Combined fluvial and pluvial urban flood hazard analysis: method development and application to Can Tho City, Mekong Delta, Vietnam

Abstract. Many urban areas experience both fluvial and pluvial floods, because locations next to rivers are preferred settlement areas, and the predominantly sealed urban surface prevents infiltration and facilitates surface inundation. The latter problem is enhanced in cities with insufficient or non-existent sewer systems. While there are a number of approaches to analyse either fluvial or pluvial flood hazard, studies of combined fluvial and pluvial flood hazard are hardly available. Thus this study aims at the analysis of fluvial and pluvial flood hazard individually, but also at developing a method for the analysis of combined pluvial and fluvial flood hazard. This combined fluvial-pluvial flood hazard analysis is performed taking Can Tho city, the largest city in the Vietnamese part of the Mekong Delta, as example. In this tropical environment the annual monsoon triggered floods of the Mekong River can coincide with heavy local convective precipitation events causing both fluvial and pluvial flooding at the same time. Fluvial flood hazard was estimated with a copula based bivariate extreme value statistic for the gauge Kratie at the upper boundary of the Mekong Delta and a large-scale hydrodynamic model of the Mekong Delta. This provided the boundaries for 2-dimensional hydrodynamic inundation simulation for Can Tho city. Pluvial hazard was estimated by a peak-over-threshold frequency estimation based on local rain gauge data, and a stochastic rain storm generator. Inundation was simulated by a 2-dimensional hydrodynamic model implemented on a Graphical Processor Unit (GPU) for time-efficient flood propagation modelling. All hazards – fluvial, pluvial and combined – were accompanied by an uncertainty estimation considering the natural variability of the flood events. This resulted in probabilistic flood hazard maps showing the maximum inundation depths for a selected set of probabilities of occurrence, with maps showing the expectation (median) and the uncertainty by percentile maps. The results are critically discussed and ways for their usage in flood risk management are outlined.

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Combined fluvial and pluvial urban flood hazard analysis: concept development and application to Can Tho city, Mekong Delta, Vietnam

Natural Hazards and Earth System Science ◽

10.5194/nhess-16-941-2016 ◽

2016 ◽

Vol 16 (4) ◽

pp. 941-961 ◽

Cited By ~ 32

Author(s):

Heiko Apel ◽

Oriol Martínez Trepat ◽

Nguyen Nghia Hung ◽

Do Thi Chinh ◽

Bruno Merz ◽

...

Keyword(s):

Hydrodynamic Model ◽

Urban Areas ◽

Flood Hazard ◽

Hazard Analysis ◽

Mekong Delta ◽

Water Levels ◽

Convective Precipitation ◽

Processing Unit ◽

Flood Events ◽

Can Tho City

Abstract. Many urban areas experience both fluvial and pluvial floods, because locations next to rivers are preferred settlement areas and the predominantly sealed urban surface prevents infiltration and facilitates surface inundation. The latter problem is enhanced in cities with insufficient or non-existent sewer systems. While there are a number of approaches to analyse either a fluvial or pluvial flood hazard, studies of a combined fluvial and pluvial flood hazard are hardly available. Thus this study aims to analyse a fluvial and a pluvial flood hazard individually, but also to develop a method for the analysis of a combined pluvial and fluvial flood hazard. This combined fluvial–pluvial flood hazard analysis is performed taking Can Tho city, the largest city in the Vietnamese part of the Mekong Delta, as an example. In this tropical environment the annual monsoon triggered floods of the Mekong River, which can coincide with heavy local convective precipitation events, causing both fluvial and pluvial flooding at the same time. The fluvial flood hazard was estimated with a copula-based bivariate extreme value statistic for the gauge Kratie at the upper boundary of the Mekong Delta and a large-scale hydrodynamic model of the Mekong Delta. This provided the boundaries for 2-dimensional hydrodynamic inundation simulation for Can Tho city. The pluvial hazard was estimated by a peak-over-threshold frequency estimation based on local rain gauge data and a stochastic rainstorm generator. Inundation for all flood scenarios was simulated by a 2-dimensional hydrodynamic model implemented on a Graphics Processing Unit (GPU) for time-efficient flood propagation modelling. The combined fluvial–pluvial flood scenarios were derived by adding rainstorms to the fluvial flood events during the highest fluvial water levels. The probabilities of occurrence of the combined events were determined assuming independence of the two flood types and taking the seasonality and probability of coincidence into account. All hazards – fluvial, pluvial and combined – were accompanied by an uncertainty estimation taking into account the natural variability of the flood events. This resulted in probabilistic flood hazard maps showing the maximum inundation depths for a selected set of probabilities of occurrence, with maps showing the expectation (median) and the uncertainty by percentile maps. The results are critically discussed and their usage in flood risk management are outlined.

