The Implementation Strategy of Flood Control Program Based on Multi Criteria Assessment in Pangkalpinang City, Bangka Belitung Islands Province

Flood event is occurred almost every year in Pangkalpinang City with the most devastating flood event was occurred in 2016 historically. Irrespective of being caused by extreme rainfall and high tides, the flooding occurred due to a decrease in the hydrological condition of the watershed due to illegal tin mining activities in the upstream area. This paper discusses the flood control strategies in Pangkalpinang City which is based on Multi Criteria Assessment (MCA). The MCA considering the technical aspects (i.e. hydraulic model) and non-technical aspects (i.e. risk assessment, economic, social, and environmental aspects) to prioritize the infrastructure conbstruction. Therefore, some areas are already identified as the focal problems that need to be applied by the concept of integrated flood control program. At the moment in 2023, the first construction priority is in PDAM-Jelana Detention Pond Flow Diversion from Pedindang River in the upstream area. For the future plan, it is planned to carry out the flood control construction according to the priority analysis namely Perak Retention Pond, River Training, Rangkui Tidal Gates, Pasir Padi Sea Wall and New-Tin Detention Pond.


Introduction
Bangka Belitung Islands Province is in the western part of Indonesia archipelago country.Pangkalpinang is the capital city of Bangka Belitung Islands Province which has the highest density of population around 1850 person per km 2 [1].The city has delta characteristic which is in the downstream area has the tidal fluctuation affect interactively (see Figure 1).In addition, Pangkalpinang City is also under development of the National Tourism Strategic Area Program in Indonesia [2].The city area is around 118,4 km 2 and the total population in 2022 is about 233,272 [1].Tin mining in Bangka Belitung has been exploited for hundred years ago.In addition, the province is the second largest tin producer in the world after China [3].Tin mining activity increases the wealth community.On the other hands, conventional and unconventional of tin mining activity yields mainly to the disadvantages impact such as decreasing of environmental stability (Figure 2) and leading to the flood event which is lowering the capacity of the river and the main drainage channel because of the acceleration of sedimentation rate [4,5,6].Another interesting point of mining issue is that the former of mining area above one hectare can be utilized as the artificial water pond.Kolong is as the local notation of artificial water pond in Bangka Belitung Islands province.These Kolong can be utilized for flood reduction and their connectivity can be optimized by having optimum water volume.Figure 3 shows the connectivity between two Kolong; Kolong Perumnas and Kolong Kacang Pedang.These two Kolong are located in the upstream area of Pangkalpinang city.At the moment, these Kolong are utilized for flood reduction purposes.Kolong Perumnas has area around 13 hectares with the depth average is around 2 meters.Therefore, the potential volume of water in this Kolong is around 260,000 m 3 .Kolong Kacang Pedang area is approximately four times of Kolong Perumnas, which is around 52 hectares, and the depth average is around 1 to 3 meters.Thus, the water volume in this Kolong can be restored around 1,200,000 m 3 .Flood event is occurred almost every year in Pangkalpinang city with the most devastating flood event was occurred in 2016 historically.The 2016 flood had similar phenomenon to 1986 but with a larger magnitude.On February 8, it rained evenly in Pangkalpinang City with a depth of 114.9 mm and continued February 9 at 183.9 mm (BMKG Pangkalpinang City).On the other hand, high tides occur at the same time, resulting in a backwater phenomenon in the river which causes the flow from upstream to be blocked, so that the water level in the river increases.The identification of flood event is carried out by previous studies, site investigation, and improving improved flood analysis.Pangkalpinang city has many causes' factors for leads to flooding flood event not withstanding heavy rainfall by global climate change and upstream land degradation by mining.The city has flat and low ground compared to the sea level which make the city vulnerable to the tidal flood.At the same time, the city is densely urbanized so that it has much unpaved area which lets the city easily inundated by rapid collecting rainwater.This finding will make good countermeasures development.This paper discusses the implementation strategy based on the consideration of technical and nontechnical aspects, which is called as the MCA criterion, leading to the physical construction plan eventually and showing current status physical construction related with future plans.The constraints of Multi Criteria Assessment in this study is only by considering the technical (i.e.hydraulic model simulation) and non-technical criterion (i.e.spatial risk analysis).

