*Corresponding author: Water Resources Engineering, Engineering Faculty, Universitas Brawijaya, Malang 65145, Indonesia E-mail address: [email protected] (Syaiful Amrie)
doi: https://doi.org/10.21776/ub.pengairan.2023.014.02.2 Received: 2022-07-08; Revised: 2022-11-16; Accepted: 2023-07-09.
Vol. 14 No. 02 (2023)
Jurnal Teknik Pengairan: Journal of Water Resources Engineering
Journal homepage: https://jurnalpengairan.ub.ac.id/index.php/jtp
Original research article
Study of Flood Control Due To Land Use Change at the Estuary of Bang River, Malang
Syaiful Amrie*, Very Dermawan ,
Endang PurwatiWater Resources Engineering Department, Engineering Faculty, Universitas Brawijaya, Malang 65145, Indonesia
A R T I C L E I N F O A B S T R A C T
Keywords:
Estuary;
Flood control;
Land use change;
Retarding basin
Water offers many advantages that are essential for the preservation of human existence.
Excessive quantities of water, such as flooding, can negatively impact human beings. The land utilization within a watershed (drainage area) plays a crucial role in the frequency and impact of flooding events. Instances of land use changes that resulted in flooding happened in the Bang Sub-watershed, located in Malang Regency. A comprehensive and up-to-date examination of flood management solutions is required. This study seeks to offer technical suggestions for flood management in the Bang Sub-watershed due to land-use alterations, utilizing the HEC-RAS software. Integrated treatment solutions are provided to mitigate the potential damage caused by future flooding. The flood simulation yielded recommendations for mitigating the impact of future floods. These recommendations include the construction of a retarding basin in the upper part of the river with a capacity of 1.4 million m3, as well as the construction of a 654 m long river embankment that is 50 cm. The alteration of the land leads to a decrease in the cross-sectional capacity of the river at the mouth of the Bang River, resulting in annual flooding occurrences. A non-technical approach to enhance the quality of the watershed is implementing reforestation measures to decrease the runoff coefficient.
1. Introduction
Natural disasters, especially floods, are often a problem in this country [1]. Floods can affect various losses [2]. Existing losses can be material or non-material losses [3]. However, the right calculation can minimize the impact [4]. East Java is one of the provinces that is prone to flooding. More than 50% of East Java province has a high potential for flood disasters, including in the study location to be discussed, namely the Bang Sub-watershed in Malang Regency [5]. The location where the research is located experiences floods almost every year [6]. One of the causes of flooding is the absence of geographical information regarding the watershed's land use [7]. It causes indiscriminate land use to change the land at risk of flooding [2]. The same problem occurred in the Bang Sub- watershed in the southern Malang Regency.
Science and technology are currently developing rapidly, and one of the concerns is developing software or software developed specifically to research flood events [8]. However, what happens to the development of technological sophistication is inversely proportional in terms of its utilization compared to the many floods that occur [9]. The development of existing software technology needs to be
utilized to analyze floods to be an accurate, comprehensive solution [10]. Analysis of flood impacts accompanied by solutions in the context of land use under the conditions at the research location needs to be carried out considering the complexity of watershed problems [11], so technological developments, one of which is in the form of software development, is a beneficial alternative in watershed management [12]. This study analyzed the effect of land change as a factor causing flooding using HEC RAS software.
Using this technology in watershed management is expected to provide a comprehensive and detailed technical study for flood control.
2. Method
2.1. Time and Research Location
This study was in the Bang Watershed area of Tamban Beach in Tambakrejo Village, Sumbermanjing Wetan district, Malang Regency, East Java province. Tamban Beach is geographically located in the southern part of the equator at coordinates 8o 26' - 8o 30' south latitude and 112o38'-112o 43' east longitude. The daily rainfall data from the Sitiarjo rain recording station as observation data from 2010 – 2019.
115
Figure 1. Map of Malang Regency [13]
The location of the study is shown in Figure 1.