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Flood hazard in the Mekong Delta – a probabilistic, bivariate, and non-stationary analysis with a short-termed future perspective

Natural Hazards and Earth System Sciences Discussions ◽

10.5194/nhessd-1-275-2013 ◽

2013 ◽

Vol 1 (1) ◽

pp. 275-322 ◽

Cited By ~ 3

Author(s):

N. V. Dung ◽

B. Merz ◽

A. Bárdossy ◽

H. Apel

Keyword(s):

Hydrodynamic Model ◽

Large Scale ◽

Flood Hazard ◽

Hazard Analysis ◽

Mekong Delta ◽

Extreme Value ◽

Peak Discharge ◽

Extreme Value Statistics ◽

Hazard Maps ◽

Flood Volume

Abstract. In this paper we present a novel approach for flood hazard analysis of the whole Mekong Delta with a particular focus on the Vietnamese part. Based on previous studies identifying the flood regime in the Mekong delta as non-stationary (Delgado et al., 2010), we develop a non-stationary approach for flood hazard analysis. Moreover, the approach is also bi-variate, as the flood severity in the Mekong Delta is determined by both maximum discharge and flood volume, which determines the flood duration. Probabilities of occurrences of peak discharge and flood volume are estimated by a copula. The flood discharges and volumes are used to derive synthetic hydrographs, which in turn constitute the upper boundary condition for a large-scale hydrodynamic model covering the whole Mekong Delta. The hydrodynamic model transforms the hydrographs into hazard maps. In addition, we extrapolate the observed trends in flood peak and volume and their associated non-stationary extreme value distributions to the year 2030 in order to give a flood hazard estimate for the near future. The uncertainty of extreme flood events in terms of different possible combinations of peak discharge and flood volume given by the copula is considered. Also, the uncertainty in flood hydrograph shape is combined with parameter uncertainty of the hydrodynamic model in a Monte Carlo framework yielding uncertainty estimates in terms of quantile flood maps. The proposed methodology sets the frame for the development of probabilistic flood hazard maps for the entire Mekong Delta. The combination of bi-variate, non-stationary extreme value statistics with large-scale flood inundation modeling and uncertainty quantification is novel in itself. Moreover, it is in particular novel for the Mekong Delta: a region where not even a standard hazard analysis based on a univariate, stationary extreme value statistic exists.

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Towards risk-based flood management in highly productive paddy rice cultivation – concept development and application to the Mekong Delta

Natural Hazards and Earth System Science ◽

10.5194/nhess-18-2859-2018 ◽

2018 ◽

Vol 18 (11) ◽

pp. 2859-2876 ◽

Cited By ~ 4

Author(s):

Nguyen Van Khanh Triet ◽

Nguyen Viet Dung ◽

Bruno Merz ◽

Heiko Apel

Keyword(s):

Land Use ◽

Flood Risk ◽

Large Scale ◽

Flood Hazard ◽

Mekong Delta ◽

Flood Management ◽

Paddy Rice ◽

Rice Cultivation ◽

Spatially Explicit ◽

Risk Maps

Abstract. Flooding is an imminent natural hazard threatening most river deltas, e.g. the Mekong Delta. An appropriate flood management is thus required for a sustainable development of the often densely populated regions. Recently, the traditional event-based hazard control shifted towards a risk management approach in many regions, driven by intensive research leading to new legal regulation on flood management. However, a large-scale flood risk assessment does not exist for the Mekong Delta. Particularly, flood risk to paddy rice cultivation, the most important economic activity in the delta, has not been performed yet. Therefore, the present study was developed to provide the very first insight into delta-scale flood damages and risks to rice cultivation. The flood hazard was quantified by probabilistic flood hazard maps of the whole delta using a bivariate extreme value statistics, synthetic flood hydrographs, and a large-scale hydraulic model. The flood risk to paddy rice was then quantified considering cropping calendars, rice phenology, and harvest times based on a time series of enhanced vegetation index (EVI) derived from MODIS satellite data, and a published rice flood damage function. The proposed concept provided flood risk maps to paddy rice for the Mekong Delta in terms of expected annual damage. The presented concept can be used as a blueprint for regions facing similar problems due to its generic approach. Furthermore, the changes in flood risk to paddy rice caused by changes in land use currently under discussion in the Mekong Delta were estimated. Two land-use scenarios either intensifying or reducing rice cropping were considered, and the changes in risk were presented in spatially explicit flood risk maps. The basic risk maps could serve as guidance for the authorities to develop spatially explicit flood management and mitigation plans for the delta. The land-use change risk maps could further be used for adaptive risk management plans and as a basis for a cost–benefit of the discussed land-use change scenarios. Additionally, the damage and risks maps may support the recently initiated agricultural insurance programme in Vietnam.

