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The problem of dam rupture has early been of interest to researchers in order to determine the factors causing the incident, the characteristics of the flood flow, the characteristics of

MINISTRY OF EDUCATION

AND TRAINING

MINISTRY OF AGRICULTURE AND RURAL DEVELOPMENT

VIETNAM ACADEMY FOR WATER RESOURCES

BUI VAN HUU

STUDY OF DAM BREAK WAVE CHARACTERISTICS

IN CASE OF A CONCRETE DAM BREAK

Specialization: Hydraulic construction engineering

Code : 9 58 02 02

ABSTRACT OF DOCTORIAL THESIS IN ENGINEERING

HANOI - 2024

The project was completed at: Vietnam Academy for Water Resources

Scientific Instructor 1: Prof Dr Le Van Nghi

Scientific Supervisor 2: Assoc Prof Dr Nguyen Nghia Hung

Critique 1: Prof Dr Pham Ngoc Quy

Critique 2: Prof Dr Nguyen Quang Hung

Critique 3: Assoc Prof Dr Tran Ngoc Anh

INTRODUCTION

Significance

Dams are water prevention and retention works built thousands of years ago When a dam breaks down, it will cause great damage to people, property and infrastructure systems According to International Rivers Magazine, as of 2021, the world has over 57,000 dams with a height of more than 15.0m, over 2,000 dam breaks, causing more than 8,000 deaths and a lot of property damage The problem of dam rupture has early been of interest to researchers in order

to determine the factors causing the incident, the characteristics of the flood flow, the characteristics of the interrupted waves when the dam breaks with many different methods from theory, experimental model, semi-experimental model, etc., and from digital models to actual observation and measurement Studies are constantly inheriting, in-depth detailed simulations, expanding boundary conditions, in order to provide quantification of dam rupture characteristics Physical models and numerical models have been studied on dam rupture but have not considered the factor of fracture size, correlation between the shape and size of the fracture with the water level upstream and downstream and the width of the downstream channel of the project Studies on 2D models are limited

in simulated space, fracture size, not taking into account cases of partial height breaks The dam rupture simulator has been arranged to move from the bottom

up by a rail and pulley system, which also affects the initial flow when the dam breaks Studies have not mentioned the disruptive waves that occur upstream when the dam bursts

Combining theoretical and experimental research on physical models and mathematical models using Flow-3D software, the thesis focuses on additional research and a more specific approach to the characteristics of interrupted waves when a concrete dam breaks including changes in fracture size; changes in the water level, downstream in three-dimensional space Clarify the characteristics and correlations between characteristics, contribute to the significance in science

as well as help managers proactively estimate the scope and extent of impact when a dam rupture occurs, support decision-making to minimize the impact on downstream Therefore, the research project on the characteristics of interrupted waves in case of a concrete dam break is necessary, scientific and practical

Purpose of the study

- Analyze and describe the shape, propagation and range of influence of the interrupted wave upstream of the rupture (reservoir); flow mode, shape, and propagation of interrupted waves downstream of the rupture (downstream conduction);

- To establish a relationship between the initial wave height and the size of the rupture and the upstream and downstream water level when the dam ruptures;

- To establish a correlation, determine the flow factor through the mf fracture according to different fracture size scenarios

Object and scope of study

Subject: the flow in case of a concrete dam break

Scope of study: Space problem; Regardless of the transformation of the channel; Flat downstream canal bottom, the canal roof coefficient of m=1

Research Methodology

The research methods applied in the thesis include: Overview research; Experiments on physical models and 3D mathematical models (Flow-3D software); Analysis of experimental data; Expert method; Dimensional analysis: applying the Buckingham method to identify experimental series and establish experimental relationships

Scientific and practical significance

Scientific significance: The thesis has further clarified the law of flow when

a concrete dam breaks; it has determined and quantified the influence of fracture size, channel width, downstream water level on the ability to escape floods through the fracture and the initial height of the interrupted wave when the dam breaks The results of the thesis contribute to complete and enrich the understanding of the interrupted waves generated when concrete dams break; is the basis for further research on other issues related to the dam rupture

Practical significance: The research results of the thesis have concretized the

shape, characteristics of interrupted waves in the upstream and downstream of the building and the transmission of dam rupture waves in a visual way Formulas, graphs and diagrams determine the flow coefficient through the rupture, the initial height, and the propagation range of the interrupted wave when the dam breaks, helping consultants and management agencies to facilitate the calculation and determination of the greatest scope of impact when an incident occurs, have a plan

to respond and act proactively and appropriately, ensuring the safety of people and property, helping to minimize risks in the management and operation of the project when an incident occurs

