Reliabilty analysis of the red river dike system in vietnam

Jommi, Technische Universiteit Delft, reservelid Keywords: Geotechnical reliability, flood defence, Red River dike, piping, uplift geotechnical engineering, hydraulic engineering Printed

ter verkrijging van de graad van doctoraan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof ir K C A M Luyben,

voorzitter van het College voor Promoties,

in het openbaar te verdedigen op woensdag 8 oktober 2014 om 12:30 uur

door

civiel ingenieurgeboren te Ha Nam, Viet Nam

Prof drs ir J.K Vrijling

Prof dr ir P H A J M van Gelder

Samenstelling promotiecommissie:

Prof drs ir J.K Vrijling, Technische Universiteit Delft, promotor

Prof dr ir P H A J M van Gelder, Technische Universiteit Delft, promotor

Prof dr T M Thu, Water Resources University of Viet Nam

Prof dr ir M Kok, Technische Universiteit Delft

Dr ir K J Bakker, Technische Universiteit Delft

Ir M R Tonneijck, Royal HaskoningDHV

Dr ir W Kanning Colorado School of Mines/Deltares

Prof dr C Jommi, Technische Universiteit Delft, reservelid

Keywords: Geotechnical reliability, flood defence, Red River dike, piping, uplift

geotechnical engineering, hydraulic engineering

Printed by: Ridderprint B.V., Ridderkerk, the Netherlands

Front & Back: Red River Dike on the right bank in Ha Noi, by Pham Anh Tuan (2010)

Cover layout by: Pham Anh Tuan

Copyright © 2014 by Pham Quang Tu

ISBN 978-90-5335-887-0

An electronic version of this dissertation is available at

http://repository.tudelft.nl/

pertaining to the dissertation

es-9 Flooding has been the issue of most concern since antiquity in the society

of Viet Nam, as shown by the classical warning “thủy, hỏa, đạo, tặc”(theVietnamese proverb):“flooding, firing, robbing, and invading”

10 The challenge of a PhD’s life is the same as that of learning a new sport butmore interesting and beneficial

These propositions are regarded as opposable and defendable, and have been approved

as such by the supervisors prof drs ir J.K Vrijling and prof dr ir P H A J M van Gelder

behorende bij het proefschrift

6 Integratie van Waterbouwkunde en Geotechniek kan tot een uiterst rijk en vruchtbaar onderzoeksterrein leiden

belang-7 De op betrouwbaarheid gebaseerde analyse in de Geotechniek is toepasbaar

in ontwikkelingslanden

8 De bijzondere klimatologische omstandigheden en de geografie van meren

en riviermondingen dragen bij aan het succes van het beheer van mingsrisico’s in Nederland

overstro-9 Sinds de oudheid zijn overstromingen het grootste probleem geweest in deVietnamese samenleving, hetgeen blijkt uit de klassieke waarschuwing“thủy,

goed-This dissertation synthesises the application of a probabilistic-based framework in nical and hydraulic engineering, for the assessment of the Red River dikes in Viet Nam Thestudy area promotes the probabilistic-based approach because of its typical natural condi-tions Lack of understanding of soil and water behaviours may lead to failures of engineer-ing design, as is proven from practice This study is intended to fill part of these gaps.

geotech-Dikes along rivers often spread over the deltaic environment and its earthen structuresare parts of a long civilian history, from hundreds to thousands of years Uncertainties ofsoil properties of the dike embankment and its foundation, and contribute to the probabil-ity of failure under a given water level To carry out an assessment of the safety of the flooddefences, both a conventional approach and a reliability analysis may be applied The for-mer relies on the factors of safety while the latter takes uncertainties of both water level(load) and soil properties (resistance) more explicitly into account The reliability-basedframework proved its benefits in important projects and in dealing with large uncertainties

in design This is demonstrated through the chapters in this thesis

Firstly, the background information of the natural conditions, the socio-economic sues and the flood defences in the Red River Delta are presented in Chapter2 The topo-graphic conditions and meteorological characteristics play an important role in the flooddefence management of the Red River Delta Around 50% of the delta area is lower than

is-2 m (above mean sea level) and the delta is partly surrounded by high mountains quently, under extreme weather conditions such as typhoons, fronts, and tropical depres-sions, flooding will affect the lowlands area On the other hand, the socio-economic issuesindicate a fast growing economy of Viet Nam Hence, new requirements of higher safetystandards for the flood defences seem advisable In fact, the flood defences in the Red RiverDelta have been established for hundreds of years and the safety standard has increasedfrom a design water level of 1/5 years (in the Imperial period ∼ the 1890s) to 1/500 years (atthe present) The current design water level is 13.4 m (above mean sea level) at the gaugingstation of Ha Noi, which will be routed to the dike stretches in the whole delta area

Conse-Secondly, the analyses of the hydraulic boundary conditions are performed in Chapter

3with a detailed description of the Red River system and an evaluation of the hydraulic rameters for the reliability analysis The Red River is formed by three tributaries (the Da,

pa-Thao, and Lo River), and the maximum observed discharge was 37,800 m3/s in August 1971

at Son Tay in the area of Ha Noi The reservoirs systems in the Da and Lo River significantlyreduce the peak flood discharge in the delta area by a storage capacity of 8.5 billion cubicmetres; as for instance in the flood with frequency of 1/500 years, the peak discharge will

decrease approximately 40%, from 48, 500 m3/s to 30, 000 m3/s By doing so, the dike

sys-tem may be severely loaded during a long duration flood of around 120 hours but the waterlevel is lower than the original Another issue is the increasing trend of the water level atthe same river discharge, which is due to the over expansion of residents in the flood plain,therefore the resulting probability of overflow will become higher in future if there are nomeasures to control such developments

Thirdly, the probabilistic-based analysis framework and its application are treated in

v

Chapter4and5, including classification of ground conditions, model uncertainties, andspatial variability of soil parameters Currently, the calculation level 1 (semi-probabilistic)has been embedded in the design codes while the calculation level 2 (e.g FORM) and 3 (e.g.Monte Carlo simulation) are used for important projects or the calibration of the level 1 de-sign codes Soils properties and ground conditions are analysed by different approaches,either by classical statistics or by a probabilistic-based framework It is hoped to increasethe engineer’s understanding about the use of probabilistic-based methods in practice Totake examples, uplift and ground coefficient are analysed by using the model factor Theformer presents a physical-based model of uplift which was calibrated for the study area byfield tests The latter discusses a new model of ground uncertainties (the ground coefficient

-α) that represents the cumulative effects of the internal erosion process in both flood and

dry season Moreover, an illustration of spatial estimation was performed on two data series

of the top-layer thickness (with the distance of sampling of around 30 m and 200 m) Thecalculation results indicate a correlation between the scales of fluctuation and the distance

of sampling It is expected that the mentioned theories and applications will be rated in Geotechnical Engineering and Flood Defences assessment, especially in Viet Nam.Fourthly, the theory of reliability analysis of a river dike and its application in the RedRiver Dikes are discussed in Chapters6and7, where piping, overflow, and instability areconsidered under a long duration flood Probabilities of overflow are predicted to be high

incorpo-in the dike stretches of the Ha Tay area due to a lower design water level incorpo-in the past fore, dike crests should be heightened to meet the same safety standards as that in the HaNoi area To evaluate piping during a long duration flood, a model of seepage length reduc-tion of piping with respect to time is developed from a basic principle: the internal erosionprocess depending on seepage gradient and ground conditions The proposed model, aftercalibrating by a historical dike failure in the study area, predicts an increase of the erosionlength from 3% to 20% (the seepage length shortening from 100% to 97% or 80% respec-tively) in one typical flood wave, which leads to the cumulative effects of piping and inter-nal erosion under the dike embankment On the other hand, the geotechnical instability isproven to be less affected during a long flood wave However, only homogeneous models