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Assessing the impact of topography and land cover data resolutions on two-dimensional HEC-RAS hydrodynamic model simulations for urban flood hazard analysis

Natural Hazards ◽

10.1007/s11069-020-03906-z ◽

2020 ◽

Vol 101 (3) ◽

pp. 995-1017 ◽

Cited By ~ 2

Author(s):

Emrah Yalcin

Keyword(s):

Land Cover ◽

Hydrodynamic Model ◽

Flood Hazard ◽

Hazard Analysis ◽

Two Dimensional ◽

Urban Flood ◽

Model Simulations ◽

Land Cover Data ◽

The Impact

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An efficient modelling approach for probabilistic assessments of present-day and future fluvial flooding

10.5194/nhess-2020-242 ◽

2020 ◽

Author(s):

Hieu Ngo ◽

Roshanka Ranasinghe ◽

Chris Zevenbergen ◽

Ebru Kirezci ◽

Dikman Maheng ◽

...

Keyword(s):

Land Subsidence ◽

Return Period ◽

Flood Risk ◽

Flood Hazard ◽

Mekong Delta ◽

Flood Risk Management ◽

Groundwater Extraction ◽

Model Simulations ◽

Can Tho City ◽

Modelling Approach

Abstract. Flood risk management and planning decisions in many parts of the world have historically utilised flood hazard or risk maps for a very limited number of hazard scenarios (e.g. river water levels), mainly due to computational challenges. With the potentially massive increase in flood risk in future due to the combination of climate change effects (increasing the hazard) and increasing population and developments in floodplains (increasing the consequence), risk-informed flood risk management, which enables balancing the risk with the reward, is now becoming more and more sought after. This requires a comprehensive and quantitative risk assessment, which in turn demands multiple (thousands of) river and flood model simulations. Performing such a large number of model simulations is a challenge, especially for large, complex river systems (e.g. Mekong) due to the associated computational and resource demands. This article presents an efficient modelling approach that combines a simplified 1D hydrodynamic model for the entire Mekong Delta with a detailed 1D/2D coupled model and demonstrates its application at Can Tho city in the Mekong Delta. Probabilistic flood hazard maps, ranging from 0.5 yr to 100 yr return period events, are obtained for the urban centre of Can Tho city under different future scenarios taking into account the impact of climate change forcing (river flow, sea-level rise, storm surge) and land subsidence. Results obtained under present conditions show that more than 12 % of the study area is inundated by the present-day 100 yr return period water level. Future projections show that, if the present rate of land subsistence continues, by 2050 (under both RCP4.5 and RCP8.5 climate scenarios), the 0.5 yr and 100 yr return period flood extents will increase by around 15-fold and 8-fold, respectively, relative to the present-day flood extent. However, without land subsidence, the projected increases in the 0.5 yr and 100 yr return period flood extents by 2050 (under RCP4.5 and RCP8.5) are limited to between a doubling to tripling of the present-day flood extent. Therefore, adaptation measures that can reduce the rate of land subsidence (e.g. limiting groundwater extraction), would substantially mitigate future flood hazards in the study area. A combination of restricted groundwater extraction and the construction of a new and more efficient urban drainage network would facilitate even further reductions in the flood hazard. The projected 15-fold increase in flood extent projected by 2050 for the twice per year (0.5 yr return period) flood event implies that the do nothing management approach is not a feasible option for Can Tho.

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Regional debris flow susceptibility analysis in mountainous peri-urban areas through morphometric and land cover indicators

Natural Hazards and Earth System Science ◽

10.5194/nhess-14-3043-2014 ◽

2014 ◽

Vol 14 (11) ◽

pp. 3043-3064 ◽

Cited By ~ 8

Author(s):

M. C. Rogelis ◽

M. Werner

Keyword(s):

