Reliability analysis of the Red River dikes system in Viet Nam

(1)

(2)

(3)

OF THE

R

ED

R

IVER

D

IKE SYSTEM IN

V

IET

N

AM

Proefschrift

ter verkrijging van de graad van doctor aan 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

Pham Q

UANG

TU

civiel ingenieur geboren te Ha Nam, Viet Nam.

(4)

Prof. drs. ir. J.K. Vrijling

Prof. dr. ir. P. H. A. J. M. van Gelder Samenstelling promotiecommissie:

Rector Magnificus, voorzitter

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

An electronic version of this dissertation is available at

(5)

pertaining to the dissertation

R

ELIABILITY ANALYSIS OF THE

R

ED

R

IVER

D

IKE SYSTEM

IN

V

IET

N

AM

by

Pham Quang T

U

1. Red River Dike safety is lower than expected, even in some so far undoubted locations (in this thesis).

2. Ground conditions contribute greatly to uncertainty in the assessment of dikes.

3. A long duration ﬂood might threaten the Red River dikes in the future when reservoirs are completed (in this thesis).

4. The highest failure probability of a dike occurs in the next few hours after the ﬂood peak (in this thesis).

5. The reduction of seepage length can be used to predict the piping process during repeated and long duration ﬂoods (in this thesis).

6. Integration of Hydraulic Engineering and Geotechnical Engineering is an extremely important and fertile research area.

7. Reliability-based analysis in Geotechnical Engineering is applicable in de-veloping countries.

8. The particular climatological conditions and the geography of lakes and es-tuaries contribute to the success of the ﬂood risk management in the Nether-lands.

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”(the Vietnamese proverb):“ﬂooding, ﬁring, robbing, and invading”.

10. The challenge of a PhD’s life is the same as that of learning a new sport but more 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.

(6)

behorende bij het proefschrift

R

ELIABILITY ANALYSIS OF THE

R

ED

R

IVER

D

IKE SYSTEM

IN

V

IET

N

AM

door

Pham Quang T

U

1. De veiligheid van de Rode Rivier Dijk is lager dan verwacht, zelfs in sommige tot nu toe onbetwijfelde locaties (dit proefschrift).

2. De grondgesteldheid draagt in grote mate bij tot onzekerheid bij de beoor-deling van dijken.

3. Een langdurige hoogwatergolf zal in de toekomst, wanneer reservoirs zijn voltooid, een bedreiging vormen voor de Rode Rivier Dijk (dit proefschrift). 4. De hoogste kans op het falen van een dijk treedt op na de piek van de

hoog-watergolf (dit proefschrift).

5. De verkorting van de kwellengte kan worden gebruikt om het pipings-proces tijdens herhaalde en langdurige overstromingen te voorspellen (dit proef-schrift).

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

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 overstro-mingsrisico’s in Nederland.

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

hỏa, đạo, tặc”(Vietnamees spreekwoord): " overstroming, brand, beroving

en invasie "

10. De uitdaging van het leven van een PhD is dezelfde als die van het leren van een nieuwe sport, maar interessanter en heilzamer.

Deze stellingen worden opponeerbaar en verdedigbaar geacht en zijn als zodanig goed-gekeurd door de promotoren prof. drs. ir. J.K. Vrijling en prof. dr. ir. P. H. A. J. M. van Gelder.

(7)

This dissertation synthesises the application of a probabilistic-based framework in geotech-nical and hydraulic engineering, for the assessment of the Red River dikes in Viet Nam. The study 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.

Dikes along rivers often spread over the deltaic environment and its earthen structures are parts of a long civilian history, from hundreds to thousands of years. Uncertainties of soil 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 flood defences, 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-based framework 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 is-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 flood defence management of the Red River Delta. Around 50% of the delta area is lower than 2 m (above mean sea level) and the delta is partly surrounded by high mountains. Conse-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 issues indicate a fast growing economy of Viet Nam. Hence, new requirements of higher safety standards for the flood defences seem advisable. In fact, the flood defences in the Red River Delta have been established for hundreds of years and the safety standard has increased from a design water level of 1/5 years (in the Imperial period ∼ the 1890s) to 1/500 years (at the present). The current design water level is 13.4 m (above mean sea level) at the gauging station of Ha Noi, which will be routed to the dike stretches in the whole delta area.