There-of embankments are mentioned in this study It is suggested that in future research moreattention should be given to the heterogeneity of dike embankments with regard to perme-ability Finally, by taking the length-effect into account, the probability of failure of dikestretches in the study area may significantly increase depending on its stretch length andthe spatial variation of soil parameters, as for instance the total probability of failure of thedike system will jump from 21%-25% to 38%-47% (by without or/and with taking length-effect into account respectively)

The findings of this research hope to contribute to a new understanding of Red RiverDike safety in Viet Nam They also open up several research directions in the combinedfield of hydraulic engineering and geotechnical engineering, and widen the applications ofprobabilistic-based approaches in Viet Nam and developing countries

In deze dissertatie wordt een synthese uitgevoerd van de toepassing van een op bilistische methoden gebaseerd kader in de geotechniek en de waterbouwkunde, ten be-hoeve van de beoordeling van de betrouwbaarheid van dijken langs de Rode Rivier (in Viet-namees: Song Hong) in Viet Nam De toepassing van een probabilistische benadering ishier met name opportuun vanwege de typisch natuurlijke omstandigheden in dit onder-zoeksgebied Een gebrekkig begrip van grond- en watercondities zou namelijk, zoals uit depraktijk gebleken is, tot mislukkingen in technische ontwerpen kunnen leiden Deze studie

proba-is bedoeld om een deel van deze lacunes te vullen

Dijken langs rivieren strekken zich uit over de delta en dergelijke grondconstructies zijndeel van een lange civiele geschiedenis, van honderden tot duizenden jaren De onzeker-heden in de grondeigenschappen van dijklichaam en fundering dragen bij aan de faalkansbij een bepaalde waterstand Om een inschatting te kunnen maken van de veiligheid vanwaterkeringen, worden vaak zowel een conventionele analyse als een betrouwbaarheids-analyse toegepast De eerste, conventionele benadering verlaat zich op veiligheidsfactoren,terwijl de tweede methode onzekerheden in waterstanden (belasting) en grondeigenschap-pen (sterkte) in acht neemt Het op betrouwbaarheid gebaseerde toetsingskader heeft zijnnut al bewezen in belangrijke projecten en bij grote ontwerponzekerheden Deze benade-ring wordt in de hoofdstukken van deze dissertatie gedemonstreerd

De achtergrondinformatie over de natuurlijke omstandigheden, de sociaaleconomischesituatie en de waterkeringen in de regio wordt in hoofdstuk 2 gepresenteerd De topogra-fische condities en de meteorologische omstandigheden spelen een belangrijke rol in dehoogwaterbescherming van de Rode Rivier Delta Ongeveer 50% van de delta ligt lager dan

2 meter (boven gemiddeld zeeniveau) en is gedeeltelijk omringd door hoge bergen halve, onder extreme weersomstandigheden, zoals tyfoons, fronten en tropische depres-sies, zal het laaggelegen land door overstromingen getroffen worden Omdat de huidigesociaaleconomische situatie van Viet Nam echter op een snel groeiende economie duidt,lijken nieuwe eisen voor hogere veiligheidsnormen voor de hoofwaterbescherming daaromraadzaam In feite, in de honderden jaren van het bestaan van waterkeringen in de Rode Ri-vier Delta zijn de veiligheidsnormen al toegenomen van een ontwerpwaterhoogte van 1/5jaar (in de imperiale periode ∼1890s) tot 1/500 jaar (2014) De huidige ontwerpwaterhoogte

Der-is 13.4 m (boven gemiddeld zeeniveau) bij het meetstation van Ha Noi, en wordt vervolgensnaar alle dijken in de gehele delta doorberekend

In hoofdstuk 3 worden de hydraulische randvoorwaarden bepaald door een leerde beschrijving van het Rode Rivier systeem en een evaluatie van de hydraulische pa-rameters voor de betrouwbaarheidsanalyse De Rode Rivier wordt gevormd door drie zijri-vieren, de Da, de Thao en de Lo rivier, die samenkomen bij Son Tay in de omgeving van Ha

gedetail-Noi, waar in augustus 1971 een maximum debiet van 37,800 m3/s werd geobserveerd De

reservoirs in de Da en de Lo rivier hebben door een opslagcapaciteit van 8.5 miljard kubiekemeter een reductie van de piekwaterafvoer in de delta tot gevolg, zoals bijvoorbeeld met be-trekking tot de overstromingsfrequentie van 1/500 jaar, zal de piekafvoer verminderen met

40%, van 48,500 m3/s tot 30,000 m3/s Het gevolg hiervan kan zijn, dat het dijksysteem

ern-vii

stig belast kan worden gedurende een langdurige hoogwaterstand van ongeveer 120 uur.Een ander aspect is een toenemend waterniveau bij gelijkblijvende afvoer, als gevolg vanbewoning in de hoogwaterbedding, welke, indien geen maatregelen worden getroffen, kanresulteren in een hogere waarschijnlijkheid van overstromen in de toekomst.

De beschrijving en toepassing van het op probabilistiek gebaseerde kader van analyse,inclusief de classificatie van grondcondities, modelonzekerheden en ruimtelijke variatievan grondparameters, wordt in hoofdstuk 4 en 5 behandeld Momenteel is het niveau 1 be-rekeningsmodel ingebed in de ontwerpnormen, terwijl berekeningsniveau 2 en 3 gebruiktworden voor grote projecten of voor de kalibratie van de niveau 1 ontwerpnormen Grond-eigenschappen en bodemcondities worden op verschillende manieren geanalyseerd, na-melijk met de klassieke statistische benadering, of met een probabilistisch gebaseerd ka-der Het is de hoopvolle verwachting dat er meer kennis zal ontstaan onder ingenieursvoor de toepassing van een probabilistische methode in het veld Als voorbeeld wordenhet opdrijven van de afdekkende laag aan de binnenzijde van de dijk en de grondcoëfficiëntgeanalyseerd met gebruikmaking van een modelfactor Het eerste voorbeeld behelst eenfysisch gebaseerd model van opwaartse druk welke in het studiegebied werd gekalibreerddoor veldonderzoek Laatstgenoemd voorbeeld beschrijft een nieuw model van grondon-zekerheid (grondcoëfficiënt -α), dat de cumulatieve effecten van het interne erosieproces

in zowel het regen- als het droge seizoen vertegenwoordigt Bovendien werd een illustratievan ruimtelijke inschatting gemaakt voor twee dataseries van de bovenlaagdikte (met eensteekproefafstand van ca 30m en 200m) De berekeningen geven een correlatie aan tussen

de schaal van fluctuatie en de afstand tussen de metingen Het wordt verwacht dat de orieën en hun toepassingen in de geotechniek en toetsing van waterkeringen, in met nameVietnam, gẹncorporeerd zullen worden

the-In hoofdstuk 6 en 7 wordt de theorie van de betrouwbaarheidsanalyse van een dijk en de toepassing daarvan op de dijken van de Rode Rivier besproken; overstromen,instabiliteit en “piping” worden bekeken onder een langdurige hoogwaterstand De kans

rivier-op overstromen wordt naar verwachting hoog in de dijkvakken in de omgeving van Ha Tayten gevolge van een lagere ontwerpwaterstand in het verleden De kruinhoogte zou daaromverhoogd moeten worden om dezelfde veiligheidsstandaard als in het gebied rond Ha Noi terealiseren Om “piping” (zandvoerende wel) gedurende een langdurige hoogwaterstand teevalueren, wordt een model van kwellengtereductie als functie van de tijd ontwikkeld van-uit een basisprincipe: namelijk het interne erosieproces dat afhangt van de kwelgradiënt engrondeigenschappen Na de kalibratie van het falen van een dijk in het onderzoeksgebiedvoorspelt het voorgestelde model een toename in de lengte van erosie van 3% tot 20% (delengte van de kwel respectievelijk verminder van 100% tot 97% of 80%) gedurende een ty-pische vloedgolf Dit leidt tot een opeenstapeling van de effecten van “piping” en interneerosie onder de dijk Anderzijds wordt bewezen dat de geotechnische instabiliteit minderwordt bedreigd tijdens een langdurige afvoergolf In dit onderzoek zijn echter slechts ho-mogene modellen van dijken bestudeerd De suggestie is dan ook dat in toekomstig onder-zoek meer aandacht wordt geschonken aan de heterogeniteit van dijken met betrekking tothun waterdoorlatendheid Tot slot, door het lengte-effect in acht te nemen, kan de waar-schijnlijkheid van een dijkbreuk in het studiegebied aanzienlijk hoger blijken te zijn danwas aangenomen, afhankelijk van de lengte en de ruimtelijke variatie in grondparameters,zodat bijvoorbeeld de totale faalkans van het dijksysteem zal toenemen van 21%-25% (zon-der lengte-effect) tot 38%-47% (met lengte-effect)