Methods
We tried to reconstruct the 2016 February 8-9 flood event in Pangkalpinang City using Rain on Grid model, utilizing JAXA satellite rain data and tidal data from PPN Sungailiat (Figure 5).Then the modeling results were verified with recorded video taken by a drone on February 9, 2016.In order to identify the spatial risk analysis, we have to combine the analysis between Hazard Component and Vulnerability Component quantitatively [7,8,9,10].
For Two-Dimensional Simulation, Rain-on-Grid module in HEC-RAS is applied.Basically, two important input data is Rainfall and Terrain topographical information.Terrain topographical information is sophiscatedly produced using the RAS Mapper function in HEC-RAS.The resultant DEM is DEM (Digital Elevation Model) officially provided RBI (Rupa Bumi Indonesia) is collected and we call it DEM1.DEM1 itself has resolution of 8 m × 8 m, so its geometric information about rivers is very poor.Therefore, river survey is carried out to know river's geometry.The surveyed river geometry is converted to DEM.We call it DEM2 to distinguish with DEM1.Thus, DEM1 represents flood plains area (more macro area) and DEM2 represents river corrider area (narrow, long and micro area).After that, we combine DEM1 and DEM2 to make DEM_combine for Inundation Analysis.In the RAS Mapper, we can also make TwoDimensional calculation Grid where the shallow water equation is solved by Finite Volume Method.This analysis is done on Baturusa basin which includes Rangkui river, Pedindang river, etc. in Pangkalpinang city.

The Modelling of Hydrology-Tidal Interaction Approach
Based on Figure 5, the rain occurred quite heavily starting on the 7th and fluctuated increasing until 9 February 2016.The peak of the rain occurred on 8 February.The peak of the tide occurs in the morning so that its gathering with concentrated runoff from upstream causes backwater to occur.Based on the Rain on Grid simulation (Figure 8), it is found that the inundated area in Pangkalpinang City on 9 February 2016 was approximately 1800 hectare.These inundated areas were due to the overflow of all the rivers in Pangkalpinang City and the drainage channels in several low-lying areas in the center of the city.To see in more detail, Figure 9 explains the results of HEC-RAS modeling on the Rangkui River using unsteady flow simulation.For the Manning coefficient, calibration has been carried out by comparing the water level modeling results with field observation points which are visible in the form of red dots in Figure 9. Based on the modeling results, it can be seen that the field observation points and the water level results from the modeling have a high level of similarity so that the model can be relied on for further analysis.

The Spatial Risk Analysis Approach
The vulnerability of delta cities to floods will increase when urbanization of coastal areas and increasing investments in cities are combined with a lack of coastal management [11].According to the spatial analysis, Tamansari area has the highest flood event numbers around 49 times in 2019 historically.Another area that has high event of flood event historically is occurred in Gerunggang area and Rangkui area with flood event 21 and 14 times respectively.The flood event numbers can be translated into the Hazard Component.Risk analysis results show that Rangkui and Taman Sari areas have the highest risk index.The analysis of risk map is logical since both areas have the strategical infrastructure and residential areas of Pangkalpinang City.This identification result is also as the consideration of decision baseline for strategical program of flood countermeasure in capital city of Bangka Belitung Islands Province.Figure 11 shows the analysis results of risk map spatially.The verification results show the similarity in the area between the model and the image captured from the recorded video (Figure 10).

