Constructing the flood hazard map for downstream of da ban reservoir, khanh hoa province, vietnam master thesis major sustainble hydraulic structures coastal engineering and management code 62 58 02 0

18 3.4.2 HEC-RAS two-dimensional flow modeling capabilities for calculation case of this study .... Gilles and Moore 2010 used the hydraulic models MIKE 11 and USACE Hydrologic Engineeri

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NICHE-CEM &TLU-ULG MASTER

PROGRAM MASTER THESIS

CONSTRUCTING THE FLOOD HAZARD MAP FOR DOWNSTREAM OF DA BAN RESERVOIR, KHANH HOA

PROVINCE, VIET NAM

Submitted by NGUYEN MANH KIEN

Ha Noi, August 2016

THUY LOI UNIVERSITY & UNIVERSITY OF LIEGE

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NICHE-CEM &TLU-ULG MASTER

PROGRAM MASTER THESIS

CONSTRUCTING THE FLOOD HAZARD MAP FOR DOWNSTREAM OF DA BAN RESERVOIR, KHANH HOA

PROVINCE, VIET NAM

Student’s Name : Nguyen Manh Kien Birthday : January 05, 1980 Student Code : 1481580203008 Email address : Kiennm4125@wru.vn

Mobile Phone Number : +84979209988 Supervisor : Assoc Prof Dr NghiemTien Lam Email : lamnt@wru.vn

Co-supervisor : Prof ir HIVER Jean-Michel Email :Jean-Michel.Hiver@ulb.ac.be

Ha Noi, August 2016

DECLARATION

I declare that this submission is my own original work, and I have not used any source

or mean without proper citation in the text Any idea from others is clearly marked This thesis contains no material published elsewhere or extracted in whole or in part from a thesis or any other degree or diploma

Ha Noi, August 2016

Nguyen Manh Kien

ACKNOWLEDGEMENT

During last six months of research and prepare of this master thesis, I have been accompanied and supported by many people I would like to thank them all for their support, guidance and encouragement throughout this work

Firstly, I would like to express my sincere gratitude to my supervisor, Assoc Prof Dr NghiemTien Lam, for his guidance, support, and provision of critical information sources for me to complete my research I also really appreciate all the support that I have received from my co-supervisor, Prof ir HIVER Jean-Michel, who provided great recommendations and lots of references for my thesis writing

Secondly, I would like to warmly thank all the lecturers in NICHE-CEM & TLU-ULG Masters’ program for providing me so much knowledge during this course

Thirdly, I would like to thank Song Da Joint Stock Company for giving me access to a lot of important data and information on my selected area for this thesis

And last but not least, I would like to thank my family and friends, who always stay by

my side and have helped me a lot to finish this program

Ha Noi, August 2016

Nguyen Manh Kien

CONTENTS

CHAPTER 1: INTRODUCTION 1

1.1 Problem statement 1

1.2 The meaning of flood map 2

1.3 Literature review 3

1.4 Objectives and methods: 4

1.5 Structure of thesis 5

CHAPTER 2: SOCIAL AND NATURAL CONDITIONS OF THE STUDY AREA 6 2.1 Natural condition of study area 6

2.1.1 Geographical location of study area 6

2.1.2 Topographical conditions 6

2.1.3 Climatic and hydrographical conditions .7

2.2 Socio-economic conditions 10

2.3 Flood risk in downstream of Da Ban reservoir 11

CHAPTER 3: THEORETICAL BASIC FOR FLOOD MAPPING 12

3.1 General overview 12

3.1.1 Background of flood mapping 12

3.1.2 Objectives of this tool 12

3.1.3 Flood mapping process 12

3.1.4 Products of flood hazard map 14

3.2 General about the hydraulic problems in river network 15

3.2.1 1D steady Flow 15

3.2.2 1D unsteady Flow 16

3.2.3 2D unsteady flow hydrodynamics 17

3.3 General about the hydraulic modeling 18

3.4 HEC-RAS model 18

3.4.1 Introduction 18

3.4.2 HEC-RAS two-dimensional flow modeling capabilities for calculation case of this study 20

3.4.3 Basic steps to modeling 21

CHAPTER 4: CONSTRUCTING THE FLOOD MAP OF STUDY AREA 23

4.1 Implementation diagram 23

4.2 Constructing database 24

4.2.1 Project data 24

4.2.2 Topography data 27

4.2.3 Landuse data 28

4.2.4 Hydrologic data 29

4.3 ARC-GIS software application to construct DEM data, cross-section data, and landcover 30

4.3.1 DEM data 30

4.3.2 Cross-sectional data of Da Ban river 30

4.3.3 Land cover data 31

4.4 HEC-RAS modeling application for flood simulation and calculation 32

4.4.1 Selection and construction of model domain 32

4.4.2 Developing a terrain model for use in 2D modeling and results in mapping 32 4.4.3 Development of a 2D model 36

4.4.4 Creating a spatially varied Manning’s roughness layer 39

4.4.5 Boundary conditions 43

4.4.6 Initial conditions 45

4.4.7 Dambreak analysis 47

4.4.8 Running the unsteady flow model 51

4.4.9 Viewing 2D output using RAS Mapper 56

4.5 Validation numerical model with analytical solution 65

4.5.1 Analytical Solution by Ritter 65

4.5.2 Modeling the 2D Dam-Break wave 67

4.5.3 Comparing the model results and the analytical solution by Ritter (1892) 71

4.6 Sensitivity testing 72

4.6.1 Introduction 72

4.6.2 Considering the factors affect the model results 73

4.6.3 Choosing position on model for analyzing results 74

4.6.4 Sensitivity testing results 75

4.6.5 Conclusion 80

4.7 Constructing flood hazard map by ArcGIS 81

CONCLUSIONS AND RECOMMENDATIONS 86

Conclusions 86

Recommendations 89

LIST OF FIGURES Figure 1: Geographical location of study area (source: Google Map) 6