De resultaten van dit onderzoek zullen hopelijk bijdragen tot een nieuw begrip van

vei-ligheid van de Rode Rivier dijken in Viet Nam De resultaten bieden nieuwe perspectieven in het interdisciplinaire veld van waterbouwkunde en geotechniek en verrui-men de toepassingsmogelijkheden van probabilistische methoden in Viet Nam en ontwik-kelende landen.

onderzoeks-Acknowledgement: Mariette van Tilburg and Mark Z Voorendt are acknowledged for

their translation this summary into the Dutch language

Luận văn này tổng hợp ứng dụng của phương pháp tính toán dựa trên lý thuyếtxác suất (probabilistic-based) trong địa kỹ thuật và thủy công để đánh giá hệ thống

đê sông Hồng ở Việt Nam Vùng nghiên cứu khuyến khích việc áp dụng phương pháptính toán dựa trên lý thuyết xác suất do những đặc thù riêng của vùng Những hiểubiết không đầy đủ về tương tác giữa đất và nước có thể dẫn tới sai xót trong tínhtoán thiết kế công trình, điều này đã được chứng minh trong thực tế Nghiên cứunày nhằm phần nào khắc phục những hạn chế vừa nêu

Đê sông thường trải dài trên vùng đồng bằng và nền đắp (thân đê) được hìnhthành trong lịch sử dài hàng trăm đến hàng nghìn năm Những bất định tiềm ẩntrong thân đê hoặc dưới nền đê có thể ảnh hưởng đến an toàn đê dưới tác độngcủa nước lũ Để tiến hành đánh giá an toàn cho đê, cả phương pháp tiếp cận truyềnthống (tất định - deterministic) và phương pháp độ tin cậy (reliability-based) thườngđược áp dụng Phương pháp truyền thống chủ yếu dựa vào hệ số an toàn trong khiphương pháp độ tin cậy xét đến sự bất định (uncertainties) của mực nước sông (tảitrọng) và chỉ tiêu cơ lý của đất nền (sức chống đỡ) Phương pháp độ tin cậy khẳngđịnh hiệu quả trong các dự án lớn, quan trọng hoặc các thiết kế có sai số lớn trongcác tham số đầu vào Những nội dung này sẽ được thể hiện qua các chương trongtoàn bộ nội dung luận án

Một, điều kiện tự nhiên, các vấn đề kinh tế xã hội và hệ thống công trình phòng

lũ được trình bày ở Chương 2 Đặc điểm địa hình và điều kiện khí tượng thủy vănđóng vai trò quan trọng trong kiểm soát và vận hành hệ thống công trình phòng lũ

ở đồng bằng sông Hồng Khoảng 50% diện tích toàn đồng bằng sông Hồng có cao

độ dưới +2.0m (so với mực nước biển) và bao quanh đồng bằng là đồi và núi Do

đó, dưới ảnh hưởng của mưa lớn (do bão, áp thấp nhiệt đới, front ) lũ lụt có thể đedọa những vùng có cao độ thấp Mặt khác, kinh tế Việt Nam đã và đang tăng trưởngnhanh, do đó một tiêu chuẩn phòng lũ cao hơn cho hệ thống đê là điều cần thiết.Trên thực tế, hệ thống đê sông Hồng được xây dựng từ hàng trăm năm trước với mực

tới mực nước tương ứng trận lũ có tần suất 1/500 năm (giai đoạn hiện tại) Mực nướcthiết kế là 13.4m tại trạm thủy văn Hà Nội và được truyền dẫn đi tới từng đoạn đêtrên toàn đồng bằng

Hai, những phân tích về điều kiện biên thủy lực được trình bày trong Chương 3với phần mô tả chi tiết hệ thống sông Hồng và phần tính toán các tham số thủy lực

để dùng trong phân tích cho các chương sau Sông Hồng gồm có ba phụ lưu (sông

Đà, sông Thao và sông Lô) và lưu lượng lớn nhất quan trắc được là 37,800 m3/s vàotháng 8 năm 1971 tại Sơn Tây, Hà Nội Hệ thống hồ chứa trên sông Đà và sông Lôgiảm đáng kể đỉnh lũ ở vùng đồng bằng với khả năng cắt lũ tổng cộng khoảng 8.5 tỷm3, ví dụ như ở trận lũ tần suất 1/500 năm, đỉnh lũ sẽ giảm khoảng 40% từ 48,500m3/s xuống còn khoảng 30,000 m3/s Bằng cách điều tiết đó, hệ thống đê sẽ phảichịu mực nước lũ cao kéo dài khoảng 120 giờ (ở cao độ +13.0m) Một vấn đề khác là

sự gia tăng mực nước ở cùng mức lưu lượng do sự gia tăng dân cư sinh sống dọc các

xi

bãi sông, điều này sẽ làm tăng xác suất tràn đê trong tương lai nếu không có biệnpháp xử lý nào được tiến hành.

Ba, phương pháp phân tích dựa vào lý thuyết xác suất (probabilistic-based) vàcác ứng dụng được trình bày trong chương 4 và 5, bao gồm phân loại điều kiện nền,

hệ số mô hình, ước lượng tham số của chỉ tiêu cơ lý của đất Hiện nay, tính toán theo

lý thuyết xác suất với mức 1 (hay còn gọi là bán xác suất) đã được sử dụng trong cáctiêu chuẩn thiết kế trong khi các tính toàn với mức 2 (ví dụ: FORM) và 3 (ví dụ: MonteCarlo simulation) thường được áp dụng cho các dự án quan trọng hoặc để hiệu chỉnhcác tiêu chuẩn thiết kế tính toán theo mức 1 Chỉ tiêu cơ lý và đặc điểm đất nền đượcphân tích với nhiều phương pháp khác nhau: phương pháp thống kê truyền thốnghoặc phương pháp tính toán dựa trên lý thuyết xác suất (probabilistic-based) Điềunày được kỳ vọng sẽ nâng cao hiểu biết của kỹ sư để sử dụng các phương pháp tínhtoán dựa trên xác suất trong thực tế thiết kế Ví dụ như hiện tượng đẩy trồi và hệ sốnền trong phân tích bằng hệ số mô hình Phần phân tích hiện tượng đẩy trồi trìnhbày mô hình dựa trên cơ chế vật lý của hiện tượng có kết hợp hiệu chỉnh bằng thínghiệm hiện trường Phần phân tích hệ số nền xem xét các sai số dưới nền đê (đặc

do ảnh hưởng của dòng thấm vào mùa lũ và mùa kiệt Ngoài ra, một ví dụ minh họaphần ước lượng trong không gian với hai chuỗi số liệu về bề dày tầng phủ (với khoảngcách mẫu là 30m và 200m) cũng được trình bày trong chương 5 Kết quả tính toánchứng minh sự phụ thuộc của tỷ lệ biến đổi (sacle of fluctuation) và khoảng cách lấymẫu Những phương pháp tính toán nêu trên kỳ vọng sẽ được áp dụng trong phântích ổn định đê, đặc biệt là ở Việt Nam

