Storm water management model to evaluate urban inundation in Lowokwaru and Blimbing sub-catchments in the city of Malang

In the city of Malang, which is a highland area, there are still floods, including Blimbing and Lowokwaru sub-districts. One of the causal factors is the change of undeveloped areas becoming relatively large built-up areas in the Upper Brantas sub-watershed, from initially 20.5% in 2000 to 69.6% 20 years later. This change also impacts the transformation of irrigation canals that once irrigated rice fields, causing them to shift their function into drainage channels that carry runoff water toward natural channels (natural drainage). Consequently, the existing canal conditions cannot accommodate the runoff water. The research employed an approach that involved analyzing floodwater management models using the Storm Water Management Model (SWMM 5.1) and surveying existing canal data. The analysis results revealed that, within the studied areas of Blimbing Village and the Blimbing Catchment Area, 10 out of the existing 170 drainage channels were incapable of handling the discharge associated with rainfall intensities designed for return periods of 5 years (112.48 mm/day) and ten years (117.09 mm/day). In the Lowokwaru Catchment Area, it was evident that 18 out of the 216 existing canals experienced overflow events with designed flood discharge for 5-year and 10-year periods.


Introduction
In the past several years, the City of Malang has experienced development along with the dynamics of its society and the authority given to the government of the City of Malang to build its city independently.The growth and development of the City of Malang has brought about effects to the entire city, one of which is the problem of urban flooding that has become frequently reoccurring in recent times.Flood-prone areas exist in both the Lowokwaru and Blimbing Sub-Districts.The identification results indicate that drainage issues in Lowokwaru Sub-District are particularly prevalent in RW (resident associations) 02, 04, 05, 09, 12, and 15.Meanwhile, the problems in the Blimbing Sub-District cover RW 5, 6, and 9 inundations.Generally, several factors trigger inundation in the area of the City of Malang [1]: 1.The occurring change in land use from open land to closed land With the population from year to year that has grown out of control, the need for settlements and economic facilities will continue to increase; consequently, conversion of land functions occurs from open spaces (fields, plains, and so on) to become closed spaces such as housing, industry, hotels, and commerce.2. The change from undeveloped areas to built-up areas in the sub-watersheds of Brantas Hulu, Bango, Amprong, Metro, and Sukun.The Bango sub-watershed, which makes up the areas of Lowokwaru and Blimbing Sub-Districts, experienced a reduction in undeveloped areas from 1967.42 ha (in 2000) to only 554.24 ha (in 2020) 3. The population growth in Malang rose in 2020 to 843,810 people from 756,982 people in 2000.4. The lack of construction of primary and secondary drainage channels leading to natural drainage and the continued extensive utilization of irrigation channels that have functionally converted into drainage channels have contributed to the issue.This research is focused on the areas of Lowokwaru (as a part of the Lowokwaru Catchment Area in Brantas Hulu Sub-Watershed) and Blimbing (as a part of the Purwantoro Catchment Area in Bango Sub-Watershed) because these areas comprise the central area of the City of Malang, which is an important area with the most significant number of inundation points, with 34 points (Lowokwaru Catchment Area) and 30 points (Purwantoro Catchment Area).
The problem of drainage is a classic phenomenon for urban areas in Indonesia from year to year that has not been managed.This problem includes the areas of Lowokwaru Sub-District and Blimbing Sub-District, which are areas in different sub-watersheds, although administratively, the two regions are close to each other.In these two regions, the problem of inundation exists at numerous points that have not received effective management.This effort requires analysis of inundation based on existing conditions, followed by analysis using an SWMM (storm water management model) to find out the reduction capabilities of existing drainage channels and to visualize inundation on the field, comparison with data of inundation locations as validation, and analysis of solutions for the management of inundation [2][3][4][5][6].This research is significant because urban flooding analysis is often not completed well due to the resolution of inundation problems not having utilized the concept of a catchment area as a sub-system.