Figure 2: Representation of terms in the energy equation 16

Figure 3: Implementation diagram 23

Figure 4: Da Ban reservoir 24

Figure 5: Headworks before upgrading 25

Figure 6: Headworks after upgrading 25

Figure 7: Inner slope of earth dam 25

Figure 8: Outer slope of dam 25

Figure 9: Existing spillway 26

Figure 10: Intake 26

Figure 11: New spillway 27

Figure 12: Da Ban river 27

Figure 13: (DEM) 30mx30m of study area 28

Figure 14: LandsatGLS\ TM_Multispectral_2000 28

Figure 15: Chart of hydrograph with frequency 0,5% 29

Figure 16: Chart of hydrograph with frequency 0,1% 29

Figure 17: DEM study area 30

Figure 18: Create cross sectional data for Da Ban river 31

Figure 19: Land cover of study area from Landsat sources 31

Figure 20: Downstream of Da Ban reservoir 32

Figure 21: RAS Mapper with a Terrain Data Layer added 33

Figure 22: RAS mapper with a channel (river) terrain data layer created 34

Figure 23: Original terrain model (top) and new terrain model with channel data (bottom) 35

Figure 24: HEC-RAS 2D modeling computational mesh terminology 36

Figure 25: 2D computational mesh 37

Figure 26: 2D flow area mesh generation editor 37

Figure 27: The storage area connected to the 2D flow area 38

Figure 28: Adding the parameter of the reservoir 38

Figure 29: SA/2D Area Hydraulic Connection editor 39

Figure 30:Parameters of a,new spillway and b, gates 39

Figure 31: RAS Mapper’s new land classification 41

Figure 32: Set Manning’s n override land cover values at some regions 42

Figure 33: Spatially varied Manning’s roughness layer 43

Figure 34: Boundary condition 44

Figure 35: Hydrograph with frequency 0,5% 44

Figure 36: Hydrograph with frequency 0,1% 45

Figure 37: Normal depth in downstream 45

Figure 38: Initial condition of Reservoir 46

Figure 39: 2D flow area computational options 47

Figure 40: Breach parameter calculator from regression equations 49

Figure 41: Shape and dimensions of calculated breach 49

Figure 42: Flow hydrographs from dam to downstream without dam break (flood frequency 0.1%) 50

Figure 43: Flow hydrographs from dam to downstream effect by dam break (flood frequency 0.1%) 51

Figure 44: Unsteady flow analysis window for a plan (plan 3) 56

Figure 45: Display results map parameters 57

Figure 46: RAS Mapper with depth results layers 57

Figure 47: RAS Mapper with velocity results layers 58

Figure 48: RAS Mapper with WSE Results Layers 59

Figure 49: Example Time Series Plot of Depth from three different Plans 60

Figure 50: Example Time Series Plot of Velocity from three different Plans 61

Figure 51: Example Time Series Plot of WSE from three different Plans 62

Figure 52: Example velocity plot with color and direction/magnitude arrows 63

Figure 53: Example of the particle tracing visualization option on top of a depth layer 63

Figure 54: Profile Line turned on and selected for plotting options 64

Figure 55: Example profile line plot of Water Surface Elevation (WSE) 64

Figure 56: Results Mapping Window 65

Figure 57: Analytical solution by Ritter (water height) 67

Figure 58: Geometry model 67

Figure 59: Hydraulic structure with a dam break 68

Figure 60: Water hight follow model at t=0s 69

Figure 61: Water depth profile along the river at t=20s 69

Figure 62: Water depth profile along the river at t=40s 70

Figure 63: Water depth profile along the river at t=60s 70

Figure 64: Analytical solution by Ritter (blue) and simulated results (orange): water depth at t=20s 71

Figure 65: Analytical solution by Ritter (blue) and simulated results (orange): water depth at t=40s 71

Figure 66: Analytical solution by Ritter (blue) and simulated results (orange): water depth at t=60s 72

Figure 67: The position on model for analyzing results 75

Figure 68: The water depth at point 3 in Sensitivity test cases of n values 76

Figure 69: The velocity at point 3 in Sensitivity test cases 77

Figure 70: The water depth at point 3 in Sensitivity test cases of mesh size 78

Figure 71: The velocity at point 3 in Sensitivity test cases of mesh size 78

Figure 72: Computed time step is 5 seconds and 2 minutes at point 3 79

Figure 73: Breach bottom elevation is 56m and 46m (Sensitivity testing) at point 1 80

