Flood Analysis on Lake Tempe, South Sulawesi

Tempe Lake is one of the largest lakes in Wajo, Sopen, and Siddenreng Rappang in southern Sulawesi. Lake Tempe is the reservoir of his five major rivers: Bilokka, Lidaratangu, Masaka, Siddenreng, Bira and Walane. The lake offers many benefits to the community such as securing water resources, flood control, agriculture, fishing, and tourism. However, approximately 20–40 cm of silt is produced annually. This can cause flooding in the surrounding areas. Therefore, the restoration of Lake Tempe is necessary to mitigate flooding caused by continuous sedimentation. The Department of Public Works and Housing has developed a phased dredging plan to improve the capacity of Lake Tempe. Later, as no disposal sites were available, dredged materials were used to build several artificial islands along the perimeter of the lake. After the reconstruction project was completed, two major floods occurred, overtopping embankments. Therefore, the design analysis of embankments is required. Analysis indicates that two flood events occur, with a 20-year return period flood and a 50-year return period flood at elevations of 8.36 m and 9.2 m. From hydraulic analysis it is found that flood embankment height of 10 m is overtopped by water level of total 9.56 m and 10.4 m respectively because of wave run up. For a 20-year return period, there is an indication that slope stability and settlement problems affect flood events. It can be concluded that flood problems occur because of extreme events and the failure of flood embankments due to overtopping.


Introduction
Lake Tempe straddles three provinces in South Sulawesi: Wajo, Siddenreng Rappang, and Soppeng.The water level of Lake Tempe is affected by a reduction in its storage capacity due to subsidence [1] [2].The erosion potential in the Lake Tempe watershed is equivalent to 600,000 m 3 /year, or a sediment thickness of 0.4 cm/year [3].This sediment causes silting, which prevents Lake Tempe from accepting incoming water during each rainy season.The water that Lake Tempe can no longer hold can eventually overflow and reach the settlements and land of residents around Lake Tempe, causing harm to those around them.
During the rainy season, the water level of Lake Tempe rises rapidly [4] [5] because of the large water supply and the lack of capacity.Because of this issue, the Ministry of Public Works and the Ministry of Public Housing are implementing restoration activities for 15 key lakes, which are national priorities, based on the National Lake Council of Indonesia, including Lake Tempe.Lake revitalization aims to restore the lake's natural function as a reservoir by dredging sediment, washing water hyacinth/water hyacinth, and creating dikes, including the structuring of the basin [6][7].Embankment construction as part of flood prevention activities for Lake Tempe.For Lake Tempe, the Ministry of Public Works and Housing developed a phased dredging plan to improve its capacity of Lake Tempe.Later, as no disposal site was available, dredged material was used to construct several artificial islands along the perimeter of the lake, and flood dikes were constructed around these islands to prevent flooding of those artificial islands.prevented.However, in May 2018 and May 2019, two recorded flood events occurred on an artificial island and a flood embankment collapsed.Therefore, it is necessary to analyze the water level of Lake Tempe, which has affected the flood levee and the stability of the flood levee due to rising water levels in Lake Tempe.This study analyzes both the hydrological and hydraulic parts of the flood to determine the cause of flooding because of the settlement on flood embankments or pure extreme conditions of the flood.

Study Location 2.1. Study Locations
The study was conducted in Lake Tempe which is one of the large lakes located in South Sulawesi Province, precisely in Wajo Regency (70% of the lake area is in this Regency), Sindenreng Rappang Regency and Soppeng Regency.The lake crosses 10 subdistricts and 51 villages.Geographically, Lake Tempe is located at 119°50'00" East Longitude -120°5'00" East Longitude and 4°00'0" -4°10'00" South Latitude [8].Based on geological characteristics, Tempe Lake is located above the continental plates of Australia and Asia and is one of the tectonic lakes in Indonesia.The river leading to the lake consists of 23 rivers, which are included in the Bila and Walanae watersheds [9].The study was conducted in Lake Tempe, one of the large lakes located in South Sulawesi Province, to be precise in Wajo (70% of the lake area is in this province), Sindenreng Rappang, and Soppeng. .The lake spans 10 subdistricts and 51 villages.Geographically, Lake Tempe is located at 119°50'00" -120°5'00" E and 4°00'0" -4°10'00" S [8].Based on its geological features, Lake Tempe is located on the continental plate of Australia and Asia and is a tectonic lake in Indonesia.The rivers leading to the lake consist of 23 rivers and are included in the Bira and Waralae Basins [9].

Figure 1. Study location schematics
For artificial islands, there are three islands inside Lake Tempe of almost the same size.These islands were created from sediment deposits excavated from Lake Tempe.For a better understanding, this is referred to as visualization.

Methods
Two major analyses were performed in this study: hydrological and hydraulic.A hydrologic analysis routes the reservoir to determine the elevation of Lake Tempe for 20-and 50-year return periods.The study also determined the initial design of an embankment that could withstand a 20-year flood.In hydraulic analysis, Water Velocity and Flood mapping analyses are performed to model the flooding incident and determine the cause of flooding.This analysis was performed to better understand what happens and to provide recommendations for the future.If there are other causes of flooding, such as geotechnical problems, this study will inspect it using slope stability analysis using the Geo5 application.For Hydrological analysis, this study used HEC-HMS modelling to perform reservoir routing at Lake Tempe.These analyses must be performed to determine the exact water level at Lake Tempe during overtopping and flooding events.Hydraulic modelling using HECRAS flood mapping.This analysis was performed to determine the flood elevation and areas impacted by the flood.HECRAS modelling using unsteady flow with hydrograph results from HEC-HMS to analyze the overtopping [11] [12].In hydrological analysis, the step is model calibration and validation to ensure that the model is correct and represents the actual result.

