Research objectives: To study rainfall-runoff changes, to downscale rainfall and surface over time and space, and to establish scientific base for calculation of design flood for transportation structures.
MINISTRY OF EDUCATION AND TRAINING MINISTRY OF AGRICULTURE AND RURAL DEVELOPMENT WATER RESOURCES UNIVERSTY DOAN THI NOI RESEARCH ON RAINFALL - RUNOFF CHANGES AND PROPOSE SCIENTIFIC METHODOLOGY TO COMPUTE DESIGNED FLOOD FOR TRANSPORTATION IN NORTHEAST MOUTAINOUS REGION OF VIET NAM Specialization: Hydrology Code No: 62-44-02-24 DOCTORAL THESIS SUMMARY HA NOI, 2016 Research has been completed at Water Resources Universty Advisor 1: Assoc Prof Ngo Le Long Advisor 2: Assoc Prof Hoang Thanh Tung Reviewer No : Prof Tran Thuc Reviewer No : Assoc Prof Tran Dinh Nghien Reviewer No : Dr Nguyen Lap Dan This Doctoral thesis will be defended at the meeting of the Universty Doctoral Committee in Room No … on… This dissertation is available at: - The National Library - The Library of Water Resources University INTRODUCTION Rationale Transportation plays a significantly role in national development, it is necessary to complete and modernize transportation system The country has largely invested annually in new construction, upgradation and expansion of transportation system for commercial, cultural and economical intercourses both locally and internationally In Vietnam, with more than 2/3 area is mountainous area, but more than 70% road length is in this area Mountainous roads are mainly III-V grades while design and construction of drainage works (bridge, culverts and ditch) have not been paid attention to as well as limitation on calculation of hydrological and hydraulic features Under the effect of tropical monsoon climate with increasing extreme rainfall that heavily damaged the transportation works One of the main courses significantly affecting sustainability and resilience of the drainage and causing falling of negative and positive talus, floating of bridge and culverts, and damage of bridge piers is effect of extreme rainfall in which incorrect or improper calculation of design flood is the main cause Vietnam Standard TCVN 9845:2013 (based on QP.TL C-6-77) introduces a number of methods for calculation of design flood based on rainfall such as: Intensity limits and Xokolopxky originated from Russian authors of which many parameters are not suitable with Vietnam conditions and their verification is difficult therefore, results of the calculation may not be suitable to the actuality and lead to dissatisfaction of apertures of the highway drainage structures The lack of rainfall and flow data, especially shorter period data, also causes difficulty during calculation of design flood Therefore, dissertation “Research on rainfall - runoff changes and propose scientific base for calculation of design flood for transportation in North-East mountainous Region of Viet nam” is urgently needed Research objectives To study rainfall-runoff changes, to downscale rainfall and surface over time and space, and to establish scientific base for calculation of design flood for transportation structures Scope of the study - Scope of study: Bac Kan and Lang Son provinces in the Northeast mountainous areas of VietNam; - Object of study: design rainfall and design flood for drainage structures on National Highway QL3, 3B, 279, 3, 4A, 4B, and 1A in the studied area 3.1 Study approach and methodology Approach To achieve the proposed target, necessary documents, general studies on rainfall-runoff changes, methods for calculation of design flood for transportation locally and internationally are collected to further select appropriate approaches that are inheritable and yet ensure creativity in research 3.2 Methodology Methods used in the dissertation include: i) analysis, statistics, and selective inheritance of related studies; ii) remote sensing and GIS has been used to establish basins in mathematical models and to develop maps as scientific base for proposal of methods for design flood calculation; iii) mathematical models and case studies have been carried out to propose suitable methods for design flood calculation transportation structures in the Northeast region of Vietnam Scientific and practical meanings Results achieved from the dissertation has high practical meaning for having partially solved the current difficulties in calculation of highway hydrology The study on rainfall-runoff changes in the Northeast region and establishment of scientific base for calculation of design flood for transportation structures have scientific meaning in approaching modern calculation methods and is premise for development of calculation procedure, which is suitable to Vietnam conditions New contributions - Improved current methods for design flood calculation for drainage structures in transportation