The Analysis of Song Putri Reservoir Storage Area on Sedimentation Rate Using Mathematical Model Approach

Song Putri Reservoir is an artificial reservoir with the aim of irrigation channels for rice fields and flood control. This reservoir is located in Eromoko District, Wonogiri Regency Sedimentation modeling in the Song Putri reservoir is needed to analyze the amount of sediment deposition against the Song Putri Reservoir. To analyze the flow patterns and sediment distribution that occurs in reservoirs, SMS (Surface-water Modeling System) 8.0 Softwarse is used. This study aims to determine the flow patterns and effects of sediment distribution on reservoir changes. The data used include Song Putri Reservoir technical data, daily rainfall data for 10 years (2009-2018), sediment data, reservoir inflow data, and bathymetry maps. To analyze the magnitude of inflow and outflow of Song Putri Reservoir, hydrological analysis using empirical methods is used, while sedimentation modeling simulation uses SMS 8.0. Software based on the simulation, result the highest flow velocity in the 50 year return period is 0.097 m / s and the smallest is 0.00 m / s. Based on the simulation results of changes in the reservoirs base for 720 hours (1 month), the values that in the return period of 50 years, 100 years and the highest 1000 years the highest was 5.795 m and the smallest of 0.001 m. Based on the calculation of prediction of sediment growth rates, it obtained the results of sediment growth rates with a percentage for a 50 year return period of 35.68% with a range of percentages between 19% - 21% and a percentage of 29.103% for a 100 year return period with a range of percentages between 22% - 24%, while the percentage growth rate of 1000 year return period sediments is 98 , 20% with a percentage range between 55% - 57%.


Background
Reservoir is a place on the surface of the ground that is intended to store / hold water when there is excess water / rainy season, then the abundant water is used for irrigation, flood control, drinking water needs and as embankments to collect runoff water from rivers to reservoirs. Song Putri Reservoir is an artificial reservoir with the aim of irrigation channels for rice fields and flood control, this reservoir is located in Eromoko District, Wonogiri Regancy Song Putri Reservoir has a topographic shape with nonuniform elevation. The non-uniform shape of the Song Putri Reservoir creates a flow pattern that results in the uneven distribution of flow velocity and sediment distribution in reservoirs. This is very influential to changes in reservoir base configuration.
Sedimentation that occurs in the Song Putri Reservoir can cause a reduction in the reservoir life span efficiency.
Sedimentation that occurs must be noticed. Particular methods are needed to determine the distribution of flow velocity and sediment distribution in the Song Putri Reservoir, so that the reservoir life control age is efficient. To find out the flow pattern and distribution of flow velocity, as well as the distribution of sediment that enters the Song Putri Reservoir, we need a way to estimate this is needed with the help of computer software. One software that can help in this research is SMS (Surface water Modeling System). This program can be used for the process of simulation of flow patterns and flow velocity distribution using RMA-2 and simulation of sediment distribution using SED2D (Anonymous, 2003).
Rainfall is the amount of water that falls on a flat surface during a certain period measured in millimeters (mm) height above the horizontal surface. Rain can also be interpreted as the height of rainwater collected in a flat, non-evaporating, non-absorbing, and non-flowing place (Suroso 2006). To determine the type of method to be used in rainfall calculations, statistical parameter analysis is performed. There are several methods for calculating the amount of rainfall design. In this study analysis of rainfall design will be carried out using the following methods: 1) Gumbel Type I distribution method, 2) Normal distribution method,

4)
Pearson Log Type III distribution method. Flow discharge is the rate of flow of water (in the form of volume of water) that passes through a cross section of the river per unit time (Asdak, 2002). The discharge unit used cubic meters per second (m3 / s). The most frequently used method for estimating discharges in a watershed where there are no observational data for discharge is the Rational Method. In this case the magnitude of the discharge is a function of the area of the watershed, the intensity of rainfall, the state of land clearing which is expressed in the runoff and slope coefficients (Loebis, 1992). The flood discharge is generic as follows: For the sake of practicality in determining the unit, then: The Haspers method used to calculate the maximum discharge is formulated as follows: HSS Nakayasu Nakayasu method used to calculate flood peak discharge is formulated as follows : 3,6 (0,3 + 0,3) Where : Qp = flood peak discharge (m 3 /sec) A = wide catchment area (km 2 ) R0 = unit rain (mm) Tp = grace period from the beginning of the flood to the peak of the flood (hours) T0,3 = time required by a decrease in discharge, from peak discharge to 30% of peak discharge (hours).

