Analysis of Land Cover Changes Impact on Design Flood Estimation Case study: Upper Cimanuk Watershed in Garut City

The unexpected flooding events that occurred in the years 2021 and 2022 in the city of Garut became unforeseen incidents. According to the National Disaster Management Agency (BNPB), the flood disasters were caused by several factors, one of which was changes in land cover. Based on this background, this research is conducted to examine the causes of changes in flood discharge that have occurred in the city of Garut due to land cover changes from the years 2000, 2010, and 2020. The evaluation of the factors causing the flood magnitude is performed by conducting a hydrological analysis in the Cimanuk Watershed. An analysis of land cover changes using the ArcGIS application to create land cover samples and HEC-HMS application was conducted to determine the percentage increase in discharge and to understand the percentage of land cover changes that occurred in the Cimanuk Watershed. The data used included satellite image maps, digital elevation models (DEM), rainfall intensity, and cross-sectional data. The accuracy of the simulation was assessed using the Nash-Sutcliffe Efficiency (NSE). The simulation results show an increase in the inflow discharge into Garut city ranging from 23.7% to 32.2% for each return period. The NSE value obtained for the model was 0.83, indicating that the HEC-HMS hydrological model performed well.


Introduction
Indonesia is a country with a tropical climate that experiences dry and rainy seasons.During the rainy season, especially, Indonesia has high annual rainfall, with rainfall increasing in mountainous areas [1].The Regional Disaster Management Agency (BPBD) states that Garut Regency is classified as an area with a high potential for flood disasters.The Garut Regency area encompasses 33 rivers and 101 tributaries, with a total river length of 1,397.34km.The Cimanuk River, which stretches 180 km and is the second-largest river in West Java, is a significant feature.
The floods of 2021 and 2022 had significant impacts, affecting around 13 sub-districts in the city of Garut.The National Disaster Management Agency (BNPB) attributed the cause of the floods to high rainfall intensity and narrowing of the river channels.The Executive Director of WALHI also stated that IOP Publishing doi:10.1088/1755-1315/1343/1/012015 2 development in Garut Regency disregarded the vulnerability of the natural conditions, resulting in mining and property development altering the land cover in the area.
Previous studies have shown that the change in land cover play the significant effect to peak discharge [2,3] .The shift in land use disrupts the function of water catchment areas and affects surface runoff [4].The increasing trend in land use change, driven by population growth and development, has led to a crisis in the upper Cimanuk watershed.This study is aimed at understanding the impact of land cover changes on design floods in Garut city.

Methods
This research utilized several methods to collect and process data.The section begins with an overview of the research area and the data collection process, providing essential information on the geographical context and the sources of data used to drive the hydrologic model.

Research Area: The Upstream of Cimanuk Watershed
The Cimanuk Watershed covers an area of 3,415 km 2 and spans four regencies as seen in Figure 1.The upstream area is in Garut Regency, the middle part includes Sumedang and Majalengka Regencies, and the downstream area is in Indramayu Regency.The shape of the Cimanuk Watershed resembles a bird's feather, with a maximum width of 39.04 km and a length of 83.12 km.The geomorphology of the Cimanuk Watershed, particularly the upstream area of focus in this research, consists of mountains and hills, with the peaks forming the watershed boundaries, and valleys and plains in the central and northern parts.The drainage patterns in the mountainous and hilly areas range from sub-radial to radial, subdendritic to dendritic, sub-parallel to parallel, sub-rectangular to rectangular, sub-trellis to trellis, and anastomotic.The Directorate General of Water Resources, Ministry of Public Works, stated that the upstream area of the Cimanuk Watershed is strategically important and requires special attention due to its significance [5].In 1992, research was conducted for the development of the multipurpose reservoir, Waduk Jatigede, in the upstream part of the Cimanuk Watershed.
To analyse the impact of land use changes in the Upper Cimanuk Watershed, the river basin is divided into 11 sub-watersheds.The division of the Upper Cimanuk Watershed into these segments is based on existing case studies, thus depicting the spatial characteristics of the river basin.The spatial distribution of watershed characteristic is important in order to better analyse the hydrological response on hydrological model [6].

Data
The following are the data and materials used in the research.