Figure 74: Flood hazard map in case of flood frequency 0.5% 83

Figure 75: Flood hazard map in case of flood frequency 0.1% 84

Figure 76: Flood hazard map in dam break case with flood frequency 0.1% 85

Figure 77: A resident area before and after covered by flood water 87

Figure 78: Velocity changes from river to floodplain 87

Figure 79: The flow through a resident area with the discharge of 500 (m3/s) 88

LIST OF TABLES

Table 1: Climate specification 8

Table 2: Climate factors of the irrigated area 8

Table 3: Maximum wind speed 8

Table 4: Evaporation 9

Table 5: Evaporation loss 9

Table 6: Rainfall 9

Table 7: Network and available surveyed factors 9

Table 8: Flow statistics at Da Ban 9

Table 9: Flow distribution at Da Ban 9

Table 10: Flow and total flood 10

Table 11: Situation of soil and forest sources 10

Table 12: Elevation volume curve 24

Table 13: Max flow and total flow in Da Ban in accordance with frequencies 29

Table 14: Manning’s n values are used for the model 40

Table 15: Eddie viscosity transverse mixing coefficients 53

Table 16: Calculation cases 55

Table 17: Calculation cases of Sensitivity test 73

CHAPTER 1: INTRODUCTION

1.1 Problem statement

Viet Nam has abundant and diverse water resources with dense river network However, water discharge is not distributed evenly over different seasons in a year Discharge in the rainy season is much greater than that in the dry season

As an agricultural country in its process of industrialization, Vietnam has built thousands of big and small reservoirs on river basins in order to regulate water flow and reduce flood's impact, to supply water for irrigation in agriculture and aquaculture, and

to generate hydropower, etc Construction of reservoirs offered tremendous efficiency

to economic sectors

Construction of reservoirs raised a major concern for safety due to potential risks to downstream areas, for example whenever a major flood appears A large flood flow or break of dam flow leads to a sudden rise of water level, and higher velocity in downstream This situation represents a serious threat to the lives and properties of people living at the project’s downstream areas

Most of the reservoirs were designed following outdated standards and are not matching for current national and international standards Additionally, nowadays serious degradation of upstream forest areas leads to more unpredictable and complicated flood patterns

Being aware of the situation, the project VWRAP has provided Vietnam Government financial assistance in strategic change with the capital loan This loan was to upgrade and modernize the irrigation system, improving irrigation services through management improvement, operation, maintenance, and financial management Notably, the effort also encouraged active participation of water users, especially farmers

In 2000, Ministry of Agriculture and Rural Development, together with World Bank’s consultants, investigated and determined 6 priority subprojects as part of the mentioned loan, as follows:

- Dau Tieng water resources subproject

- Yen Lap water resources subproject

- Ke Go water resources subproject

- Cam Son - Cau Son water resources subproject

- Da Ban water resources subproject

- Phu Ninh water resources subproject

Along with upgrade and modernization of these six dam areas, safe operation and management were also considered in VWRAP study

With the aim to reduce damages caused by flood, to propose the solution of flood prevention through predicting probable inundation area with different flood scenarios (upstream floods and dam breaking), this thesis focuses on “Constructing the flood hazard map for downstream of Da Ban reservoir, Khanh Hoa province" The results will provide inputs to manage reservoir, develop resident relocation plans and specific responses

1.2 The meaning of flood map

A flood map is a visual tool that allows its users to know about inundation level corresponding to the forecasted water surface elevation at a certain position of the study area This is very important for decision makers in the cases of emergency The aims of flood mapping is:

1 To show the inundation area, water depth, flow velocity and other parameters at any position in flood region when input data is known;

2 To provide a foundation for selecting or associating flood prevention methods;

3 To support for performing land management of floodplains;

4 To serve as a basis for studying flood prevention methods in basic construction;

5 To provide information for designing and operating flood prevention works in the future

1.3 Literature review

Flood is one of the natural disaster types which caused serious damages to people Management and assessment of floods’ impacts on people's lives through simulation, development of flood map have now become more and more familiar with a research topic Recently, one of the most common methods is by applying a hydraulic model to develop flood map causing rainfall and dam breaking Some typical studies in this field and approaches will be mentioned briefly in the following section

Gilles and Moore (2010) used the hydraulic models MIKE 11 and USACE Hydrologic Engineering Center’s River Analysis System (HEC-RAS) to simulate floods in Netherlands, Belgium and the United Kingdom Their study used model aiming at flow management, forecast, and construction of national flood forecasting system

Vanderkimpen and Peeters (2008) simulated flood by MIKE FLOOD model application

to setup an evacuation plan in a timely manner for a coastal delta region of Belgium

By using MIKE FLOOD model, several effects of floods to the inundation area and to evaluate damages were able to indentify

Xiong (2011) analyzed dam break using HEC-RAS This study described the dam break from both theoretical and modeling application aspects Breach parameters prediction, understanding of dam break mechanics, and peak outflow prediction were all shown to

be essential for the analysis of dam break, and eventually helped to determine the loss

or damages He also used an application example of Foster Joseph Sayers Dam break, further modeled and analyzed using HEC-RAS model based on available geometry data