taking into account of rainfall - runoff changes and of detailed ground surface conditions by application of modern techniques such as hydrological models and GIS; - Developed calculation program for design floods for drainage structures in transportation Contents of the dissertation In addition to the Introduction, Conclusions and Recommendations, the dissertation consists of 03 chapters: Chapter 1: Literature reviews on calculation of design flood for transportation works Chapter 2: Development of scientific base for calculation of design floods for transportation works in the Northeast region of Vietnam Chapter 3: Case studies and recommendation of design flood methods for transportation works CHAPTER LITERATURE REVIEWS ON CALCULATION OF DESIGN FLOOD FOR TRANSPORTATION WORKS 1.1 1.1.1 Literature reviews on design flood calculation Studies in the World Calculation of design flood spanned a long period of study is inheritable, developed, refined and modernized to serve construction of safety works in rainy season, especially in the condition of climate change A number of typical studies on design floods can be mentioned are Chow and Shaw (1964), Chow and Maidment (1988), Vijay (2002), Raghunath (2006) studies and others Basically, methods for calculation are based on principle theory of runoff and efficient transformations of rainfall into runoff However, from the last two decades, outstanding development of computer, remote and GIS technology has allowed scientists to analyze, experiment, and update modern technology for refinement of parameters that had not been able to be developed by earlier methods 1.1.2 Studies in Vietnam Studies for calculation of design floods in Vietnam are mentioned in standards, norms, text book, projects and dissertation such as: QP.TL C-6-77 (1977); Engineering Hydrology of Do Cao Dam et al (1990), Doctorate dissertation - a study on extreme rainfalls and floods in Vietnam by Le Dinh Thanh (1997); Principle of Hydrology and Design hydrology calculation of Le Van Nghinh (2000 & 2003); Ministerial scientific study report “study on forecast and warning on exceeding design flood – additional spillway solution” of Pham Ngoc Quy et al (2005), Text book on Engineering hydrology of Ha Van Khoi et al (2012); National scientific project on “Study on proposed scientific base for flood and sea wall design standards in the conditions of climate change and sea level rise in Vietnam and solutions for disaster risks mitigation and resilience” chaired by Ngo Le Long The studies present methods for calculation of current design flood, hydrographical models for flow calculation, development of computer program for calculation of design flood or proposal of method for calculation of design flood for reservoirs in consideration climate change ‟s impacts 1.2 Literature reviews on design flood calculation for transportation works 1.2.1 Studies in the World Calculation of design flood for transportation works in the world is summarized in guideline and design standards namely: Technical guideline for planning and design (flood control section) in Japan; Guideline on bridge and road design -technical standards of national road transportation agent in England; Guideline on drainage structure calculation of American Association of State Highway and Transportation Officials (AASHTO) in US; HEC introduction of hydrological model; Highway hydrology (FHWA); Technical guideline (TR55) or design standards; “ Guideline on and standards of bridge design” in Columbia; Procedures BCH 63-67- procedures on survey and design of railway and highway crossing river in Russia and other documents Based on general results, it is possible to classify calculation methods into two main groups: i) Methods used in Western countries, Japan and US and ii) methods used in Eastern Europe Russia (Figure 1.1) and Figure 1.1 Methods for calculation of design flood for transportation works 1.2.1 Studies in Vietnam Calculation of design floods in transportation in Vietnam is summarized and mentioned in Vietnam Standards TCVN 9845:2013; Handbook of calculation of hydrology and hydraulic in bridge and road (2006) of Ministry of Transport; Studies of Mai Anh Tuan (2003), Tran Dinh Nghiên (2003), Nguyen Quang Chieu & Tran Tuan Hiep (2004), Nguyen Xuan Truc (2009), Doan Nhu Thai Duong (2012), Nguyen Tien Cuong (2012), and most recently in doctorate dissertation of Nguyen Anh Tuan (2014) Methods for calculation of design flood in Vietnam are subject to area, status of hydrological data and importance of works (level of works) It is possible to classify into two groups: i) statistics analysis and ii) caused and effected analysis (Figure 1.2) based on researches of Russia for each area 1.3 Figure 1.2 Methods for calculation of design flood in transportation in Vietnam Limitations in design flood