Bốn, lý thuyết phân tích ổn định đê và ứng dụng cho đê sông Hồng được trình bàytrong chương 6 và 7, trong đó xói ngầm, tràn đê và mất ổn định được xem xét dướiđiều kiện lũ kéo dài Xác suất tràn đê được dự báo là cao ở các đoạn đê của Hà Tây

cũ do mực nước thiết kế cũ thấp hơn (so với mực nước thiết kế của đê thuộc Hà Nộicũ) Do đó, đỉnh đê cần được nâng cao để đạt cùng tiêu chuẩn an toàn với hệ thống

đê trên toàn Hà Nội mới Để đánh giá an toàn hệ thống đê trong điều kiện lũ kéo dài,

mô hình đường thấm giảm dẫn theo thời gian được đề xuất dựa trên các nguyên tắc

cơ bản: xói ngầm phụ thuộc vào gradient thấm, điều kiện đất nền và chiều dài đườngthấm ban đầu Mô hình chiều dài đường thấm giảm dần theo thời gian, sau khi hiệuchỉnh bằng một trường hợp vỡ đê ở vùng nghiên cứu, dự báo chiều dài xói ngầm

có thể phát triển từ 3% đến 20% (tương ứng chiều dài đường thấm giảm từ 100%xuống 97% và 80%) trong một trận lũ điển hình Điều này sẽ dẫn đến hiệu ứng tíchlũy các nguy cơ tiềm ẩn dưới nền đê do xói ngầm và cát chảy Mặt khác, phân tích ổnđịnh nền đê chứng minh hệ số an toàn ít bị ảnh hưởng do mực nước sông dâng cao,nhưng chỉ có phân tích cho thân đê đồng nhất được tiến hành Trong tương lai, cầnđánh giá sự ảnh hưởng này trong trường hợp thân đê không đồng nhất, đặc biệt là vềtính thấm Cuối cùng, bằng việc xét đến sự ảnh hưởng theo chiều dài (length-efect),xác suất phá hủy của từng đoạn và toàn hệ thống đê tăng lên đáng kể, do ảnh hưởngcủa chiều dài mỗi đoạn đê và sự biến đổi chỉ tiêu cơ lý của đất nền trong không gian.Chẳng hạn, xác suất phá hủy của toàn hệ thống đê tăng từ 21%-25% khi chưa xét đếnlength-effect tăng lên 38%-47% khi xét đến length- effect

Những phát hiện trong nghiên cứu này được hy vọng sẽ góp phần hiểu biết tốthơn về hệ thống đê sông Hồng ở Việt Nam Từ đó, những hướng nghiên cứu mớicũng được mở ra trng lĩnh vực đa ngành giữa địa kỹ thuật và thủy công, đồng thời

cũng mở ra những triển vọng ứng dụng phương pháp tính toán dựa vào xác suất ởViệt Nam và các nước đang phát triển khác.

Summary v

1.1 Background and problem statement 1

1.2 Objectives 2

1.3 Research questions 2

1.4 Approaches and methods 3

1.5 Contributions and scope 3

1.6 Organization of the dissertation 4

2 Flood Defences Management in the Red River Delta 5 2.1 Natural conditions 5

2.1.1 Topographic characteristics 6

2.1.2 Meteorological conditions 8

2.1.3 River systems 11

2.2 Socio-economic characteristics 12

2.2.1 Relevant social characteristics 12

2.2.2 Economic issues 14

2.3 Development of flood defences and safety standard 15

2.3.1 Imperial period 15

2.3.2 French period 15

2.3.3 Viet Nam war period 16

2.3.4 Doimoi period 16

2.3.5 Review of dike engineering in four periods 16

2.3.6 Typical floods in Viet Nam from the 1900s 17

2.4 Current management of flood defence in the Red River Delta 19

2.4.1 Institutional framework 19

2.4.2 Flood defence management 20

2.5 Discussion 21

3 Hydraulic Boundary Conditions of the Red River Delta 23 3.1 System description 23

3.2 Hydraulic boundary conditions 27

3.2.1 Literature review 27

3.2.2 River discharge analysis 28

3.2.3 Flood routing 33

3.3 Challenges for flood defence in the near future 38

3.4 Discussion 39

xv

4 Probabilistic-based Analysis in Geotechnical Engineering 41

4.1 Introduction 41

4.2 Theory of reliability analysis 41

4.2.1 Levels of calculation 42

4.2.2 System reliability 43

4.2.3 Risk-based analysis 44

4.2.4 Relations of reliability-based and conventional design 46

4.3 Uncertainties in Geotechnical Engineering 47

4.3.1 Understanding of uncertainty in geotechnical engineering 47

4.3.2 Analysis of uncertainty 50

4.4 Site characterization in geotechnical engineering 50

4.5 Classical estimation of soil properties 52

4.5.1 Regression analysis 52

4.5.2 Estimation of independent variables 53

4.6 Spatial variation of soil properties 54

4.6.1 Random field 54

4.6.2 Spatial variability of soil properties 59

4.6.3 Geostatistics 61

4.7 Discussion 62

5 Analysis of Geotechnical Conditions of the Red River Dikes 65 5.1 Introduction 65

5.2 Geological conditions 65

5.2.1 Geological formations 65

5.2.2 Groundwater 67

5.3 Analysis of geotechnical soil properties 70

5.3.1 Index properties 70

5.3.2 Strength properties by direct shear test 70

5.3.3 Permeability 72

5.4 Classification of ground conditions 72

5.4.1 Introduction 72

5.4.2 Application of CART method in classification of the Red River dikes 74

5.4.3 Analysis results 74

5.5 Statistical analysis of the observed sand boils at Sen Chieu 76

5.5.1 Introduction 76

5.5.2 Probabilistic classification method 76

5.5.3 Analysis results 77

5.6 Probabilistic analysis of the field test on the Thaibinh formation 77

5.6.1 Introduction 77

5.6.2 Understandings of phenomena 78

5.6.3 Probabilistic-based analysis method 81

5.6.4 Results and discussion 84

5.7 Probabilistic analysis of ground coefficient 84

5.7.1 Introduction 84

5.7.2 Probabilistic analysis of the ground uncertainties 85

5.7.3 Results and discussion 86

5.8 Spatial Estimation of top-layer thickness 87

5.8.1 Introduction 87

5.8.2 Spatial estimation of soil thickness 88

5.8.3 Results 88

5.9 Discussion 88

6 Reliability Analysis of River Dikes During a Long Duration Flood Wave 91 6.1 Introduction 91

6.2 General framework to assess a dike section 92

6.3 Overflow analysis of a dike section 93

6.4 Analysis of piping of a dike section 94

6.4.1 Current approaches 94

6.4.2 Experiment-based model of piping 97

6.4.3 Decay and delay of piezometric head effects to piping 104

6.5 Instability analysis of a dike section 105

6.5.1 General approach 105

6.5.2 Influence factors to instability of dikes in a long duration flood 106

6.6 Assessment of a dike system 108

6.6.1 General overview 108

6.6.2 Length-effect 108

6.6.3 System boundary issue 109

6.7 Discussion 111

7 Probabilistic Analysis of the Red River Dike in Viet Nam 113 7.1 Introduction 113

7.2 Dike conditions classification 113

7.3 Reliability analysis of a dike stretch 118

7.3.1 Overflow 118

7.3.2 Piping 120

7.3.3 Instability 122

7.4 Reliability analysis of a dike system 125

7.5 Discussion 128

8 Conclusions and Recommendations 129 8.1 Conclusions 129

8.1.1 Conclusions concerning Chapter 2 129

8.1.2 Conclusions concerning Chapter 3 130

8.1.3 Conclusions concerning Chapter 4 130

8.1.4 Conclusions concerning Chapter 5 130

8.1.5 Conclusions concerning Chapter 6 and 7 131

8.2 Recommendations 132

D Soil Property along the Red River dike 153

THERed River Delta (RRD), located in northern Viet Nam with an area of around 15, 500

