Design Flood Discharge of 50 Years in Garang River Using Nakasayu Synthetic Unit Hydrograph Method

Various kinds of buildings in civil engineering require a careful planning. For example, the planning of a water building needed a method to calculate the design flood discharge before starting to plan the dimensions of the building to meet the effectiveness of the water structure. Design flood discharge can be determined using several hydrograph methods that have been used in water building planning in Indonesia. One of the popular hydrograph methods is the Nakayasu Synthetic Unit Hydrograph method. In this case, the design flood discharge is in the Garang watershed, precisely in Semarang, Central Java province, using rainfall data for the past 16 years. Hydrological analysis is carried out first before determining the design flood discharge with a return period of 2, 5, 10, 25, and 50 years. The results of the design flood discharge using Nakayasu method respectively were 305,522 m3/s, 390,742 m3/s, 447,783 m3/s, 520,560 m3/s, and 574,912 m3/s.


INTRODUCTION
The calculation of design flood discharge is the most important aspect of planning the water structure. Design flood discharge is a component which is needed to determine the magnitude of peak flow discharge in a Watershed. This flood discharge will be used in calculating the dimensions of water structures such as dams, groundsill, and so on. Design flood discharge can be calculated using rational methods and several hydrograph methods that previously have been used in the planning of water structures in Indonesia.
Hydrograph is a method that uses diagrams to illustrate the relationship between flow rate and time. A hydrograph must be adjusted by observing and analyzing hydrology to determine the characteristics in a watershed. Some popular hydrograph methods include ITB, GAMA-1, SCS, ITS-1, ITS-2 and Nakayasu [1]. The method used in this study is Nakayasu Synthetic Unit Hydrograph method.
The calculated flood discharge is 2, 5, 10, 25, and 50 years. The location chosen for research is in the Garang watershed, starting from the head of the river to the coordinates 7° 1' 40.444" S and 110° 24' 7.999" E where it is known with its fast flow in a short time [2]. Therefore, the determination of design flood discharge is needed especially for the advantage of the local society if they want to plan the building's construction around the river. The research location is shown in the figure 1: Before drawing a hydrograph curve, the first step is to look for its constituent components, such as rainfall intensity and base flow. Rain intensity is the level of rainfall per unit of the time, it can be calculated using the Manonobe method which is described as follows [3]: where : I = rain intensity (mm/hour) t = rain duration (hours) R24 = maximum rainfall (mm) The base flow which has a meaning as groundwater flow due to rainfall that come through infiltration and percolation can be searched with the following formula The Nakayasu Synthetic Unit Hydrograph method is the first hydrograph method developed in Japan [5]. This method has been applied several times in water structures in East Java. Sutapa [6] opined that until now the use of the Nakayasu method has given satisfactory results. The equation used to draw a hydrograph is as follows [7]: Meanwhile, drawing Nakayasu hydrograph curve is divided into 3 conditions which are described as follows: where : Qp = flood peak flow rate (m 3 /s) C = runoff coefficient A = watershed area (km 2 ) Ro = unit rain (1 mm) Tp = time interval from the beginning of the rain until the flood's peak unit Tg = concentration time Tr = rain time unit T0,3 = the time required by a decrease of peak discharge up to 30% of peak discharge.

METHODOLOGY
This case used several methods including the method of observation by direct observation of the study site to determine the conditions around the area and other components needed in the study. Documentation methods were also carried out by collecting data, such as rainfall data from the three nearest rain stations and concerning data of watersheds such as topography, area, and length. Last is the literature method by taking references from journals, modules, books that are supporting research.
The initial step to solve this case is to observe the research location and data collection. The step is continued by proceeding data such as regional rainfall analysis, frequency and probability analysis, data compatibility test, rainfall intensity analysis and design flood discharge analysis.

RESULT AND DISCUSSION
The rainfall data used 16 years of data from the nearest rain station, which is Sumurjurang, Simongan, and Gunungpati using the polygon-Thiessen method. The results of the regional rainfall analysis are provided below: The distribution selection results using the log-Pearson III distribution method with rain return periods of 2, 5, 10, 25 and 50 years are described as follows: The results of the data validity test using the chi-square method by dividing class intervals of 5 classes are presented in the following table: The value of the chi-square test was 4, while the critical value in the table was 7,81. Then, it can be concluded that the tested data represent some or all the existing data.
While the results of rainfall intensity analysis using the manonobe method for 24 hours are described as follows: The width of the watershed and the length of the river are obtained through the analysis of the ArcGIS software. So, the results obtained from the base flow that have been calculated are described as follows: Design flood discharge analysis curve using Nakayasu Synthetic Unit Hydrograph with a return period of 2, 5, 10, 25, and 50 years is presented in the following figure:

CONCLUSION
Based on the results of the discussion, it can be concluded that in the case of Garang watershed using rainfall data for 16 years with several hydrograph methods. One of them is using the Nakayasu Synthetic Unit Hydrograph. The values obtained were 305.522 m 3 /s for the 2-years return period, 390.742 m 3 /s for the 5-years return period, 447.783 m 3 /s for the 10-years return period, 520.560 m 3 /s for the 25-years return period and 574.912 m 3 /s for the 50-years return period.