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 pa-rameters for the reliability analysis. The Red River is formed by three tributaries (the Da, 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 significantly reduce the peak flood discharge in the delta area by a storage capacity of 8.5 billion cubic metres; 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 water level is lower than the original. Another issue is the increasing trend of the water level at the 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 no measures to control such developments.

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

(8)

Chapter4and5, including classification of ground conditions, model uncertainties, and spatial 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 increase the engineer’s understanding about the use of probabilistic-based methods in practice. To take examples, uplift and ground coefficient are analysed by using the model factor. The former presents a physical-based model of uplift which was calibrated for the study area by field 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). The calculation 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 incorpo-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 Red River Dikes are discussed in Chapters6and7, where piping, overflow, and instability are considered under a long duration flood. Probabilities of overflow are predicted to be high in the dike stretches of the Ha Tay area due to a lower design water level in the past. There-fore, dike crests should be heightened to meet the same safety standards as that in the Ha Noi 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 erosion process depending on seepage gradient and ground conditions. The proposed model, after calibrating by a historical dike failure in the study area, predicts an increase of the erosion length 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 is proven to be less affected during a long flood wave. However, only homogeneous models of embankments are mentioned in this study. It is suggested that in future research more attention 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 dike stretches in the study area may significantly increase depending on its stretch length and the spatial variation of soil parameters, as for instance the total probability of failure of the dike 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 River Dike safety in Viet Nam. They also open up several research directions in the combined field of hydraulic engineering and geotechnical engineering, and widen the applications of probabilistic-based approaches in Viet Nam and developing countries.

(9)

In deze dissertatie wordt een synthese uitgevoerd van de toepassing van een op proba-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 is hier met name opportuun vanwege de typisch natuurlijke omstandigheden in dit onder-zoeksgebied. Een gebrekkig begrip van grond- en watercondities zou namelijk, zoals uit de praktijk gebleken is, tot mislukkingen in technische ontwerpen kunnen leiden. Deze studie is bedoeld om een deel van deze lacunes te vullen.

Dijken langs rivieren strekken zich uit over de delta en dergelijke grondconstructies zijn deel van een lange civiele geschiedenis, van honderden tot duizenden jaren. De onzeker-heden in de grondeigenschappen van dijklichaam en fundering dragen bij aan de faalkans bij een bepaalde waterstand. Om een inschatting te kunnen maken van de veiligheid van waterkeringen, 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 zijn nut 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 sociaaleconomische situatie en de waterkeringen in de regio wordt in hoofdstuk 2 gepresenteerd. De topogra-fische condities en de meteorologische omstandigheden spelen een belangrijke rol in de hoogwaterbescherming 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. Der-halve, onder extreme weersomstandigheden, zoals tyfoons, fronten en tropische depres-sies, zal het laaggelegen land door overstromingen getroffen worden. Omdat de huidige sociaaleconomische situatie van Viet Nam echter op een snel groeiende economie duidt, lijken nieuwe eisen voor hogere veiligheidsnormen voor de hoofwaterbescherming daarom raadzaam. 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 1/5 jaar (in de imperiale periode ∼1890s) tot 1/500 jaar (2014). De huidige ontwerpwaterhoogte is 13.4 m (boven gemiddeld zeeniveau) bij het meetstation van Ha Noi, en wordt vervolgens naar alle dijken in de gehele delta doorberekend.