Ackerman and Brunner (2011) did a research on the flood that causes dam break by using HEC-RAS model and HEC-GeoRAS They have shown practical combination between HEC-RAS model and HEC-GeoRAS tool to develop a dam breaking model and understand effects of a flood caused by the event HEC-GeoRAS will export the geometry data from topography map system and transfer that data into HEC-RAS model HEC-RAS will simulate unsteady flow from the dam break process This result associates with GIS technology to establish the flood mapping for flood preparedness and prevention

Currently, in Vietnam, there are many hydraulic models that have been applied in studying floods by of government agencies and research institutes, such as MIKE FLOOD, HEC-RAS, WMS, etc Each model has its own advantages and the features can be used separately to complete, depending on the subject that the researchers chose Some study cases can be listed as follows:

Luu Duy Vu and Nguyen Phuoc Sinh (2012) applied WMS model to forecast flood for downstream of Da Nang city In this study, the authors used WMS to simulate the exceptionally large floods in 2007 and 2009 to find out the model parameters and validation, thereby creating flood scenarios for Da Nang city The WMS was chosen because it can simulate flood well Especially, this model can be combined with another free model such as HEC-RAS, HEC-HMS, TR-20, etc

In developing flood maps for downstream of the Vu Gia-Thu Bon river, Tran Van Tinh (2013) applied HEC models including HEC-HMS, HEC-RAS, and HEC-GeoRAS in combination with GIS data to simulate inundation area and depth at lower Vu Gia-Thu Bon river basin with floods in 2009 and another flood of design frequencies of 1%, 2%, 5%

It can be clearly seen that using the hydraulic model to simulate flow and flood mapping

is very common nowaday The combination of HEC-RAS model and GIS is a suitable approach for this problem Especially in the recent time, HEC-RAS model has developed a two-dimensional hydraulic model This development will bring a lot of advantages in flooding simulations With this model, we can simulate well the flow in the river as well as on the flood plain It is necessary for the study area downstream of

Da Ban Reservoir

1.4 Objectives and methods:

The objective of this study is to apply the hydraulic modeling to develop a flood hazard map for downstream area of Da Ban reservoir, Ninh Hoa district, Khanh Hoa province, Viet Nam with different scenarios of upstream floods and dam breaking The results will provide inputs for decision making processes in reservoir management and the development of resident relocation plans and specific responses

In present, there are two major methods which are used worldwide for flood mapping including:

a Flood mapping based on the investigation of the high flood in history

b Flood mapping by mean of numerical simulation using hydraulic models

This study the second method will be used, focusing on the application the hydraulic modeling and GIS software and data for the flood mapping

1.5 Structure of thesis

The thesis is organized with the following parts:

Chapter 1: Introduction

Chapter 2: Social and natural condition of the study area

Chapter 3: Theoretical basis for flood mapping construction

Chapter 4: Development of the flood map of study area

Conclusions and recommendations

CHAPTER 2: SOCIAL AND NATURAL

CONDITIONS OF THE STUDY AREA

2.1 Natural condition of study area

2.1.1 Geographical location of study area

Figure 1: Geographical location of study area (source: Google Map)

2.1.2 Topographical conditions

The Da Ban river basin situated in Ninh Hoa district, Khanh Hoa province The

Trang city, and 10 km west of 1A National Highway

The main flow direction of the Da Ban river is Northern-Southern The river is originated from the Da Den mountain, Truong Son mountain range, from an elevation

slope The Da Ban river is one of the main distributaries of the Ninh Hoa river with the conjunction near Lac Hoa

The river basion has low vegetation cover with the forest area holds between 28% and 35% The forests are one of the monsoons tropical forests with large leaves and are prevailed at elevation +100 m In the dry season, there are many falling leaves (the land degraded after the forest was destroyed to make milpa in a long time), so the water – retaining capacity is low

Major penology is yellow, red laterite soil, eroded easily, especially in the forest destroyed to make milpa

Since the construction of the Da Ban Reservoir, downstream area has been changed significantly due to sedimentation

2.1.3 Climatic and hydrographical conditions

The climatic and hydrographical conditions of the river basin have been studies and reviewed by the consultant of the Join Adventure between Nippon Koei (Japan) and Royal Haskoning (Holland) in a feasibility study Based on this study, we sum up the main characteristics of the climatic and hydrographical conditions

2.1.3.1 Climatic condition

The climatic characteristics of the Ba Ban river basin in particular and another area in Vietnam, in general, have a dry season with rainfall lower than evaporation and a water excess in the rainy season

The most notable characteristic of the basin is the ocean curing intersection so temperature and moisture lower than same latitude area but a deep seat in the mainland

On the contrary, this area is under flood effect with average wind speed, maximum wind speed, annual rainfall, maximum rainfall rather high

Near Da Ban reservoir, there are meteorological stations of Ninh Hoa, Da Ban, Dong Trang, Nha Trang, Ea Krong Hin which are recording data of all meterological parameters over a long period

- Measurement network and factors of climate in the basin

Surveyed stations: Da Ban station, Dong Trang station, Ninh Hoa station, Nha Trang station and Ea Krong Hin station

Surveyed factors: Air temperature, moisture, evaporation, rainfall

- The climate specifications are shown in Table 1:

