Some hydrodynamic problems for flood forecasting and flood control in the red river system

Vietnam Journal of Mechanics, VAST, Vol. 29 , No. 3 (2007) , pp. 271 ·· 283 Special Issue Dedicated to the Memory of Prof. Nguyen Van Dao SOME HYDRODYNAMIC PROBLEMS FOR FLOOD FORECASTING AND FLOOD CONTROL IN THE RED RIVER SYSTEM NGUYEN VAN DIEP Institute of Mechani cs, VAST Abstract. My research & development acti vity in t he field of environmental & natura l fluid mechanics ha5 been started a fter one year 's working visit , proposed by Prof. Nguyen Van Dao, at the Laboratoire National d 'Hydraulique de France in C hatou , France ( 1979- 1980) . Until now this activity is still a most important one. In the paper it is presented some selected scientific results in one of hydrodynamic problems for flood forecasting and flood control : developing of the lD hydraulic model, lD & quasi 2D model, ID hydraulic model for dam break flow , 2D hydraul ic model, coupling of lD and 2D hydraulic models and some theirs applica tions for flood forecas ti ng and flood control in the Red River System. This paper is dedicated to the memory of Prof. Nguyen Van Dao, wi t h whom 1 had a big chance to work and to collaborate during about 30 years, to whom I would like to express my heartfelt thanks. 1. PROF. NGUYEN VAN DAO AND MY RESEARCH AND DEVELOPMENT ACTIVITIES IN THE ENVIRONMENTAL AND NATURAL FLUID MECHANICS In 1977 year I have finished my thesis for obtaining the degree of Doctor of Science in the Moscow University and returned to Vietnam. At this time Prof. Nguyen Van Dao has been the General Secretary of the Vietnam Institute of Science. With his invitation and by his suggestion I have been appointed to a position of the Chief of Laboratory of Mechanics. During my long stay in Moscow University - a palace of fundamental sciences uot only in the former Soviet Union, but also in the vVorld, I tried to learn and to find out about perspective trends in modern development of mechanics as a branch of fundamental sciences. In my understandings and conclusions , t he most perspective t rend in modern development of mechanics is following: mechanics needs to be originated from the teclmi- cal and technological developments, from the problems of exploitation and protect ion of ,natural resources and environment [1]. At this time, I have been interested only in two problems of mechanics: mechanical problems for oil & gas exploitation & t ransportat ion and aero-dynamical problems for military technique developments. But suddenly, in 1979 Prof. Nguyen Van Dao has proposed me to take a working visit at Laboratoire National d'Hydraulique de France in Chatou, France (1979-1980) . This visit has been organised by Dr. Nguyen Quoc Son and Dr. Dang Van Ky, research directors of Laboratoire de Mecanique des Solides, L'Ecole Polytechnique de Palaiseau, P aris. During one year 's st ay in this laboratory, I tried t o learn how to apply fundamental 272 Nguyen Van Diep problems of hydromechanics for solving practical issues in water resources management in Vietnam. From this time, the environmental & natural mechanics became one of my most important research and development activities. With my initiative and efforts the Institute of Mechanics became one of strongest research units working in the field of Environmental & Natural Fluid Mechanics, and is responsible for realizing different research projects in the level of national and international programs in this field. The main of these are: - Research Project on Salinity Intrusion Prediction Methods for Mekong River Sys- tem (National Research Program on Mekong Delta Water Recourses Research, 1981-1985, Ministry of Science and Technique). - Research Project on Litho-hydrodynamic Processes (National Marine Research Pro·- gram, 1986-1990, Ministry of Science, Technology and Environment). - Research Project on Creation of Numerical Modeling Technology for Flood Control in the Red River System (National Research Program on Flood Control for the Red River System, 1999-2000, Ministry of Agriculture and Rural Development). - Research Project on the Scientific Bases for Flood Forecasting and Flood Control in the Red River System (National Research Program on Environmental Protection and Nakral Disaster Prevention, 2001-2004, Ministry