km2and a population of nearly 20 million people, is protected by approximately 3000

km of dikes which are classified into four grades from III, II, I to “special”, indicating theincrease of the safety levels In the past, the main cause of dike failure was overflow due

to limited height Of the 38 cases of dike failures in the 19t h century most were related tooverflow, as for instance was the case in the floods in 1910, 1915, 1925, 1945, 1971 (Khanh

et al.,1995;GDM,1995) Currently, the flood mitigating measures, which are expected tolower flood risk in the RRD, includes the reforesting in the upper basins, the constructingdams and reservoirs to store flood water, the strengthening of dikes, and the raising of pub-lic awareness of flood protection By doing so, the safety of flood defence in the delta hasbeen significantly improved, indicated by the design water level that has increased from 1/5

to 1/500 years for one hundred year

The assessment of flood defences in Viet Nam currently has some limitations, istic evaluation and inadequate hydraulic boundary conditions The deterministic evalua-

determin-tion is the so-called “deterministic framework” that treats load and resistance as standard

values This approach also relies on the Factor of Safety (FS) but the determination of thevalues of FS is still implicit The inadequate hydraulic boundary conditions refer the assess-ment of flood defence at the highest water level in the flood frequency from 1/50 years to1/250 years in the past, while the current designed flood frequency is 1/500 years (IWRP,

2009;Khoi,2010) Therefore, the safety margin of the Red River dike system in the newboundary conditions fascinates many researchers in this field

To resolve the mentioned problems, we apply the “reliability-based framework” to

as-sess the Red River dike system, in which uncertainties of load and resistances are taken intoaccount, and the flood defences are considered in the new hydraulic boundary conditions(van Gelder,2000;Vrijling and van Gelder,2002) In the reliability-based approach, waterlevel and soil properties, which respectively represent the load and strength in the assess-ment, are modelled as random processes and these can be integrated in the risk-based anal-ysis The new hydraulic boundary is also investigated in the flood frequency of 1/500 yearsthat will threat the dike system with a long duration flood wave From such input boundary

1

1 conditions, the reliability indices of the whole dike system can be figured out probabilisti-cally Figures1.1&1.2illustrate the differences between the deterministic and probabilistic

Figure 1.1: Deterministic approach, of which load

and resistance are chosen from the mean values

and factors of safety are empirically given.

Figure 1.2: Probabilistic approach, of which the

char-acteristic values, or probability of failure is

- To understand the background conditions and related issues, including natural ditions, socio-economic issue, and flood defences development;

con To clarify the hydraulic boundary conditions in a flood frequency of 1/500 years;

- To synthesise the applicable probabilistic-based framework in geotechnical ing and to analyses the geotechnical conditions of the Red River dikes in the area of

prob-lems, and the suggested solutions, we addressed the main question as follows: “How to apply the reliability-based analysis framework to evaluate the safety of the Red River dikes in the flood frequency of 1/500 years?”

In order to answer the main research question, the following key questions are

gener-ated:

1 What are the natural and socio-economical conditions these influence the

assess-ment of the Red River dike in the Ha Noi area?

2 What are the hydraulic boundary conditions of the Red River in the area of Ha Noi?

3 How does the probabilistic-based approach take the uncertainties in geotechnical

en-gineering into account?

4 How do the geotechnical conditions influence the assessment of the Red River dikes

in the Ha Noi area?

5 How to assess the reliability of the Red River dike during a long duration flood?

6 How safe is the Red River dike during the flood with a frequency of 1/500 years?

TOobtain the objectives, we devised a numerical modelling approach to apply to the

hy-draulic boundary conditions, the geotechnical engineering conditions, and the

reliabil-ity analyses The hydraulic boundary conditions were modelled to figure out the stage

dis-charge relationship from observed data, and to predict the flood frequency of 1/500 years

On the other hand, the geotechnical data, gathered in different geological investigations

from both in situ and laboratory, were statistically analysed The reliability analysis was

based on the Monte Carlo simulations

Sections in Chapters5,6and7concern the hypotheses on the spatial variation of soil

properties and the piping failure mechanism Soil data in the study area are inadequate to

elaborate the spatial distributions, therefore several assumptions have been made to

sim-plify the ground conditions Nevertheless, the developed framework in this research can be

applied to another case studies if the in-situ tests are adequately obtained

THEstudy demonstrates a reliability-based assessment framework for the Red River dike

system with special consideration of the hydraulic boundary condition and spatial

vari-ation of soil properties The flood frequency of 1/500 years was elaborated to figure out its

effects to the safety margin of the Red River dike The spatial variability of soil

parame-ters was probabilistically evaluated to model the inherent uncertainties from ground

con-ditions

Several models have been applied to elaborate the geotechnical failure mechanisms in

dike assessment (e.g piping, uplift, internal erosion, and instability) Both theoretical and

experiment method in combination with observed data have been adopted to calibrate the

new models, becoming the reliable and applicable models

The assessment framework was adapted for the case study in the Red River and it also

es-tablishes a new research direction in an integrated field (hydraulic-geotechnical

engineer-ing) from reliability-based point of view The research focuses on the dike system itself

ex-cept other hydraulic structures such as dams, reservoirs, weirs, etc A risk-based framework,

with attention to consequences of failure of the flood defence system, can in the future be

investigated based on these research results

1 1.6. O RGANIZATION OF THE DISSERTATION

THEdissertation is organized into eight chapters, and the detailed outline is given in ure1.3 Chapter2synthesises the background conditions of the Red River Delta, includ-ing natural conditions, socio-economic issues, and flood defences development Chapter

Fig-3presents the analysis of hydraulic boundary conditions, by taking the system operation

of reservoirs and combination of flood discharge of three tributaries into account The sulting water level and flood duration will be integrated in the calculations in Chapters6

re-and7 Chapters4and5, in turn, demonstrate the theoretical probabilistic-based works in geotechnical engineering and the applications to the geotechnical conditions inthe Red River dikes Aims of Chapters6and7is to develop an assessment framework forriver dikes during a long duration flood, and to apply to the Red River dikes in the area of

frame-Ha Noi Finally, Chapters1and8contain the introduction, and the conclusions and mendations respectively

recom-Introduction (Chapter 1) INTRODUCTION

Analysis of Geotechnical Conditions

of the Red River dikes (Chapter 5)

Hydraulic Boundary Conditions

of the Red River Delta (Chapter 3)

Reliability Analysis of River Dikes During a Long Duration Flood Wave

The Red River Delta (RRD) is the second largest delta in Viet Nam with an area of around

15, 5001km2and a population of nearly 20 million The flood defences in the RRD havebeen established for several centuries with thousands of kilometres of dikes along the RedRiver system

This chapter provides an overview of the flood defence and water management in theRRD Section2.1describes the natural conditions, including the meteorological and thetopographic conditions Section2.2presents several socio-economic issues which couldinfluence the dike assessment in term of flood risk analysis Section2.3shows how the flooddefence and its safety standard have been improved over time; the current management

of flood defence will be presented in Section2.4 This chapter will be concluded with adiscussion section

be-tween latitudes from 8o270 to 23o230N, and longitudes from 102o80 to 109o270E The

total mainland area is around 330, 000 km2, which is divided into eight regions, namely theNortheast2, the Northwest, the Red River Delta, the North Central Coast, the South centralCoast, the Central Highlands, the Southeast and the Mekong River Delta, with a total of fiftyeight local provinces and five municipalities The Red River Delta is located in Northern

Viet Nam with an area of around 15, 500 km2, its latitude and longitude varies from 19o050

to 21o340and from 105o170to 107o070respectively, see Figures2.1and2.2

The Red River delta is the second biggest delta in Viet Nam, after the Mekong River Delta

in the South, but it is also the most crowded area in the country, including eleven provincesnamely Vinh Phuc, Ha Noi, Bac Ninh, Ha Nam, Hung Yen, Nam Dinh, Thai Binh, Hai Duong,

and Ninh Binh provinces.