Table 1: Climate specification

Month Air temperature

( 0 C)

Relative humidity (%)

Wind speed (m/s)

Evaporation (mm)

- Climatic conditions of the irrigated area are shown in Table 2

Table 2: Climate factors of the irrigated area

Month Temperature

( 0 C)

Relative humidity (%)

Wind speed (m/s)

Evaporation (mm)

Sunshine hours (h)

- Annual rainfall at Da Ban: X0lv= 1950 mm

- Maximum wind speed shown in Table 3

Table 3: Maximum wind speed

Direction North Northeast East Southeast South Southwest West Northwest

V (m/s) 26.97 25.54 22.61 14.77 17.42 22.80 16.28 18.98

3 Increment of evaporation due to reservoir : Z mm 604

- Loss of raised evaporation Z in Table 5

Table 5: Evaporation loss

No Factors Unit Value Remarks

1 Annual rainfall, X 0 mm 1950 Thiessen method for 2 stations: Ea

Krong Hin and Da Ban

2 Max rainfall in one day,

X 1 , P 0,5 %

mm 688 Arithmetic average, 2 stations: Ea

Krong Hin and Da Ban

3 Max rainfall in 3 days,

X 3 , P 0,5 %

mm 952 Same as above

2.1.3.2 Hydrological condition of the basin

- Hydrological network and available measurement factors see in Table 7

Table 7: Network and available surveyed factors

No Station name River name Basin

area,

km 2

Recording period

Years of record

1 Dong Trang Cai river in Nha Trang 1445 18832001 19

- Hydrological specifications are shown in Table 8

Table 8: Flow statistics at Da Ban Specification Annual flow

Q 0 (m 3 /s)

Coefficient of variation, Cv

Coefficient of skewness, Cs

Q 75% (m 3 /s)

- Flow distribution at Da Ban see Table 9

Table 9: Flow distribution at Da Ban

Q 75 %(m 3 /s) 1.48 1.40 0.85 2.58 7.71 12.20 8.62 0.63 0.10 0.84 0.72 1.15 2.94

Q 50 %(m 3 /s) 1.89 1.42 1.11 0.99 1.75 2.12 2.58 1.42 5.43 8.49 16.87 3.20 3.94

- Flow and total flood see in Table 10

Table 10: Flow and total flood

No Specifications Unit Design frequency, P %

0,50 0,10 0,01

2 Design flood volume, Wp 10 6 m 3 54.44 67.58 84.79

2.1.3 Ecological specification of animal, vegetation

The basin surface mostly has weathered soil with acrisols, ferralsols, and histosols

Along the stream banks, there is alluvium fluvisol with the firm mechanical part, high

fertility, suitable for rice and farm production in a small area The arid soil has a large

area with the heavy mechanical part which is impoverished and the ferals type with firm

components, suitable with fruits, industrial trees, and medical plants Leptosol is

popular on slope soil and high mountain with low richness, suitable with secondary

forest The Table 11 indicates situation of soil and forest sources

Table 11: Situation of soil and forest sources

No Soil type Area (ha) Total (ha)

Ninh Hoa Van Ninh

Khanh Hoa province which was implemented by Song Da Consulting Joint Stock

Company in 2010 The area belongs to Ninh Son commune, Ninh Hoa District, Khanh

Hoa Province includes the ethnic groups of Kinh, Dao, Tay, and Day Thai with the

population of 40,000 people in 2006 However, the Kinh ethnic group holds 90%,

almost of them are immigrants from other areas, in about 1980s The major economy is

agriculture, main products are maize, tapioca, and rice In the area around Da Ban

Reservoir, people also plant flowers and transports it to the North or South provinces

Presently, average food income per capita is low (200 kg/capita/year) They are dreeding cattle and poultry for their own usage Other sectors such as handicraft and

VND/capita/year The poor and hungry rate is over 36% (in which more than 10% households are ever-hungry) In the areas downstream of the dam (Village 5, Ninh Son commune) the economy is mainly agricultural production with simple tools, and low productivity, poor material facilities Crop plants are based on rainfall or water source from irrigation canal systems

Culture and society: The rate of households with electricity power supply is 85% (almost households in the village have a television) Other services such as telephone, computer and other culture-spirit living in the area nearly not developed yet Healthcare service lacks equipment with only two physicians and three nurses

In the commune, there is an elementary school and high school with poor equipment Nowadays, Da Ban Water Resource System is one of tourist attractive place but it has not received tourists frequently yet At the weekend, some tourists go fishing here, however, the service is poor

2.3 Flood risk in downstream of Da Ban reservoir

Since the Da Ban Reservoir was built, inundation issues were reduced with the regulation ability of reservoir However, the flood risk in downstream can occur in cases

of the reservoir discharge flow with design flood or rare-frequency flood The restricted river channel and high-density resident along the river banks will affect the inundation situation in case of severe floods

CHAPTER 3: THEORETICAL BASIC FOR FLOOD

MAPPING

3.1 General overview

3.1.1 Background of flood mapping

According to WMO ( 2013), a map shows flood hazards, flood-prone areas, and related spatial information are necessary parts for an effective approach to integrated flood management This is especially important when discussing the problems of space, such

as land use planning in flood management framework Flood maps can help people to imagine from flood assessments The flood assessments and flood mapping have a close relationship There are a lot of difference formats of flood mapping and flood assessment such as risk map, hazard map, etc Flood maps are preliminary and detailed which depend on technical expertise as well as human and financial resources