calculation in transportation in Vietnam From studying, analysis and assessment, it can be seen that limitations in design flood calculation for transportation works in Vietnam as the followings: i) At present, selection of frequency is based on levels of road but not consideration of other negative conditions such as natural and hydrometeorological conditions of drainage catchment area that led to unqualification and/or the works had been damaged in monsoon; ii) Calculation of design flood is only paid attention to the peak discharge but not consideration to the flood volume that led to overflow of drainage works because of improper design, flow accumulation in upper pool will cause formation of air pressure damaging drainage tails/supports and breakdown of drain-body of the road; Consultation tables as scientific base for calculation of design floods for transportation works are old and based on old Russian studies and developed from limited data in Vietnam (small scale maps and short time-series data), in addition to traditional supporting and outdated instruments for the calculation that led to unavoidable and undesired tolerance of the results and subjected to experience of the calculators 1.4 Study approach and methodology From limitations in calculation of design flood in transportation in Vietnam and pre-eminence of a number of methods for calculation of design flood used in Western countries, Japan and US, the dissertation chooses approach as illustrated in figure 1.3 including: i) Studies on selected methods, applied conditions, data requirements, and scientific base; ii) Study on characteristics of rainfall: weather formations causing heavy rains in the Northeast region and historical floods with statistical damages of transportation have been presented for urgent needs of the study The study on changes of rainfalls over space and time has been carried out using statistical methods, remote sensing and GIS, in which Mann-Kendal and Sen have been used for assessment of rainfall changes over time, frequency and recurrent analysis have been used for development of IDF curves for different sub-area/regions in the studied region as well as downscaling daily rainfall to smaller duration rainfall such as hourly rainfall Remote sensing and GIS have been used to update information and to develop rainfall intensity, Cv contour maps; iii) Study on analysis of ground surface conditions of the Northeast region: spatial data from maps and remote sensed data have been used to delineate and calculate characteristics for more than 40 drainage subcatchments in the studied areas, to develop CN, runoff coefficient, and roughness coefficient maps for flood design calculation; iv) Experimental calculation and assessment of the results: analysis on base and conditions of application of each selected method to propose appropriate methods for each type of works and in complying with characteristics of the studied areas; v) Development of computer program in order to integrate all achieved results and procedures for guidance on calculation of design flood for transportation works in the Northeast region with the hope that such program solves a number of limitations in current calculation methods and at the same time leave it open for the researchers‟ further update and improvement Literature reviews Rational method: - Runoff coefficient C - Rainfall intensity I - Catchment area A SCS-CN method - CN map; - Catchment area A - DU Study on catchment surface factors Study on rainfall factors Changes over space: - Changes of CV; Changes over time: Mann kendall and Sen test for trend Regional recurrent equation method - Catchment area A - Catchment slope S - Mưa Calculation of design rainfall: - Downscaling daily rainfall to short duration rainfall - Develop IDF curves - Calculate design rainfall Delineati on and calculatio n of subcatch ment characteri stics Deve lopm ent of CN maps Develo pment of runoff coeffic ient maps Develop ment of roughness maps Case studies - Drainage catchment structures A< 5km2 - Drainage catchment structures A=5-30 km2 - Drainage catchment structures A=30-100 km2 - Drainage catchment structures A>100km2 Recommended suitable methods for the North-East region Figure 1.3 Diagram of study approach 1.5 Introduction of Studied Areas The Northeast region of Vietnam (figure 2.1) is next to China, southeast looks into Northern Bay, South is limited by Tam Dao Mountain and Red River Figure 2.11 CV Map of the studied area Figure 2.24 Isohyet map (I-1-100) of the studied area Figure 2.23 IDF curves at Lang Son and Dinh Lap stations Table 2.6 Coeficients of IDF curves, I=a.Dn T=5 Trạm T = 10 T = 25 a n a n a n Bac Kan 50,9 -1,25 63,0 -1,25 79,1 -1,25 Bac Son 59,3 -1,32 74,9 -1,38 95,9 -1,45 Cho Ra 38,2 -1,18 47,5 -1,17 59,9 -1,16 Đinh Lap 54,8 -1,14 71,1 -1,14 93,8 -1,14 