In hoofdstuk 3 worden de hydraulische randvoorwaarden bepaald door een gedetail-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 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 kubieke meter 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

(10)

stig belast kan worden gedurende een langdurige hoogwaterstand van ongeveer 120 uur. Een ander aspect is een toenemend waterniveau bij gelijkblijvende afvoer, als gevolg van bewoning in de hoogwaterbedding, welke, indien geen maatregelen worden getroffen, kan resulteren 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 variatie van grondparameters, wordt in hoofdstuk 4 en 5 behandeld. Momenteel is het niveau 1 be-rekeningsmodel ingebed in de ontwerpnormen, terwijl berekeningsniveau 2 en 3 gebruikt worden 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 ingenieurs voor de toepassing van een probabilistische methode in het veld. Als voorbeeld worden het opdrijven van de afdekkende laag aan de binnenzijde van de dijk en de grondcoëfficiënt geanalyseerd met gebruikmaking van een modelfactor. Het eerste voorbeeld behelst een fysisch gebaseerd model van opwaartse druk welke in het studiegebied werd gekalibreerd door 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 illustratie van ruimtelijke inschatting gemaakt voor twee dataseries van de bovenlaagdikte (met een steekproefafstand 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 the-orieën en hun toepassingen in de geotechniek en toetsing van waterkeringen, in met name Vietnam, geïncorporeerd zullen worden.

In hoofdstuk 6 en 7 wordt de theorie van de betrouwbaarheidsanalyse van een rivier-dijk en de toepassing daarvan op de rivier-dijken van de Rode Rivier besproken; overstromen, instabiliteit en “piping” worden bekeken onder een langdurige hoogwaterstand. De kans op overstromen wordt naar verwachting hoog in de dijkvakken in de omgeving van Ha Tay ten gevolge van een lagere ontwerpwaterstand in het verleden. De kruinhoogte zou daarom verhoogd moeten worden om dezelfde veiligheidsstandaard als in het gebied rond Ha Noi te realiseren. Om “piping” (zandvoerende wel) gedurende een langdurige hoogwaterstand te evalueren, wordt een model van kwellengtereductie als functie van de tijd ontwikkeld van-uit een basisprincipe: namelijk het interne erosieproces dat afhangt van de kwelgradiënt en grondeigenschappen. Na de kalibratie van het falen van een dijk in het onderzoeksgebied voorspelt het voorgestelde model een toename in de lengte van erosie van 3% tot 20% (de lengte 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 interne erosie onder de dijk. Anderzijds wordt bewezen dat de geotechnische instabiliteit minder wordt 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 tot hun 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 dan was 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).

(11)

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

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

(12)

(13)

Luận văn này tổng hợp ứng dụng của phương pháp tính toán dựa trên lý thuyết xác 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 pháp tính toán dựa trên lý thuyết xác suất do những đặc thù riêng của vùng. Những hiểu biết không đầy đủ về tương tác giữa đất và nước có thể dẫn tới sai xót trong tính toán thiết kế công trình, điều này đã được chứng minh trong thực tế. Nghiên cứu này 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 hình thành trong lịch sử dài hàng trăm đến hàng nghìn năm. Những bất định tiềm ẩn trong thân đê hoặc dưới nền đê có thể ảnh hưởng đến an toàn đê dưới tác động của nước lũ. Để tiến hành đánh giá an toàn cho đê, cả phương pháp tiếp cận truyền thống (tất định - deterministic) và phương pháp độ tin cậy (reliability-based) thường được áp dụng. Phương pháp truyền thống chủ yếu dựa vào hệ số an toàn trong khi phương pháp độ tin cậy xét đến sự bất định (uncertainties) của mực nước sông (tải trọng) và chỉ tiêu cơ lý của đất nền (sức chống đỡ). Phương pháp độ tin cậy khẳng định hiệu quả trong các dự án lớn, quan trọng hoặc các thiết kế có sai số lớn trong các tham số đầu vào. Những nội dung này sẽ được thể hiện qua các chương trong toàn 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 văn đóng 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 đe dọa những vùng có cao độ thấp. Mặt khác, kinh tế Việt Nam đã và đang tăng trưởng nhanh, 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 nước thiết kế tương ứng trận lũ có tần suất 1/5 năm (giai đoạn phong kiến∼1890s) tới mực nước tương ứng trận lũ có tần suất 1/500 năm (giai đoạn hiện tại). Mực nước thiết kế là 13.4m tại trạm thủy văn Hà Nội và được truyền dẫn đi tới từng đoạn đê trên toàn đồng bằng.