2.2. Data Collection
In preparing this study, data that supports both primary and secondary data are needed. What is meant by secondary data is data sourced from related agencies and has been measured, while primary data is obtained based on direct measurements in the field.
In general, the necessary data in this study are : 1. The digital map, which are :
a. A topographic map with a scale of 1:25.000 from BAKOSURTANAL
b. Land use map for two different years (2001 and 2019) from BAKOSURTANAL
2. Government Regulation in the form of a Regional Spatial Plan in Malang Regency and East Java Province.
3. Hydrologic data
Data in the form of daily rainfall of rain measuring stations in the Bang Sub-watershed from 2010 – 2019 was obtained by the Sitiarjo rain station. The rain station in this study only used 1 (one) rain station because the rain station was the closest to the research location, where there were 3 (three) other rain stations (Dampit, Kemuan, and Bantur rain stations). Details of the location of each rain station are described in the Thiessen Polygon drawing in Figure 2 below.
2.3. Research Stages
The procedure for the stages of research and the calculations and analysis carried out in this study are as follows:
1. Perform data abnormality tests and perform outlier tests 2. Perform data consistency test with RAPS (Rescaled
Adjusted Partial Sums) calculation
3. Dividing the Bang River Watershed into five parts as desired to be included in boundary conditions
4. Calculating design rainfall using the Normal, Log Normal, Gumbel, or Log Pearson III methods with a predetermined lifetime, the method chosen is the one with the smallest deviation value
5. Testing distribution suitability with the Smirnov Kolmogorov Test and Chi-Square Test
6. Determination of different drainage coefficients according to land use in different years 2001 and 2019 in each sub watershed
7. Determination of rain intensity in the form of rain hours and net rainfall
8. Determination of design flood discharge by synthetic unit hydrograph Nakayasu Method for design flood Q2, Q5, Q10, Q25, dan Q50
9. Analysis of the study location, including watershed analysis in contour maps and determination of river flow and cross-sectional dimensions.
10.Hydraulic analysis
11.The software used is HEC-RAS 6.1 to determine the flow pattern [14]. The steps to solve it are as follows :
a. Starting HEC-RAS 6.1 b. Entering Geometry Data
c. Entering Unsteady Flow and other flow boundary conditions
d. Processing Data (Running Data)
e. Output Data Processing Results (Output Data)
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Figure 2. Thiessen Polygon hydrologic station for research location
Figure 3. Bang Watershed apportionment 12.Alternative Planning Concepts
13.The selection of this alternative is intended to obtain the benefits of the most effective, efficient, and optimal solution patterns, namely by simulating various
alternatives that have been made.
14.A review of existing solution patterns still relevant to current conditions will also be considered an alternative treatment.
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Figure 4. Land Use in Sub DAS Bang (2001)
Figure 5. Land Use in Sub DAS Bang (2019)
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Table 1. Land use in Bang Sub-watershed in 2001
No. Land use C Area (A)
% A C x % A km2
1 Rice field 0.15 1.82 7.32 0.01
2 Dryland farming 0.1 4.05 16.28 0.02
3 Plantation 0.4 5.87 23.59 0.09
4 Inland waters 0.1 0.04 0.16 0.00
5 Jungles 0.2 11.87 47.71 0.10
6 Housing 0.6 1.23 4.94 0.03
Total 24.88 100.00 0.25
Table 2. Land use in Bang Sub-watershed in 2019
No. Land use C Area (A)
% A C x % A km2
1 Rice field 0.15 3.60 14.47 0.02
2 Dryland farming 0.1 1.65 6.63 0.01
3 Plantation 0.4 6.07 24.40 0.10
4 Inland waters 0.1 0.03 0.12 0.00
5 Jungles 0.2 9.20 36.98 0.07
6 Housing 0.6 4.33 17.40 0.10
Total 24.88 100.00 0.30
3. Results
3.1. Watershed Apportionment
To simulated to get close to real conditions in the field, the design flood calculation is divided into 5 sub-watersheds.
