Probabilistic Design of Coastal Flood Defences in Vietnam Probabilistic Design of Coastal Flood Defences in Vietnam Proefschrift ter verkrijging van de graad van doctor aan de Technische Universiteit Delft, op gezag van de Rector Magnificus professor ir. K.C.A.M. Luyben, voorzitter van het College voor Promoties, in het openbaar te verdedigen op dinsdag 21 september 2010 om 10.00 uur door MAI VAN CONG Master of Science in Hydraulic Engineering geboren te Ninh Binh, Vietnam Dit proefschrift is goedgekeurd door de promotor: Prof. drs. ir. J.K. Vrijling Copromotor: Dr. ir. P.H.A.J.M. van Gelder Samenstelling promotiecommissie: Rector Magnificus, Prof. drs. ir. J.K. Vrijling Dr. ir. P.H.A.J.M. van Gelder Prof. dr. ir. M.J.F. Stive Prof. dr. H. Hanson Prof. dr. J.W. Hall Ir. K.W. Pilarczyk Dr. ir. S.N. Jonkman Prof. dr. W.S.J. Uijttewaal voorzitter Technical University of Delft, promotor Technical University of Delft, copromotor Technical University of Delft Lund University, Sweden Newcastle University, UK Formerly Rijkswaterstaat Technical University of Delft Technical University of Delft, reservelid Copyright © 2010 by Mai Van Cong All rights reserved. No part of this book may be reproduced in any form or by any means including photocopy, without written permission from the copyright holder. ISBN 978-90-9025648-1 Printed by: Sieca Repro, Delft, the Netherlands Cover layout: Pham Quang Dieu Cover image: Violent ocean wave (source: Ocean Waves Wallpapers) to my family i SUMMARY Probabilistic design of coastal flood defences in Vietnam The year 2005 brought hurricane Katrina to New Orleans (U.S), it was also a historical year for Vietnam because of the disastrous impact of typhoons on sea flood defences and coastal regions. In total, eight typhoons hit the Vietnamese coast that year resulting in human casualties and enormous economic damage. In response to that, both the central and local governments of Vietnam have paid the highest-ever attention to the rehabilita- tion of sea dike systems and the improvement of the safety of its coastal regions. The improvement of the flood defence systems of Vietnam calls for the use of the latest available knowledge at all levels. Vietnam has a profound practical experience in the field of flood protection; however, there is a lack of up-to-date knowledge in the field of dike design and flood risk management, especially regarding to coastal flood defences. This study focuses on coastal flood defences in Vietnam and its objectives are to evaluate the current level of safety and reliability, to develop safety standards and design practices and to formulate a proposal for the improvement of the flood defences. To meet the study objectives, the research presented in this thesis has focused on firstly reviewing, developing and widening the application of probabilistic design and state of the art of risk assessment and risk evaluation in the field of flood defences (theory part). Secondly, as a demonstration of knowledge transfer, the proposed approach has been ap- plied to the coastal flood defence system of Nam Dinh province in Vietnam which is con- sidered as a representative coastal system in the country (application part). The main contents covered in this thesis are summarized below. Chapter 2 gives an overview of the coastal area, existing approaches for flood defence in Vietnam and a description of the case study area. The coastal zone in the Red River Delta of Vietnam, where the case study was carried out, is under a potential threat of fluvial flooding, coastal flooding and coastal erosion. In the region, sea dikes have been used as a predominant countermeasure with two intended functions, i.e. protecting low- lying areas from coastal flooding and reducing risk for the hinterland caused by coastal erosion. However, as was also experienced during recent typhoons in the last years, the sea dikes do not seem to function very well and their strength is insufficient to withstand the hydraulic loads with a sufficiently low frequency of occurrence. This is due to the underestimation of the hydraulic boundary conditions and imperfections in the design, construction and maintenance of the dike systems. A qualitative multi-criteria analysis has been developed for proposing a suitable coastal protection strategy for the study area. It is proposed to use a combination of sea dike systems to provide flood protection, Summary ii and beach erosion control measures, such as groins and/or sand nourishments to preserve a shallow foreshore. To have a better understanding of the actual hydraulic load conditions in the coastal area and to derive input information for the reliability analysis, methods for the statisti- cal analysis of extreme hydraulic load conditions have been developed and applied (Chapter 3). These include data management techniques (i.e. trend analysis, stationarity tests, seasonality analysis and peaks over threshold analysis); and statistical methods to model the occurrence probabilities of extreme values (the value which is exceeded by the random environmental variable with very small probability p). Hydraulic observations for the coast Vietnam are available for a period of 35 years and on that basis an estimate needs to be derived for a design sea load, i.e. waves and storm surge levels, corresponding to a 100 to 10,000 years return period. The methods of Regional Frequency Analysis (RFA) and Mixed Distribution Fitting (MDF) are used in this study for such a sea load. In order to ensure that the statistically predicted values of the interested sea loads will be as close as possible to the true values, recommendations are made to compare these values with physical hindcast models which are based on historical information of ty- phoon characteristics such as wind speed. Inspection of monthly sea water level data at Nam Dinh coast reveals that there exists a quadratic rising trend, with periodicity of 6 months, 12 months and 224 months in the data. The 6-month periodicity reflects well the influence of two monsoons per year to the fluctuation of sea water level; the 12-month periodicity is explained by the influence of yearly typhoons in the region. The 224-month periodicity, which is approximately 18.6 years, is almost exactly equal to a tidal circle. Although sea level rise (SLR) is not a focus of this study, based on a detected linear trend of the observed water level data the rate of SLR can be estimated approximately at 0.68 cm per year at Nam Dinh coast. In addition, this study recommends that estima- tion of SLR should be based on not only mathematical extrapolations, but also the long- term nature of sea level fluctuation. The theoretical background of probabilistic design and reliability analysis of flood de- fences is reviewed in Chapter 4. Models for safety assessment and reliability based design are developed for the specific application to coastal flood defence systems. In the safety assessment model, present probabilistic calculation techniques are applied to quantify the failure probability of the system and the system components. The effect of the length of the dike system on the system reliability was deployed in the model as well. The reliabil- ity-based design model is used to search for an optimal geometry of the dike system given an admissible failure probability. Chapter 5 demonstrates the application of the models proposed in the previous chapter to Vietnam. The case study area is the Nam Dinh coastal flood defence system in the Red River delta of Vietnam. It is found that the existing coastal flood defence system is less safe than its required safety standards. According to the standards the dikes should be designed to provide a target safety of 1/20 per year. However, the total failure prob- ability of the dike system is estimated to be much higher, about 0.15 per year. This cor- responds to the occurrence of a failure once in 7 years instead of once in 20 years as in- tended. Excessive wave overtopping induced dike failure is the dominant failure mode, [...]... Historical loss of life due to storms and floods in Vietnam .170 7.2.5 Discussion 172 7.3 Loss of life due to flooding in Vietnam .173 7.3.1 Individual risk for coastal flooding in Vietnam 173 7.3.2 Societal risk due to flood on a country level 174 7.4 Case study: analysis of flood risk in Nam Dinh coastal zone 179 7.4.1 Loss of life estimates for Nam Dinh coastal. .. recommendations of the research are presented in Chapter 9 This includes remarks regarding the methodology: strong and weak points of present research range of applicability and cautions; as well as remarks about the application case of Vietnam 25 Introduction Probabilistic design of coastal flood defences in Vietnam Introduction Chapter 1 Introduction, background and fundamentals Chapter 2 Water defences in Vietnam. .. River Delta of Vietnam and assessing its present protective measures The coastal zone in the Red River Delta of Vietnam is under large threat due to fluvial flooding, coastal flooding and coastal erosion In the region, sea dikes have been used as the predominant counter measure with two intended functions, viz protecting low-lying areas from coastal flooding; and reducing risk for the hinterland caused... of the activity (benefit, voluntariness) and baseline mortality statistics in a country In this assessment, first the mortality due to natural causes and daily activities such as traffic is involved Based on Vietnamese statistics, baseline mortality has been determined including the risks of traffic and living in a flood- prone area in Vietnam An FN-curve for flooding in Vietnam is constructed showing... for the Vietnamese situation in flood defences, in general, and sea flood defences, in particular 1.5 PROBABILISTIC VS DETERMINISTIC APPROACH The conventional design method in civil engineering design is a deterministic approach The basis of the deterministic approach is the so-called design values for the loads and the strength parameters Loads for instance are the design water level and the design. .. presented in this thesis is focused on developing and widening the application of probabilistic design, safety and reliability assessment, risk assessment and evaluation and risk based design in the fields of flood defences An application is done for the case of a coastal flood defence system in Vietnam On basis of the study objectives, the thesis includes 9 chapters, as below A schematic outline of this... occurred in 1953), proposed a new way of looking at the safety of the Netherlands against flooding This committee was aware of the fact that not all the knowledge needed to implement this new approach was available at that point, but the new way of thinking enabled the development of the scientific approach of the flood defences One of the important changes was the development of probabilistic techniques in. .. extension of existing probabilistic reliability models for flood defence systems regarding length effects of the system (see also Mai Van & Calle 2009); - investigation of present situation of coastal flood defences and coastal protections in the Red River delta of Vietnam (Mai Van et al 2009f, Mai Van & Pilarczyk 2005); - developing a generic probabilistic and risk-based design model for flood defence... definition In low-lying coastal regions, such as in the Netherlands, Japan, Vietnam, and Bangladesh, the coastal flood defence systems (CFDS) are very important to protect the lowlying regions from sea flood Commonly, this system contains sea dikes, dunes, estuarine river dikes, dams, sluices, pumping stations, etc (see Figure 1.1) In principle, failure of any component of the system leads to flooding... the flooding risks taking into account the multiple failure mechanisms of a dike or a structure Dutch hydraulic engineers were among the first to apply this theory in the practical design of structures Reliability models were first used during the design and the construction of a storm surge barrier in the Easternscheldt in 1976 and later in the design of a storm surge barrier in the Nieuwe Waterweg In . Probabilistic Design of Coastal Flood Defences in Vietnam Probabilistic Design of Coastal Flood Defences in Vietnam . is involved. Based on Vietnamese statistics, baseline mortality has been determined including the risks of traffic and living in a flood- prone area in Vietnam. An FN-curve for flooding in Vietnam. up-to-date knowledge in the field of dike design and flood risk management, especially regarding to coastal flood defences. This study focuses on coastal flood defences in Vietnam and its objectives