Satellite Imagery Processing.
In creating land cover samples, satellite imagery is employed and assisted by the ArcGIS application using the Image Classification method.This method involves generating samples of land cover types that align with the satellite imagery.Once the samples are generated, the ArcGIS application will produce a land cover file corresponding to the upper Cimanuk watershed area.Due to limitations in data availability and the quality of the utilized satellite imagery, the range of feasible land cover classification types is restricted.The employed land cover classification types are divided into 5 categories, as listed in Table 1, along with their respective Curve Number (CN) values.
Table 1 The conventional CN data was previously established based on standardized land cover descriptions using assumptions regarding the specific land use details [7].

Hydrological Data.
This study makes use of daily rainfall information collected from various stations located within and around the Cimanuk Watershed.The data is sourced from a single entity: the Cimanuk Cisanggarung River Basin Agency (BBWS Cimanuk Cisanggarung).The managed and utilized stations encompass Cikajang, Boyongbong, Leuwigoong, Cipasang, Leuweungtiis, Pamulihan, and Darmaraja.Additionally, in order to assess the hydrological model, cross-sectional data for the Cimanuk River is necessary.This particular data is also supplied by BBWS Cimanuk Cisanggarung.

Design Rainfall Analysis
Rainfall data from each station is initially distributed using the normal, Gumbel, and Log-Pearson Type III distributions.This is done to depict the distribution of the rainfall data.Both the Chi-Square test and the Smirnov-Kolmogorov test must be conducted to ascertain whether the distribution of the available data can be reconciled with the theoretical distribution used.The Thiessen polygon method is employed to determine regional rainfall, with each sub-watershed being influenced by different stations.

SCS-CN Method
The Soil Conservation Service is one method used for conducting flood design analysis.This method is based on the relationship between the amount of rainfall that falls during each rain event and the soil's infiltration capacity [7].Initial abstraction   covers all the initial losses that occur before surface runoff initiates.  comprises water held in surface indentations, water caught by plants, evaporation, and infiltration [8].The extent of   fluctuates significantly, typically aligning with soil and land cover characteristics.Through the analysis of various small-scale experimental watersheds, a practical connection between initial abstraction   and maximum retention potential S was established.This relationship facilitates the calculation of direct runoff value   based solely on precipitation depth  and retention potential  [8].Equation (1) and equation ( 2) were utilized to calculate the value of initial abstraction and direct runoff.
Soil retention potential  is a parameter in the hydrological analysis of the SCS-CN method that is related to land use in a watershed or catchment area.Equation (3) was used to determine S value.The Curve Number or CN is a measure of the relative value of soil retention potential based on the land use present in the field, with a scale of values ranging from 0 to 100.By plotting data for  and   from many watersheds, SCS found the standardized curve shown in Figure 2.
The primary factors influencing the CN value include Hydrologic Soil Group (HSG), land cover, land management practices, hydrological circumstances, and Antecedent Moisture Condition (AMC).The CN value in Figure 2 pertains to AMC II, which represents a typical antecedent moisture condition.In contrast, AMC I signify a state of minimal runoff potential in the soil, while AMC II indicates the highest runoff potential.The CN value for AMC I and AMC III can be ascertained using the equation (4) and equation ( 5) [8]. And

HEC-HMS
The HEC-HMS software application is designed to model one-dimensional flow in rivers.This application was developed by the Hydraulic Engineering Center (HEC), one of the divisions within the Institute for Water Resources (IWR) under the US Army Corps of Engineers (USACE).In its operation, the HEC-HMS application requires several input parameters.The HEC-HMS hydrological modeling system produces data such as flow discharge, streamflow hydrographs, and rainfall runoff simulations.Peak discharge calculations are based on the catchment area in the watershed using the Unit Hydrograph method.In the utilization of the HEC-HMS application, rainfall data serves as input, which is processed through the watershed system and generates outputs in the form of discharge and hydrographs.The main components in the HEC-HMS model are as follows: • Basin model -containing watershed elements.
• Meteorologic model -specifying the start and end times of computations.
• Control specification -indicating the start and end times of computations.
• Time series data -inputting rainfall and discharge data • Paired data -containing paired data such as unit hydrographs.

