FLOOD INUNDATION AND RISK ASSESSMENT
IN THE WELOLO RIVER BASIN,TIMOR-LESTE
Zeferino De Fatima Tilman

Supervisor: Professor OHARA Miho 
Dr. NAITO Kensuke
Professor KATAYAMA Koji

MEE22718

ABSTRACTThe main objective of this study was to propose and assess the effectiveness of flood control measures,to minimize the impact of floods in the Welolo River basin using the Rainfall-Runoff-Inundation model.
The Welolo River is the second-largest river in the Viqueque municipality, Timor-Leste, and its
catchment has been suffering from frequent floods due to a tropical cyclone event of 2021. In this study,
flood inundation scenarios were simulated with and without considering the influence ofcountermeasures. The total damage was estimated through analyzing the targeted 100-year flood return
periods, and scenarios with and without the proposed countermeasure conditions were compared. The
results showed that the levee was the most effective structural flood countermeasure.
Keywords: Welolo river basin, RRI Model, Flood inundation, Flood risk management
INTRODUCTION
Timor-Leste is a mountainous Southeast Asian Island country that lies at the eastern end of theIndonesian archipelago, 400 km north of Australia, across the Timor Sea, and on the Lesser Sunda
Islands. The total area is 14,874 km2, and the population (based on the 2022 census) is approximately
1.34 million. Administratively, the island comprises 13 municipalities (Figure 1.1). Geographically, thecountry is vulnerable to several kinds of natural hazards, and floods are the most common natural hazard
among all kinds of hazards, caused by a combination of heavy rainfall, steep topography, and widespread
deforestation. The impact of natural hazards in this country is grave, including losses of human lives
and properties. Most recently, the heavy rainfall resulting from Tropical Cyclone Seroja in April 2021
caused large-scale landslides and floods, with a total of 25,022 households affected by the floods acrossall 13 municipalities. In Dili municipality alone, 82%, or 11,558, households were affected, and 46
fatalities were recorded (UNICEF Timor-Leste Floods Response, 2021). Therefore, risk assessments
and appropriate countermeasures are important for effective flood mitigation and risk management.
The main objective of the research is to propose and assess the effectiveness of countermeasures toreduce flood risk. This research encompasses a comprehensive approach to flood management in theWelolo River basin such as Developing a rainfall-runoff-inundation (RRI) model simulation to facilitate
effective decision-making for flood mitigation and management in the basin, conducting acomprehensive assessment of the consequences resulting from flooding on households and agriculturalland, and propose countermeasures, such as the implementation of levee construction and riverbed
dredging, as potential strategies to mitigate flood impacts.

1. Assistant Officer of GIS, Institute of Petroleum and Geology (IPG.IP) Timor-Leste.
2. Professor, ICHARM, PWRI/GRIPS, University of Tokyo, Japan
3. Research Specialist, ICHARM at PWRI, Japan
4. Professor, GRIPS, Tokyo, Japan1

Figure 1 Location of Welolo River Basin (WRB)
STUDY AREA
The Welolo River basin is located in the southeastern part of Timor-Leste, and the low-lying area
includes three sub-districts such as Viqueque, Ossu, and Lacluta. This basin is situated close to Buicaren
and Bibileo villages in Viqueque sub-district. The WRB extends over 242.060 km2 located between
latitude 8° 49' 0.346" South and Longitude 126° 16' 46.237" East. There are two rain gauge stations
close to the catchment: Lacluta and Ossu Stations. The Welolo River basin receives an annual
precipitation of approximately 2500 mm. During the flood event in April 2021, the catchmentencountered a critical situation characterized by a peak flow rate of approximately 1600 m3/s. In thestudy area, agriculture plays a central role as the primary source of income. Around 80% of thepopulation in Buicaren village depend on agriculture to sustain their livelihoods. The primary focus ofagricultural activities in this area centers around the cultivation of paddy, which serves as a staple crop.
METHODOLOGY
Based on the analysis of flood inundation in the Welolo River basin, I used topographic data such as thedigital elevation model (DEM), flow direction (DIR), flow accumulation (ACC), land cover, soil data,building footprint and observed rainfall data from two stations. Furthermore, rainfall data andtopographic data such as the digital elevation model (DEM), flow accumulation (ACC), flow direction,
land cover (DIR), and soil data are used as inputs to the rainfall-runoff-inundation (RRI) modelsimulation.
In this study, the Rainfall-Runoff-Inundation (RRI) model is employed to simulate both the depth anddistribution of inundation. Using The RRI model in this research is vital to comprehensively address thedynamics of flood occurrences within the Welolo River basin. For the process of model calibration, thesimulation result by the RRI model is compared with the actual inundation map of the Welolo Riverbasin, this approach is taken due to the absence of available discharge data. This process of model
calibration ensures that the RRI model effectively meets the agreement between the simulation result2and the actual inundation map. Frequency analysis is conducted with the ground-based observed rainfall
data of 10 years from 2012 - 2022 by Gumbel and GEV analysis. The results predicted rainfall of the25, 50, 100- year return periods are obtained. To obtain the damage assessment and flood risk maps, theRRI model's output for the 2021 flood, along with building footprint and land cover data, is imported
into ArcGIS software to estimate the affected household and agriculture land under different depths ofinundation for 2021 flood event. Similarly, RRI simulation results for 100-year return period, along withbuilding footprint and land cover data are added to ArcGIS software to estimate the inundation ofhousehold and agricultural land under different inundation depths for 100-year return periods flood.