Hai, những phân tích về điều kiện biên thủy lực được trình bày trong Chương 3 với 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 vào tháng 8 năm 1971 tại Sơn Tây, Hà Nội. Hệ thống hồ chứa trên sông Đà và sông Lô giảm đáng kể đỉnh lũ ở vùng đồng bằng với khả năng cắt lũ tổng cộng khoảng 8.5 tỷ m3, ví dụ như ở trận lũ tần suất 1/500 năm, đỉnh lũ sẽ giảm khoảng 40% từ 48,500 m3/s xuống còn khoảng 30,000 m3/s. Bằng cách điều tiết đó, hệ thống đê sẽ phải chịu 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

(14)

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

Ba, phương pháp phân tích dựa vào lý thuyết xác suất (probabilistic-based) và các 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 các tiêu chuẩn thiết kế trong khi các tính toàn với mức 2 (ví dụ: FORM) và 3 (ví dụ: Monte Carlo simulation) thường được áp dụng cho các dự án quan trọng hoặc để hiệu chỉnh các tiêu chuẩn thiết kế tính toán theo mức 1. Chỉ tiêu cơ lý và đặc điểm đất nền được phân tích với nhiều phương pháp khác nhau: phương pháp thống kê truyền thống hoặc phương pháp tính toán dựa trên lý thuyết xác suất (probabilistic-based). Điều này được kỳ vọng sẽ nâng cao hiểu biết của kỹ sư để sử dụng các phương pháp tính toán dựa trên xác suất trong thực tế thiết kế. Ví dụ như hiện tượng đẩy trồi và hệ số nền trong phân tích bằng hệ số mô hình. Phần phân tích hiện tượng đẩy trồi trình bày mô hình dựa trên cơ chế vật lý của hiện tượng có kết hợp hiệu chỉnh bằng thí nghiệm hiện trường. Phần phân tích hệ số nền xem xét các sai số dưới nền đê (đặc trưng là hệ sốα), hệ số này cũng có thể dùng để diễn tả hiện tượng tích lũy xói ngầm do ảnh hưởng của dòng thấm vào mùa lũ và mùa kiệt. Ngoài ra, một ví dụ minh họa phần ước lượng trong không gian với hai chuỗi số liệu về bề dày tầng phủ (với khoảng cách mẫu là 30m và 200m) cũng được trình bày trong chương 5. Kết quả tính toán chứng minh sự phụ thuộc của tỷ lệ biến đổi (sacle of fluctuation) và khoảng cách lấy mẫu. Những phương pháp tính toán nêu trên kỳ vọng sẽ được áp dụng trong phân tích 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 bày trong chương 6 và 7, trong đó xói ngầm, tràn đê và mất ổn định được xem xét dưới điều 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 Nội cũ). 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 đường thấm ban đầu. Mô hình chiều dài đường thấm giảm dần theo thời gian, sau khi hiệu chỉnh 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 tích lũy các nguy cơ tiềm ẩn dưới nền đê do xói ngầm và cát chảy. Mặt khác, phân tích ổn định 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, cần đánh giá sự ảnh hưởng này trong trường hợp thân đê không đồng nhất, đặc biệt là về tính 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 hưởng của 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 đến length-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 tốt hơn về hệ thống đê sông Hồng ở Việt Nam. Từ đó, những hướng nghiên cứu mới cũng được mở ra trng lĩnh vực đa ngành giữa địa kỹ thuật và thủy công, đồng thời

(15)

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

(16)

(17)

Summary v

Samenvatting vii

Tom tat xi

1 Introduction 1

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

(18)

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

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

5.5.2 Probabilistic classification method. . . 76

5.5.3 Analysis results. . . 77

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

5.6.2 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

5.7.2 Probabilistic analysis of the ground uncertainties . . . 85

(19)

