Paper 3-4-3
BEST PRACTICE GUIDELINES FOR FLOOD RISK ASSESSMENT IN THE
LOWER MEKONG RIVER BASIN
GERT SLUIMER 1 , HENK OGINK 2 , FERDINAND DIERMANSE 2 , FRANK KEUKELAAR 1 , BAS JONKMAN 1
, TRAN KIM THANH 3
AND TES SOPHARITH 4
1
Royal Haskoning, Nijmegen, the Netherlands
2
Deltares, Delft, the Netherlands
3
VinaMekong Engineering Consultants, Ho Chi Minh City, Viet Nam
4
Consultant, Phnom Penh, Cambodia
ABSTRACT
As part of Component 2 of the Flood Mitigation and Management Program (FMMP) of the Mekong River Commission (MRC) guidelines for flood risk assessment have been developed. This paper summarizes the contents of these best practice guidelines for flood risk assessment and presents examples of results of flood risk assessments (FRA) for a case study area in the Lower Mekong River Basin. Within the FRA there needs to be insight in the flood hazards (flood levels and their probability of occurrence), the flood damages and both need to be combined to estimate the level of risk. A sound knowledge of the flood hazards, potential damages and risks is fundamental for the development of a balanced mix of measures aiming at the reduction of the risks.
INTRODUCTION
Component 2 of the Mekong River Commission (MRC) Flood Mitigation and Management Program (FMMP) aims, among others, at the development of guidelines for the preparation of flood risk management plans and for the evaluation of the impacts of flood risk management measures. A good knowledge of the flood hazards, potential damages and risks is fundamental for the development of a balanced mix of measures aiming at the reduction of the risks. The paper presents the best practice guidelines of flood risk assessment as developed in the FMMP-C2 for application in the MRC Member States for the Lower Mekong River Basin (LMB). The guidelines are intended for policy makers, basin planners, practitioners involved in flood risk management related projects and those active in dialogues regarding trans-boundary impacts of projects. The best practice guideline was prepared on the basis of a review of other guidelines and best practice documents in the field of flood risk management (e.g. the EU Floodsite and Excimap projects) and existing guidelines and flood risk assessments for the LMB countries. The objective of this paper is to summarize the main contents of the guidelines and show examples of the application of flood risk assessment for case study areas in the Lower Mekong Basin.
Integrated flood risk management (IFRM) is an approach to identify, analyse, evaluate, control and manage the flood risks in a given system. Flood risk assessment (FRA) is part of IFRM and involves the identification of hazards, damages and risks for a given system or area. As such, FRA provides a basis for evaluation of the acceptability of a risk level and the analysis of the effects of risk reduction measures. The general steps in flood risk assessment are shown
in Figure 1 and discussed below. The first step is the definition of the system and area studied and the collection of relevant data for this area (e.g. data basic regarding elevation of the terrain and the hydraulic processes). Flood hazard analysis is the second step and it includes an analysis of meteorological events that may eventually lead to flooding. In connection with the characteristics of the respective watershed the hydrological hazards (peak discharges, volumes) are assessed that may create the flooding. In connection with the characteristics of river channel and floodplain the flood hazard can be determined in terms of inundation area, depth etcetera. The results can be eventually displayed by means of flood (hazard) maps. The third step is the analysis of potential damages in the areas prone to flooding based on socio-economic data and a vulnerability / damage model. In the final step the risk is determined by combining the results of the flood hazard analysis, that gives insight in the probability of a certain hazard, and the results of the damage assessment. These results can be displayed in different forms, e.g. by means of risk maps, graphs or risk numbers that give insight in the yearly expected damage.
Flood hazard maps
System definition and collection of basic data
Flood damage maps Flood hazard analysis Vulnerability and damage assessment Risk determination and flood risk
mapping
Figure 1. General scheme for flood risk assessment
The following sections describe the contents of flood hazard analysis, flood damage assessment and risk assessment. To illustrate the potential outputs of the steps examples are included from the results of assessments that were made for the so-called focal areas as part of stage 1 of the FMMP project (Royal Haskoning et al., 2008). Analyses were made for areas in Thailand, Lao PDR, Vietnam and Cambodia and in this article examples of the results for the latter country are presented. Section 5 summarizes concluding remarks regarding the guidelines, their application and their further development.