The distribution of design flood calculations based on sub- watersheds is shown in Figure 3. The division of the Sub- watershed is based on the river land used in the simulation.
Each Sub DAS represents the discharge input as boundary conditions or boundary conditions that will be used for simulation [15].
The picture above shows each sub-watershed divided into five regions according to the conditions and location of tributaries in the field. This division will later be used as boundary conditions for the next calculation step. The area values for each sub-watershed sequentially starting from sub-watersheds 1 to 5 are 8.57 km2, 10.17 km2, 1,872 km2, 0.427 km2, and 1,404 km2.
3.2. Land Use Comparison
According to data from the land use map of the Sendang Biru estuary based on data obtained from BAKOSURTANAL, the types of land in the Bang Sub- watershed can be divided into six types. To obtain a comparison of the flow coefficient value (C), it is necessary to distinguish the land use used in 2001 and 2019 as follows.
Figures 4 and Figure 5 show differences in land use change between 2001 and 2019. The method used in drawing land use maps is digitizing the original map using GIS Software.
The following compares the land use values of the Bang Sub- watershed between 2001 and 2019.
Table 1 and Table 2 show detailed information in Figure 4 and Figure 5 that there is a difference in the value of the land use coefficient, which was previously 0.25 in 2001 compared to 0.3 in 2019. After dividing the Subwatershed and comparing the land use value each year, the next step is calculating the design flood discharge with the following results.
Table 3 and Table 4 show the stark difference in calculation results between 2001 and 2019. It indicates that design flood calculations using land use in 2001 must be updated again to draft flood discharge using land use data in 2019 [16]. Thus, it can be concluded that to anticipate the consequences of floods caused as stated in the research objectives, it is necessary to design flood management using flood simulations at high discharges, namely in the calculation of design flood discharge using land use data in 2019, as shown in Table 4.
3.3. Research Modelling
Several things need to be considered in doing flood modeling, including :
1. Boundary conditions 2. Calibrate the model 3. Existing flood simulation 4. Simulation of flood dike solution 5. Simulated flood solution retarding basin 6. Simulated flood solution combination
The modeling details of this study are explained as follows.
Boundary Conditions
Boundary conditions or boundary conditions in this modeling are generally divided into 3 (three) parts:
upstream, middle, and downstream, with detailed schemes in Figure 6. The determination of boundary conditions in the models is not only on the sub-watershed area but also on flood discharge entering from each of these areas is carried out with the following limitations:
a.The boundary conditions in the upstream and middle are discharge inputs at 5 points representing each sub- watershed division at each station point according to survey data in the field.
b.The boundary condition downstream is at the river's end with a hydrograph stage value in tidal elevation.
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Table 3. Calculation of design flood discharge with land use in 2001 Design
Flood
Design Flood Discharge with Land Use in 2001 (m3/dt)
Sub DAS 1 Sub DAS 2 Sub DAS 3 Sub DAS 4 Sub DAS 5
Q2 15.47 18.36 3.38 1.14 2.53
Q5 19.81 23.51 4.33 1.46 3.24
Q10 22.66 26.9 4.95 1.13 3.71
Q25 23.81 28.26 5.2 1.19 3.9
Q50 26.27 31.18 5.74 1.31 4.3
Q100 28.94 34.35 6.32 1.44 4.74
Table 4. Calculation of design flood discharge with land use in 2019 Design
Flood
Design Flood Discharge with Land Use in 2001 (m3/dt)
Sub DAS 1 Sub DAS 2 Sub DAS 3 Sub DAS 4 Sub DAS 5
Q2 30.57 32.53 10.59 2.91 7.92
Q5 42.31 44.96 14.65 3.87 10.89
Q10 50.13 53.23 17.35 4.52 12.87
Q25 54.53 57.89 18.87 4.88 13.98
Q50 61.33 65.09 21.22 5.44 15.7
Q100 67.49 71.61 23.34 5.95 17.26
Figure 6. Boundary conditions for research modeling
Figure 7. Flood simulation for two years design (Q2)
120
Figure 8. Flooding area of Q2
Figure 9. Q25 simulation without any construction (existing)
3.4. Calibration of Model
Model calibration is not intended for design planning.