5.8 Spatial Estimation of top-layer thickness . . . 87

5.8.2 Spatial estimation of soil thickness. . . 88

5.8.3 Results. . . 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

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

8 Conclusions and Recommendations 129 8.1 Conclusions. . . 129

8.1.1 Conclusions concerning Chapter 2. . . 129

8.1.5 Conclusions concerning Chapter 6 and 7 . . . 131

8.2 Recommendations . . . 132

References 133

Appendices 143

A Historical Dike Failures in the Red River Delta 145 B Multiple Linear Regression 149 C Kriging Estimation - BLUE 151

(20)

D Soil Property along the Red River dike 153 E Observed Sand Boils at Sen Chieu in the Red River 161 F Model Factor for Uplift Test on the Thaibinh Formation 165 G Reduction of Seepage Length During High Water Level 167 H Solutions for Groundwater Flow 171 I Decay and Delay of Groundwater Flow 177 J Observed Phenomena During Flood Seasons in the Red River Dike 181

Acknowledgements 187

Loi cam on 189

Curriculum Vitæ 191

(21)

1

I

NTRODUCTION

1.1. BACKGROUND AND PROBLEM STATEMENT

T

HERed 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 the increase 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 to overflow, 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 to lower flood risk in the RRD, includes the reforesting in the upper basins, the constructing dams 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 has been 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, determin-istic evaluation and inadequate hydraulic boundary conditions. The determindetermin-istic evalua-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 the values 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 to 1/250 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 new boundary 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 into account, 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, water level 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 years that will threat the dike system with a long duration flood wave. From such input boundary

(22)

1

conditions, the reliability indices of the whole dike system can be figured out probabilisti-_{cally. Figures}_1.1_&_1.2_{illustrate the differences between the deterministic and probabilistic} approach. Variables L,R Frequency µR µL Resistance Load

Figure 1.1: Deterministic approach, of which load and resistance are chosen from the mean values and factors of safety are empirically given.

Variables L,R

Probability density function: pdf

µL µR

Load Resistance

Mean safety margin

R_char L char P f L_char.γL<Rchar/γR L* R*

Figure 1.2: Probabilistic approach, of which the

de-sign values (R∗and L∗) are chosen from the

char-acteristic values, or probability of failure is

calcu-lated in this form Pf= Î

Z <0

fRL(R < L)dRdL.

1.2. OBJECTIVES

T

He aim of this research is to develop a probabilistic-based framework to evaluate the safety of the Red River dikes system derived from the current state of the art in flood risk analysis. This is also a chance to transfer and to apply the most advanced knowledge developed at the top-world level (in the Netherlands) to the developing countries (in Viet Nam). Scientifically, this research is expected to fill the gaps between geotechnical engi-neering and hydraulic engiengi-neering, and to create new research directions: dike engiengi-neering and flood risk. Therefore, the following objectives of the assessment of the Red River dikes are derived:

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

- To clarify the hydraulic boundary conditions in a flood frequency of 1/500 years; - To synthesise the applicable probabilistic-based framework in geotechnical

engineer-ing and to analyses the geotechnical conditions of the Red River dikes in the area of Ha Noi;

- To create an assessment framework of the Red River dikes during a long duration flood wave and to evaluate the safety margin of the Red River dikes in the area of Ha Noi.

1.3. RESEARCH QUESTIONS

C

ONCLUDINGfrom the above mentioned objectives, the background knowledge, the

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?”

(23)

1

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?

1.4. APPROACHES AND METHODS

T

Oobtain 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.

1.5. CONTRIBUTIONS AND SCOPE

T

HEstudy 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.