RESEARCH OVERVIEW OF DAM RUPTURE AND

WAVE DISRUPTION 1.1 Definition and classification

Dam is a work built to offer water or together with related works to create a water reservoir Concrete dam is a type of dam where the dam building material

is concrete There are many ways to classify dams, some common classifications such as: according to hydraulic mode (dams to allow water to overflow, dams to prevent water from overflowing), according to dam construction materials

(erosable materials – dams of local materials, non-erosable materials – concrete, steel, composite dams, etc.), according to the use of dams (dams to retain water, diversion) and according to the dam structure Concrete dams are classified into serveral types: gravity concrete dams, arch concrete dams, concrete dams, etc Dam rupture is a phenomenon in which a dam loses control and releases a

"cannot be actively controlled" amount of water or a dam is destroyed, suddenly and quickly releases a large volume of water downstream According to dam building materials: local material dam breakage (earth dam, rock dam) and concrete dam breakage (gravity concrete dam and arch dam)

Figure 1.2 Diagram when the dam

breaks

Figure 1.3 Fracture parameters

(section A-A) Interrupted waves are unstable movements that change rapidly and appear at

a "break" point in the waveform A forward wave is a wave that travels in the same direction as the direction of the flow An inverse wave is a wave that travels

in the opposite direction to the direction of the flow Positive waves are waves that travel to increase the level of water flow Sound waves are transmitted waves that reduce the height of the water level of the flow A positive wave is a wave that travels in the same direction as the direction of the flow and increases the level of the water level Positive-negative waves are waves that travel in the same direction as the flow direction and reduce the height of the water level Inverse waves – positive waves are waves that travel in the opposite direction to the direction of flow and increase the height of the water level of the flow An inverse-negative wave is a wave that travels in the opposite direction of the flow and reduces the height of the water level Flathead waves are waves caused by dam rupture in the case of no water downstream (dry downstream) Researchers based on the direction of movement and the impact of increasing or decreasing the initial flow water level to classify the interrupted waves Accordingly, disruptive waves are divided into 4 types: Forward - positive waves; Inverse waves - positive; Forward wave - negative; Inverse wave – negative

1.2 Dam rupture and causes of dam rupture

Since recorded, the statistics is showing that the rate of dam breakage in the world has decreased over the periods The highest rate of dam failures: by life - dams under 5 years old, by height - dams with a height of less than 15.0 m, by reservoir capacity - capacity less than 100 million m3, by the period of the incident

- from 1900 to 1929 In Vietnam, as of 2022, there have been no recorded concrete dam breakage incidents, the dam breakage incidents that occur are the incidents of local material dams (earthen dams, fallen stone dams, etc.) such as the unfinished dam Cua Dat – Thanh Hoa (2007); Dam Ha Dong - Quang Ninh (2014)

The causes of dam breakage incidents can be due to: Floods exceeding the design (the main cause, accounting for 1/3 of the incidents); Weak geology; Deterioration of the quality of the work (increase in the life of the work); Construction issues; Human factors: errors in the design and construction of works, errors in operation, due to war or deliberate destruction by humans; Some other causes such as earthquakes, landslides, ice melt, avalanches, seepage flows, etc For concrete dams, the main cause of 73.2% of incidents is due to the quality

of construction and floods exceeding the discharge capacity

1.3 Study on the shape of concrete dam fractures

Each type of material and type of dam will have a different shape and size

of fracture when a problem occurs With earth-dams: the form of the fracture can

be trapezoidal, triangular With concrete dams: they are rectangular and break into many blocks, the dam rupture site usually occurs in the middle (accounting for 30%) Therefore, when simulating the concrete dam rupture incident, the thesis chooses a rectangular, multi-block fracture

1.4 Dam rupture and wave disruption research

The problem of unstable flow hydraulics and quick changes on the open conductor (dam rupture) is a difficult and complicated problem Dam rupture and intermittent wave characteristics have been of interest to researchers since the end

of the 19th century, typical studies include: Ritter (1892) conducted an experimental study on a rectangular prismatic canal with a flat bottom, downstream without water (downstream without obstacles) Stoker (1957), developed and expanded the boundary conditions that affect the characteristics of dam rupture flow when considering the downstream water level factor Theoretical research on the Navier-Stokes equation system written for stratigraphic flow with the addition