Still, it is needed only to validate whether our flood model is close to the real conditions on the ground that experience flooding almost every year. The results of the approach to the model using a 2-year return period discharge are shown in Figure 7. It shows the simulation results using Q2 discharge.
The results of this simulation are verified with conditions in the field based on survey data, namely data at the review point (village bridge). Detailed information obtained from the results of field surveys is that at the time of the flood, the connecting bridge in the village (sta. 10447) was submerged with an estimated depth of between 20 – 25 cm. The results of the Q2 simulation also showed overflow at some point. The identification of flooded locations based on surveys shows three flood points, as shown in Figure 8.
The picture above shows the simulation results using Q2
in 3 possible flood points. The simulation results need to be verified with survey data in the field. It is verified by
information from residents that there are also frequent floods almost every year around that point. Based on the suitability between the simulation model and the real conditions in the field survey results, this shows that the modeling is valid and acceptable. The next step is to simulate an existing flood.
3.5. Existing Flood Simulation
As described in Table 3 and Table 4 in Subchapter 3.2, the flood value in 2019 has significant differences, so the flood management simulation must be carried out using the 2019 design flood discharge value only because it anticipates the worst possible impact of flood events [17]. So, the simulation was carried out in Q25 annually with the results on modeling, as shown in Figure 9. The picture above shows how flooding occurs without treatment. Inundation occurred at sta.12309 and 12009 and between sta.11177 and sta.10331 with a maximum height of 2.13 m above the dam on sta.10641.
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Table 5. The need for the embankment of the Bang River River
Sta
W.S.
Elev (m)
Discharge (m3/s) Elevation (m) Hydr Depth (m) Embankment needed (m) Left Channel Right Left Right Left Channel Right Left Right
12309 15.63 113.7 15.63 15.87 0.01 1.92
11177 14.04 0.2 114.43 0.33 13.46 13.47 0.58 1.77 0.56 1.38 1.36
10641 14 0.22 115.31 0.09 12.65 12.93 1.33 2.87 1.04 2.13 1.84
10447 13.78 0.28 115.19 0.39 12.56 12.62 1.22 2.94 1.17 2.02 1.97
10396 13.76 0.22 115.31 0.39 12.56 12.62 1.2 2.91 1.14 2 1.94
10348 13.73 0.07 115.51 0.41 12.56 12.62 1.17 2.89 1.11 1.97 1.91 8249 11.16 0.21 133.39 0.07 10.67 10.69 0.47 2.26 0.47 1.27 1.27 7872 10.89 0.09 133.96 0.01 10.15 10.12 0.74 2.55 0.77 1.54 1.57
7713 10.6 0.01 134.32 0.01 10 10 0.6 2.56 0.6 1.4 1.4
7410 10.5 0.83 133.99 0.08 9.51 9.45 0.99 2.07 1.05 1.79 1.85
7289 10.27 2.22 132.65 0.26 9.2 9.2 1.01 2.62 1.07 1.81 1.87
7132 10.05 0.53 134.83 0.08 8.88 9.03 1.16 2.56 1.02 1.96 1.82
7014 9.9 0.11 135.32 0.23 8.98 8.67 0.91 2.62 1.22 1.71 2.02
4734 7.41 139.91 0.04 7.47 6.88 2.35 0.52 1.32
4530 7.15 0.01 140.3 0.03 7.01 6.99 0.14 2.13 0.16 0.94 0.96
4268 6.82 0.14 140.54 0.12 6.23 5.94 0.59 2.28 0.88 1.39 1.68
Runoff also occurs between sta.10266 to sta.6496 with a maximum height of 1.96 m above the dam on sta.7132 as well as between sta.4734 to sta.4268 with a maximum height of 1.68 m above the dam on sta.4268. Seeing the impact of floods that occur in such a way, this needs to be handled. The first alternative form of treatment is the construction of guard dikes as needed based on the height of runoff that occurs.