(24)

1

1.6. ORGANIZATION OF THE DISSERTATION

T

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

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 re-sulting water level and flood duration will be integrated in the calculations in Chapters6

and7. Chapters4and5, in turn, demonstrate the theoretical probabilistic-based frame-works in geotechnical engineering and the applications to the geotechnical conditions in the Red River dikes. Aims of Chapters6and7is to develop an assessment framework for river dikes during a long duration flood, and to apply to the Red River dikes in the area of Ha Noi. Finally, Chapters1and8contain the introduction, and the conclusions and recom-mendations respectively. Introduction (Chapter 1) INTRODUCTION BACKGROUND BOUNDARY CONDITIONS THEORY APPLICATION CONCLUSION

Flood Defences Management in the Red River Delta (Chapter 2)

Conclusions and Recommendations (Chapter 8)

Probabilistic-based Analysis in Geotechnical Engineering (Chapter 4)

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 (Chapter 6)

Probabilistic Analysis of the Red River Dike of Viet Nam (Chapter 7)

(25)

2

F

LOOD

D

EFENCES

M

ANAGEMENT IN

THE

R

ED

R

IVER

D

ELTA

The Red River Delta (RRD) is the second largest delta in Viet Nam with an area of around 15, 5001km2_{and a population of nearly 20 million. The flood defences in the RRD have}

been established for several centuries with thousands of kilometres of dikes along the Red River system.

This chapter provides an overview of the flood defence and water management in the RRD. Section2.1describes the natural conditions, including the meteorological and the topographic conditions. Section2.2presents several socio-economic issues which could influence the dike assessment in term of flood risk analysis. Section2.3shows how the flood defence 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 a discussion section.

2.1. N

ATURAL CONDITIONS

V

IETNAM, a country in Southeast Asia, has a shape of the letter “S”, and is located

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 the Northeast2, the Northwest, the Red River Delta, the North Central Coast, the South central Coast, the Central Highlands, the Southeast and the Mekong River Delta, with a total of fifty eight 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 provinces namely Vinh Phuc, Ha Noi, Bac Ninh, Ha Nam, Hung Yen, Nam Dinh, Thai Binh, Hai Duong,

1_{The total area of eleven provinces in the RRD is around 21, 000 km}2_{including the mountain areas of Quang Ninh}

and Ninh Binh provinces.

2_{In the yearbook of the General Statistics Office of Viet Nam, the Northeast and Northwest are merged into the}

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

(26)

2

Hai Phong, Ninh Binh, and Quang Ninh. Ha Noi is the heart of the Delta of enormous po-litical, cultural and economic value, created through centuries of history. In 1010, a king named 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 is still a political, economical centre of Viet Nam.

Figure 2.1: Viet Nam from space, adapted fromHeinimann and Andreas(2004).

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

(27)

catch-2

Tuyen Quang Reservoir Hoa Muc Reservoir Thac Ba

Thao River Lo River

Da River Da River

Red River

Hoa Binh Reservoir

Day River

Dao River Ninh Co River Red River Thai Binh River

Kinh Thay River Cau River

Pha Lai

Thuong River

Luc Nam River

Red River Tra Ly River Luoc River Duong River Reservoir Cam River Han River

Van Uc RiverLach Tray River

10 20 Hanoi area LEGEND Protected area River Ha Noi 0 20 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 3 10 4 7 26 28 27 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/).

3_{All the elevations mentioned here is above the mean sea level, which is known as the national datum at the Hon}

(28)

2

Figure 2.3: Topography of the study area in Red River Delta, from the Landsat 4 image.

2.1.2. M

ETEOROLOGICAL CONDITIONS

Weather patterns, including rainfall, evaporation, temperature, and wind speed, will affect the ﬂood management in the study area. Precipitation due to different sources (e.g. con-ﬂict front, tropical depression, and typhoon) is distributed in the rainy season, from May to October, but mainly in the period of July - September. The monthly average rainfall varies from 200 mm to 300 mm and the annual average from 1500 mm to 2000 mm. The propor-tion of rainfall in the rainy season is over 80% of the total annual rainfall. In some extreme climate conditions, the maximum rainfall in different periods of the river basin is relatively high, see Table2.1for more details.

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 in July and the average minimum temperature is 14 degree Celsius in January. The dominant wind directions in the rainy season are south and south-east, and the average monthly wind speed 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 affect the hydraulic boundary conditions in the Red River, for instance the peak discharge and the increased rate of the water level in a ﬂood wave. The Da River reaches its peak discharge in July, while the peak discharge of the Thao and Lo River are in August (see Figure3.3). This leads to a peak ﬂood discharge in the Red River in August as well. The lag time of the peak discharge is due to the differences of topographic characteristics in the upper basin and the climate patterns, which are also changed due to the regional atmospheric conditions

(29)

2

(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.