North mountain area, similarly the North Central Coast and the South Central Coast are also merged into the Central Coast area.

5

Hai Phong, Ninh Binh, and Quang Ninh Ha Noi is the heart of the Delta of enormous litical, cultural and economic value, created through centuries of history In 1010, a kingnamed Ly Thai To chose Thang Long (the former name of Ha Noi) as the capital of the coun-try at that time There have been many changes during these thousands years, but this area

po-is still a political, economical centre of Viet Nam

2.1.1. TOPOGRAPHIC CHARACTERISTICS

Two main river systems, the Red River and the Thai Binh River (see Figures2.2,2.5&2.6),affect the topographical conditions in the RRD due to the characteristics of their catch-

Tuyen Quang Reservoir Hoa Muc Reservoir Thac Ba

Thao River Lo River

Da River Da River

Red River

Hoa Binh Reservoir

Day River

Day River

Dao River

Ninh Co River

Red River Thai Binh River

Kinh Thay River

Ha Noi

kilometres

HANOI IN REDRIVER DELTA AREA

Gulf of Tonkin (East Sea)

20 18 19

21 13

17 22

2 15

14 16

1 24

5 11

23 35

9

8

36 29

37

6 30

34

31 33

32 38

Nam Dinh Thai Binh

Hai Phong

N

Figure 2.2: Study area in the Red River Delta

ments areas In the upstream part of the Red River, over 47% of the catchment area is

high mountains, of which in the area of Viet Nam, the elevation is higher in the northwest

(over 1000 m3) and lower in the southeast (around 600 − 700 m) It is similar in the

north-west basin of the Thai Binh River with over 60% area of low hills (elevation in a range of

50−150 m) In the delta area, elevation varies from 7−15 m in the area of Ha Noi to 0.5−2 m

on the coast, and around 50% of the whole delta is lower than 2.0 m However, there remains

several rocky hills with an elevation of around 20 − 50 m in eight out of eleven provinces in

the RRD (except Thai Binh and Hung Yen province) The rocky hills lead to the variation of

ground conditions in the Red River dikes, for more details seeIWRP(2009);Khoi(2010) and

Figure2.3(adapted from the data of U.S Geological Survey http://glovis.usgs.gov/)

Dau station.

The annual evaporation rate is relatively high, around 1000 mm, with a maximum value

in June of 100 mm and a minimum in February of 56 mm The annual average temperature

is around 24 degree Celsius, but the average maximum temperature is 33 degree Celsius inJuly and the average minimum temperature is 14 degree Celsius in January The dominantwind directions in the rainy season are south and south-east, and the average monthly windspeed is around 1.6-2.1 m/s See Figure2.4andQCVN-2:2009/BXD(2009) for more infor-mation

As previously mentioned, the characteristics of topography and meteorology will affectthe hydraulic boundary conditions in the Red River, for instance the peak discharge and theincreased rate of the water level in a flood wave The Da River reaches its peak discharge inJuly, while the peak discharge of the Thao and Lo River are in August (see Figure3.3) Thisleads to a peak flood discharge in the Red River in August as well The lag time of the peakdischarge is due to the differences of topographic characteristics in the upper basin andthe climate patterns, which are also changed due to the regional atmospheric conditions

(IWRP,2009;Khoi,2010) The increase rate of water level in a flood wave is relatively high

in the upstream river with a range of 3−7 m per day, while it is around 0.5−1.5 m per day in

the delta area Recently, this increasing rate has changed due to the operation of reservoir

systems

NE NE SE SE SE SE SE SE SE NE NE NE

0 5 10 15 20 25 30 35

0 50 100 150 200 250 300 350

It is believed that the most critical situation will coincide when there is an extreme flood

in all three rivers, in combination with heavy rain in the downstream and a high storm surge

on the coast; however, the probability of this event is not investigated in this study, and even

it is expected to be very small (Khoi,2010)

2.1.3. RIVER SYSTEMS

The Red River and the Thai Binh River systems influence the water management in the

RRD with their total annual water volume of around 120 billion cubic metres The Red River

system constitutes the major part4of the delta, which has a total length of 1150 km and

a basin area of 143, 0005km2at Son Tay, including the Da River, Thao River, and Lo River

The characteristics of these rivers will be discussed in more detail in Chapter3 In the Thai

Binh River system, the total length is around 388 km and basin area is about 15, 180 km2,

including the Cau River, Thuong River, and Luc Nam River, which forms a minor part of

the delta Interestingly, the Thai Binh River system is named after Thai Binh although only

around 5 km of river flows through the area of Thai Binh province, see Figure2.5and Table

2.2

From the Red River, water will be diverted into the Duong River and the Luoc via the

Thai Binh river system The diversion rate at the Duong River is around 28 − 30% while

that of the Luoc River is around 10 − 14% of the total discharge from Son Tay Recently,

these diversion rates have been increased and led the dike system in the Thai Binh River

to overload with flood water (Khoi,2010;IWRP,2009) Typical flood discharges in different

gauge stations along the Red River indicate the decreasing discharge rate from the Ha Noi

area to the downstream, see Figure2.6for more information

at Pha Lai (three tributaries of the Thai Binh River, including the Cau River, Thuong River, and Luc Nam River,

nam area, 48% is in the China area and 1% in the Laos area.

a Ri ve r

T ha

ver

L

o Ri ve r

Red Ri ver

R

ed Rir

i Bi

nh Ri ve r

T ha

i B

inh Ri r

Dao River 1

Luoc River Thai Binh Tra LyRi

ver

1

2

7 6

5

4 3

Red River basin Ninh Co River 2

Cau River 3 Thuong River 4 Luc Nam River 5

Kinh Thay River 6

Figure 2.5: Simplified scheme of the Red River and Thai Binh River systems in the Red River Delta.

Gauging

River**

(*) the predicted values by taking the dike failures into account

2.2.1. RELEVANT SOCIAL CHARACTERISTICS

As a result of the practising agriculture and the cultivation of water-rice in the RRD, the Vietpeople have close-knit communities During thousand of years of fighting against water,

Figure 2.6: Distribution of river discharge in the Red River and Thai Binh River system.

people have organised themselves and have created a modus of living with water, from a

small embankment to protect the low land for cultivation to a complete flood defence

sys-tem

The population of Viet nam, in 2010, was around 87 million people; of which in the Red

River Delta around 19.7 million, and in Ha Noi over 6.7 million people The population

growth rate is around 1.03% per year and around 60% of population is in the age group of

15 − 60 years old The young population is a potential labour resource, but the population

pressure may also lead to the unemployment of the young people, seeGSO(2011) and Table

2.3for more details

During the economic reform, the urbanization has been expanded through areas with

different industrial zones, factories, and resident areas As a result, there will be

signifi-cant changes in land use and infrastructure, and the real estate will become more valuable

Therefore, a demand of a higher safety standard for the flood defences has been increasing

in the public

To raise the awareness of flood protection, education is supposed to be the key measure

providing not only public education but also higher level training for staff and professionals

A number of training projects have been conducted to help the Vietnamese Government to

strengthen their administration, governorship and research work (Binnie-Partners,1994b,a;

Dijkman et al.,1996)

( 2011 ).