Lang Son 46,9 -1,24 56,2 -1,24 68 -1,24 That Khe 58,2 -1,32 72,5 -1,31 91,5 -1,29 Ngan Son 57,7 -1,25 71,8 -1,27 89,6 -1,3 Huu Lung 67,5 -1,36 83,9 -1,39 105,9 -1,41 Figure 2.25 Mass 24h rainfall curves at Bac Kan and Bac Son stations 12 Results of the study shows that if consideration of the Northeast area as one of the 18 zones of Vietnam using reduction factor and a maximum daily rainfall for calculation of design flood in transportation, results are unreasonable due to large fluctuation of rainfall both in space and time among the areas (table 2.9 and figure 2.17) Therefore, downscaling (elaboration) of daily rainfall into hourly rainfall and development of daily rainfall distribution map, and IDF curves for different locations in the studied area are necessary 2.2.2 Development of surface database Development of surface database for calculation of design flood for transportation in the studied area includes catchmentdelineration, determination of catchment characteristics (area, length, slope etc), preparation of CN, runoff coefficient, roughness coeficient maps (tables) In this research, remote sensing and GIS were applied for updating of information, for spatial analysis, and for development of the above mentioned maps and tables for flood calculation Figure 2.29 Diagram for development of CN map 2.2.2.1 Development of Curve Number Map Development of Curve Number Map for the studied areas is performed according to diagram 2.29 The result is illustrated in Figure 2.32 13 Figure 2.32 Curve Number map of the Bac Kan - Lang Song region 2.2.2.2 Development of runoff coefficient map Development of runoff coefficient map for the studied areas is performed according to diagram 2.33 The result is illustrated in figure 2.34 Figure 2.33 Diagram for development of roughness coefficient map 2.2.2.3 Figure 2.34 Runoff coefficient map of Bac Kan and Lang Son provinces Development of roughness factor coefficient (Manning n) map Development of n roughness factor map for the studied areas is performed according to diagram 2.35 The result is illustrated in figure 2.36 Figure 2.35 Diagram for development of roughness coefficient map Hình 2.36 Roughness coefficient map of Bắc Kạn Lạng Sơn provinces 14 2.2.2.4 Delineation and determination of catchment features from DEM It is very necessary to calculate characteristics of catchment such as averaged elevation, runoff direction are used to estimate time of concentration, catchment area More than 40 drainage catchments of the Northeast Region of Vietnam have been delineated and its characteristics have been determined Sample results are briefly shown in Figure 2.37 and Table 2.17 Figure 2.37 Map of Bac Khuong bridge catchment Bảng 2.17 Characteristics of cầu Bắc Khương bridge catchment No 2.3 Characteristics of Bac Khuong Bridge Design probability Catchment area Main flow length Total flow length Catchment length Catchment averaged width Catchment roughness Catchment averaged slope River bed slope Symbol P F L ∑Li Llv B n J(Sb) % Js(Sr)% Unit % km2 km km km km2/km Value 506,5 27,3 40,1 56,2 9,0 2,0 23,6 0,4 Conclusion of chapter II It has been seen from the research results that if the North - East Region is assumed as one of 18 rainfall regions in Vietnam as specified in TCVN 9845:20013, is not suitable in calculation of design floods because of large changes of rainfall over space Thus, for calculation of design floods for small drainage catchments in transportation, it is suitable to downscale daily to hourly rainfall, to develop IDF curves, to develop 24 hour accumulation rainfall distribution curves and to develop rainfall contour maps In addition, the 15 downscaling and updating of catchment surface such as catchment delineation and characteristics determination, development of CN, runoff coefficient, roughness coefficient grid maps are also important in establishing scientific base (database) for application of selected flood design calculation method in this dissertation CHAPTER CASE STUDIES AND RECOMMENDATION OF DESIGN FLOOD METHODS FOR TRANSPORTATION 3.1 Classification of drainage structures in calculation of design floods Drainage structures are classified based on the followings: + Applied condition of calculation method: the rational method is applied for catchments with area less than 65 km2; SCS method is applied for catchments with area range from to 500 km2; while the regional recurrent method is applied based on the data details; + Data collection on drainage structures (location, route, quantity); + Calculation parameters (soil types, land cover, land used, CN, runoff coefficient, roughness coefficient, slope etc) This research has classified drainage catchments for calculation of design flood into groups: The first group: structures having small drainage catchment area (A 100 Design SCS-CN Catchment size (km2) A