(30)

10 2 . F L O O D D E F E N C E S M A N A G E M E N T IN T H E R E D R IV E R D E L T A

Table 2.1: Maximum rainfall in one day at stations in the catchment areas of the Red River system, data fromQCVN-2:2009/BXD(2009).

Meteorology

River Maximum rainfall (mm) in different periods of time (minute)

Station 10 30 60 90 120 240 480 720 1440

Lai Chau Da 23.3/1962 46.9/1962 68.4/1971 83.1/1971 95.6/1971 116/1971 151/1971 174/1971 229/1971

Dien Bien River 22.1/1972 51.4/1975 69.7/1972 89.6/1975 94.3/1975 126/1975 168/1975 175/1975 200/1975

Son La - 29.1/1979 59.8/1979 85.4/1979 114/1979 128/1979 135/1979 137/1979 138/1979 171/1979

Hoa Binh - 31.8/1963 64.3/1963 86.2/1965 96.7/1965 101/1965 146/1962 203/1962 238/1962 283/1975

Lao Cai Thao 29.6/1961 71.1/1961 93.7/1960 103/1960 105/1960 151/1971 185/1971 185/1971 191/1971

Sa Pa River 29/1983 68.3/1963 106/1963 N/A 141/1963 160/1963 163/1963 196/1971 300/1968

Yen Bai - 29.9/1961 70.1/1961 94.3/1961 101/1961 130/1964 149/1964 175.9/1973 187/1973 198.9/1966

Ha Giang Lo 26.8/1974 58/1979 77.6/1979 95.4/1979 114/1966 144/1973 183/1961 200/1965 239/1965

Tuyen Quang River 32.1/1969 61.5/1969 89.4/1969 98.9/1969 108/1964 122/1964 130/1964 158/1961 211/1960

Viet Tri - 31.4/1973 55.8/1977 82.4/1975 99.7/1975 106/1975 128/1975 190/1975 292/1973 372/1976

Son Tay Red 30.7/1970 62.5/1970 89.7/1970 120/1970 133/1966 232/1971 281/1971 412/1971 508/1971

Ha Noi River 35.2/1968 56.8/1968 94/1967 114/1967 116/1967 130/1972 174/1972 180/1972 234/1972

Nam Dinh - 30.7/1965 63.1/1965 151/1977 181/1977 191/1977 218/1977 222/1977 236/1977 250/1975

Ninh Binh - 40/1974 80.9/1974 114/1978 160/1978 192/1978 232/1978 248/1978 340/1978 529/1978

(31)

2

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 1 2 3 4 5 6 7 8 9 10 11 12 Te m p, (De g) /W in sp ee d (m/ s) R ain fa ll (m m ), H um ini ty, (% ) Month Rainfall Huminity T_average T_max T_min Wind speed NE: Northeast dominant wind direction

Figure 2.4: Meteorograph at the Lang station of the Ha Noi area, fromQCVN-2:2009/BXD(2009).

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.

4_{In the flood season, the peak discharge at Son Tay (the Red River) is about 12 − 16 times of the peak discharge}

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

exclusive the diverted discharge from the Duong River, see Figure2.5).

5_{The total basin area of the Red River and the Thai Binh River is around 169, 000 km}2_{, of which 51% is in the Viet}

(32)

2

CHINA VIET NAM Viet Tri Lai Chau Son La Hoa Binh Tuyen Quang Thac Ba D a Ri ve r T ha o Ri ve_r L o Ri ve r Red Ri ver R ed Ri ve r Duong River D ay R ive r D ay Ri ve r Son Tay Ha Noi LEGEND

Flood gate at the Day river Gauging station in study area

4 Border

Dam and reservoir

Re d Ri ver N Gulf of Tonkin D ay R iv er Van Uc River Pha Lai T ha i Bi nh Ri ve r T ha i B in_h Ri ve r Dao River 1 Luoc River Thai Binh Tra Ly_Ri 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

7

Thai Binh River basin

Nam Dinh Hai Phong L A O S V IE T N A M Borde r

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

Table 2.2: Maximum flood discharge and water level in the Red River system, data fromKhoi(2010);IWRP(2009).