Research Methodology
The data of the study include Song Putri reservoir technical data, daily rainfall data for 10 years (2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018), sediment data, reservoir inflow data, and bathymetry map obtained from BBWS Bengawan Solo. To get good and direct results, a flowchart is made of the plan and the work steps carried out, as shown in the Figure 1.

Figure 1 Flow Chart
The steps for data processing in general are as follows: 1) Hydrological analysis using empirical methods. The following processing flow: a. Hydrological analysis uses maximum daily rainfall data for 10 years (2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018) 2) Surface-water modeling system 8.0 is done using 3 steps, among others as follows: Geometric modeling, The process of simulating flow patterns and flow velocity distribution with RMA-2 and simulation of sediment distribution with SED2D.

Research Result
Rainfall analysis using Arithmetic/Algebra Method. In this study there are three rain gauge stations which are considered relevant to represent rain observations in the study area (Song Putri Rain Station, Nawangan Rain Station and Parangjoho Rain Station). Frequency distribution analysis is intended to obtain the amount of rainfall that is determined based on certain design benchmarks. For the purposes of the analysis set with return periods of 5, 10, 25, 50, 100, and 1000 years, as shown in the Table 3.
Below is a graph of the intensity of rainfall in the Alang watershed. In the flow tracking method which is carried out using the reservoir tracking method, a volume of runoff will be obtained from the reduction in the value of the inflow and the outflow of water as the result of data processing by the Nakayasu HSS. The overflow of water Is water volume units which can later be used as design plan for storage wolume of polder ponds (storage) to be used.
Recapitulation of Flood Discharge Calculation Results, as show in Table 4.   Based on the results of the analysis of the flow tracking that has been done, it can be shown the flow of incoming water (Inflow) and the outflow of water (outflow) that occurs so that it can be described in graphical form as this Figure 3.

Time (hours)
Inflow Q50 (m3/dt) Outflow Q 50 (m3/dt) Inflow Q100 (m3/dt) Outflow Q100(m3/dt) Inflow Q1000 (m3/dt) Outflow Q1000 (m3/dt) From the results of the simulation of flow patterns in the form of vectors, it can be seen the characteristics of flow patterns at points A, B, and C at each return period by obtaining the same flow pattern characteristics. In condition A, the flow of water entering the reservoir is not similar because the flow velocity changes with distance due to changes in appearance from small to large. Hence the flow direction spread to fill every part of the reservoir. In condition B, the water flow is trapped on one side of the reservoir so that the direction of flow tends to spin. These conditions can be caused by differences in the viscosity of the water so that there is a change in speed that causes the water to move irregularly to form a vortex. And in condition C, the water flow is not uniform because the flow velocity changes with distance due to containment in the reservoir downstream.
There is no significant difference between the characteristics of flow patterns at each return period, but the difference can be seen based on the magnitude of the velocity distribution resulting from the simulation results. The recapitulation of the horizontal flow velocity distribution of the simulation results at each review point on the return period of 50 years, 100 years, and 1000 year for 24 hours can be seen in table 8.  The results of sedimentation simulation in the form of changes in the base of the Song Putri Reservoir in the return period of 50 years, 100 years, and 1000 years during 720 can be seen in the following figure 8. Changes in the base of the Song Putri Reservoir (bed change) in the 50 year return period.
Recapitulation of the horizontal flow velocity distribution of simulation results at each point of view in the return period of 50 years, 100 years, and 1000 years for 24 hours can be seen in table 9.   Based on flood measurement data (Q) and sediment concentration (Cs), suspension discharge (Qs) can be comma and the suspension discharge curve is made (graph of the relationship between Q and Qs). Using these equations, the prediction of sediment growth rate can be calculated which is presented in Table 10. period with a percentage range between 22% -24%, while the percentage of sediment growth rates for 1000 year return periods is 98.20% with a range of percentages ranging from 55% -57% ( Figure 11 and Table 11).  Based on the calculation of predicted sediment growth rates the results of sediment growth comma with a percentage for a 50 year return period of 35.68% with a range of percentages between 19% -21% and a percentage of 29.103% for a 100 year return period with a percentage range between 22% -24%, while the percentage of sediment growth rates for 1000 year return periods is 98.20% with a range of percentages ranging from 55% -57%. Thus it can be concluded that the greater the year increase, the greater the percentage of sediment growth rate in the Song Putri reservoir.