Pangkalpinang City Flood Management Masterplan
Based on hydraulic, tidal and spatial risk analysis approach, a flood management master plan is determined to countermeasure floods based on their causes.To cope with floods caused by tides, it is planned to build a sea wall and tidal gates.The sea wall functions as a barrier to tides and waves entering the land, while the tidal floodgates function as a regulator of the water level in the river so that it can be controlled and not affected by the tide level.Meanwhile, to deal with flooding due to runoff, it is planned to reduce flood discharge and increase the capacity of the river conveyance.See Figure 12 for the river scheme of flood management in Pangkalpinang city.Flow reduction is implemented by reducing the amount of flood discharge using a retention pond or diverting the flow to a temporary shelter such as detention pond, which will later be returned to the river when the water level in the river has dropped.To increase the capacity of the river conveyance, implemented by the construction of levee in several river segments so that the flood water level does not overflow onto the land.See Figure 13 for the location of retention pond, see Figure 14 for the location of flow diversion and also see Table for

Construction Priority
There are several parameters to be considered in determining priority analysis, including technical aspect (flood inundation reduction), social aspect (land acquisition), environmental aspect (effects on social facilities, government, and the environment) and economic aspect (feasible or not feasible).Each aspect has the highest score of 100 and the lowest score of 0. To carry out the priority analysis, the Regulation of the Minister of Public Works and Public Housing Number 12/PRT/M/2014 is used to determine scores for each aspect (technical, social, environmental and economic).Based on the priority analysis carried out and after scoring, the result is that the most prioritized construction is flow reduction scheme (PDAM-Jelana Detention Pond flow diversion.The second priority is River Training and the third priority is Rangkui Tidal Gate and the others is for last priority.See Table 2 for the priority scale.In current conditions, the first construction priority (PDAM-Jelana Detention Pond Flow Diversion from Pedindang River) is in the construction phase and has entered the preparation stage for backpile and sheetpile installation as well as inlet construction.

Future Plan of Flood Reduction Program
For future plans, it is planned to carry out the flood control construction according to the priority analysis in Table 2, namely Perak Retention Pond, River Training, Rangkui Tidal Gates, Pasir Padi Sea Wall and New-Tin Detention Pond.With this development, it is hoped that there will be no more inundation due to runoff of the Rangkui River, Pedindang River, and other rivers and main channels that cross Pangkalpinang City.

Conclusions
Flood event is occurred almost every year in this Pangkalpinang City with the most devastating flood event is occurred in 2016 historically.Irrespective of being caused by extreme rainfall and high tides, the flooding occurred due to a decrease in the hydrological condition of the watershed due to illegal tin mining activities in the upstream area.This paper discusses the flood control strategies in Pangkalpinang City which is based on hydrological-oceanographic modeling and a spatial risk analysis approach.The hydraulic model is employed to identify the water flow behaviour by reconstructing the extreme flood event in 2016.Simulation results show that there is an indication of the bottle neck affect in these three main waters shed of Rangkui River, Pedindang River, and Lantai River.At the same time, the tidal fluctuation in the Bayur Bay is in high tide period.The maximum inundation at some points of the city is around two meter and the simulation results are validated by field observation.In the implementation strategy of flood control program, the priority scale of physical construction is already identified based on the consideration of technical, economic, social, and environmental aspects.At the moment in 2023, the first construction priority is in PDAM-Jelana Detention Pond Flow Diversion from Pedindang River.For the future plan, it is planned to carry out the flood control construction according to the priority analysis namely Perak Retention Pond, River Training, Rangkui Tidal Gates, Pasir Padi Sea Wall and New-Tin Detention Pond.

Figure 2 . 3 Figure 3 .
Figure 2. One of the mining activity spots in Bangka Island

Figure 4
shows the flood inundation depth at several points around Pangkalpinang City during the February 2016 extreme flood.There has been no official report on the February 2016 flood losses in Pangkalpinang City.

Figure 4 .
Figure 4. Flood inundation at several points in Pangkalpinang during 2016 extreme flood.

Figure 5 .
Figure 5. Rainfall on 6-10 Feb 2016 in Pangkalpinang City from JAXA Satellite and tidal data from PPN Sungailiat station

Figure 8 .
Figure 8. Map of Flood Modelling Results for Pangkalpinang City (February 2016)

Figure 10 .Figure 11 .
Figure 10.Image capture from rain on grid modelling and drone video for verification

9 Figure 13 .
Figure 13.Location Map of Perak Retention Pond

Table 1 .
Summary for Pangkalpinang City Flood Control

Table 2 .
Construction Priority for Flood Management Pangkalpinang City