Study Location
The research was conducted in specific geographical areas within the Lowokwaru and Blimbing Sub-District, located within Malang.These two regions were chosen due to their hydrological characteristics and fall within the boundaries of the Lowokwaru Catchment Area and Purwantoro Catchment Area, as defined by hydrological demarcations.These catchment areas delineate the regions where surface water runoff and drainage patterns converge, making them pertinent for studying urban flooding dynamics and hydrological management strategies.

Data Collection
The following data are required to be collected: 1.Primary Data a.The data on existing drainage channels, including dimensions, materials, coordinate locations and channel cross-section shapes, were obtained through field surveys.b.The data on inundation points were collected from BPBD data and through the results of a Focus Group Discussion (FGD) with the people.

Method
The stages of this study can be explained as follows: Data collection, Digital mapping of Catchment/Drainage Areas, hydrological analysis of rainfall and inundation with a design return period of 5 years [1] [7], SWMM analysis for urban flooding analysis [8][9][10] [15], Analysis of the hydrological capacity of existing channels, and establishing alternatives for inundation management by increasing channel capacity

Hydrological Analysis of Maximum Rainfall
The calculation of maximum rainfall involved the analysis of maximum daily rainfall data collected from the nearest climatology stations, namely, the Blimbing Rain Station and the Lowokwaru Rain Station.The calculations of the mean regional rainfall utilized the algebraic average method for both stations [11].Table 1 shows the calculation results for Mean Regional Rainfall with the data range from 2012-2021 (10 years).

Frequency Analysis
The Gumbel Method and Log Pearson III Method were used in conducting frequency analysis, after which the results were compared based on parameter calculations for statistical distribution selection to determine which method had the criteria of being statistically acceptable [7].The results of calculations for frequency analysis with the Gumbel Method and Log Pearson III Method are presented in Table 2.
The calculations showed that the design rainfall with a 5-year return period was 114.71 mm and 112.37 mm.

Distribution Goodness-of-Fit Test
Testing of distribution goodness-of-fit must be conducted to determine the acceptability of the frequency distribution for the sample data based on the two selected methods and which of the Gumbel and Log Pearson III methods fulfills the requirements.Testing of distribution goodness-of-fit in this research utilized the Kolmogorov-Smirnov Test for vertical deviation and the Chi-Squared Test for horizontal deviation [10][11][12] [13].The summary of calculation results can be seen in Table 3.As a result of a comprehensive analysis, it was determined that the Log Pearson III Method, chosen as the most suitable method based on rigorous evaluation, was employed as the primary methodology for deriving design rainfall.

Analysis of Hourly Rainfall Distribution for SWMM Input
Hourly rainfall intensity data is crucial as input parameters for the rain gauge in SWMM [3][4][5][6][7][8][9].The determining of this parameter utilized the Mononobe method, which is a method that is still often used for the analysis of urban flooding in Indonesia.Further, hourly rainfall intensity data on the field was unavailable because Blimbing Rain Station and Lowokwaru Rain Station have rain gauges (ombrometer) that utilize a manual daily recording system.Table 4 shows the results of hourly rainfall intensities using the Mononobe method for a 5-year return period.The 5-year return period was determined from the criteria for determining return periods based on urban typology [14], as the area of the study location being analyzed has a total area of 123 ha (within the area interval of 100 ha -500 ha) and belongs to the large city typology, the 5-year return period was used (Table 5).

