Tóm tắt Luận án Development of a suitable model for hydraulic simulation of the integrated urban draingage system in tidal area

MINISTRY OF EDUCATION MINISTRY OF AGRICULTURE AND TRAINNING AND RURAL DEVELOPMENT VIETNAM ACADEMY FOR WATER RESOURCES SOUTHERN INSTITUTE FOR WATER RESOURCES RESEARCH ==================== DANG THANH LAM DEVELOPMENT OF A SUITABLE MODEL FOR HYDRAULIC SIMULATION OF THE INTEGRATED URBAN DRAINGAGE SYSTEM IN TIDAL AREA Research Sector: Water Resources Engineering Sector Code: 62 58 02 12 PHD THESIS SUMMARY Ho Chi Minh city-2015 Study is done at: Southern Institute for Water Resources Research Supervisor: Prof.Dr Nguyen Tat Dac Review Expert 1: Review Expert 2: Review Expert 3: Thesis will be presented at Evaluation Board of the Academy organized at : At hour of the date month year 2015 Thesis is available at: National Library of Viet Nam Library of the VN Academy for Water Resources Research Library of the Southern Institute for Water Resources Research 1 INTRODUCTION Scientific research and practical application of effective drainage and solving inundation depends on several factors of which the application of urban hydrological and hydraulic modelling is a very important one for planning and design of drainage system. A technical tool namely DELTA-P has developed to compute complex interactions of river and sewerage system under the influences of rainfall, flood and tide. Study purpose: To find scientific solutions for urban drainage issues and to improve river and pipe hydraulic coupling model for suitable urban application under specific conditions of strong tidal impact, heavy rainfall and rapid urbanization in Viet Nam. Findings: (1) Development of a computer module P for urban hydraulic modelling; (2) Coupling of module P and existing river hydraulic model DELTA; (3) Simultaneously run model for surface rainfall-runoff, pipe flow and tidal river hydraulic computation; (4) Capable application of coupling model for an actual drainage network. Scientific values: The research study has developed a new model DELTA-P for coupling model on urban surface-pipe-river simulation. The model has been successfully tested. Practical values: Provide a new capable modelling tool for solving urban drainage problems. 2 CHAPTER 1 : OVERVIEW OF URBAN FLOODING URBAN DRAINAGE MODEL 1.1. FLOODING AND MITIGATION MEASURES FOR CITIES It is said that the urban drainage work began in thousands of years ago. By the 1980’s, application of computational technique for simulation of drainage network had been improved economic benefit and more technical transparent. However, statistical reports shows still rapidly flood damages in the world . Annual flood damage estimate is at 100 millions USD in the first half of the XX century and it reaches 1 billions USD in the second half, especially about 10 billions in the latest 10 years. Urban flooding in Viet Nam is mainly caused by seasonal heavy rainfall, tropical storm and high tide. Some historical events are as follows: - Serious flood damage of Ha Noi capital city in November 2008. - Hue city is annually flooded 3-4 times with highest depth about 4 meters. - Da Nang city is often flooded at about 30 locations during heavy rain. - Can Tho in the Mekong delta is suffering from flooding in wet season. - Largest Ho Chi Minh is now hardly fighting against flood problems caused by upstream flood, heavy rain and high tide (Figure 1-1). Causes of flood: - Objective causes: low land, heavy rain, big upstream flood, 3 degradation of sewerage and rapid urbanization; - Subjective causes: ineffective urban planning, urban river encroach, protective forest degradation. Figure 0-1: Main causes for urban flooding of Ho Chi Minh city Mitigation measure: Urban flood mitigation planning is developing with active contribution of scientists, public and technology. Application of a capable model is defined in the integrated urban drainage strategy as follows: - Comprehensive analysis on causes, impact and mitigation 4 measures on urban flooding; - Providing detailed information on impact of each factor on sewerage; - Strong support on infrastructure development plan and drainage works; - Evaluating the past damage and predicting future risk; - Development of emergency action plan; - Analysis on impact of drainage system on rivers; - Climate change and sea level rise adaptation. Hydrological and hydraulic models are developed to simulate single or complex issues as follows: - Under ground pipe and open river interaction; - River and flood plain hydraulic; - Surface runoff, under ground pipe and open river interaction; - Surface and ground water interaction; - Under ground pipe flow and tide interaction. The surface runoff, under ground pipe and open river interaction package is strongly recommended for integrated