Impact of Landuse Change on The Inundations in The Ciberes Watershed

Whenever the rainy season approaches, the Ciberes River always inundates a number of regions. This research was conducted to determine the effect of land use change on the inundation in the Ciberes Watershed, given the aforementioned context. Using land cover change data from 2000 to 2023, a hydrological and hydrodynamic analysis of the Ciberes Watershed is conducted to determine the impact of landuse change. This investigation employs HEC-HMS for hydrological modeling and HEC-RAS for 1D and 2D hydrodynamic modeling. The simulation results demonstrate that the change in CN value is directly proportional to the flood discharge, and the flood discharge is directly proportional to the height of the flood inundation. Based on these findings, it can be concluded that the river’s cross section can accommodate flood discharges with a return period of 50 years, except in coastal areas. whereas the floods that occurred in 2018 and 2022 have a 10-year return period. It can be concluded that the topographical condition of the river in the downstream segment, which has a higher elevation than the surrounding land and flat soil conditions, is the primary cause of inundation in the Ciberes Watershed.


Introduction
Flood issues are a common problem in Indonesia, with annual floods affecting various regions throughout the country.The causes of flooding are not solely attributed to high rainfall intensity but also to other factors, such as the degradation of river basins (DAS), improper river channel development planning, and other contributing elements [1].In the eastern part of the Cirebon Regency, particularly within the Ciberes River Basin in the Ambit Village of the Waled Subdistrict, there is an area that consistently experiences flooding every year, especially in recent times This region is located in the downstream Ciberes River Basin, from the Ambit Weir to the Ciberes River's estuary in the Gebang Subdistrict.Throughout the year 2018, a total of 13 (thirteen) subdistricts (72.2 percent) in this area were affected by floods, with varying floodwater heights reaching up to 2 (two) meters.The impacted subdistricts include Losari, Ciledug, Pasaleman, Pabuaran, Pabedilan, Waled, Pangenan, Babakan, Astanajapura, Mundu, Lemahabang, Susukan Lebak, and Gebang.These floods affected around 50,000 IOP Publishing doi:10.1088/1755-1315/1343/1/012016 2 people, submerged 20,000 homes, and endangered 431 hectares of agricultural land [2,3].Based on the background mentioned, this research is conducted to examine the causes of flood inundation that occurs in the Ciberes Watershed.The evaluation of flood inundation causes is carried out through hydrological and hydrodynamic analyses occurring in the Ciberes Watershed.The hydrological analysis will be conducted using the assistance of HEC-HMS software, which functions to convert rainfall data into streamflow.Meanwhile, for the hydrodynamic analysis, the assistance of HEC-RAS 1D and HEC-RAS 2D software will be utilized.HEC-RAS 1D serves the purpose of modeling river channels to determine water surface elevations within the channels, enabling the assessment of the river's conveyance capacity.On the other hand, HEC-RAS 2D is employed to model potential inundation areas within the watershed based on the topography of those areas.Additionally, flood analysis is conducted using land cover change data from the years 2000, 2010, 2012, 2018, 2022, and 2023, with the aim of understanding the influence of land cover changes on the flood inundation that occurs in the Ciberes Watershed.cropfailure.The estimated losses incurred from this situation are around 55 billion in local currency.

Method
The impact of landuse change on the inundations mapping in the Ciberes watershed was performed using hydrological and hydrodynamic simulations using HEC-HMS, HEC-RAS 1D and HEC-RAS 2D, respectively.This research incorporated several methods to gather and process data.

Research Area: Ciberes Watershed
The research location discussed in this report is situated in the Ciberes Watershed, which is one of the watersheds located on the Java Island, encompassing the Cirebon area and falling within the Cimanuk-Cisanggarung River Basin.Geographically, the Ciberes Watershed is flanked by two other watersheds on its right and left, namely the Cikalapu Watershed and the Cisanggarung Watershed.The Ciberes Watershed covers an area of 105.56 km² with the longest river spanning 45.78 km².The topography of Ciberes Watershed is predominantly flat, which impedes the effective gravity flow of water.Consequently, the frequency of local flooding in the region increased.