Region Red River Northen Northern - Central Eastern Mekong Country

Delta Mountain - Central Coast Highland South Delta Population, mil 19,770 11,169.3 18,935.5 5,214.2 14,566.5 17,272.2 86,927.7 Area, sq km 21,063.1 95,338.8 95,885.1 54,640.6 23,605.2 40,518.5 331,051.3

G.O.C products, bill VND 185,286.1 51,703.7 106,689.5 21,138.1 128,663.0 55,238.6 548,719.0

FDI - tol reg cap, mil USD 47,443.2 2,856.5 41,458.0 772.8 93,694.0 10,257.5 196,482.0 A.M.I per capita, thousand VND 1,580 905 1,018 1,088 2,304 1,247 1,387 G.O.C ∼ Gross output construction mil ∼ million bil ∼ billion

FDI ∼ Foreign Direct Investment num ∼ number tol reg cap ∼ Number of registered capital A.M.I Average monthly income sq km ∼ square kilometre

2.2.2. ECONOMIC ISSUES

After 1986, the Doi Moi renovation helped to expand both size and quality of the economy

in Viet Nam, from the majority of state owned enterprises to the dominant non-state ownedsectors The economic value varies in proportion between agriculture, service, and industry,for instance agriculture values 19%, followed by service 38% and industry 43% respectively.The GDP growth rate and GDP per capita are illustrated in Figure2.7

From the previous discussion, it is apparent that the growth rate of the economy has

0 2 4 6 8 10 12

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Figure 2.7: Economic growth in the past 25 years in the period of 1986 − 2012, data from World Bank http://data.worldbank.org/country/vietnam.

increased the standard of living of the citizens in the RRD, but it has also lead to problems

in terms of water management People in the RRD now have a higher income, and the frastructure has been expanded into villages The values of real estate and other properties

in-in the protected regions have also in-increased Furthermore, the politically and economicallyimportant role of the capital of Ha Noi will also lead to a catastrophic consequence if thearea is flooded Therefore, the demand of a higher standard of safety in this area is created.However, the over-expansion of residents on the flood plain leads to the increase of waterlevel at the same discharge, which will threaten the dikes by overflow, see Chapter3

2.3.1. IMPERIAL PERIOD

The imperial period of Viet Nam started in the 11t h century6 However, the ancient Viet

already knew, in the centuries BC, how to build dikes to protect themselves in the low-lying

land of the RRD, as described in the Chinese historical documents (Khanh,1981)

Dur-ing the approximately one thousand years under Chinese rule, there was no noticeable

im-provement of the water management From the 1000s, the Ly emperors encouraged people

to construct dikes around the area of Ha Noi, the new country capital, and along the rivers

Subsequently, the following dynasties continued their fights against flooding in

combina-tion with protecting their country from the Northern invasions

Significant progress in hydraulic knowledge can be noted from the beginning of the 19t h

century during the Nguyen dynasties, when people measured the water levels and

strength-ened the dikes after each flood Hundreds of kilometres of dikes were built throughout this

period, and most of them located in the major municipal areas in the Red River Delta In

1857, Mandarin Nguyen Tu Gian proposed an integrated measures for flood protection,

in-cluding dams, reservoirs, and river dredging instead of heightening the dike crest level only

(Khanh,1981;Khanh et al.,1995)

A number of dike construction programs were undertaken by the Nguyen Emperors,

and therefore, the last geometry measurements of dikes and the typical flood in 1893 were

selected for the calculation of this period There were 38 dike-failures years during the 19t h

century with catastrophic economic damage and fatalities; in contrast, only 8 years of river

dike-failures were observed in the 20t hcentury At the end of this period, the dike systems in

the RRD were partly constructed, but at least it could protect against a water level of 10.5 m

equal to a flood frequency of 1/2–1/5 years (seeKhanh,1981;Khanh et al.,1995;FPD,2000)

2.3.2. FRENCH PERIOD

The French period is from the 1890s to 1945 when France fully established their rule in Viet

Nam At that time, the French were concerned with the construction of hydraulic structures

to protect cities from flooding, especially in the Ha Noi area The French engineers were

involved in the water management plan at that time, namely Norinandin, Peytavin, Rouen,

whose plans set a foundation for the flood mitigation in the Red River Delta Hundreds of

kilometres of dikes were built and strengthened following the dike improvement programs,

mainly from 1915 to 1926 such as the 1st and 2nd dike programs (Gauthier,1931;GDM,

1995;Khanh,1981)

The amount of earthwork carried out in the French period is the second biggest compare

to all four rehabilitation strategies, see Figure2.8 Consequently, the noticeable progress in

flood protection was made during this period, for instance the increase of the dike crests

from 10 5m to 13.0 m, and the construction of the Day flood gate to divert water to the

Day River in 1937 On the other hand, the safety levels were not equally established for

the whole delta, as for instance the flood in 1945 leading to dike failures in many locations

outside the Ha Noi area The French engineers also realized that it should be important to

deal with different safety criteria for each of the protected regions, but these plans had not

been completed because of the Vietnamese revolution in 1945 In other words, the dike

programs were accomplished as far as possible in 1945

2.3.3. VIET NAM WAR PERIOD

The third period is the Viet Nam war7, which resulted in the anomalies of the river dikes;the dikes, at that time, could be bomb targets or were excavated to slow down the enemies’assaults In terms of dike engineering, the anomalies of dike embankment came not onlyfrom its very construction, but also from such war activities Although, many rehabilitationprojects have since then been carried out in different dike systems in the RRD, they werestill threaten in high flood waves After the declaration of a new Viet Nam (by Ho Chi Minhand the pro-communist people), the new government also paid attention to strengthen theflood defences with the design water level equivalent to a flood frequency of 1/50 years.Millions of man-days had been mobilised for the earthworks from 1954 to 1965 It led theamount of the earthwork in this period to the first proportion, see Figure2.8

In 1971, the flood frequency of approximately 1/75 − 1/100 years, affected the Red RiverDelta with serious damage and fatalities, but the dikes around Ha Noi8were safe Afterthat, the Vietnamese engineers developed a plan to strengthen the flood defences by theconstruction of dams and reservoirs to redistribute river water discharge This is supposed

to be the most important measure of the flood mitigation in this delta, because the dikecrest heightening is limited due to its soft foundation and land-use in the highly populateddelta

The Hoa Binh hydro-power station was constructed in 1979 under the support fromthe Soviet Union It is estimated to cut down the flood discharge to 7 billion cubic metres(Khoi,2010) Consequently, the safety standard was established, for the Red River dikes,

equivalent to the design water level of 13.3 m in Ha Noi (around the water level in Ha Noi in

the 1971 flood)

2.3.4. DOIMOI PERIOD

The last period is the Renovation-of-Economy period (known as Doi Moi in Viet Nam) from

1986 till now Economically, the non-state enterprises were established and equally treated

as the state-owned sectors As a result, new technologies have been applied to the dikesafety in the projects funded by the Asian Development Bank (ADB) and by local govern-ments The safety level of the flood defence system has been significantly improved due

to the application of effective measures such as the reforestation in upper basins, the struction of dams and reservoirs to store water, the rehabilitation of dikes, and the raisingpublic awareness of flood protection The Hoabinh hydro-power station was completed in

con-1989 and others finished in 2007 and in 2012, which upgraded the total maximum storagevolume to around 8.5 billion cubic meters In short, the Red River dikes are now able towithstand a flood frequency of 1/250 years to 1/500 years compared to 1/50 years in theViet Nam war period (Khoi,2010)