strategy. General approach for drainage system modelling is defined as (i) One dimensional (1D) model for in bank flow and (ii) Two dimensional model (2D) for over land flow. The river and pipe flow are typically 1D and surface flow is possibly 1D or 2D. The world wide modelling tools are available such as SWMM (US), WASSP (UK), MOUSE and MIKE (Denmark), SOBEK URBAN (The Netherlands). Some of them are applied in urban drainage studies for Ho Chi Minh city. In Viet Nam the F28 model is developing and testing. Currently practical application of separated single module for each river or pipe modelling for the studies mentioned above is big disadvantages. Technical and practical situations of Viet Nam 5 require an integrated package of coupling model for ‘outer layer’ (rivers) and ‘inner layer’ (under ground pipes) simulation. 1.2. MODELS FOR URBAN DRAINAGE a) Basic definitions Impact of urbanization on hydrological cycle A complete hydrological cycle on the earth is described by the water path from rainfall, runoff and river flow routing to the ocean. In the urban catchment natural flow routing is significantly modified by drainage work, man-made storage and concreted surface causing more lag time and less infiltration rate. Urban catchment characteristics Urban catchment boundary is both naturally defined by topography and artificially separated by human made structures such as roads. 1.3. URBAN HYDROLOGICAL MODEL Some methods applied for rainfall-runoff computation are including: a/ Experimental equations: - The Rational Method and Unit Hydrograph. b/ Popular mathematical models: - Mike RR package contains NAM, SMAP, FEH and DriFt modules - HEC-HMS model. Remaks: The Urban-B type is most suitable for Viet Nam urban modelling with full data set and Urban-A type for lack of data. 1.4. URBAN HYDRAULIC MODEL Urban flow: Runoff forms street flows, enters manholes, flows through under ground pipe and discharge into river. Street flow plays 6 an important role as a main flow path and temporary storage basin in the case of extreme rainfall. A standard urban hydraulic model must contain and couple rainfall runoff, pipe routing and river flow modules. SWMM model: SWMM model computes pipe and river flow by using Saint- Venant equation for unsteady 1D condition. Numerical method for pipe network is presented in detailed in the model manual . Many hydraulic structures (pump, weir, gate) can specifically be schematized. The model was applied to simulate a pilot drainage system of Tham Luong-Ben Cat in Ho Chi Minh city. The pilot model showed rain water drainage in the pipes, but no tidal impact. MIKE model package: The MIKE package contains single MOUSE, MIKE11 and MIKE21 model or coupling sets of MOUSE-MIKE21 (MIKE URBAN) and MIKE11-MIKE21 (MIKE FLOOD) Examples of pilot studies of MIKE package are as follows: - MIKE FLOOD application for flood simulation of Nhue-Day rivers in Ha Noi. - Urban drainage model for Dhaka city in Bangladesh using MOUSE simulates surface flow and under ground pipes. F28 model: The model contains rainfall runoff, street and pipe flow modules. Pilot model application for urban area of Thu Thiem (Ho Chi Minh city) simulates pipe system and river network and generates urban flooding caused by heavy rain. VRSAP model: VRSAP model is widely applied for flood control study in sub- 7 urban and delta area. The model is typical 1D hydrodynamic routing for open rivers. CHAPTER 2: DELTA-P MODEL FOR COUPLING PIPE AND RIVER HYDRAULIC DELTA-P model (a river hydraulic model DELTA and additional pipe model P) is developed with clear theoretical basic and applied for a pilot study to test the model functions and model outputs. Pilot study is done to verify model simulation and to analyze structural urban drainage measures utilizing pipe and river network. Modelling activities are as follows:: - Calibration of the model by using simulated and observed data for both river stage and Tan Hoa-Lo Gom catchment inundation. River network covers entire lower Dong Nai-Sai Gon river basin from downstream of Dau Tieng, Phuoc Hoa and Tri An dam to the sea and is capable to simulate upstream flood and tidal impact, local flood plain regulation. - Urban drainage model network covers underground pipes of Tan Hoa-Lo Gom catchment and is capable to simulate rain water discharge and tidal impact on pipes. - Testing simulation of pipe network and canal improvement to reduce urban flooding. - Capable simulation of integrated river tidal barriers and underground pipes. 