Data
This research used various data sources, including daily rainfall data, DEM, sea level, river cross section.The DEM data was employed to define the boundaries of Ciberes watershed and as terrain data for HEC-RAS 2D simulations.In addition, land use data were obtained through Citra Landsat imagery, which played a crucial role in calculating rainfall runoff and estimating the design flood using the SCS-CN method.Hydrodynamic data used in this research include sea level data.Sea level data was acquired from sensor data provided by [PasangLaut] [4].These datasets used to analyze tides using the Admiralty method.The daily rainfall data from 2000 to 2022 was obtained from the Cimanuk Cisanggarung River Basin Development Agency.This data was essential for determining the maximum daily precipitation throughout the year and calculating design rainfall.River cross section was obtained from Cimanuk Cisanggarung River Basin Development Agency.This data used for HEC-RAS 1D simulations.

Hydrology Model: SCS-CN Method
In this research, hydrology model calculations are planned to use the Soil Conservation Service -Curve Number (SCS-CN) method, which is an empirical approach commonly employed for direct runoff from transformed rainfall, starting from a diverse land cover watershed, including agricultural land, forests, urban or urbanized areas, and it can integrate various characteristics of different catchment areas.In this research HEC-HMS software used to simulates precipitation and runoff processes within a watershed.This software offers a range of methods that can be tailored to specific watershed locations and conditions.[5]For urban areas such as ciberes, the time lag routing method and SCS-CN are considered suitable.[6]The Soil Conservation Service -Curve Number (SCS-CN) method connects land cover with soil characteristics and generates a curve number (CN) or runoff coefficient.The CN value, a key parameter in the SCS-CN method, is closely associated with the antecedent moisture conditions (AMC), which reflects the moisture content of the soil.The CN value varies depending on the soil conditions.This research assumes that the soil conditions is AMC II representing an average moisture content.Based on research by [Chow], the following are the equations used in the SCS-CN method: [7]   = (−   ) 2   −   + (1) Where P, Pe, Ia, S and CN denote the precipitation (mm), direct runoff (mm), initial abstraction (mm), maximum retention potential (mm), and curve number, respectively.The basic assumption of this method is that runoff occurred after the initial abstraction was fulfilled.This abstraction comprises interceptions, surface storage, and infiltration.Retention potential is a parameter dependent on the catchment's land use.According to the Soil Conservation Service, retention is a function of CN, a relative measure of retention based on land use with a value between 0 and 100.The value itself is determined based on the soil type and previous moisture conditions (AMC).
In this study, the Wanny method is employed to determine rainfall intensities for various time durations.Rainfall duration in this study is assumed to be 6 hours long.

Sea Level
Tides are a visible natural occurrence in the sea characterized by the vertical movement of seawater mass, ranging from the surface to the deepest parts of the seabed [8].These ocean tides are primarily caused by gravitational forces and play a crucial role in modelling extreme conditions within estuary.Furthermore, [Araújo] stated that high tides are important to consider in modelling coastal flooding [9].

Hydrodinamic Model: HEC-RAS 1D & HEC-RAS 2D
HEC-RAS, another software developed by the US Army Corps of Engineers, is used for modelling the extent and depth of inundation.River cross section data is needed to employ the one-dimensional model effectively and Accurate terrain data is needed to employ the two-dimensional model effectively.Flood IOP Publishing doi:10.1088/1755-1315/1343/1/0120164 events were simulated in HEC-RAS using the Saint-Venant equations under unsteady flow conditions [10].

Hydrology Simulation
HEC-HMS is used to conduct hydrology simulation of the Ciberes Watershed.The model configuration used for this simulation is shown in Figure 9.

Figure 9. Ciberes Watershed Schematic
The model defined each of the subwatersheds, junctions, and reaches.The SCS-CN method was used to estimate the parameter.Meanwhile, the methods for the transform parameter are SCS unit hydrograph for each sub-watershed and reach, respectively.In using these methods, several input parameters must be considered.The observed precipitation data from the rain gauge was analyzed, leading to the conclusion that it follows a Type 3 Log Pearson distribution.This distribution was then used to calculate design rainfall for various return periods, as shown in Figure 10.Based on the urban channel design plan, which considers return periods of 10, 25, and 50 years, these return periods will be used to calculate flood discharge.

Figure 11. Peak Flow in Ciberes Watershed
Based on the simulation results from HEC HMS, it can be observed that using the same rainfall amount, the resulting discharge will vary for each year under consideration due to differences in land cover or curve number (CN) values for each year.A higher curve number (CN) value indicates a larger discharge value.