of Science and Technology) [2]. - Research Project on Decision Support System for ecosystem upgrading and flood control of a sustainable development in the Red River System (China, Vietnam), Pilot Phase (supported by the European Commission, 2001-2004) [3]. - Research Project on building regulation rules for operation of Hoa Binh, Thac Ba, Tuyen Quang and Son La reservoirs to guarantee safety to flood protection in the Red River Basin (National Research Program on Multipurpose Multiple Reservoir Operation, 2006-2007, Ministry of Agriculture and Rural Development). In realizing these projects many scientific research results have been transferred to and used in some organizations, for example, the Central Committee for Flood and Storm Pre- vention, National Centre for Meteor-Hydrological Forecasting and other water resources management and planning organizations. To dedicate to the memory of Prof. Nguyen Van Dao, in this paper there will be presented some my research activities in flood forecasting and flood control in the Red River Basin [2-9]. 2. FLOOD FORECASTING AND FLOOD CONTROL IN THE RED RIVER SYSTEM Red - Thai Binh River System is the second biggest river system in Vietnam, after Mekong River. In recent years, big floods frequently happened in Vietnam, and flood disaster causes massive losses of human life and immense damages. To reduce these damages caused by floods , for short-term and long-term flood prevention and control in the Red - Thai Binh River basin followings measures are taken: strengthening dike systems, clearing river flows for flood discharge, building reservoirs to reduce floods in upstream of big rivers, diverting and retaining floods, reforesting and protecting watersheds, intensify dike management and protection [2, 3]. Some hydrodynamic problems for flood forecasting ... 273 Before taking any of these measures, important information and data must be provided or predicted, and advanced modeling technologies are a privileged tool for such decision - making. The Institute of Mechanics has been involved in many research projects concerning the problem of integrated water resources management in the Red River Basin. Here it is mentioned some projects supported by European Commission, by the Ministry of Science & Technology, Vietnamese Academy of Science & Technology and Ministry of Agriculture & Rural Development as indicated in the Introduction. ·.• CH fN .1\ Legend Nation boundary River basin boundary River Sea Fig. 1. Red River Basin (source: VIWRP, MARD) In participating in these projects, the Institute of Mechanics has collected and created a data base, has developed and used different modeling tools - first elements of DSS for flood control & management in the RRB [2 - 9]: - Database - Hydrological model - One and quasi two dimensional hydraulic model - Flood forecasting model using a hydrological & one and quasi two dimensional models - Two dimensional hydraulic model - Flash flood forecasting model using a hydrological, one and two dimensional models - One and two dimensional hydraulic dam & dike break flow model 274 Nguyen Van Diep - Socio-economic model for evaluating damages caused by flood - Pilot Decision Support System coupling above indicated models [Fig. 2] So{'fo- t'('.On:9nl ic G!S, R~.m<~ l~ St~llSiu~ Fig. 2. Pilot Decision System for Flood Control in the Red River Basin In the following parts it is selected only some investigat ions of the Institute of Me- chanics in developing hydraulic models for flood forecasting and flood control in the Red River System [2-6]. 3. ONE- AND QUASI-TWO DIMENSIONAL HYDRAULIC MODEL FOR THE COMPLEX RIVER NETWORK - VALIDATION AND APPLICATION IN THE RED RIVER SYSTEM 3.1. Basic Equations The free surface flow in a single branch of a river network can be described by t he so-called Saint-Venant equations [2 , 3] under t he assumption of a hydrostatic pres_surc and uniform distribution of the velocity along the vertical axis. In practice, the flow in the main channel and flood plain arc quite different due to different frictions. So the following type of equations is often used: aAs aQ _ O at + ax - ' (3. 