Summary of doctoral thesis: Building numerical model to simulate the flow and sediment transport on small and medium watershed

Objectives: Research on the scientific basis to a construct numerical model to simulate the flow and sediment transport on watershed. Application of the built numerical model in some small and medium watersheds.

MINISTRY OF EDUCATION

AND TRAINING

MINISTRY OF AGRICULTURE AND RURUAL DEVELOPMENT

THUYLOI UNIVERSITY

DAO TAN QUY

BUILDING A NUMERICAL MODEL SIMULATING WATER FLOW AND SEDIMENT TRANSPORT IN SMALL

AND MEDIUM WATERSHEDS

Specialization: Hydrology Code no: 62 44 90 01

SUMMARY OF DOCTORAL THESIS

HA NOI, 2017

This scientific work has been accomplished at: ThuyLoi University

Advisor 1: Assoc.Prof Dr Pham Thi Lan Huong

Advisor 2: Assoc.Prof Dr Ngo Le Long

Review No.1: Assoc.Prof Dr Nguyen Ba Quy

Review No.2: Dr Nguyen Lap Dan

Review No.3: Assoc.Prof Dr Nguyen Hoang Son

This Doctoral Thesis will be defended at the meeting of the University Doctoral Committee at:

at ………… on………

This thesis is available at:

- The National Library

- The Library of Thuyloi University

INTRODUCTION

1 Problems statement

In recent years, under the negative impact of natural factors as well as the global climate change have caused erosion, sediment transport and land degradation in watersheds, especially inhillslopes

Vietnam is situated in the region of tropical monsoon climate with mountains account forabout 3/4 of the total area, hence erosion is considered as a major threat to the earthen slopes in Vietnam.If we do not have measures to prevent erosion, hundreds of tons of soil and nutrients are lost every year leading tolands become degraded and can no longer cultivated

Therefore, the “building numerical model to simulate the flow and sediment transport on small and medium watershed” is necessary and urgent to apply in

calculating the flow and sediment transport on Vietnam’s watersheds

3 Objects and Scope

- Objects of the study: The numerical models simulating flow and sediment transport

- Scope of the study: Small and medium watersheds

4 Study contents

- Provide an overview of the numerical models simulating erosion and sediment transport in the medium and small watersheds in the world as well as in Vietnam, then evaluate the technical limitation and point out the issues that the thesis should concentrate

- Apply the theoretical basis of the mechanism of erosion and sediment transport to develop anumericalmodel simulating erosion and sediment transport in small and medium watersheds

- Apply the developed numerical model to some small and medium watersheds

in Son La province

5 Methodology

- Literature review;

- Studying on the basic theory and inheriting other related studies;

- Remote sensing and Geographic Information System (GIS);

- Mathematical Modelling;

- The study results were reported and discussed in many conferences

6 Implications

Scientific significance: The research results of this thesis will reconfirm that

buildingnumerical models simulating sediment transport applying remote sensing and GIS is very effective and necessary in the present period

Practical significance: The results of this thesis will provide local data sources

and tools that can monitor, evaluate, look up information and monitoring the impact of erosion and sediment transport to the activities of exploitation and using land and water, thenprovideperfectly reasonable solutionsfor water resource

planning, and for land use planning

7 New contributions

- Construction of a new numerical model simulating the flow and sediment transport on small and medium watershedsthat usesLax-Friedrich Scheme and addsmore elements of time and space to it to solve flow and sediment transport equation

- Construction of the correlation equationbetweenthe resultsof inter-rill erosion and rill erosion that can predictthe amount of sediment eroded and transported

in watershedsbased on the intensity of rainfall

8 Contents: The thesis consists of 3 main chapters

Chapter I:Overview of models simulating sediment transport in small and

medium watershed

Chapter II:The scientific basis to develop a numerical model simulating

sediment transport in small and medium watersheds

Chapter III: Application of the developed numerical model to simulate flow

and sediment transport in several small and medium watersheds

CHAPTER 1 OVERVIEW OF MODELS SIMULATING SEDIMENT TRANSPORT IN SMALL AND MEDIUM WATERSHED

1.1 Overview of erosion and sediment transport inwatershed

1.1.1 Definitions and basic concepts

1.1.1.1 Watershed erosion

Erosion is a phenomenon where piece elements, clods, and some time the whole land surface to be eroded and transported away by the wind and water power

Water erosion depends on the energy of the water flow and the resistance to erosion of the soil where water flows

1.1.1.2 Sediment and deposition

Suspended sediment is small-sized sediment particles floatingand driftingdown

in the water The speed of it equals the speed of water flow

Deposition is a process where the soil particles detached by erosion then deposited in the ground or inside the water such as lakes, streams and wetlands