Figure 2 Schematic diagram of the study approach
DATA
In this study, the rainfall data were obtained from the National Authority of Water and Sanitation (Figure3 (a). The topographic data, such as the 80-meter resolution data of the Digital Elevation Model (DEM),
Land cover, and soil data, were provided by the Institute of Petroleum and Geology, as shown in Figures
3 (b) and 3 (c). In addition, the building footprint data was downloaded from the Open Street Mapwebsite.

acb

Figure 3 (a) Meteorological station and river network, 3 (b) digital elevation model (DEM), and 3(c)land cover map.
3RESULTS AND DISCUSSION
In this research study, the process of calibrating and validating the model's simulations is conducted bycomparing the simulated results with the actual flood inundation maps. This approach is taken due tothe absence of available discharge data. Furthermore, the development of the actual inundation map waspredicated on a field survey undertaken during April 2021 flood event in the Welolo River basin. For
the calibration process, The RRI input parameters had been modified as a result of the adjustments madefollowing the 2021 flood event. Rainfall data observed at two stations from 2012 to 2022 was taken forfrequency analysis by Gumbel and GEV methods. The catchment's average yearly maximum rainfall of
10 years was considered for the frequency analysis. The projected rainfall for a 100-year return period
was estimated at 430.4 mm. I chose to use the GEV Method for a two-day probability scenario with aconversion factor of 1.6, aiming to create a custom hyetograph that shows a 100-year return period
event. The inundation analysis In Figures 4 (a) and 4 (b), respectively demonstrated that the proposed
countermeasure, involving levee construction, considerable reduced flood inundation in the entire study
area.

abFigure 4 (a) 100-year return period without countermeasure and 4 (b) 100-year return period withcountermeasure.
Table 1 shows the inundation area and affected households for 100 years return period without theproposed countermeasure.
Inundation Depth inInundation Area in
Affected
Affected(m)(ha)
householdPopulation
0.4 - 1382
124620
1 - 1.5
239
297
1485
1.5 - 2
122
114
570
>2
100
100500Total Inundated
842
635
3.177
Total Overall
1225
1288

4

Table 2 shows the inundation area of paddy fields and cropland for 100 years return period without theproposed countermeasure.
Inundation Depth
0.25 - 0.50
0.50 - 0.75> 0.75
Total Inundated Area
Total Area off Paddy and CroplandPaddy field in (ha)
15.95
20.73
48.96
85.64
234.02

Cropland
(ha)
28.52
21.06
57.56
107.14354.47

The total damage of households, paddy fields, and croplands for 100-year return period floods ispresented in Table 1 the comprehensive assessment of the cumulative impact, revealing a total inundated
area of 842 hectares, with 635 households affected and an approximate population of 3,177 people
adversely affected. Moreover, paddy fields experienced total damage of 85.64 hectares, and cropland
was impacted over an area of approximately 107 hectares, as detailed in Table 2.
Table 3 shows the inundation area and affected households for 100 years return period with the proposed
countermeasure.
Inundation Depth in(m)0.4 - 1
1 - 1.5
1.5 - 2
>2
Total Inundated
Total Overall