SWMM Analysis for the Blimbing Sub-Catchment
The analyzed area of the Blimbing Sub-District, which belongs to the Blimbing Sub-Catchment, can be seen in the schema below.Construction of the model for existing drainage channels utilized the technique of on-screen digitalization on the latest Google Earth map to model Nodes and Junctions and to divide sub-sub-catchments.Red shading on polygons illustrates land that experiences inundation when rain simulation is performed.Then, solution analysis was performed based on related field information, land limitations, channel inclination slope, dimensions, narrowing buildings, etc.The results of the SWMM simulation with the 5-year return period can be seen in Figure 1.Utilizing the outcomes derived from the SWMM analysis, we were able to extract valuable insights regarding the maximum water levels within the channels and the corresponding water velocities, enabling us to ascertain whether overflow incidents occurred.Figure 1 depicts the simulation results specifically relevant to the channels experiencing overflow events with a 5-year return period.
Out of 170 drainage channels in Blimbing, ten channels overflowed and could not contain inundation with a 5-year return period (Table 6).These results were appropriate to the information on inundation points found on the inundation map, indicated by field survey results and data from the Department of Public Works of the City of Malang.Suggestions for channel dimension changes are given to provide a management solution; the results are the following new channel dimensions (Table 7).After modifications were made to the channel dimensions, the previously overflowing drainage channels no longer experienced overflow events (Table 7).This improvement is evident in these channels' computed discharge containment capacities, which were subsequently compared to the design discharge.These comparisons demonstrate that each drainage channel has a discharge containment capacity that effectively manages rainfall events with a 5-year return period [15].

Analysis of Lowokwaru Sub-Catchment
The following is the water level profile for existing conditions with a 5-year return period from crossing points of simulation results, which can be seen in the figure below.From the results of the simulation, as shown in Table 7, 16 channels were evaluated and found to exceed their capacities, which are located in Jalan Cempaka Kuning, Jalan Nusa Indah, Jalan Anggrek Vanda, Jalan Mawar, Jalan Mawar IV, Jalan Bungur, Jalan Tirtonadi, and Jalan Sarangan.
According to SWMM 5.1, if the maximum height of a channel in the simulation results is the same as or greater than the height of the existing channel, then the point is identified to overflow, or in other words, the drainage channel capacity is exceeded.Following the results of SWMM simulations, the areas affected by drainage channel overflow were validated using survey results and data from the BPBD (Regional Disaster Management Agency).Consequently, the necessity arose to recommend changes, including adjusting dimensions for various existing channels and implementing additional measures, such as sediment dredging.After implementing dimensional changes to the channels (Table 8), the next step was evaluating the channel flood discharge (Q channel) using a rational method based on dimensional changes.In Table 9, it can be seen that the capacity of the drainage channel met the Q design.3 demonstrates that the crossing and channel could not contain the discharge, resulting in flooding.The distinct bright blue waterline near the channel's mouth, denoted as an indicator between J109 and J110 within channel C134, unequivocally confirms that the channel experienced an overflow event.
Figure 4 shows the effects of dimensional changes on channels C34 (Jalan Cempaka Kuning) and C64 (Jalan Anggrek Vanda).Notably, these modifications prevented the channels from overflowing, as evidenced by the blue line (indicating water pressure) and the green line (representing the deepest crosssection) remaining below the current elevation line.-Channel C151, located in Jalan Tirtonadi, Lowokwaru Sub-District.
-Channels C159 and C160 are located in Jalan Sarangan, Lowokwaru Sub-District.2. The following alternatives are to be conducted to manage inundation in the Lowokwaru Sub-District: -Channels on Jalan Nusa Indah and Jalan Mawar are to be cleaned up.
-Channel inlets are to be added on Jalan Tirtonadi and Jalan Sarangan.
-Channels C31, C32, and C34 on Jalan Cempaka Kuning are to be dredged of sediment to increase their heights to 1 m.-Channels C35, C39, and C41 on Jalan Nusa Indah will be dimensioned to a channel width of 0.6 m. -Channel C64 on Jalan Anggrek Vanda will be dimensioned to a width of 0.8 m.
-Channel C94 on Jalan Mawar will be dimensioned to a channel width of 0.6 m.
-Channels C104, C134, and C137 on Jalan Mawar IV are to be dredged of sediment to increase their heights to 0.6 m. -Channels C121, C124, and C170 on Jalan Bungur will be dimensioned to a channel width of 0.8 m. -Channels C150 and C151 on Jalan Tirtonadi are to be increased in dimension to a channel width of 1 m.

