Flood Mapping Potential Areas Using HEC-RAS Software (Case Study: Kota Lama of Semarang)

Semarang has experienced floods every year, with a peak in the rainy season. At the end of 2022, there was heavy rain which caused the East Flood Canal River to overflow. As a result, the floods inundated several areas in Semarang City, including Kota Lama of Semarang. This study aims to determine the value of the designed discharge, the area of the flood inundation that occurred and the flood inundation map for return periods of 2, 5, 10, 25, 50 and 100 years. The methodology of this research consist of hydrological analysis, hydrological modelling with HEC-HMS and hydraulics modelling with HEC-RAS and integration with ArcGIS. In this study, it was found that for a return period of 2 years the planned flood discharge value was 153.7 m3/second with an inundation area of 406 Ha and a flood height of 17 cm to 68 cm, up to a return period of 100 years with a planned flood discharge value of 455.7 m3/second with an inundation area of 796 Ha and a flood height of 83 cm to 180 cm.


Introduction
Semarang East Flood Canal River is one of the flood control river systems in Semarang City, which is located in the eastern part of Semarang City.Created in 1889 during the Dutch colonial period to overcome the floods that occurred at that time.Flooding is a condition where water runoff has a height above the normal water level, causing stagnant water on a lower surface on the river side.Some of the factors that cause flooding are sea tides and high-intensity rainfall.
At end of December 2022, rain with high intensity caused several areas in Semarang City to be flooded.The recorded rainfall on December 31, 2022 was 92.7 mm.Every year, Semarang has experienced several flood disasters due to high rainfall.This can be proven by the 10-year rainfall data that occurs with an average rainfall intensity including very heavy rain (100 -150 mm/day).In this study an analysis of flood inundation mapping will be carried out in Semarang Regency which focuses on the Kota Lama of Semarang.Flood inundation mapping analysis will be carried out with the ArcGIS program to display geographical conditions, HEC-HMS for hydrological analysis and modeling and HEC-RAS for hydraulics modeling for two-dimensional river flows.

Study Literature 2.1 Hydrological Cycle
The hydrologic cycle or water cycle is a process in which water originating from the earth's atmosphere will continuously return to the atmosphere and occur repeatedly with the aim of maintaining the availability and amount of water on earth to remain stable.The stages of the hydrologic cycle are evaporation, transpiration, evapotranspiration, sublimation, condensation, advection, precipitation, runoff, infiltration, and conduction [1].

Rivers and Watersheds
A river is a container or channel where water flows from upstream to downstream and is bounded by a boundary line on the right and left.In general, the water in the river flow comes from springs, dew, rain, and snow.The flow of water moves from a high place to a lower place due to the force of gravity.Some of the functions of the river are agricultural irrigation, tourism objects and water reserves.
A watershed is a water system area that is scientifically formed and bounded by hills and mountains which receives, collects, and distributes rainwater that falls into a watershed through a river system in the area and is discharged at one outlet point.[2].

Flood
Flooding is a condition where water runoff has a height above the normal water level, causing puddles of water on a lower surface on the river side [3].

Designed Flood
Designed flood is the maximum flood discharge that is expected to occur during a predetermined return period.The value of the planned flood discharge is determined according to an analysis of the frequency of rainfall during a predetermined return period [4].

Data Testing
Outlier test is performed to determine the existence of data with significant differences in value among other data.The Grubbs & Beck method sets the upper limit (XH) and lower limit (XL) where the test is accepted if there is no data that exceeds the upper outliers and is smaller than the lower outliers [5].

Frequency Distribution
The frequency distribution is a calculation analysis to obtain the probability of the occurrence of a hydrological event.For hydrological analysis, at least 2 years of hourly rainfall data or at least 10 years of annual rainfall data is required [5].

Chi-square test
The chi-square test is a test of the fit of the distribution function based on the weight of the sum of the squares of the difference between the observations and the theory which is divided into several subgroups [5].

Smirnov Kolmogorov test
The Smirnov Kolmogorov test is a distribution function compatibility test based on the maximum vertical distance between the observations and the theoretical sample distribution [5].

Rain Distribution
The distribution of rain is the pattern of rain in a certain area with various distributions of hourly percentage values.Calculation of hourly plan rain distribution is recommended to be carried out in accordance with the "Technical Guidelines for Dam Design Flood Calculations".Design rainfall distribution is calculated using PSA-007 cumulative rainfall distribution [5].

ArcGIS
ArcGIS is a geographic information system (GIS) processing tool that displays and processes data from actual geographic conditions and functions to display and process relevant geographic information systems and create or edit maps [6] [7].

HEC-HMS
HEC-HMS (Hydrologic Engineering Centre -Hydrologic Modelling) is a program from the US Army Corps of Engineers designed to operate the simulation of hydrological processes and routing processes in river areas [8].
The parameter needed in calculating effective rainfall is the curve number (CN) value.The CN value describes the characteristics of a watershed such as land use, soil type, humidity, and vegetation conditions.The CN value will affect the amount of rainfall lost in the form of infiltration [9].Based on the watershed modelling that has been done with ArcGIS which is shown in Figure 1, it is found that the area of the watershed is 4984 ha, the slope value is 0.2% and the length of the river path is 13.32 km.In this study, the validation of inundation height calculations cannot be carried out by direct surveys in the field, so what can be done is to carry out hydrological and hydraulics modelling using rainfall data that occurred on December 31, 2022.The news attachment shows flooding around Kota Lama Semarang ranging from between 15 cm to 90 cm which can be seen from Figure 3 and 4. Based on the results of the hydrological and hydraulics modelling that has been done, the inundation height around the Kota Lama of Semarang is 54 cm so that it can be stated that the calculation and modelling of flooding in the Kota Lama Semarang can be validated because it is in accordance with the incident news.Table 2 is the result of the calculation of the frequency distribution which has been tested for chisquare and Smirnov Kolmogorov and the terms of each distribution.Based on these calculations, the rainfall plan used is the rainfall from the Log Pearson III method.Table 3 is the value of the 6-hour rain distribution for return periods of 2, 5, 10, 25, 50 and 100 years.The rainfall used is the result of table 1 calculations.From this hydraulics modelling, the results of flood mapping and flood depth around the study area were obtained.so it is necessary to solve this flooding problems because of the area is important for intercity transportation.

Conclusion
From the results of hydrological modelling using the HEC-HMS with the SCS hydrograph method, the design flood value for each period is: 5. Suggestion 1.The use of rainfall data obtained through the BMKG website needs to be calculated with data at other nearby meteorological stations to obtain more accurate results; 2. The determination of the perimeter in hydraulics modelling in the HEC-RAS program should be enlarged so that it can cover a more actual inundation area.

2. 8
HEC-RAS HEC-RAS (Hydrologic Engineering Centre -River Analysis System) is a program of the US Army Corps of Engineers designed to perform river flow modelling in one and two dimensions, sediment analysis, water quality analysis and hydraulics building functions [10][11][12][13].

Table 1 .
Distribution of Rain PSA-007 Rain 6 Hours Validation

Table 1
is the value of the 6-hour rain distribution for validation calculations.The rainfall used is the daily rainfall value on December 31, 2022, taken from the Ahmad Yani meteorological station.

Table 2 .
Distribution Terms Test Results

Table 3 .
Distribution of Rain PSA-007 Rain 6 hours