Inundation Area in(ha)923900531225

Affectedhousehold16800201288

Affected
Population
80
4000120

Table 4 shows the inundation area of paddy fields and cropland for 100 years return period with proposed
countermeasure
Inundation Depth0.25 - 0.50
0.50 - 0.75
> 0.75
Total Inundated Area
Total Area off Paddy andCroplandPaddy field in (ha)
2.73
3.43
6.16
21.32

Crop land (ha)
14.54
18.28
32.64
65.46

234.02354.47

The analysis presented in Tables 3 and 4 illustrates the effects of 100-year return period floods andproposes countermeasures. The result reveals notable reductions in inundated areas, contracting from842 ha to 53 ha; affected paddy fields experience a reduction from 85.64 hectares to 21.32 ha; andimpacted cropland diminishes from 107.14 he to 65.46 ha.
CONCLUSIONS
Flood inundation maps with historical data were developed for flood risk management, preparedness,and mitigation against the design return period of flood events. Using simulation results from the RRI
model simulation, the effectiveness of the proposed countermeasures was looked at in terms of the areaof flooding and the depth of flooding. The simulation of a 100-year return period of flood as per the2021 flood pattern was conducted with the levee construction condition, and the total damage is5

estimated using the same assumption and information used for the non-levee condition. It is observedthat the total damage is estimated to reduce by almost 85% by constructing levees.
RECOMMENDATION
The levee construction is strongly recommended as a fundamental measure for enhancing flood
protection and flood management. Needed a discharge station in the downstream area for effectively
monitoring and measuring river discharge, and needed to Perform a comprehensive detailed survey
within the designated survey area to verify and confirm the obtained results.
ACKNOWLEDGMENT
I wish to extend my heartfelt gratitude to my esteemed supervisors, Professor Ohara Miho and Dr.
Kensuke Naito, for generously dedicating their time and offering invaluable advice, unwavering support,and continuous encouragement. Your insightful discussion and guidance have significantly contributed
to the improvement of my thesis. Furthermore, I would like to express my sincere thanks to Dr.
Kakinuma and Dr. Shrestha for their kind support and invaluable guidance throughout this thesis. Yourassistance has been instrumental in shaping the outcomes of my research, and I am truly grateful foryour contributions. I am greatly indebted to my friend and colleague who were generous with theiradvice during my study period. Social thanks Institution of Petroleum and Geology for providing mewith an opportunity to practice in this course. I am great full to the International Centre for Water Hazardand Risk Management (ICHARM), Public Work Research Institute (PWRI), National Graduate Institutefor Policy Studie (GRIPS), and Japan International Cooperation Agency (JICA) for giving me theopportunity to participate in this program and supporting financially through my study period in Japan.
REFERENCES
Tin Myint Aung, September 2013, Flood Inundation Analysis for Effective Countermeasures in theBago River Basin.
United Nations Office for Disaster Risk Reduction (UNISDR), Database Disaster Information
ManagementSystem(Desinventar),
Timor-LestePeriod1992to2017
http://www.desinventar.net/DesInventar/index.jsp
Takahiro SAYAMA, 2016, Rainfall-Runoff-Inundation (RRI) Model Ver.1.4.2 User's Manual“International Center for Water Hazard and Risk Management (ICHARM), Public Works ResearchInstitute (PWRI), Disaster Prevention Research Institute (DPRI), Kyoto University
E.J. Gumble, Juli/August, Probability-interpretation of the observed return-periods of floods,
https://doi.org/10.1029/TR022i003p00836
Aye Aye Nainhg, August 2015, Flood Inundation Analysis by Using RRI Model for Chindwin RiverBasin, Myanmar.
Akhlaque Ahmad Siddiki, September 2013, Flood Inundation Analysis and Risk Assessment of Bagmati
River Basin, Nepal
UNDP , 1994 , Vulnerability and risk Assessment , 2nd Edition, Cambridge , United Kingdom UN-ISDR,(s.d) International Strategy for Disaster Reduction-Terminology of disaster risk reduction,
http://www.unisdr.org/eng/library/lib-terminology-eng%20home.htm,
University,SOURCE
No.4/2006; pp . 8-14, 48-50
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