manage the potential capability of raindrops through its kinetic energy and hydraulic gradients at the surface of the ground so that ground conductivity by water mass can be reduced or prevented [6]. The process of natural sediment transfer is greatly influenced by the conditions of the hydrological cycle. There are two understandings of the phenomenon of the hydrological cycle namely the ancient and modern views (Mays, 1996 in T. Marhendi, 2014) [11] [27]. According to ancient concepts, the condition of the hydrological cycle is something that is naturally awakened or given. While the modern concept states that human activity is very influential on the conditions of the hydrological cycle. this shows that there is a significant change in land cover in the Serayu Hulu watershed and this has led to increased land erosion and sedimentation entering the river.

Research Location
The research location is in the upper of the Serayu Barrage and Serayu River.

Research Data
Data in this study include [3]

Hydrologic and Sediment Prediction Analysis
Data analysis conducted in this study included hydrological analysis, sedimentation analysis of sediment transport models, predictive analysis of sedimentation development [8] [9] [10] [11] [13]. Hydrological analysis is used for the analysis of discharge increased and analysis of sediment increased based on flow discharge [18] [19] [20] [21]. Hydrological analysis was carried out to obtain the Serayu River flow hydrograph, if discharge data were not available in the data year. The stages of the implementation of hydrological analysis are carried out according to the availability of data using the Mock model [1]. In general, debit data recording is often not available well enough, in the sense of: a. There is no record of discharge to the river to be analyzed b. The location for recording debits does not fit the analysis control point c. Recording data is not long enough d. Recording data only records peak debits e. No data are available for recording debit and rain pairs at the same time and the recording duration is relatively short (for example hours).
To overcome this, flow hydrograph analysis can be done using the rain-flow transformation method. Sedimentation analysis of the sediment transport model is used to determine the development of sedimentation in the upstream of the Serayu weir movement.
While the prediction analysis of the development of sedimentation is used to predict the development of sedimentation upstream of the srayu weir using statistical models. Analyzes were performed using the statistical (regression) method for several different equations using 2005-2016 data. Several regression models are used such as the linear regression model and the 2nd order polynomial regression model [1]. One prediction analysis of sedimentation increased upstream of the Serayu Barrage was carried out by regression analysis. Regression analysis is one of the statistical tools based on the properties of the relationship between two variables or several variables. This regression analysis is done to get a prediction equation from a set of data or variables so that it can estimate its value based on one other variable or several other variables. In this case the variable used is a quantitative variable. In this study a regression analysis is used to obtain the selected equation that will be used to predict the magnitude of the increase in sedimentation in the upstream of the Serayu Barrage in the future [8] [9] [10] [11].

Transport Sediment Analysis
Sediment transport analysis is intended to calculate the increase in sediment upper Serayu Barrage. Sediments transported upstream of the Serayu Barrage through the Serayu river channel will be partially deposited along the Serayu River channel, and some of which will be transported upstream of the weir [3] [15] [16] [17] [22]. The volume of sediment that settles upstream of the weir includes suspension sediment and bottom load through the river channel and the volume of sediment that enters the dam upstream directly. To calculate the volume of suspension sediment entering the weir, the following formula is used: with, Qs = suspension sediment discharge (ton / day) QW = river flow discharge (m3 / sec) a, b = constant (depending on field measurement data) To find out or determine the volume of sediment that settles, the methods of Shen and Hung (1971) [

Analysis Prediction of Sediment Increased
Based on the analysis of sediment predictions using statistics, the results of the analysis in Figure 6 and Table  3 are as follows. Referring to Figure 8, using a second order polynomial regression model, it appears that the equation can be used to predict the development of sediment in the upper reaches of the Serayu Barrage with an R2 value of 0.8182. This figure showed that the model can be used to predict the development of sediment in the upper reaches of the Serayu Barrage considering its value is above 0.8. Based on this equation, the results of the prediction of sediment development until 2036 can be seen in Table 3. Table 3