Conclusion
Several recommendations have been formulated based on a comprehensive assessment of the drainage channels within the study area of Lowokwaru Sub-District, City of Malang, utilizing the latest hydrological analysis and the Storm Water Management Model, coupled with field inundation validation.First, it is recommended to increase the channel dimensions along Jalan Cempaka Kuning (to Jalan Anggrek Vanda) through dredging, raising the channel height to 1 meter.Second, enhancing the channel width along Jalan Nusa Indah and Jalan Anggrek Vanda to 0.6 meters and 0.8 meters, respectively, will help alleviate drainage issues.Third, it is crucial to address channel overflow problems on Jalan Mawar by widening it to 0.6 meters.Additionally, mitigating overflow concerns on Jalan Mawar IV (to Letjen Sutoyo IV) through sediment dredging and elevating the channel height to 0.6 meters is recommended.Lastly, addressing overflow problems in Jalan Bungur, Jalan Tirtonadi, and Jalan Sarangan by increasing their widths to 0.8 meters, 1.0 meters, and 1.2 meters, respectively, will to more effective flood management in the area.The following recommendations have been proposed to evaluate drainage channels in the study area of Blimbing Sub-District, City of Malang.First, it is advisable to expand the dimensions of the channel along Jalan Laksda Adi Sucipto Gang Setia, increasing its width from 0.25 meters to 0.75 meters.Second, extending the dimensions of the channel along Jalan Laksda Adi Sucipto Gang XII, increasing its width from 0.25 meters to 0.75 meters, will improve drainage capacity.Third, enlarging the channels near the Blimbing Sub-District Office on Jalan Tenaga Utara is recommended, widening them from 0.55 meters and 0.50 meters to 1.2 meters and 1 meter, respectively.Lastly, expanding the channels on Jalan A. Yani near Carrefour, increasing their widths from 0.45 meters and 0.50 meters to 1 meter each, will enhance the overall drainage system in the area, mitigating flood-related issues.

Figure 2 .
Figure 2. Situational Map after Running 5-Year Return Period Simulation for Lowokwaru Catchment

Figure 3 .
Figure 3. Profile Cross-Section Cutaway of the Water Level for the Existing Drainage Channel

Figure
Figure3demonstrates that the crossing and channel could not contain the discharge, resulting in flooding.The distinct bright blue waterline near the channel's mouth, denoted as an indicator between J109 and J110 within channel C134, unequivocally confirms that the channel experienced an overflow event.Figure4shows the effects of dimensional changes on channels C34 (Jalan Cempaka Kuning) and C64 (Jalan Anggrek Vanda).Notably, these modifications prevented the channels from overflowing, as evidenced by the blue line (indicating water pressure) and the green line (representing the deepest crosssection) remaining below the current elevation line.

Figure 4 :
Figure 4: Profile Cross-Section Cutaway of the Water Level after Dimensional Changes Based on the results of evaluating drainage channels at the study location of Lowokwaru Sub-District, City of Malang, by using hydrological calculations and modeling using Storm Water Management Model (SWMM 5.1) software and based on data of field observations, the following analysis results were obtained:

Table 1 .
Calculation of Mean Regional Rainfall

Table 2 .
Calculations of Frequency Analysis with the Gumbel and Log Pearson III Methods

Table 3 .
Summary of Calculations for the Kolmogorov-Smirnov Test and Chi-Squared Test

Table 4 .
Calculation of Hourly Rainfall Intensity with a 5-year Return Period

Table 5 .
Determining Return Periods Based on Urban Typology

Table 6 .
Simulation Results of Overflowing Channels with a 5-Year Return Period

Table 7 .
Comparison of Channel Discharge with Design Discharge after Dimensional Changes with a 5-Year Return Period

Table 8 .
Summary of New Dimensions for Drainage Channels Channels C157, C158, C159, and C160 on Jalan Sarangan are to be increased in dimension to a channel width of 1.2 m. 10-