Gauging

River**

Basin area Maximum flood

Station (km2) Qmax(m3/s) Hmax(m) Month-year

Lai Chau Da 33,800 14,200 190.00* 8-1945

Hoa Binh Da 51,800 22,700 24.35 8-1996

Lao cai Thao 41,000 8,430 86.85 11-1908

Yen Bai Thao 48,000 10,300 34.86 8-1971

Thac Ba Chay 6,170 3,590 29.60* 8-1971

Ha Giang Lo 8,330 4,010 106.04 8-1969

Tuyen Quang Lo 29,800 12,000 31.87 8-1971

Son Tay Hong (Red) 143,700 37,800* 16.83 8-1971

Ha Noi Hong (Red) 25,500* 14.80 8-1971

Thuong Cat Duong 10,800* 14.30 8-1971

Nam Dinh Dao 7,750 5.77 8-1971

Trieu Duong Luoc 2,650* 7.77 8-1971

Quyet Chien Tra Ly 3.140* 6.75 8-1971

Phu Hao Hong 11,900 5.97 8-1971

Truc Phuong Ninh Co 2.270* 3.92 8-1971

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

(**) see the simplified scheme of the River systems in Figure2.5

2.2. SOCIO-ECONOMIC CHARACTERISTICS

2.2.1. RELEVANT SOCIAL CHARACTERISTICS

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

(33)

2

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;

(34)

2

Table 2.3: Summary of socio-economic characteristics of different regions in Viet Nam in 2010, data fromGSO

(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 Density, person per sq km 939 117 197 95 617 426 263 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 - num. of project 3,682 345 809 135 7,746 678 13,395 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 owned sectors. 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 1985 1990 1995 2000 2005 2010 G D P g ro wt h , (% ) G DP per ca p it a, ( U SD) ,G DP, (10 ^8 U SD) Year GDP GDP_Capita GDP_growth

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 in-frastructure has been expanded into villages. The values of real estate and other properties in the protected regions have also increased. Furthermore, the politically and economically important role of the capital of Ha Noi will also lead to a catastrophic consequence if the area 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 water level at the same discharge, which will threaten the dikes by overflow, see Chapter3.

(35)

2

2.3. D

EVELOPMENT OF FLOOD DEFENCES AND SAFETY STANDARD

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.

(36)

2

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 only from its very construction, but also from such war activities. Although, many rehabilitation projects have since then been carried out in different dike systems in the RRD, they were still threaten in high flood waves. After the declaration of a new Viet Nam (by Ho Chi Minh and the pro-communist people), the new government also paid attention to strengthen the flood 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 the amount 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 River Delta with serious damage and fatalities, but the dikes around Ha Noi8were safe. After that, the Vietnamese engineers developed a plan to strengthen the flood defences by the construction 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 dike crest heightening is limited due to its soft foundation and land-use in the highly populated delta.

The Hoa Binh hydro-power station was constructed in 1979 under the support from the 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 dike safety 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 con-struction of dams and reservoirs to store water, the rehabilitation of dikes, and the raising public awareness of flood protection. The Hoabinh hydro-power station was completed in 1989 and others finished in 2007 and in 2012, which upgraded the total maximum storage volume to around 8.5 billion cubic meters. In short, the Red River dikes are now able to withstand a flood frequency of 1/250 years to 1/500 years compared to 1/50 years in the Viet 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, which came from the limited height of the dike systems, caused many dike failures. From the imperial period until now, the dike embankments have been heightened many times, and

7_{The Viet Nam war is widely recognized in the period of 1955−1975; hereafter, we use this definition for the periods}

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)