1) aQ a (Q2 ) az Q IQI at+ ax AJ + gAJ ax + gAJ I<2 = O, (3.2) where Z - water elevation, Q - discharge, As and A f - wet cross sectional areas for the main and total flow, I< - conveyance, t - time, x - space coordinate. Some hydrodynamic problems for flood f orecasting ... The continuity equation for storage cells is following dV = """'Q dt ~ where V is the water volume, Q is in- and out- going discharges. 275 (3.3) The flow must be conservative, so at confluences or tributaries the sum of all discharges must be zero. At hydraulic structures flow rate is defined by the empirical formul a: Q = f (Za , Zht , a) (3.4) where Za, Zht are the upstream and downstream water levels and a is characteristic parameter of structures. A structure may be modeled by one of two types: a spillway and a sluice. Finally the initial and boundary conditions must be added . 3.2. Method of solution To get numerical solutions the considered river network is split into river branches, separated by nodes [2, 3]. A node is point in a river system where the Saint-Venant equations are not valid. A confluence or a tributary is a node. For a hydraulic structure in rivers it is associated with two nodes- upstream and downstream due to di fferent water levels. If the network consists of storage cells, that make the problem to be quasi-two dimen- sional, they are nodes also. Therefore structures that link rivers and storage cells have two nodes too. The numerical method is based on the implicit 4-points Preissmann method for river branches, implicit finite difference scheme for storage cells and linking discharges of hy- draulic structures. 3.3. Validation of the model by test-cases developed in European hydraulic laboratories To verify 1-D hydraulic model, one can use the test cases developed by the European hydraulic laboratories. These 12 test cases are divided into 3 groups as follows: Test case No 1: Released wave in a rectangular channel , No 2: Steady sub-critical flow in a rectangular channel branch, No 3: Steady flow in a network with sub discharges, No 4: Steady flow through a structure, No 5: Dynamic wave, No 6: Diffusion wave, No 7: Kinematical wave, No 8: Wave through a reservoir, No 9: Local disturbance in steady flow, No 10: Steady flow for non-uniform geometry, No 11 : Unsteady How in complex riverbed, No 12: Tributaries. The model has been verified by the 12 above mentioned test cases [2-3]. 3.4. One and quasi-two dimensional hydraulic model for Red River System For the application of the model to the Red-Thai Dinh river network, it is necessary to develop 1 & quasi-2D hydraulic model [2-3] . This model is constructed to adapt the existing flood protection rules: - Simulating flood in river network, - Simulating flood into Day river through the flood diversion structure of the Van Coe Sluice and the Day Dam when t he water level at Hanoi is above 13.Gm, - Simulating flood into retention area if the flood diversion does not reduce the water level at Hanoi bellow 13.6 m. 276 Nguyen Van Diep The calculation network is shown in Figure 3. It is quasi-two dimensional and consists of a river network, flood diversion areas in the Day river catchments, retention areas Tam Thanh, Lap Thach, Luong Phu, Vinh Tuong. Discharge boundaries of the network are given at stations: Hoa Binh on the Da river , Yen Bai on the Thao River, Thac Ba on the Chay river , Tuyen Quang on the Lo river, Lien Son on the Pho Day river, Phu Cuong on the Ca Lo river, Thac Huong on the Cau river , Cau Son on the Thuong river, Chu on the Luc Nam river, Chi Thuy on the Tich river and Hoang Long on the Hung Thi river. Water level boundaries are given on 9 estuaries: Day, Ninh Co, Ba Lat, Tra Ly, Thai Binh, Van Uc, Lach Tray, Cam, Da Bach. The cross sections (topographical data) are measured in period 1999-2000 years. For the Red - Thai Binh river network special treatments are needed to simulate flow through the Van Coe sluice and the Day dam because of their special operational rules and also to simulate flows from the river to the retention areas. In the case of big floods , the Reel River Delta is split split into 278 storage cells using the DEM of 1/50,000 scale (see Fig. 3). Fig. 3. The Red River Delta split by cells Thecomputational schema of lD hydraulic model for the Reel River System is presented in Fig. 4 Some hydrodynamic problems for flood forecasting .. . Fig. 4. Computational schema of lD hydraulic model for the Red River