1.1.2 The main causes of erosion and factors affecting erosion

1.1.2.1 The main causes of erosion

a Group of rainfall factors: Rainfall, rainfall intensity and distribution

will decide to the forces dispersing the particles of soil, to the amount of water and to the velocity of runoff Rainfall in a short time will limit erosion due to the insufficiency of water to form flow When it rains with a greatintensity in a long time, the very serious erosion will occur

b Group of soil mechanical component factors: For soil with heavy

mechanical components, the particles of soil are always small, smooth, cohesive and hard to break up, thus the risk of erosion is low For soil with medium mechanical components, the particles of soil have moderate size and links, and to

be porous, hence it is easy to be eroded by surface runoff Therefore, the risk of erosion is high For soil with light mechanical components, although having unstable structure but it has a large particle size that is difficult to transport, thus the erosion risk is not high This soil has good permeability but bad water retention

1.1.2.2 The factors affecting erosion

There are 5 main factors affecting soil erosion include topography, soil type,

vegetation, climate and humanity

1.1.3 Classification of watershed erosion

Splash erosion, sheet erosion, rill erosion, gully erosion

1.1.4 Sediment transport in the watershed

Transport is the washing and movement of the soild particles from high lands through rivers and ultimately to the ocean Sediment transport process in the watershed is complex that depends on the amount of flow, erosion, transport and deposition

1.2 Studies in the world

1.2.1 Studies on overall assessment of erosion

Current erosion process is associated with agricultural activities Many have said that the land was exploited to become exhausted could be the reason causing past civilizations lost Therefore, together with land degradation, erosion exists as a problem throughout the development process of humanity

1.2.2 Studies on models simulating the process of erosion and sediment transport

1.2.2.1 The empirical model

The empirical model is primarily based on the analysis of observations and relationships from measured data The value of parameters in the empirical model identified through the model calibration process, but usually determined from the verification of the actual monitoring data

The empirical model is not to mention the depositionissues in the watershed and not to calculate a specific rainfall, and also do not consider the erosion in rills, channels and see the depth of the overland flow to be constant

1.2.2.2 Physically based model

Unlike the empirical model, the physically based modelwas developed based on the understanding of the laws of motion and the physical mechanism of the process of erosion, it means that this model based on the understandinghas been theorized as the laws of physics or equation The physical processes of erosion, including soil particle detachment, transferring and sediment transport

The physically based model simulating erosion and sediment transport is constructed based on the mathematical equationsdescribing the physical phenomena of soil erosion process The mathematical basis of the model is the

continuity equation of Bennett Continuity equation is commonly used in modeling the dynamics of soil erosion as follows:

1.2.3 Algorithmsin watershed scale erosion and sediment transport models

The watershed scale erosion and sediment transport models are based on the continuity equation of Bennet These are solved by numerical methods such as CREAM, EUROSEMmodels, while othersto be solved by analytical methods such as SWAT, WEPP modelsetc.In a case the models solved by analytical methods, the slopes must be uniform and have rainfall excess Ina general case the problem is usually solved by numerical methods, using the finite element schemes and a specific grid of time and space

1.3 Studies in Vietnam

1.3.1 Studies on overall assessment of erosion

Vietnam is located in the region of tropical monsoon climate with relatively heavy rainfall (average from 1800 - 2000 mm), but unevenly distributed and concentrated mainly in the rainy season Heavy rainfall concentrates water into flow ofa great intensity, thus this is a major cause of soil erosion in Vietnam

1.3.2 Studiesand applications of erosion and sediment transport models

The studieson factors causing erosion and on the predictabilityof erosion on hillslopes gave the erosion indicators of rainfall and the erosion coefficient of soil(K) to proceed to apply the universal soil loss equation (USLE) by Wischmeier and Smith, and to predict soil erosion and to preliminary showerosion potential by rainfall on small scale maps However, these studies did not consider other factors causing erosion such as slope, slope length, diversified cropping systems, soil protection factor Results of these are still at the level of a forecast of generalized erosion

Besides, the previous studies only used the universal soil loss equation combined with theRemote sensing and Geographic Information System (GIS)

to assess the soil erosion potential and to analyse the impacts on agricultural production This wayresulted ingreat efficiency, helped to save time and costs Study results in the form of GIS data,so they are very intuitive, easy to use and calibrate However, the K and Pcoefficients were taken in the reference, hence they would partly affect calculation results

1.4 Gaps in the studies of erosion and sediment transport - Research orientation in the study

1.4.1 Gaps in the studies of erosion and sediment transport

The researches in Vietnam is mainly apply available mathematical models of erosion and sediment transport