FLOOD HAZARD ASSESSMENT
In general the flood hazard assessment starts with an initial assessment of flood types in the area. Within the LMB several flood types can occur including: tributary floods, main stream floods, combined floods, floods in Cambodian flood plain, and floods in the Mekong delta. To develop an appropriate assessment of flood hazards the following steps have to be followed.
Data collection and analysis. This includes data gathering, data storage and validation
(checking) of the data. Relevant data sources include: soil, land use and elevation maps, meteorological data (rainfall, evaporation), hydrological data (water levels, discharges), hydraulic infrastructure, river geometry (cross sections) and historical flood data (flood marks). Data storage should preferably be done in a GIS (geographic information system) environment. An important step that is often forgotten is data validation. The guidelines describe how the steps can be filled in for various situations that vary regarding the type of flooding (e.g. tributary floods, delta floods etc.) and the available data sources (e.g. rainfall data, observed flows, regional flood statistics).
Hydraulic modelling. In general hydraulic data will only be available for a limited
number of points/locations in an area. A hydraulic model is needed to assess data on the flooding, e.g. the water depth, for a larger area, and it uses physical laws and area-specific input information to do so. The guidelines describe which kind of modelling approaches can be chosen for several flood types and geographic situations. For example, for the Mekong delta the hydrodynamic ISIS software has been used to simulate the flows and water depths for several locations in the delta. A hydraulic model is also useful to evaluate the effects of measures on the hydraulics and flood depths in the area.
Flood frequency analysis involves the calculation of the frequency or probability of a
flood with a certain magnitude. Information on flooding probabilities is needed to assess the level of risk. Depending on the type of flood considered, the available time series of data, the flood frequency analysis can be done in different ways. It can be based on observed flows, or local rainfall statistics in combination with a runoff model. The guidelines describe several techniques and indicate for which situations these can be utilized.
Flood hazard mapping. The results of flood hazard assessments for different return
periods can be translated to flood maps. It is therefore needed that the model results for water levels at several (point) locations are included in a georeferenced data table. This table can be attached to an elevation map of the area. By combining information on the water levels and the terrain elevation and using interpolation techniques flood maps can be created. These show the extent of the flooded area and the local flood depth. Figure 2 shows an example of a flood depth map for the trans-boundary area of Cambodia and Vietnam.
Figure 2. Flood depth map for the trans-boundary area of Cambodia and Vietnam for the 25 year return period
FLOOD DAMAGE ASSESSMENT
The consequences of a flood encompass multiple types of damage. An overview of different types of damages / consequences is given in Table 1. The damage is divided into tangible and intangible damage, depending on whether or not the losses can be assessed in monetary values. Another distinction is made between the direct damage, caused by physical contact with floodwaters, and damage indirectly following from the flood. Indirect damages are damages caused by disruption of physical and economic linkages of the economy, and the extra costs of
emergency and other actions taken to prevent flood damage and other losses, see e.g. (Parker et
al., 1987). This includes, for example, the loss of production of companies affected by the
flooding, induced production losses of their suppliers and customers, the costs of traffic disruption or the costs of emergency services. Flooding in the Mekong basin causes not only damages, but benefits as well. These include beneficial effects for fishery, agricultural production and biodiversity. For a complete assessment these would have to be included as well. Table 1. Classification of flood damages
Tangible Intangible
Direct Physical damage to:
• housing, structure and assets • infrastructure and public utilities • agriculture
• loss of life • health effects
• environmental damages Indirect • temporary relocation
• cleaning & sanitation • loss of income
• industrial production losses • others
• societal disruption
• increased vulnerability of survivors
The general approach for the assessment of direct damage is shown in Figure 3 and to estimate the flood damage there needs to be insight in the following factors:
• the flood characteristics (depth, velocity, duration, affected area, etc.) • the number and type of land use functions affected and their value; • the susceptibility of land use functions against flooding.
Figure 3. General approach for damage estimation
Generally, a so-called (stage) damage function is used to relate the level of damage to the flood characteristics. These functions have been developed in different countries, see e.g. (Penning-Rowsell and Chatterton, 1977; Dutta et al., 2003; Jonkman et al., 2008) for further background. Most damage functions take into account water depth as the main factor. However, for some damage types, e.g. damage to paddies, it is also important to take into account the duration of the flooding and / or the season when the flooding occurs.