System 4. SOME NUMERICAL METHODS FOR SOLVING THE 1-D SAINT-VENANT EQUATIONS OF GENERAL FLOW REGIME - APPLICATION TO DAM BREAK FLOWS 4.1. Equations and numerical methods Let us consider the Saint-Venant equations of the form [4, 5] where aA aQ at+ ax= q, aQ + ~(Q2 +gli) = sd at ax A ' 277 (4.1) (4 .2) And t is time, x - space coordinate, A - wet cross sectional area, Q - discharge, g - gravity, Ii, h - account for pressure forces, S0 - bed slope, Sf = l ;J~ -bed friction, K - conveyance, q - literal unit discharge. By using the change rule of derivatives for P(x, A) = gfi (x, A) one gets another forms of source terms: aPI aP aAI Sd= - +- - -gASt ax A=const aA ax z= const ) (4 .3) (ali ah) Sd=gA(So-S1)+ --A- , ax ax ( 4.4 ) 278 Nguyen Van Diep (aii az) Sd = -gASJ + - - A- , ax ax (4.5) where z is the water level. In the vector form one can write: au aF(x, U) = S( U) at + ax x, ' (4.6) where U = U(x, t) = ( ~ ) , F(x, U) = ( Q2 Q ) , A+gli S(x, U) = ( L ) . Numerical methods for conservation laws are mainly developed for the homogenous equations, where the source terms are identical to zero. For the non-homogenous equations, like the Saint Venant equations, one uses numerical methods for conservation laws to solve the homogenous part, then uses the pointwise or upwind approaches to include the source terms. The homogenous part of the equations is solved by numerical methods for conservation laws: the Lax-Friedrichs, the Self-adjusting Hybrid, the Nessyahu-Tedmor, and the Roe's approximation methods. The source terms can be discretized following the pointwise, upwind or mixed approaches. The mixed approach for discretization of source terms is recommended for balancing the flux and source terms. 4.2. Verification of the numerical methods by Test-Cases There has been done a verification of 4 numerical methods for solving the Saint- Venant equations: the Lax-Friedrichs, the Self-adjusting Hybrid, the Nessyahu-Tedmor, and the Roe's approximation methods. The source terms has be discretized following the pointwise, up-wind or mixed approaches. By the numerical tests it is recommended that upwind and the mixed approaches are more appropriated to the Saint-Venant equations, the Roe's approximation is an efficient method and can be used with all the source term approaches of discretization. The Roe's approximation with the up-wind and mixed technique for the source terms has been tested in detail by following test-cases, covering all of three flow regimes: sub-, trans- , and super-critical: the water at rest problem, steady flow through a bump, the dry bed dam break problem, the wet bed dam break problem, the dry bed dam break problem with friction, the dam break problem with a local constriction, and the hydrodynamic wave, the diffusion wave, the dynamic wave problems. The Roe's approximation with up-winding and mixed technique for the source terms finally has been chosen for dam break modeling. 5. SOME NUMERICAL METHODS FOR SOLVING THE 2-D SAINT-VENANT EQUATIONS AND THE 1 AND 2D LINKED MODEL FOR THE RED RIVER SYSTEM 5.1. The 2-D Saint -Venant Equations The water flow in a 2D domain can be described by the so-called the shallow water equations (the 2D Saint-Venant equations) [2-3]. In a conservative form they are written Some hydrodynamic problems for flood forecasting ... as follows: with H = H(1) + H(2) , au aE ac _ H at +ax+ ay - n<1l = ( s 0 o ) • n<2 l = ' ~x ' ghSo ,y 279 (5.1) (5.2) where h , qx, qy, are unknown functions, h = h(x, y, t) is the flow depth, qx = qx(x , y, t) and qy = qy(x, y, t) are the unit-width discharge components (qx = uh and qy = vh with u, v are the depth-averaged velocities) in x and y directions, respectively, g is the gravity acceleration, So ,x, So,y are the bed slopes, F(qx), F(qy) are the bed shear stresses in x and y directions, respectively: v'u2+v2 F(qx) = -gqx C2 h,x v'u2 + v2 F(qy) = -gqy c2 h,y (5.3) Ch,x = Ks ,xh2l3 ' ch,y = Ks,yh213 ' where Ch,x' Ch,y and KS ,Xl Ks ,y are Chezy and Strickler coefficients. For the system of equations ( 4.6) the initial and boundary conditions must be added. The numerical method for this system is the finite volume one. 5.2. The numerical methods and their validations by the test-cases Method of finite volume has been used for solving the equation system (5.1)- (5.3) together with the initial and boundary conditions. First of all the Green's theorem technique and the Chorin's projection method have been applied to solve the Saint-Venant equations following the successive steps: convection- diffusion, wave propagation and velocity correction. For modelling 2D dam break flows the Roe's approximation method has been used. The developed models have been validated by several academic tests: Total dam-break problem, Partial dam-break problem, Dam-break in a channel with a local constriction, Dam-break in a channel with a non symmetrical flood plain. 