3.1.2 Objectives of this tool

WMO (2013) also indicates that flood maps play an important role in flood management The basic aims of flood maps are to provide information about shape, size, speed of flood in the past as well as their impacts, which help in decision-making

on various aspects of integrated management of floods Objectives of flood maps include changing land uses and climate change; land use regulations and building codes; impacts of urbanization; emergency response; asset management; or overall public awareness

3.1.3 Flood mapping process

The flood mapping is a process which depends on local conditions The process and production often implement following steps:

Implementation of flood assessments and development of flood maps which requires some relevant considerations before the programme launch

3.1.3.1 Objectives of a flood mapping programme

WMO (2013) suppose that objective of flood mapping programme includes purpose, target audience, and target area, so it has to answer the following questions:

- What are these maps produced for?

- Who is using the maps?

- Which areas are covered? (river basin, particular flood plain, river reach, particular settlement, a whole province, etc.)

3.1.3.2 Type of maps

There are different types of maps such as event map, hazard map, vulnerability map and risk map It depends on the objectives of the project, the resources available and the potential benefit achievable

For example, in this study, a hazard map is considered

3.1.3.3 Mapping approach and methodology

In this part, I only present for flood hazard map Three different approaches for developing flood hazard maps:

- The historic approach is based on past flood events: To develop flood zones,

the documents can be used including written reports, old maps or photographs

or any other documents These documents may provide relevant information The historic approach is often used for the calibration of the detailed mapping stage or preliminary and general purpose flood assessments

- Geomorphologic approach: This approach uses the distinct marks that flood

leaves in the landscape to develop flood maps As a historic approach, the geomorphologic approach also serves the preliminary and general purpose flood assessment and their respective maps It is often used for the calibration of the detailed mapping stage

- Modeling approach: In this approach, hydraulic models are applied to simulate

floods of a particular magnitude occurring in area study The modeling approach often serves the detailed flood assessment

The choice of approach depends on the stage of mapping, purpose, and available data

In this thesis, the hydraulic modeling approach is applied for the area study

3.1.3.4 Data needs and availability

A good flood hazard mapping involves various data sets:

- Topographic data: Topographic maps with contours, digital elevations models,

river cross-sections and similar data

- The magnitude of hazard: this data includes rainfall, stream gauge, and

hydraulic data such as channel geometry, bed roughness, etc

- Exposure: this is the data of socio-economic activities such as works,

population, residents, industries, and the economic value of exposed assets Such data may be not always readily available

- Vulnerability: Often such data are not available Vulnerability classes have to

be attributed to land-use classes, such as housing estate, industrial complex, transport infrastructure, etc

3.1.3.5 Implementation and update process

According to WMO (2013), the updating process has to be defined before the mapping starts Updating should occur regularly every 10 to 15 years or after we get new information

3.1.4 Products of flood hazard map

According to WMO (2013), the standard scale of hazard maps is varying from 1:5,000

to 1:25,000 A scale of 1:10,000 is a good scale to identify the features which inundated The hazard map should superpose on the available topographic map with ground elevations and physical features

3.1.4.3 Purpose and use

As mentioned previously, the flood hazard maps provide basic information for various floodplain management issues and help different stakeholders including local governments make decisions in flood management Flood hazard maps play an essential role in assessing of flood risk, development of flood mitigation plans, preparing flood risk management schemes, and in particular for local urban planning Flood hazard maps form the basis for the flood risk maps, flood emergency maps, and other related maps

3.2 General about the hydraulic problems in river network

In fact, the flow in the river system or another area is the complex spatial flow Nowadays to resolve the hydraulic problems in the channel, river network, one often calculates using equations of one-dimensional (1D) steady flow, or one-dimensional unsteady flow, or two-dimensional (2D) unsteady flow, or event three-dimensional (3D) unsteady flow

3.2.1 1D steady Flow

3.2.1.1 Equations for basic profile calculations

Water surface profile is computed from one cross section to next by resolving the energy equation which is written as follow:

e h g

V Y Z g

V

Y

22

2 1 1 1 1

Figure 2: Representation of terms in the energy equation

3.2.1.2 Limitations of 1D steady flow

There are limitations of the 1D steady flow such as:

- Flow is steady

- Flow is gradually varied

- Flow is one dimensional

- Rivers have small slopes

z gA x

V: average velocity of section (m/s);

z: water level at calculation section (m);

3.2.3 2D unsteady flow hydrodynamics

Similar to 1D unsteady flow problem, the basic system of equations for 2D unsteady flow has one continuity equation and two motion equations

The mass (continuity) conservation

0)

()(

hv x

The momentum conservation

When the horizontal length scales are much larger than the vertical length scale, volume conservation implies that the vertical velocity is small The Navier-Stokes vertical momentum equation can be used to justify that pressure is nearly hydrostatic In the absence of baroclinic pressure gradients, strong wind forcing, and non-hydrostatic pressure, a vertically-averaged version of the momentum equation is adequate Vertical velocity and vertical derivative terms can be safely neglected The shallow water equations are obtained

v f

y

u x

u v x

H g y

u v

2

2

)()()