Hydrologic Analysis
As mentioned in the Methods section, this study was divided into two parts: hydrological analysis and hydraulic analysis.The first part was a hydrological analysis to determine the water level of Lake Tempe at 20-year and 50-year intervals.To determine the goal of finding water level, this study modeled the Lake Tempe watershed hydrologically.Determining the correct water level for the return period requires tuning and validating the hydrological model to ensure that the water level is correct.[13].The first step in hydrological analysis is to calibrate the hydrological model.In this step, there are two flood events that he used, May 2002 and June 2019.The calibration results from the modelling process are as follows:  As can be seen in Figures 3 and 4, the results of model calibration using two flood events in May 2002 and June 2019.The red line shows the modelling results, and the black line shows the observed elevation.From Figure 3, we can see that the model results were excellent and that the difference between the observed and modelled results was small.The highest modelled pool elevation was 10.4m, while the observed pool elevation was 10.15m, with a 2.4% difference between the model and observed pool elevation.Figure 4 also shows a great result, the modelling peak pool has an elevation of 8.36 m, while the observation pool has an elevation of 8.63 m, a 3.1% difference in elevation between the model and observation pools.On average, the difference in elevation between the model and observation pool was 2.75%, suggesting that the model was already well adjusted.The next step was the validation of the model.This step is done to ensure that the calibrated model is validated and can be said to be correct.Another flood event that occurred in May 2018 was used for the validation process, which was the current highest flood that occurred after the dredging and reclamation processes.Here in model verification result shown in Figure 5. Figure 5 shows that the validation process yielded excellent results.The modelling peak pool elevation is 9.12 m, while the observation pool elevation is 9.2 m, giving a 0.8% difference between the model and observation pool elevations.The results show that the model has already been calibrated and validated and can be used to detect 20-and 50-year floods.To determine the 20-year and 50-year flows, we assumed the frequency analysis results of the rainfall post used in the modelling.The frequency analysis results are shown in Figure 6 and 7. Therefore, if the rainfall post data for the real event are missing, the frequency analysis rainfall will replace the data during the calibration and validation processes.Thus, the June 2019 flood event represents a 20-year return period, and May 2018 represents a 50-year return period.Therefore, we can conclude that 8.36 m equals the elevation of the 20-year flood pool and 9.12 m equals the elevation of the 50-year flood pool.

Hydraulic Analysis
The results of the Hydrological analysis were used for hydraulic analysis.A flood hydrograph was used to produce a flood map of the reclaimed island.Here, represents the result of the flood map.As shown in Figure 8, one of the reclaimed islands was flooded.From the water surface elevation, it's the same as expected from Hydrological analysis 8.36 m for 20-year return period and 9.12 for 50-year return period, but this elevation not accounting the wave run up.As shown in Figure 9, the graph of with the fetch length F related to the investigated point on the reservoir bank was initially considered as the longest straight-line distance from the opposite bank of the water surface in the direction of the wind [14], the wave run-up height is approximately 1.2 m which resulting in total water elevation of 9.56 m and 10.4 m respectively.
If we look closer to total water surface elevation is 9.56 m and 10.4 m respectively is still lower from flood embankment elevation of 10 m. this is make no sense because from real life accident 20-year return period flood makes flooding problem in reclamation island.This indicates that there is another possible cause of island flooding.A possible reason for the embankment and the entire island made of soil products of sediment dredging of Lake Tempe is that there are major possibilities that the embankment is failing due to settlement and the slope is failure; therefore, the next step is to check the slope with the flood water level effect on the flood embankment.The results are shown in Figure 10.As shown in Figure 10, the slope safety factor was only 0.847.This is lower than the safety standard of 1.25.For slope stability analysis 1.25 is the bare minimum of the safety factor lower than that there is huge potential for sliding [16].This result indicates that the original design of the flood embankment is not sufficiently safe to lean towards failure.So, it can be concluded that flood happens because of extreme rainfall which resulting of high-water surface elevation makes overtopping of the flood embankment and there is problem of slope stability that the original design stability is inadequate making the water at normal flood able to cause flood on the island.

Conclusions
Several conclusions can be drawn based on this analysis.First, 20-year return period and 50-year return period flood causing Lake Tempe elevation rising to 8.36 m and 9.12 m respectively.Secondly, due to wave run up of 1.2 m, Lake Tempe total water elevation of 9.56 m and 10.4 m respectively which make overtopping incident on flood embankment on 50-year return period flood.For a 20-year return period, failure due to flood embankment failure can be seen from its slope stability of 0.847, which indicates that slope failure causes flood embankment failure and flooding on the island.

Figure 6 .Figure 7 . 6 From
Figure 6.Comparison between actual rainfall and 20-year return period rainfall

Figure 8 .
Figure 8. Flood map on Lake Tempe, South Sulawesi