Foreign models are usually commercial and costlyones with closed source, thus applying to river watersheds in Vietnam will not avoid certain errors for the parameters in the model calibrated according to abroad data

1.4.2 Research orientation in the study

Research on the theoretical basis of modeling flow, erosion and sediment transport in the medium and small watershed Then construction of the diagram

of forming flow and sediment transport, and the diagram simulating process flow of sediment in the watershed

The study suggests new stable algorithms with the convergence for continuity flow equation, momentum equation and erosion and sediment transport equation

Conclusions of chapter 1:

Author provides an overview of simulation models of sediment transport in the watershed in Vietnam and worldwide, then points out that in Vietnam, we mainly use available mathematical models of erosion and sediment transport which are commercial with high cost and closed source Moreover, the parameters of these are constructed based on the physical characteristics of abroadwatersheds, so when applying to the watersheds in Vietnam will lead to certain errors Therefore, to overcome the limitations of the models mentioned above, author will concentrateon constructing a new model simulating sediment transport in Vietnam’s small and medium watersheds

CHAPTER 2 THE SCIENTIFIC BASIS TO DEVELOP A NUMERICAL MODEL SIMULATING SEDIMENT TRANSPORT IN SMALL AND MEDIUM WATERSHEDS

2.1 Theoretical foundations – Research and propose algorithm

2.1.1 Theoretical foundations

2.1.1.1 Equation for infiltration

The infiltration process is calculated from a equation calculating losses of infiltration during rainfall The Green - Ampt Mein - Larson equation is written

2.1.1.2 Equations for Flow

a Equations for Overland Flow

The continuity equation: h (uh)

b Equations for channel/river flow

The continuity equation: ( )

2.1.1.3 Equations for simulation of flow

a Equations for erosion and sediment transport in watershed

Model simulating erosion and sediment transport is constructed based on mathematical equations describing the physical phenomena of erosion and sediment transport The mathematical basis of physical phenomena isthe continuity equation of Bennett The continuity equation for erosion and

sediment transport is written as follows:

2.1.2 Research and propose algorithm

Currently, there are many methods to solve equations for flow and equations for erosion and sediment transport In this study,the thesis uses Lax-Friedrichs scheme adding time and space components and it called finite difference Lax-Friedrichs-Weightscheme (LFW) to solve a system of equations for flow and equations for erosion and sediment transport The algorithm was proved by giving the evidence of convergence for the above equations

2.1.2.1 The algorithm for solving equations for overland flow

a.The algorithm: With x j = jΔx, tn = nΔt, the Lax-Friedrichs scheme (LF) approximates to the derivative of function u = u(x,t)at the point (xj, tn):

Applying LFW for equation (2-6), then we get the differential equation of (2-6)

as follows:

1 1 1

b.The convergence of algorithm

Study on the stability of LFW difference algorithm for equations (2-6) corresponds with the stability of LFW difference algorithm for the following equation:

a.The algorithm: There are many methods of solving equation (2-14), but

to limit the errors, many used the implicit or explicit finite difference schemes The author used the Lax - Friedrichsscheme adding more time and space

components to approximate the derivative of functionu = u(x,t) at a point (xj,

tn) with xj = jΔx, tn = nΔt Then we have:

b.The convergence of algorithm:

The LFW algorithm is stable for equation (2-14), it is easy to inspect the stability of the algorithm by the way similar to the steps in the process of checking the stability condition of the LFW difference algorithm to equation (2-26)

2.1.2.4 The algorithm for solving equations for erosion and sediment transport inchannel/river

The LFW scheme is applied to erosion and sediment transport equation in

2.2 Construction of the model components

Based on the theoretical analysis of the formation of flow and sediment transport in the watershed Thethesis constructsa diagram describing the flow of sediment in the river basin as Figure 2-3

Figure 2-3 Diagram of calculation of erosion and sediment transport

2.3.1 Inter-rill process

2.3.1.1 The potential of inter-rill erosion

Calculation of erosion and sediment transport in the watershed is very complicated, thus to simplify it the author chose the following equation:

2.3.1.2 Inter-rill sediment transport

In this model, the author use the Wischmeier - Meyer equations to define rill sediment transport capability as follows:

Ei = Ciqs KCslr (2-74)

2.3.2.2 Rill sediment transport

There are many equations were built to calculate sediment transport capacity, but the applicability of each equation suit each model The author chose the following equation to calculate:

2.3.3 Processes in channel/river

2.3.3.1 Sediment transport capacityin channel/river

The sediment transport capacity is calculated by the Parsons et al equation as follows:

Q

il sl

V

q t A E



2.3.3.2 Sediment transport in channel/river

Sediment transport in river is modeled by the Engelund - Hansen equation as follows:

1/ 2 3/ 2 0