Land Cover ◽

Debris Flow ◽

Flood Risk ◽

Urban Areas ◽

Debris Flows ◽

Flood Hazard ◽

Flow Direction ◽

Hazard Analysis ◽

Principal Component ◽

Debris Flow Susceptibility

Abstract. A method for assessing regional debris flow susceptibility at the watershed scale, based on an index composed of a morphometric indicator and a land cover indicator, is proposed and applied in 106 peri-urban mountainous watersheds in Bogotá, Colombia. The indicator of debris flow susceptibility is obtained from readily available information common to most peri-urban mountainous areas and can be used to prioritise watersheds that can subsequently be subjected to detailed hazard analysis. Susceptibility is considered to increase with flashiness and the possibility of debris flows occurring. Morphological variables recognised in the literature to significantly influence flashiness and occurrence of debris flows are used to construct the morphometric indicator by applying principal component analysis. Subsequently, this indicator is compared with the results of debris flow propagation to assess its capacity in identifying the morphological conditions of a watershed that make it able to transport debris flows. Propagation of debris flows was carried out using the Modified Single Flow Direction algorithm, following identification of source areas by applying thresholds identified in the slope–area curve of the watersheds. Results show that the morphometric variables can be grouped into four indicators: size, shape, hypsometry and (potential) energy, with energy being the component that best explains the capability of a watershed to transport debris flows. However, the morphometric indicator was found to not sufficiently explain the records of past floods in the study area. Combining the morphometric indicator with land cover indicators improved the agreement and provided a more reliable assessment of debris flow susceptibility in the study area. The analysis shows that, even if morphometric parameters identify a high disposition to the occurrence of debris flow, improving land cover can reduce the susceptibility. However, if favourable morphometric conditions are present but deterioration of the land cover in the watershed takes place, then the susceptibility to debris flow events increases. The indicator of debris flow susceptibility is useful in the identification of flood type, which is a crucial step in flood risk assessment especially in mountainous environments, and it can be used as input for prioritisation of flood risk management strategies at regional level and for the prioritisation and identification of detailed flood hazard analysis. The indicator is regional in scope, and therefore it is not intended to constitute a detailed assessment but to highlight watersheds that could potentially be more susceptible to damaging floods than others in the same region.

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Assessment of the Hydraulic Performance of the Urban Drainage System due to Climate Change using DHI MIKE URBAN

Journal of Biomedical Research & Environmental Sciences ◽

10.37871/jbres1222 ◽

2021 ◽

Vol 2 (4) ◽

pp. 261-267

Author(s):

M Maryam ◽

R Kumar ◽

N Thahaby

Keyword(s):

Hydrodynamic Model ◽

Urban Areas ◽

Flood Hazard ◽

Climatic Variability ◽

Drainage System ◽

Urban Drainage ◽

Urban Flooding ◽

Present Context ◽

Flooding Events

Changes in climate, waterlogging hazards and regional floods are more prominent in present context. The paper reviews potential of flood hazard in dense urban areas, using GIS-based 1-D hydrodynamic model (MIKE URBAN). The major factor contributing to the urban waterlogging in recent decades is the climatic variability and thus the long-term variations of precipitation and drainage system of an urban area were evaluated. MIKE URBAN (1-D) hydrodynamic model can be used to comprehensively simulate inundation processes. The model simulates the processes of rainfall and runoff, urban drainage, and flooding. MIKE URBAN can be used to appraise the potential immersion dangers of any planned drainage system. This paper reviews the increasingly urban flooding events expected in the future for the different cities across the globe. Thus, the surface runoff processes of cities need to examine the regional drainage system.

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Handling uncertainty in bivariate quantile estimation – An application to flood hazard analysis in the Mekong Delta

Journal of Hydrology ◽

10.1016/j.jhydrol.2015.05.033 ◽

2015 ◽

Vol 527 ◽

pp. 704-717 ◽

Cited By ~ 42

Author(s):

Nguyen Viet Dung ◽

B. Merz ◽

A. Bárdossy ◽

H. Apel

Keyword(s):

Flood Hazard ◽

Hazard Analysis ◽

Mekong Delta ◽

Quantile Estimation ◽

Bivariate Quantile

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Large-scale stochastic flood hazard analysis applied to the Po River

Natural Hazards ◽

10.1007/s11069-020-04260-w ◽

2020 ◽

Vol 104 (3) ◽

pp. 2027-2049

Author(s):

A. Curran ◽

Karin De Bruijn ◽

Alessio Domeneghetti ◽

Federica Bianchi ◽

M. Kok ◽

...

Keyword(s):

Risk Management ◽

Flood Risk ◽

Large Scale ◽

Extreme Event ◽

Flood Hazard ◽

Hazard Analysis ◽

Flood Risk Management ◽

Water Levels ◽

Stochastic Methods ◽

Po River

Abstract Reliable hazard analysis is crucial in the flood risk management of river basins. For the floodplains of large, developed rivers, flood hazard analysis often needs to account for the complex hydrology of multiple tributaries and the potential failure of dikes. Estimating this hazard using deterministic methods ignores two major aspects of large-scale risk analysis: the spatial–temporal variability of extreme events caused by tributaries, and the uncertainty of dike breach development. Innovative stochastic methods are here developed to account for these uncertainties and are applied to the Po River in Italy. The effects of using these stochastic methods are compared against deterministic equivalents, and the methods are combined to demonstrate applications for an overall stochastic hazard analysis. The results show these uncertainties can impact extreme event water levels by more than 2 m at certain channel locations, and also affect inundation and breaching patterns. The combined hazard analysis allows for probability distributions of flood hazard and dike failure to be developed, which can be used to assess future flood risk management measures.

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