3.6. Flood Simulation with Embankment
Looking at the simulation of the existing Q25 model, the first alternative treatment suggested was the construction of dikes at points that experienced flooding. The levee elevation needed to secure areas prone to flooding, then the calculation of dike needs in the Bang River is shown in Table 5.
Table 5 shows the results of HECRAS software simulations related to how high the overflowing water exceeded the existing dams on the right and left of the river.
Based on the existing Q25 in the table above, the calculation of dike needs shows that the highest dike requirement is 2.13 m at sta.11177 and sta.4268. The required height of the dam is rounded to 2.2 m along the flood points, with the total length of the dam that needs to be built being 6909 m.
The next alternative treatment that needs to be tried to be simulated to overcome flooding at the research site is to build a retarding basin.
3.7. Flood Simulation with Retarding Basin
The construction of retarding basins as a form of flood management at the study site is in the upper reaches of the river. It is due to several considerations, including :
1. Construction purpose
The purpose of the construction of this retarding basin is to reduce the amount of peak flood discharge so that the flood hydrograph curve that occurs becomes more gentle.
2. Land availability [18]
At the study site, there was not so much land available to construct retarding basins with a large capacity.
3. Operational procedure
The operation of retarding basins requires the simplest operation pattern because the study location is a rural area far from the city center and related agencies that usually carry out operation and maintenance activities.
A map of the construction site of the retarding basin is shown in Figure 10. The volume of the retarding basin plan is 1.4 million m3. This volume was obtained by trial and error because there is no documentation or analysis of flood inundation at the research site. The second consideration of the development of retarding basins with such dimensions is the availability of land for development in the upstream area of the river. Therefore, constructing this retarding basin must optimize the available land on the upstream side of the research site so that a value retarding basin volume worth 1.4 million m3 is obtained in the upper reaches of the river [19].
Figure 11 shows the impact of retarding basin development as a form of flood management. Overall, there was a drop in water levels of 82.4 cm. However, overflow was above the dam at several critical points, including in sta.
4268 is 9 cm tall, between sta. 7289 to sta. 6769 with a height of 18 cm. It requires a solution by combining retarding basins and dikes [20].
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Figure 10. Map of the location of the retarding basin construction plan
Figure 11. Results of flood simulation with retarding basin development
Figure 12. Flood simulation results with combination solution
123 3.8. Flood Simulation with Combination
Seeing that the simulation of the retarding basin model still has several problems at critical points in the form of overflow, it is necessary to build dikes at critical points of flooding as high as 50 cm. The simulation results after the construction of retarding basins and dikes are shown in Figure 12. It shows that there is no longer a runoff water flow due to the construction of a 50 cm high embankment. Thus, it can be concluded that the solution by the conditions in the field is to build a retarding basin, which has a storage capacity of 1.4 million m3 combined with building a river dam as high as 50 cm along critical points starting from sta.
7289 to sta. 6632.5, which is 654 m.
4. Conclusion
The results of the flood simulation study at the mouth of the Bang River are divided into two types; the first is model calibration to find out how the simulation is similar to real conditions in the field using the Q2 design flood discharge.
The second simulation aims to determine the form of flood solution at the research location, which in this simulation does not use Q2 but uses design flood discharge 25 years (Q25). Land change causes a reduction in the cross-sectional capacity of the river at the mouth of the Bang River, which results in floods that occur almost every year. The best alternative flood control analysis that can be done at the mouth of the Bang River is to build a 0.5 m barrier to secure the entire critical area along with the retarding basin.
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