5.3. Coupling between the 1-D and 2-D Hydraulic Models In many cases, the coupling between the lD and 2D Hydraulic Models is necessary for flood control in the Red River Delta. The following schema for the coupling lD & 2D is developed and used: - With the known water level at time step n from 2D Model, at the time step n + 1, use the lD Model for calculating the discharge in the connecting section. - Using the discharge at time step n + 1 from lD Model, calculate the water level in the connecting section at time step n + 2 by the 2D Model. - The iteration process will be finished when the difference between the water levels from two models is small enough. 280 Nguyen Van Diep 6. SOME APPLICATIONS OF HYDRAULIC MODELS FOR FLOOD FORECASTING AND FLOOD CONTROL IN THE RED RIVER SYSTElVI lVfany oht ained scientific results have been transferred 1 o ctnd used in some orgaui- zations, for example, t he Central Committee for Flood aud Storm Prevent ion , ;'\atio11nl Centre for l\'lcteor-Hydrological Forecasting and other water resources rnana.gemc11t aud planning organizations [2-3] . Here it will be presented two examples. 6.1. Flood Forecasting in the Red River System Dy request of"the Centnll Committee for F lood and Storm Prevention, frorn the 2002- yea.r flood season , Inst itute of l\lcchanics, every day (from June, lG until September, 15), forecas ts the discharge into Hoa Dinh reservoir and the water level in lln noi for 48 homs by using t he Hydrological l\fodel, lD Hydrauli c Model and Hcservoir 01wrntion l\locle l. -- Mc:isurcrn cnt n - - · Forecas t 5 _ ... " 0 Time (h ) Pig . 5. Predicted & measured clisc:lmrge from Iloa Dinh reservoir According to Vietnamese regulations, these values arc the main a.11d t!lost illlportrn1t paramclt'rs for ta.king ar;y flood control measure. Some fo recasting results arc prcsC'u! cd i11 Fig. 5-li. Dy eva luation of the Central Commi ttee th<'S<' forernstiug rcsult- s 11rc ncccprnblc. Some hydrodynamic problems for flood forecasting ... W a tor le v e I in Hano ls ta tlo n from 1 6 / 612 0 0 3 lo 1 6 19 12 0 0 3 __ .: -==-----'-'"""'" '"" "., T lm e ( ll our) --~~~~~~~~ Fig. 6. Predicted & measured water level at Hanoi 6.2. Evaluating of Flood Control Measures in the Red River System Table 1. The effectiveness of Hood red uctio11 for 500-year flood of l 99G-.vcar shape Scenario Zmax Ha Noi (m) Qmax Ha Noi (m3 /s) Zrnax Son Tay ( m) Qmax Son Tay (m3 /s) No protection Hoa Binh measures 15.53 40.475 18.62 49.470 operation 14.58 31.3Gl 17.34 38.G32 Hoa Dinh + Day darn operation 13.97 2G.071 1G.G9 3G.952 15.43 All Flood protection measures 13.92 25.GG5 lG.Gl 35 .849 15.43 Zmax Va Coe ( n1) ~~~~~~~~~~~~~~~~~~~~~~~~~~~· Qmax Day Dam (m /s) Zmax Tam Thanh (m) Zmax Lap Thach (m) 4.19 E3 21 .90 13.26 4 .02 E3 21.42 18.08 281 ~~~~~~~~~~--~~~~~~~~~~~~~~~~~- Z 111 ax Luong Phu (m) 16.lG 18.80 V Day Dam 1.36 E9 1.1 E9 Zmax Hoa Binh reservoir (m) 115.60 The 1-D & quasi 2-D IMECH model is used for evaluating the effectiveness of fioocl reduction by taking fiood protection measures (when thrcre arc only Hoa Di11li and Time Ba reservoirs) for the floods with return period of 500-year and 1000-ycar To solve this problem it is necessary to: - Evaluate Upstream Discharges for the Red River System for extreme Hoods wit l1 return period of 500 and 1000 years. - Evaluate prepared Hood control measures (reservoir operation, di version (l Jl(l clcte11- tio11 zones) to protect these extreme floods. - Evaluate the role of new measure - emergency spi ll ways to protect t besc extreme Hoods. 