For the assessment of damage for the LMB two approaches are distinguished. The first approach is the absolute approach and the damage functions describe the relationship between
the water level and the absolute damage value for a larger area, e.g. a district. The relationship between water level and damage to individual objects is not known. This approach is particularly applicable when damage data is only available for a larger area in combination with general information on historical flood levels in the river. The second approach is the relative
approach and in this approach the damage function gives the relationship between water depth
and the damage as a fraction or percentage of the maximum damage value. Relative damage functions can be derived if more detailed survey information on local water depths and damage to individual houses, or smaller units of infrastructure and agricultural land use are available. Examples of the two types of damage functions that were derived in stage 1 of the FMMP-C2 project are shown in Figure 4. Which of the two approaches is used depends on the availability of information and resources and the scope and desired level of detail of the analysis. As part of the damage assessment also a choice has to be made regarding which damage categories are included in the analysis. In general, those categories are analysed that are expected to determine the largest part of the damage. For the case study areas this concerned damage to housing, agriculture and infrastructure.
Koh Andeth: Direct & indirect damages Housing
0.00 0.01 0.02 0.03 0.04 3.0 3.5 4.0 4.5
max water level at Borey Cholsar (M+MSL)
Da m a g e ( M ill io n US D) damage fit
House damage curve - Koh Andet District (Takeo)
y = 0.0351e0.9444x R2 = 0.9881 0% 10% 20% 30% 40% 50% 60% 70% 0.50 1.00 1.50 2.00 2.50 3.00 3.50
Flood water depth in yard (m)
Da m a g e s Temporary Semi-temporary Overall
Figure 4. Absolute (left) and relative (right) damage functions for housing for the area Koh Andet in Cambodia
Various approaches are available for the assessment of indirect damages. In some countries approaches have been developed that include a model of the linkages in the economic system. Within stage 1 of the FMMP project the indirect damage has been assessed by means of surveys. Consequently the indirect damages have been related to the direct damages and a ratio between the two types of damage has been derived. The guidelines further describe how the damage functions can be elaborated and further elaborate on necessary steps such as the processing of flood data, economic aspects in the analysis (e.g. conversion from current to fixed prices).
To enable consistent damage assessments, standardized computer models for damage assessment have been developed in several countries. Examples are the HAZUS model developed in the United States and the Damage and Casualties Assessment (DACA) Module developed in the Netherlands. The application of the DACA model to parts of the Lower Mekong Basin has been investigated for an area in Thailand (Leenders et al., 2009). The development of such a computer model becomes particularly attractive when a) large amounts of underlying (GIS) data is available; b) there is good information to derive damage functions and these functions are applicable to various regions; c) a large number of calculations has to be made.
The results of the damage assessment can be presented in different ways. Possible outputs are numbers, such as the total damage number or the damage by category, or damage maps that indicate the level of damage for different areas. As an example the results are presented for the Koh Andet area in Cambodia. Table 2 displays the damages for events with different probabilities. As an example of a damage map Figure 5 shows the damage to paddies by commune for the flowing with a 100 year return period. In the guidelines it is described in detail
how several types of flood damage maps can be created by combining information on the flood hazards, land use and damage functions.
Figure 5. Damage map for paddies for flooding with a 100 year return period for case study areas in Cambodia (map is based on the relative damage functions)
Table 2. Damage data for different return periods for Koh Andet district. Damage is shown in million US$
Probability (1/year) Damage
1% 5% 10% 50%
Housing 0.05 0.04 0.03 0.00
Infrastructure and relief 1.14 0.75 0.69 0.10
Agriculture 2.15 1.58 1.48 0.35
Total 3.35 2.37 2.20 0.45
In addition to the direct and indirect damages that can be expressed by means of monetary values, other damage types could be relevant for the analysis as well. The guidelines summarize existing approaches for the estimation of loss of life and health effects.
RISK ASSESSMENT
In the risk assessment the results of the hazard and damage assessments are combined. Flood risk is generally defined as the product of the probability of flooding and damage, but other ways of presenting the risk are possible. Often information is available for the economic damage for a limited number of flood scenarios that cover a range of probabilities (see table 2). The relationship between the damage value and the probability can be plotted in a graph, which is also called the probability-damage curve. Figure 6 gives an example of the probability damage curve for three case study areas in Cambodia (see Figure 5 for locations of the areas). Alternatively it is also possible to plot an equivalent relationship between the return period and the damage level, as the return period equals 1 / probability.