2.1. NUMERICAL BASIC OF RIVER HYRDAULIC MODEL Numerical method of river hydraulic model DELTA-P presents strong mathematical and fluid dynamic basic. Recent improvement on database, GIS application and user interface were made. 8 2.2. NUMERICAL BASIC OF PIPE HYRDAULIC MODEL Drainage pipe is applied for long round and rectangle shape types. Main regimes of pipe flow are schematized as follows: - Pressure flow for full pipe; - Semi-pressure flow for submerged upstream inlet and free downstream outlet; - Non-pressure flow for un-submerged upstream inlet or non- full pipe. Non-pressure flow can be submerge or free mode which depends on water depth (higher or lower than critical depth respectively). Flow equations presented on Figure 2-1 contain function of discharge (Q) and water head variables Q = f(H,h). The equations are solved by differential and numerical methods in DELTA-P. Figure 2-2 illustrates the pipe flow regimes. The linear relation between Q and water head (H in two pipe inlets or pipe outlet and river) is formed at pipe inlets or outlets. By using reduction algorithm for Q we can obtain the systems of linear algebraic equations for variable H. Figure 2-1: Scheme of pipe flow regimes and respective equations 9 The DELTA-P has an advantage by using same algorithm for all types of connections of river sections and pipes. Figure 2-3 illustrates a connection of a pipe and river junction (I). Figure 2-2: Scheme of pipe flow regime Rainfall-runoff simulation in P module: Runoff generated in the catchment is stored in basins, routing along streets and drain away via manholes. Drainage discharge depends on street width, manhole area and water depth. Variation of water depth above manholes is expressed by the equation (2-1): ,( ) ( , ) i i i i k i k k dZ F R t Q Z h dt (2-1) Where Fi is storage area, Ri (t) is rainfall, Qi,k (Zi, hk) is discharge of sub-catchment k draining via manhole i. 10 Water level at junctions or manholes is a combination of river stage and flood depth Z then it can present inundation depth caused by rain or tide. Figure 2-3: Pipe connection to river junction I CHAPTER 3 : APPLICATION OF DELTA-P FOR MODELLING RIVERS OF HO CHI MINH AREA AND DRAINAGE PIPES OF TAN HOA- LO GOM CATCHMENT 3.1. URBAN FLOODING IN HO CHI MINH CITY Ho Chi Minh is the biggest city in Viet Nam with existing population of 8.6 million people and land area of 2095 square kilometers. Topography: A large part of area is low at about less 1.0 meter above mean sea level (MSL) and surrounded by dense branches of lower Dong Nai river. 11 Climate: Annual temperature and humidity are high. There are two seasons of the year: wet and dry. Average annual rainfall is 1915 mm and rainfall amount in wet season shares 93% of annual total. River system and hydrological regime: All of river branches of Ho Chi Minh city are influenced by semi-diurnal tide from east sea. The high tide level is at 1.55-1.60 meter above MSL. Main rivers: including Dong Nai and Sai Gon branches those flow through the central part of the city and Vam Co, Long Tau, Thi Vai branches surround the south-east side. Urban rivers/canals: including Nhieu Loc-Thi Nghe, Tan Hoa- Lo Gom, Tau Hu-Ben Nghe, Kenh Doi-Kenh Te, Nuoc Len-ben Cat and Cau Son-Vam Tat branches. Sub-urban rivers/canals: several branches in Thu Duc and southern city area. Sewer pipes: existing under ground pipes for domestic waste and rain water drainage. Classification of sewerage is as follows: Primarily/class I: are rivers and canals. Class II, III and IV: are underground pipes with about total 1000 km long. Urban flooding In period 2007-2008 there were about 4011 ha of flooded area in both central and sub-urban zones. Of which, 2861 ha of construction land and 1150 ha of agriculture land. Flooding in central city is 100cm deep and 24 hours long. After 2008 to present the flood points and flooded area are decreased by 2/3 and 1/2 respectively. Flood problem is most critical in the case of heavy rain and high tide. 12 Sub-urban flooding: Occurred in the case of: high tide, heavy rain and big upstream flood. Flooded area is 4348 ha mostly in Nha Be and Can Gio districts. Annual flood damage is 371 billion VND and highest is 500 billions VND. Causes of flooding: In central part: land level below 1.5m above MSL; high rainfall intensity; high tidal level in river; lack of and poor drainage pipes. In surrounding part: low land level less 1.0m above MSL; low crest of dyke; high tide and big upstream flood discharge. Flood mitigation measures: Are defined by the Ho Chi Minh city urban drainage planning study as follows: Improvement of sewerage system from every building to the rivers with high design rainfall intensity. Construction of complete ring dykes and tidal barriers. Flood mitigation projects of HCM City There are available studies at two levels: master plan and local projects, listed as follows: - Master plan and Feasible study on waste and rain water drainage for Ho Chi Minh city by 2020. - Water work planning study for flood mitigation of Ho Chi Minh city. - National research study on flood mitigation measures for Ho Chi Minh city. - Pre-feasibility study for environmental improvement of Tau Hu-Ben Nghe and Kenh Doi-Kenh Te canals. - Project on sanitation and infrastructure improvement of Tan 13 Hoa-Lo Gom canal. - Project on environmental improvement of Hang Bang canal. 