()

()

(

(2-5)

u f

y

v x

v v y

H g y

v v

2

2

)()()

()

()

(

(2-6)

Where u and v are the velocities in the Cartesian directions, g is the gravitational

f is the Coriolis parameter

3.3 General about the hydraulic modeling

With the development of science, especially computer science, there are many numerical models have been developed to solve the above hydraulic problems

There are a lot of numerical models which can solve 1D steady flow and 1D unsteady

flow such as MIKE 11, VRSAP, HEC-RAS

The numerical models which can solve 2D unsteady flow problems are very common including MIKE 21, MIKE 21 HD, MIKE FLOOD, HEC-RAS, etc

In this study, HEC-RAS has been selected to solve the 1D and 2D flooding problems

3.4 HEC-RAS model

3.4.1 Introduction

The U.S ArmyCorps of Engineers’ River Analysis System(HEC-RAS) is software that allows user to perform one-dimensional steady flow hydraulics; one and two-

dimensional unsteady flow river hydraulics calculations; quasi-unsteady and full unsteady flow, sediment transport – mobile bed modeling; water temperature analysis; and generalized water quality modeling (nutrient fate and transport)

The HEC-RAS software was developed at the Hydrologic Engineering Center (HEC), which is a division of the Institute for Water Resources (IWR), U.S Army Corps of Engineers The software was designed by Mr Gary W Brunner, leader of the HEC-RAS development team The two-dimensional unsteady flow modeling capabilities were developed by GaryW Brunner, Mark R Jensen, Steve S Piper, Ben Chacon (Resource Management Consultants, RMA), and Alex J Kennedy

According to USACE (2016) beside the one-dimensional (1D) version, nowadays HEC has had two-dimensional (2D) hydrodynamic routing within the unsteady flow analysis portion of HEC-RAS Users can now perform one- dimensional (1D) unsteady flow modeling, two-dimensional (2D) unsteady flow modeling (Saint Venant equations or Diffusion Wave equations), as well as combined 1D and 2D unsteady flow routing The 2D flow areas in HEC-RAS can be used in a number of ways

The follow this guideline book, HEC-RAS can perform:

- Detailed 2D channel modeling

- Detailed 2D channel and floodplain modeling

- Combined 1D channels with 2D floodplain areas

- Combined 1D channels/floodplains with 2D flow areas behind levees

- Directly connect 1D reaches into and out of 2D flow areas

- Directly connect a 2D flow area to 1D Storage Area with a hydraulic structure

- Multiple 2D flow areas in the same geometry

- Directly connect multiple 2D flow areas with hydraulic structures

- Simplified to very detailed Dam Breach analyses

- Simplified to very detailed Levee Breach analyses

- Mixed flow regime The 2D capability (as well as the 1D) can handle the supercritical and subcritical flow, as well as the flow transitions from subcritical

to supercritical and supercritical to subcritical (hydraulic jumps)

3.4.2 HEC-RAS two-dimensional flow modeling capabilities for calculation case of this study

The 2D flow routing capabilities in HEC-RAS have been developed to allow the user

to perform 2D or combined 1D/2D modeling The 2D flow modeling algorithm in RAS has the following capabilities:

HEC-1 Can perform 1D, 2D, and combined 1D and 2D modeling HEC-RAS can perform

1D modeling, 2D modeling, and combined 1D and 2D modeling

2 Saint-Venant or Diffusion Wave Equations in 2D The program solves either the

2D Saint Venant equations (with optional momentum additions for turbulence and Coriolis effects) or the 2D Diffusion Wave equations In general, the 2D Diffusion Wave equations allow the software to run faster and have greater stability properties

3 Implicit Finite Volume Solution Algorithm The 2D unsteady flow equations solver

uses an Implicit Finite Volume algorithm The implicit solution algorithm allows for larger computational time steps than explicit methods This method improves this stability and robustness 2D flow areas can start completely dry, and handle a sudden rush of water into the area Additionally, the algorithm can handle subcritical, supercritical, and mixed flow regimes (flow passing through critical depth, such as a hydraulic jump)

4 Connecting between lateral structure, storages area and 2D flow area

Algorithms of software allow user can connect easily to many components such as storage area, structures, and 2D flow area Besides, the software has a tool which allows

and support in dam break calculation, so it is very suitable for a case study of the thesis

5 Unstructured or Structured Computational Meshes The software was designed

to use unstructured computational meshes, but can also handle structured meshes The computational cells can be triangles, squares, rectangles, or even five and six- sided elements (the model is limited to elements with up to eight sides) The mesh can be a mixture of cell shapes and sizes

6 Detailed Flood Mapping and Flood Animations Mapping of the inundated area,

Mapper features The mapping of the 2D flow areas is based on the detailed underlying terrain and not the computational mesh cell size

7 Tightly combine with other software HEC-RAS can combine with many software

to use input data or analysis results ARC-GIS is a good combination and was used in this study