Model Performance Evaluation: Nash-Sutcliffe Efficiency (NSE)
The Nash-Sutcliffe efficiency test is one of the statistical methods used to evaluate the performance of a model utilized for predicting hydrological variables such as river discharge, rainfall, and the like.This test is based on comparing the predicted model outcomes with previously known empirical data [13].

HEC-HMS Model Implementation
Watershed delineation is conducted using data from the United States Geological Survey (USGS) obtained from the USGS website.Sub-watershed division is based on the branching of the Cimanuk River as the main river, and specific sub-watersheds are created to determine flood discharge in the research area.Figure 3 and figure 4 illustrate sub watersheds division and polygon Thiessen on the upstream of Cimanuk watershed.Table 3 shows the return period rainfall.The use of this period is based on the flood events in 2021 and 2022, where the rainfall is matched with a return period that closely corresponds to those rainfall events.
The rainfall information incorporated into the HEC-HMS application is customized to match the significant stations within a particular sub-watershed.Consequently, each sub-watershed contains unique rainfall values that align with the relevant influential stations.

Land Cover Data Samples
In creating land cover samples, satellite imagery is utilized alongside the ArcGIS application using the Image Classification method.This method involves generating sample land cover types that correspond to the satellite image.Once the samples are established, the ArcGIS application will subsequently produce a land cover file that aligns with the extent of the Cimanuk Upper Watershed.Figure 5 to figure 7 shows land cover sample in the specified years, while figure 8 shows a comparison of the area of land cover that has changed.The composite CN and Percentage of impervious land were summarized in table 4.

Design Flood Simulation
The simulation conducted utilizes land cover data corresponding to the review years, namely, the land cover data for the years 2000, 2010, and 2020.The displayed discharge samples are located at Junction 1 and Junction 2. Junction 1 serves as an observation point positioned upstream of the Cimanuk River's entry into Garut City, while Junction 2 is an observation point placed downstream of the Cimanuk River's passage through Garut City. Figure 9 to figure 16 shows the discharge hydrograph with the specified return period at specified location.Based on the simulation results of HEC-HMS, the percentage increase in inflow discharge into Garut city for the 2-year return period is 32.2%, for the 5-

Hydrological Model Performance
The model validation was obtained by comparing the peak discharge simulated results with the observed peak discharge data.Table 5 shows the comparison of observed and modeled peak discharge.The empirical validation data was collected from the Automatic Water Level Recorder (AWLR) at the Cipasang location on several dates in the year 2020.

Conclusion
Hydrological models are necessary for simulations due to the complexity and uncertainty of hydrological processes.The NSE value for the Cimanuk Upper Watershed hydrological model is 0.83, indicating good performance.Based on the simulation results of HEC-HMS, the percentage increase in inflow discharge into Garut city for the 2-year return period is 32.2%, for the 5-year return period is 27.5%, for the 10-year return period is 25.5%, and for the 25-year return period is 23.7%.This signifies those changes in land use leading to an increased percentage of impervious surfaces result in higher runoff discharge.Significant changes in land cover occurred in the built-up land class with an increase of 10.43% and a decrease of 15.47% in the rice field class.

Figure 1 .
Figure 1.Delineation of the Upstream of Cimanuk Watershed.

Figure 2 .
Figure 2. Relationship of  with   [9]2.5.ArcGISArcGIS is a Geographic Information System (GIS) software application developed by the Environmental Systems Research Institute (ESRI).It was first commercially published in 2000.ArcGIS offers several features for data processing, including ArcMap, ArcInfo, ArcEditor, ArcView, and ArcCatalog.The ArcGIS software application greatly aids in the watershed delineation process due to its elevation data capabilities, enabling the generation of accurate and complex watershed boundaries[10].Within the field of water resources, watershed delineation plays a crucial role and contributes to hydrological modeling.This analysis can be utilized for flood prediction[11].The precision of delineating a watershed depends on the accuracy and resolution of the available Digital Elevation Model (DEM)[12].

Table 3 .
Return Period Rainfall

Table 4 .
Percent Impervious Change

Table 5 .
Discharge comparison at AWLR Cipasang Based on the calculations above, the NSE value at AWLR Cipasang is 0.83, falling into the "very good" category.This indicates that the performance of the HEC-HMS hydrological model is quite satisfactory for conducting flood simulations.