2.3.5. REVIEW OF DIKE ENGINEERING IN FOUR PERIODS

During the past hundred years, overflow and the resulting erosion of the inner slope, whichcame from the limited height of the dike systems, caused many dike failures From theimperial period until now, the dike embankments have been heightened many times, and

of both 1945−1954 (the French restored their colonial rule till the end of the Dien Bien Phu Battle), and 1955−1975 (involvement of American Army - after the Geneva Conference to the fall of the South Viet Nam government)

its compaction ratio of the filling materials, K9, has varied considerably In addition, soil

properties and their distributions, which play important roles in the dike safety assessment,

also change underneath the dikes, particularly during a long flood wave

As can be seen from Figure2.8, the earthwork has been undertaken for hundreds of

years From 1955 to 1975, most of the earthwork had been done, followed by that of the

French period (1905 − 1945); the rest took place in the Doi Moi and the Imperial period

Figure2.9demonstrates how a dike has been constructed in different periods, from a very

low dike to a current massive dike (Gauthier,1931;GDM,1995;Khanh,1981;Khanh et al.,

1995) Figure2.10presents the decrease of the flood frequency and flood water levels, in

different periods of dike rehabilitations at a dike section at K m54 + 200, from 91% in the

1890s to 0.2% in 2000 SeeTu et al.(2012b) for more details

20.0 111.0

168.0

16.1

0.16 2.1

0.27 0

20 40 60 80 100 120 140 160 180

( 2000 ); GDM ( 1995 ); Khanh ( 1981 ); Khanh et al ( 1995 ).

5.0 2.6

5.0 3.0

5.0 1.5

20.0

5.0 1.9 4.5

2.5

5.5

Figure 2.9: An example of dike strengthening from the past to the present at Km 82 on the Red River Dikes in Ha

2.3.6. TYPICAL FLOODS IN VIET NAM FROM THE 1900S

There are several typical floods such as the flood in 1905, 1915, 1925, 1939, 1945, 1968, 1969,

1971, 1986 and 1996, which lead to the catastrophic damage for the RRD In this study, we

describe the details of the flood in 1971 (before the operation of the Hoabinh reservoir) and

the flood in 1986 (after the operation of the Hoabinh reservoir), which also caused the two

dike failures at Cong Thon and Van Coc on the Red River dike system in the area of Ha Noi

a density ∼ moisture curve in compaction laboratory test (ASTM D698 & ASTM D1557 ).

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

00 10 20 30 40 50 60 70 80 90 100

Figure 2.10: Decrease of flood frequency in different periods and factor of safety of dike embankment, data from

Tu et al ( 2012b ).

THE1971FLOOD

The flood in 1971 was the most severe flood in the 20t hcentury, as a result of an extremeflood in the Da River in combination with the medium large floods in both the Lo and ThaoRiver The total rainfall in August 1971 were relatively high, as consequence of the conflict-ing front and typhoon precipitation, in both catchment areas of three tributaries (e.g 843

mm at Ha Giang) and in the delta area (e.g 443 mm at Ha Noi) See Tables2.4&2.5for more

details The observed peak discharge at Son Tay was 34, 302 m3/s and it was predicted to

be 37, 800 m3/s after the restitution analysis, by taking the dike failures into account The

flood frequency was around 1/75 − 1/100 years and the measured water level at Ha Noi was13.9 m The Red River dike protected zone 1 (see Figure2.2) was safe but the dikes at otherslocations failed (e.g Khe Thuong – the Da River, Cong Thon – The Duong River, and Nhat

Trai – the Thai Binh River) The inundated areas were 2500 km2and resulted in 594 ties, affecting of around 2.7 million people (seeTinh(2000) and FigureA.2

fatali-The dike failure at Cong Thon in Gia Lam district of the city of Ha Noi took place at 20:00

on August 23, 1971, lagged 18 hours after the peak flood, and the water level was decreased

by 22 cm, see Figure2.11& Table2.5 Termite activity and piping were believed to be themain causes of this failure In Gia Lam district, the catastrophic damage included the in-

undation area of 9.1 km2, and 20 fatalities, see AppendixAandDMD-HN(2002) for moredetails

THE1986FLOOD

The flood in 1986 was a result of the very large floods in both the Thao and Lo rivers and

in combination with the medium flood in the Da River Consequently, 165 people lost theirlives, and a large area of the Phu Tho, Vinh Phuc, Bac Ninh, Bac Giang, and Ha Noi provinceswere inundated

The dike failure at Van Coc happened at 1:25 on July 28, 1986, when the water level

was 12 cm lower and 17 hours earlier than the peak of flood (see Figure2.11) Piping wasobserved and believed to be the cause of the dike failure The consequences of failure were

the inundation of an area of 22.9 km2with a huge damage See AppendixAfor more details

Table 2.4: Total monthly rainfall in the 1971 flood and 1986

flood.

Meteorology

12 13 14 15 16 17 18

Figure 2.11: Flood wave in the periods of August 14 − 28, 1971 and July 22 - August 9, 1986 at Son Tay gauging

station with indications of the dike failures at Cong Thon and Van Coc.

2.4.1. INSTITUTIONAL FRAMEWORK

On the state level, the Ministry of Agriculture and Rural Development (MARD) has full

re-sponsibilities for the water management and flood defence in the flood season The

Min-ister of MARD is also the vice-president of the National Committee for Flood and Storm

Control (NCFSC)10which will carry out the management of emergency actions to deal with

flooding and storms in the rainy season The General Water Management Office (GWMO)

under MARD, and the Department of Dike Management and Flood Control (DDMFC) under

GWMO, are responsible for dike management and technical support of the flood defences

in Viet Nam The DDMFC is also incharge of the new research and construction projects for

the dike improvement and flood protection

On the provincial level, the governor of each province will create the same system todeal with disaster and water management The Provincial Committee for Flood and StormControl (PCFSC) is set up from the different representatives of Departments on the provin-cial levels The Dike Management Division (DMD) under the Department of Agricultureand Rural Development (DARD) has the similar duties and responsibilities as the DDMFC,but in the lower level DMD may carry out new projects for dike improvement under theapproval of the DDMFC or the provincial government

On the district level, similar schemes will be established for both flood defence ment (dike brigade) and the District Committee for Flood and Storm Control (DCFSC) Ineach dike brigade, engineers are incharge of the local flood defence management and themaintenance work The dike brigade may be divided into smaller groups to deal with theactual situations during the flood season

manage-On the commune level, the Commune Committee for Flood and Storm Control (CCFSC)

is also established but there is no organization of dike management at this level However,the local authorities have responsibility to coordinate the local dike brigade (if present).Figure2.12presents a framework of the water management, disaster management systems;seeVrolijk(2002);IWRP(2009) for more information

2.4.2. FLOOD DEFENCE MANAGEMENT

Having discussed in the previous sections, the strengthening of the dike systems in the RRDhas been mentioned in the management of flood defences, considering it as a part of thewater management plan (e.g surface water, ground water, waste water, and drinking wa-ter) The flood risk in the RRD may come from both rivers and coast, as a result of heavyrainfall from typhoons, fronts, and tropical depressions However, high precipitation alsocomes from downstream areas, which leads to inundation inside the protected regions TheRed River flood defence system is defined as a system of dikes, and hydraulic structures (e.g.culverts, dams, reservoirs, flood gate, flood retention area) In this study, we will pay atten-tion to the system of dikes; the hydraulic structures are discussed in other research projects.Currently, the management of flood defences in the RRD follows the guideline in14TCN-

122(2002) with the following features:

- Design water levels of the dike in Ha Noi are equivalent to a discharge with frequency

of 1/500 at the Son Tay gauging station, under the system operation of a series ofreservoirs in the upstream rivers;

- In case of an extreme condition, water will be diverted into the Day River (if the waterlevel at Ha Noi is predicted to be higher than 13.4m and the storage capacities of thereservoirs reach their limits);

- The flood mitigation measures for the RRD include:

+ Strengthening dike system;

+ Dredging river bed (to increase flow discharge);

+ Constructing reservoirs;

+ Diverting flood discharge;

+ Reforesting in the catchment;

+ Intensifying awareness of flood protection