3.4.3 Basic steps to modeling

The following are the basic steps for performing 2D modeling within HEC-RAS (Brunner, Warner, Wolfe, Piper, & Marston, 2016):

1 Establish a horizontal coordinate projection to use for the model, from within RAS Mapper

HEC-2 Develop a terrain model in HEC-RAS Mapper The terrain model is used to establish the geometric and hydraulic properties of the 2D cells and cell faces itis also needed in order to perform any inundation mapping in HEC-RAS Mapper

3 Develop a land classification data set within HEC-RAS Mapper in order to establish Manning’s n values within the 2D Flow Areas Additionally, HEC-RAS has an option for user defined polygons that can be used to override the land classification data or as calibration zones

4 Add any additional mapping layers that may be needed for visualization, such as aerial photography, levee locations, road networks, etc

5 From within the Geometry Editor, draw a boundary polygon for each of the 2D Flow Areas to be modeled Or we can import the X, Y boundary coordinates from another source

6 Layout any break lines within the 2D flow area to represent significant barriers to flow, such as levees, roads, natural embankments, high ground between the main channel and overbank areas, hydraulic structures, etc

7 Using the 2D Flow Area editor, create the 2D computational mesh for each 2D Flow Area

8 Edit the 2D Flow Area mesh in order to improve it, such as: add additional break lines; increase or decrease cell density as needed

9 Run the 2D geometric pre-processor from RAS Mapper in order to create the cell and face hydraulic property tables

10 Connect the 2D Flow Areas to 1D Hydraulic elements (river reaches, lateral structures, storage area/2D flow area hydraulic connections) as needed

11 Add any necessary hydraulic structures inside of a 2D Flow Area

12 From the Geometric Data editor, draw any external boundary condition lines along the perimeter of the 2D flow areas

13 Enter all of the necessary boundary and initial condition data for the 2D flow areas

in the Unsteady Flow data editor

14 From the Unsteady Flow Simulation window, set any necessary computational options and settings needed for the 2D flow areas

15 Run the Unsteady flow simulation

16 Review the combined 1D/2D output in RAS Mapper, as well as using the existing output capabilities for the 1D portions of the model

CHAPTER 4: CONSTRUCTING THE FLOOD MAP OF

Figure 4: Da Ban reservoir Table 12: Elevation volume curve

The main part is an earth dam with the length of 347,50 m, the height of 42,50 m, the crest width of 12 m, the crest elevation of 67 m, the outer slope of 3:1, and the outer slope of 2,5:1 (Figure 5 to Figure 8)

Figure 5: Headworks before upgrading

(Source: Google Map)

Figure 6: Headworks after upgrading

(Source: Google Map)

- The spillway (Figure 9) is made by reinforced concrete, 3 radial gates operated manually using a capstan The dimension of the radial gate is B×H = 3×6 m

Figure 9: Existing spillway

- The intake is made of reinforced concrete (see Figure 10), located directly under the spillway The service gate is a vertical rising one, located at upstream The operating gate is radial gate located at downstream, operated using a screw elevator

Figure 10: Intake

- The new free spillway (Figure 11) was built to the right of the earth dam with a total width of 100 m and the sill elevation is 63 m

Figure 11: New spillway (Source: Google Map)

4.2.2 Topography data

- Topographic data of the river downstream of Da Ban with the scale 1:10.000 are cross-sectional data surveyed by NIPPONKOIER and OSAKA in May 2006 (Figure 12)

Figure 12: Da Ban river

A cross-section of river

- The Digital elevation model (DEM) 30 m×30 m of study area is developed using Figure 13the topographic maps of the scale 1:10.000 (Figure 13)

23.8 24 24.2 24.4 24.6 24.8 25

Figure 13: (DEM) 30mx30m of study area

4.2.4 Hydrologic data

The inflows to the reservoir have been calculated by NIPPONKOIER and OSAKA (2006) The results are presented in Table 13 and ( Figure 15 and Figure 16)

Table 13: Max flow and total flow in Da Ban in accordance with frequencies

No Specifications Unit

Frequency P %

0,50 0,10 0,01

1 Design peak flow, Qp m 3 /s 1650 2200 2790

2 Design flood volume, Wp 10 6 m 3 54,44 67,58 84,79

Figure 15: Chart of hydrograph with frequency 0,5%

Figure 16: Chart of hydrograph with frequency 0,1%

0.00 500.00 1000.00 1500.00 2000.00

4.3 ARC-GIS software application to construct DEM data, cross-section data,

and landcover

4.3.1 DEM data

Based on the DEM developed for Khanh Hoa province, the boundary of the study area,

we can create DEM data for the study area as in Figure 17

Figure 17: DEM study area

4.3.2 Cross-sectional data of Da Ban river

The cross sectional data was created from the topographic data of the area by GeoRAS tool in ArcGIS software (Figure 18)

HEC-The topographic data is used to create triangulated irregular network (TIN) before making the cross-section data

Figure 18: Create cross sectional data for Da Ban river

4.3.3 Land cover data

Land cover data is created from Landsat data sources in order to estimate the Manning’s

n values into to 2D flow area properties tables

In this part, the classification tool in ArcGIS is used to analysis data from Landsat data (Figure 19)

Figure 19: Land cover of study area from Landsat sources