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0% 10% 20% 30% 40% 50% 60% 70% Probability of Execedance Da m a ge ( M illion U S D)
Koh Andaet District Koh Thum District Kampong Trabaek District
Figure 6. Probability damage curve for three Cambodian districts
The risk/expected damage can be determined by calculating the area under the curve that shows the relationship between probability of exceedance and the damage. The risk value is also referred to as the (yearly) expected value of economic damages and it has the unit of US$ per year. For the Koh Andet case study area the risk value equals US$ 0.77 million per year. The results of the risk calculations can also be visualized. Figure 7 shows the results for the flood risk calculations for the focal areas in Cambodia in Vietnam. The size of the circle shows the magnitude of the risk for the district and within the circle the distribution over the three damage categories (housing, infrastructure & relief, agriculture) is shown. It becomes clear that the risk values and the distribution of risk over the damage categories vary between districts. For Koh Andet the largest part of the risk is determined by the category agriculture.
The results of the risk assessment can be used to evaluate the effects of measures on the risk. Basically there are two types of measures, i.e. measures to reduce the probability and measures to reduce the consequences. Figure 8 shows these two types and schematically shows the effects on the probability - damage curve. Based on the results a cost benefit analysis of measures can be carried out in which the investments in measures are compared with the benefits (i.e. the reduction of the risk). A related technique is the economic optimization and it provides insight in the optimal level of risk reduction. The guidelines provide more information
on cost-benefit analysis and optimization and it is also discussed how the risk to life can be evaluated.
Figure 7. Flood risks by district for selected areas in Cambodia and Vietnam
Measures to reduce flooding probability: • strengthening of dikes and embankments • river widening (‘room for rivers’) • retention and emergency storage
Measures to reduce flooding consequences: • land use planning
• secondary flood defences
• flood proof buildings (e.g. raising)
Damage ($) Probability of exceedance (1/year) 1/100 1/10 1/5 1/2 Damage ($) Probability of exceedance (1/year) 1/100 1/10 1/5 1/2
Figure 8. Effects of measures on flood risk.
CONCLUDING REMARKS
The best practice guidelines are intended to enable a consistent and uniform approach for flood risk assessment for the Lower Mekong Basin. The guidelines describe the theory and make the connection to the practice in the LMB by including examples of results of flood hazard-, damage- and risk assessments for a case study area. These guidelines do not fully prescribe the methods and models for the steps in the risk assessment. Rather they provide the general principles and give an overview of the steps that need to be made for a complete risk
assessment. The exact elaboration and application of the risk assessment will highly depend on the local situation. In this context the type of flooding will be an important aspect as the main flood and damage characteristics differ between the types of flooding that can occur in the Lower Mekong Basin. For all steps in the risk assessment the availability of resources, the (spatial) scope of the analysis and the desired level of detail of the output and its application in risk evaluation are important factors. The exact elaboration of the steps in the risk assessment process will thus rely on the above aspects. In these guidelines first guidance is given on useful approaches for several situations.
For a further development of the guidelines and the risk assessment process in the Lower Mekong Basin it is recommended to apply the guidelines to (new) case study areas in various parts of the Lower Mekong Basin. In addition it could be investigated if approaches and models could be further harmonized for the LMB and to what extent the development of standardized and automated approaches, e.g. for damage assessment, is feasible and needed.
REFERENCES
Dutta, D., Herath S., Musiake K., 2003. A mathematical flood loss estimation, Journal of Hydrology, No. 277:pp. 24 –49.
Jonkman S.N., Bockarjova M., Kok M., Bernardini P., 2008. Integrated Hydrodynamic and Economic Modelling of Flood Damage in the Netherlands, Ecological Economics 66, pp. 77-90
Leenders J.K., Roelevink A., Parodi G., Wagemaker J., 2009a. Development and Demonstration of DACA (Damage and Casualties Assessment) in the Chiang Rai Region, Lelystad: HKV Consultants and ITC
Parker D.J., Green C.H., Thompson P.M., 1987. Urban flood protection benefits – a project appraisal
guide. Gower Technical press.
Penning-Rowsell, E.C., Chatterton, J.B., 1977. The benefits of flood alleviation – a manual of assessment techniques, Saxon House, ISBN 0566001908.
Royal Haskoning, 2008. Flood damages and flood risks in focal areas. Annex 2 to the draft stage 1 evaluation report, August 2008.
Royal Haskoning, Deltares and UNESCO-IHE, 2009. Guidelines for flood risk assessment. Draft final report prepared for the Mekong River Commission Secretariat.