3.2. RIVER HYDRAULIC MODEL Model schematization of DELTA-P for downstream branches of Dong Nai-Sai Gon rivers in Ho Chi Minh city area is developed with sufficient available topographic data, river stage time-series and boundary conditions (upstream discharges and sea tidal level time- series). Model is calibrated against river water level in October 2005 (example of hydrograph is presented in Figure 3-1) High computational speed is a big advantage of DELTA-P for simulating large scale network. Figure 3-1: Hydrograph of observed and simulated water level time series at Phu An station in October 20015 3.3. PIPE HYDRAULIC MODEL (1) Model extent Sewerage system of Tan Hoa–Lo Gom (THLG) covering an area of 2500 ha is one of five drainage systems in central part of Ho Chi Minh city. The main Tan Hoa-Lo Gom canal is about 7600m long. 14 Sewerage pipes are installed along main streets and they connect to main canal. Diameter of pipes is in range of 60-100cm. (2) Flood issues and mitigation measures Flooding is caused by some main reasons such as big rainfall, poor sewerage pipes, obstacle of drainage canal and low land elevation. Inundation mitigation measures are proposed as follows: - Improvement of main drainage canal of THLG; - Installation of new pipes to increase drainage discharge. (3) Topographical data There are available canal cross-sections, location map and dimension of pipes. (4) Existing inundation: There area 11 inundation sites with an area of 58 ha (location map of sites is presented in Figure 3-2). Flood duration is about 10 hours and flood depth is 25-30cm, in average. (5) Boundary conditions: Model boundary conditions are upstream discharge at Dau Tieng, Tri An, Phuoc Hoa dams and sea level at Vung Tau. Rainfall data is at Tan Son Nhat (airport) station. Model is calibrated against water level at Nha Be, Phu An and Bien Hoa stations. However, there are lack of observed water level and discharge data for THLG canal. (6) Pipe network The pipe model consists of 106 sections with determined flow directions, junctions, pipe diameter and inlet/outlet elevations. (7) Rainfall Testing simulation uses 10-year design rainfall in 120 minutes of Tan Son Nhat station. 15 (8) Simulation of flood in October 2005 Model simulation is run for a 15-day period from October 01 st 2005 to October 16 th 2005 covering one tidal cycle. Rainfall occurred at 3am to 6am in October 01 st 2005 during low tidal period. Model simulation generated flood depth and flood duration caused by rain and tide (Figure 3-3). Figure 3-2: Location map of investigated inundation sites of TH-LG Land slope of THLG is in direction of upstream to downstream and it makes easy drainage condition. However, some high elevation sites upstream such as model node 232 (Tan Phu district) and node 237 (Binh Thoi, District 11) were flooded. The outlet basin of THLG was flooded by both rainfall and tide due to low land elevation below 1m (below MSL). Figure 3-4 presents existing flood extent and 16 depth. The largest flood depth is 40cm and occurred in Binh Thoi (district 11) and is followed by Phu Lam (district 6) with flood depth of 30cm. Flood duration is about 01 hour in average. Figure 3-3: Hydrograph of canal water level and street inundation depth of THLG Simulation also shows regular flood in Cho Lon area (district 6) with maximum flood depth of 20-30cm and total flood duration of 20 hours in a period of half of a month. Generated flood map is fit to actual flood extent for overall catchment and flood depth in district 6, district 11. Simulated flood duration of 1-2 hours is also fit to actual duration. 17 Figure 3-4: Flood map caused by past rainfall event in October 2005 (9) Simulation of flood mitigation Alternative 1 (PA1): Widening and excavation of TH-LG canal Tan Hoa-Lo Gom canal is assumed to enlarge cross-section at 30m wide and -2.5m deep for upstream and middle sections (existing 10-20m wide and +1.5m deep). Existing wide and deep downstream section is remained. Simulated maximum water level (Hmax) upstream is decreased by 1m (from 2.3m down to 1.3m). Downstream water level is insignificantly decreased by 0.1-0.2m. 18 Figure 3-5: Location map of flood area and depth for PA1 Street flood depth along upstream canal section is decreased by 7-8cm and along downstream section about 7-10cm, except for downstream low land area (example of node 248 and 249) is increased about 10cm. Simulated flood map for PA1 is presented in Figure 3-5. (10) Simulation of flood mitigation Alternative 2 (PA2):