282 Nguyen Van Diep In the Table 1 it is presented the result of evaluating the effectiveness of flood reduction for 500-year flood of 1996-year shape when no emergency spillways are used. 7. CONCLUSION AND ACKNOWLEDGMENTS As indicated in the Introduction, I started my research & development activity in the field of environmental and natural fluid mechanics from the 1979 year, when, by the proposal of Prof. Nguyen Van Dao, I had a one year's stay at the Laboratoire National d'Hydraulique de France, Chatou, France. Until now, this activity is still my most impor- tant one. Together with my colleagues from the Institute of Mechanics, we have received some research & development results in the problems of environmental and natural fluid mechanics, especially in the problems of hydromechanics for flood forecasting and flood control in the Red River System. To express my personal gratitude, I select some recent scientific results, obtained by my group in the Institute of Mechanics, to publish in the special issue of the Vietnam Journal of Mechanics, dedicated to the memory of Prof. Nguyen Van Dao, with whom I had a big chance to contact, to work and to collaborate during about 30 years. In this occasion my acknowledgments are also expressed to the VAST, MOST, MARD and European Commission for financial support to realize many scientific projects in the field of water resources research. REFERENCES 1. Nguyen Van Diep, Some Features in the Development of Modern Mechanics, Collection of Research Works, Laboratory of Mechanics, Vietnam Institute of Sciences, pp. 7-15, 1978 (in Vietnamese). 2. Nguyen Van Diep, National project KC08-13 "Studying the Scientific Basis for Flood Forecast- ing and Flood Control in the Red River Basin", Final Repo~t, 2005 (in Vietnamese). 3. Nguyen Van Diep, International project "Decision Support System for Ecosystem Upgrad- ing and Flood Control of a Sustainable Development in the Red River System-Pilot Phase" - FLOCODS, European Commission, 2002-2004, IMECH Technical Report, 2004. 4. Nguyen Van Hanh, Nguyen Van Diep, Ngo Huy Can, On Some Numerical Methods for Solving the 1-D Saint-Venant Equations of General Flow Regime. Part 1: Numerical methods, Vietnam Journal of Mechanics 24 (4) (2002). 5. Nguyen Van Hanh, Nguyen Van Diep, Ngo Huy Can, On Some Numerical Methods for Solving the 1-D Saint-Venant Equations of General Flow Regime. Part 2: Verification and Application, Vietnam Journal of Mechanics 25 (1) 2003. 6. Dan Nguyen, Dartus D., Diep Nguyen Van, New Development of Overland and Hydraulic Models for Ffood Controls in the Red River Delta, J. of Advances in Natural Sciences 5 (2) (2004). 7. Nguyen Van Diep, Ngo Huy Can, Hoang Van Lai, Nguyen Hong Khanh, First Elements of the Decision Support System for Flood Control in the Red - Thai Binh River System, Proceedings of the International Workshop on Flood Prevention and Control in the Yangtze River (FOCYR), Wuhan, China, 2004. 8. Nguyen Van Diep, Ngo Huy Can, Nguyen Kim Dan, Development and Application of Deci- sion Support System for Flood Control in the Red River Delta, International Conference on Reservoir Operation fj River Management (!CROM), Guangzhou and Three Gorges, China, 2005. Some hydrodynamic problems for flood forecasting .. . 283 9. Nguyen Van Diep, Ngo Huy Can, Numerical Modelling Tools for Flood Forecasting and Flood Control in the Red River Basin, The 7-h Int. Conj. on Hydroscience and Engineering (ICHE- 2006), Sep. 10 - Sep. 13, Philadelphia, USA, 2006. Received July 29, 2007. MQT s6 v AN DE THUY DQNG LVC TRONG vr$c nv BAO v .A KIEM ,, - - ,,.! - ......... SOAT LU LlJT H~ THONG SONG HONG Hoc Chat 16ng t\}' nhien va moi tmang duqc bat dau tl.r sau chuyen cong tac 1 nam t<?-i vi~n ThUy l\}'C Quoc gia, Phap (1979-1980) theo de nghj cua Giao SU Nguyen Van D00. Cho den nay, ho0t d(mg nay v§.n la ho0t d(mg quan tr9ng nhat cua toi. . Trong bai bao trlnh bay mc lva ch9n cua m<)t trong nhung van de Thuy dc phl,lC V\l vi~ d\}' bao va kiem soat lu 11,lt: phat trien mo hlnh thuy l\l'C 1 chieu, mo hlnh thuy l\l'C 1 va gia 2 chieu, mo hlnh thuy l\l'C m<)t chieu cho dong chay do v& d~p, mo hlnh thuy lvc 2 chieu, ket noi mo hlnh thuy lvc 1 va 2 chieu va m<)t so ung dl,lng cua chung trong dv bao va kiem soat lu 11,lt h~ thong song Hong. Bai bao nham tucmg ni~m Giao SU Nguyen Van D00, nguM ma toi c6 djp duqc lam vi~c va c<)ng tac gan 30 nam qua, va toi muon bay t6 long cam c:Yn chan thanh cua mlnh t&i Giao su.