Sea Level
The Gumbel distributions were used to analyzed and determine the design wave, with the result shown in Table 2.These distributions play a crucial role in understanding and modelling the characteristics of the design wave for downstream parameter.3.5 Hydrodynamic Simulation 3.5.1 HEC RAS 1D.Hydrodynamic model was developed using HECRAS 1D, incorporating river cross section data and river schematic.The cross-section data was adjusted to align with the MSL to be consistent with other information used in the research.The river schematic data represented as a 1D river Area.This comprehensive representation ensuring an accurate simulation of flow dynamics in the model.Hydrodynamic model also used two boundary conditions, namely flow and stage hydrographs, for the upstream and downstream boundaries, respectively.In order to determine the stage hydrograph, the 50-year design wave was added to the HHWL elevation.The boundaries were selected to depict extreme conditions.Figure 12 shows hydrodynamic model used and Figure 13 shows the result of simulation.In order to determine the stage hydrograph, the 50-year design wave was added to the HHWL elevation.The boundaries were selected to depict extreme conditions.Figure 20 shows hydrodynamic model used, clearly representing its configuration and components.HEC-RAS two-dimensional analysis generated an inundation map and depth results shown in Figure 14 to Figure 19, and Table 3.Based on the simulation results conducted using HEC-RAS 1D and 2D software, it can be stated that flood inundation occurring in the Ciberes Watershed experiences changes in water levels based on variations in land cover and Curve Number (CN) values.When the obtained discharge is higher, the water levels within the channel increase.It can be inferred that the magnitude of the CN value influences the height of flood inundation within the watershed.Furthermore, the floods in the Ciberes Watershed, particularly in the downstream area, are primarily attributed to the topography of the surrounding region, which is lower than the elevation of the river.This situation prevents floodwater from naturally flowing into the river.This can be evidenced by the water level conditions in the HEC-RAS 1D simulation, which show that the river cross-section can still accommodate flood discharges with return periods of 25 and 50 years.In contrast, during the flood events in 2018 and 2022, the flood discharges fall within the 10-year return period, resulting in smaller discharges compared to the 25 and 50-year return periods.

Conclusion
In conclusion, this study utilizes HEC-HMS, HEC-RAS 1D, and HEC-RAS 2D to assess the impact of land cover changes on flooding in the Ciberes watershed.
The land cover changes that occurred in the review years of 2000, 2010, 2012, 2018, 2022, and 2023, primarily affected the forested and built-up areas, where the built-up land increased by 9.24% and the forest decreased by 12.14%.The elevation and extent of inundation have increased as the flood flow discharge in the Ciberes watershed has risen due to the higher Curve Number (CN) values.
The floods that occurred in 2018 and 2022 were a result of the topography of the Ciberes watershed, where the land elevation around the river is lower than the river's elevation, causing floodwaters to be unable to drain into the river.Downstream flooding in the Ciberes watershed, especially near the coast, was caused by the rising sea level along the coastline, leading to the intrusion of seawater into the river.
The flow rate during the 50-year recurrence interval, which is 705.2 m³/s, can still be accommodated within the Ciberes river channel.Hec-Ras modeling indicates that the water surface elevation does not exceed half of the channel depth, except in areas with lower channel elevations compared to the upstream channel sections and near the coast, where the channel height is lower than the high tide water level.

Disclaimer
The authors declare no conflict of interest.

3. 1
Parameter and Scenario Definition Prior to conducting the simulations, it is crucial to determine the parameters and scenario.Each scenario should accurately represent Ciberes Watersheed.This research used six scenarios.The scenario simulated based on land cover change is conducted using land cover data from the years 2000, 2010, 2012, 2018, 2022, and 2023.The parameters used are, upstream, and downstream boundaries.All scenarios use 10, 25, and the 50-year design flood as the upstream boundaries.Meanwhile, the downstream boundaries comprise the sea level elevation is used as a boundary condition with return periods of 10, 25, and 50 years.

3. 2
Curve NumberIn this research, land cover samples are classified using satellite imagery data with the assistance of ArcGIS software using the satellite image data to identified different land cover types.The land cover types used for classification are divided into 4 categories with soil characteristics falling under category A, B, C, D and the soil in the Cirebon Regency area is predominantly categorized as type B.

Table 1 .
Landuse Change in Ciberes 5Figure 8. Landuse Change in CiberesIn Figure8, it can be observed that over time, residential and cultivated crops are expanding due to population growth in the vicinity of the Ciberes Watershed.This has led to a reduction in the extent of open space and forests, resulting in an increase in the Curve Number value in the Ciberes Watershed.The changing CN values from year to year can be observed in table 1.