of a tangle coefficient with Ritter (1892); Whitham (1955), Stoker (1957), Hubert (1984, 1989) In Vietnam, there are studies by Nguyen Canh Cam (2006), Le Thi Thu Hien (2015) using simple analysis, characteristic lines or latent difference

diagrams to solve the dam rupture problem A number of formulas were published for determining the flow of escape through the fracture, the initial height of the interrupted wave according to the upstream water head H Researches on the experimental models were mainly conducted on the glass trough (2D model) as by Chen Yang et al (2010); Hatice Ozmen-Cagatay et al (2010, 2012 and 2020), Hubert Chanson (2008), Hunt,B (1987), Ritter, A (1892), Stoker, J J (1957) and Wenjun Liu et al Vietnamese studies focused on ruptures for dikes and dams built with local materials such as Nguyen Tuan Anh, Luu Nhu Phu (2003), Nguyen Doanh Oanh et al (2006), Pham Ngoc Ru et al (2011), Le Van Nghi, Bui Van Huu

et al (2015); Pham Thu Huong (2018) The numerical model has obtained many breakthrough results in identifying the characteristics of the interrupted wave shape

1.5 The largest instantaneous flow escaping through the rupture

Studies have shown that the greatest instantaneous flow through the rupture (Qp) depends on factors such as: dam type, type of embankment material, dam rupture time (tf), water level in the reservoir, and many other parameters Therefore, to simplify, the studies only consider a few key factors such as dam height (Hđ), the lake’s presure head before the rupture, and water volume on the rupture (H and Vf)

1.6 Chapter 1’s Conclusions

The thesis has summarized the studies and incidents of major dam ruptures that occurred in the world and Vietnam, highlighted the process of occurrence, the causes and the damages caused by the incident, thereby, provided analysis and synthesis of the causes of dam rupture in general and concrete dams in particular

The thesis has also generalized the existing studies that used flow characteristics of concrete dam break method including theorical researches, experimental researches, semi-experimental researches and digital model - CFD

It lastly provided analysis and synthesis on the achievements, limitations of existing studies on the shape characteristics of the positive interrupted wave and the interrupted wave height which are the main factors affecting the highest instantaneous flow through the rupture Thereby, a focused reserach direction will be eshtablished for the thesis

SCIENTIFIC BASIS FOR IDENTIFYING THE CHARACTERISTICS OF INTERRUPTED WAVES WHEN A

CONCRETE DAM BREAKS 2.1 Theoretical basis for calculating the flow through the rupture

For each type of material or dam type, the fractures will have different shapes, sizes, and positions With local material dams, the fractures are usually trapezoidal

or triangular With concrete dams - the object of research of the thesis - the fractures are rectangular, fragmental and occur in the middle of the dams The flow through the fracture has similar characteristics to the flow through the pragmatic spillway with a polygonal cross-section In case the height of the rupture is small (less than 0.1H), the flow runs down to the dam foot along the downstream roof toward the downstream The air doesn’t exist under the water layer – the state of non-vacuum flow In case the height of the rupture increases (0.1H< hf <0.75H), the flow is large, similar to the flow through the wide-crested spillway , the flow out of the rupture area separates from the downstream dam surface, and pours into the downstream channel Air exists under the water flowing through the rupture - the state vacuum flow

Assuming that the flow through the rupture is the same as the flow through the traditional overflow ; and the formula for determining the flow through the rupture is as shown in (2.2), when the flow through the rupture is free flow, n = 1.0

Qn= σnmfBf√2gH3/2 (2.2)

2.2 Formulation of experimental research equations

Develop an empirical equation to determine independent and dependent quantities The experimental data are synthesized for finding a close correlation

between the quantities: Input Quantity → Dependent relation (experimental regression equation) → Output Quantity

The relevant factors, affecting the ability to drain the flood through the fracture and the characteristics of the interrupted wave, include the engineering the flow, which are divided into input and output data groups

The input data is intentionally controlled according to the intention of the experiment, including: parameters of the dam and fracture , flow input into the model, and the depth of the downstream water level The output data is the flow escaping through the rupture, the initial height of the interrupted wave, the horizontal wave length upstream, and the vertical wave length upstream

The Buckingham equation presenting variable quantities is used to describe the hydrodynamic phenomenon to be studied in a function From the quantities (12 independent variable quantities), we select 3 basic dimensions: time [T], length [L], mass [M] The remaining independent variable quantities (n-3 = 12-3

= 9) are transformed into a dimensionless relation, the Pi function Given the function f(1, 2, 3 ) = 0 (2.10)

General equation considering the flow characteristics through the dam rupture:

+ Determination of the initial height of the wave:

Parameters at the upstream; the flow coefficient, which is independent of the wave elements downstream, so (2.16) is rewritten as Lsn

+ Considering the transmission of waves on the upstream reservoir:

Wave propagation at the upstream reservoir depends on the ability to drain the flood through the rupture and the timing of the incident The flow through the rupture, calculated based on the width of the rupture and the water level H

(accounting approaching flow rate), the downstream water level (hh), the initial wave height downstream (hw), the wave transmission velocity (vs) and the downstream conductor width, does not affect the wave transmission upstream, therefore (2.16) is rewritten as:

If we consider the transmission of waves in two directions to be independent

of each other, we have:

2.3 Research Model

2.3.1 Physical Models

The physical model was built and experimented according to the National Standard TCVN 8214-2009 on testing hydraulic models of irrigation and hydropower works

Design and construction ensured mechanical similarity between the actual work and the model, including geometric analogy, kinematic analogy and dynamic analogy The flow through the fracture is an open flow, the force acting

on the flow has gravity playing a major role, so the model is applied a standard similar to gravity, or the Froude standard

Fixed bed undisorted general model (3D model) (Figure 2.3) Model scale l

= 125 The model is 20.0m long; the reservoir is 12.0m wide, the channel is 8.0m wide and 1.6m high; equivalent to the actual model size of 2,500.0m long; reservoir width of 1,500.0m, downstream channel width of 800.0m and height of 200.0m (the main channel is 150.0m high) The joints, the upstream and downstream channel are plastered with cement mortar to ensure the same roughness as of the real work to be simulated

The facture simulation during a dam break is implemneted by Concrete Dam Simulation System The system has been patented by the National Office of Intellectual Property No.2043: Equipment to simulate the process of concrete dam breakage according to Decision No.35545/QD-SHTT dated 09/05/2019 (Figure 2.7.b)

Arrangement of measuring equipment; Model error assessment; Evaluation of the appropriateness of the experimental data The interupted wave signal is collected by the Duck electronic measuring head system located on the model conductor The measuring head system is connected to a computer and a dedicated data collector For each mearurement, 10 signals are taken per second with maximum measuring time of 180 seconds

Determine the flow through the fracture on the model by arranging 01 trapezoidal trough located at the exit (end of the model) 8.0m from the fracture location, equivalent to the actual 1000m from the fracture, the guide section has

a relatively flat terrain, a small slope of <1.0%

Figure 2.3 Physical model of concrete

dam breakage

Figure 2.7.b Dam rupture

simulator installed on the model

2.3.2 CFD Digital Models

In order to add more experimental cases and scenarios for PM, the thesis simulates a dam rupture on a 3D mathematical model using Flow 3D software The model is set up including 3 simulated areas including area 1 - upstream reservoir, area 2 - dam + fracture and area 3 - downstream aqueduct (with the prototype being Son La dam)

The calculation grid for the model is divided in the form of rectangular grid blocks with uniform grid step: x = y = z = m (Figure 2.15) The conformity and quality of the mesh are checked by using the Simulation Pre-check tool in Flow-3D (Mesh Quality) The grid steps are performed several times to find the mesh size divided by the result that matches the physical model and the Flow-3D model and ensure that the model run time is not too long for a computational scenario

From the existing studies on the tangled flow simulation method, the thesis chooses the method of solving the RNG k-  model to simulate the dam rupture problem The model was tested and calibrated with data obtained from the physical model for the case of upstream water level of 217.83m, fracture width (Bf) of 187.5m and fracture height (hf) of 45.0m

Mesuring result comparing location is 400m from the dam foot, in the middle

of the downstream channel The results showed that the difference between the physical model and the Flow-3D model was more than 6.0%

Therefore, the Flow-3D model set up for the research problem ensures reliability for calculating different scenarios of the thesis

Figure 2.12 Simulation areas set up

and created in SketchUp software

Figure 2.15 Calculation grid plan

Figure 2.16 Boundary conditions of

the model Figure 2.17 Position determined on the physical model at x=400m

a Wave shape on a physical model b Wave shape on Flow-3D model

Figure 2.18 Downstream Interrupt Wave Shape on the physical model and

Flow-3D model

Depth of Flow Flow Rate

Figure 2.19 Depth and flow rate at point x=400m

2.3.3 Combination of physical model and CFD numerical model

The results on the physical model and the numerical model are complementary and mutually supportive The numerical model uses 01 set of data on the physical model as a set of data to verify the mathematical model, in contrast to 80 scenarios on the numerical model to clarify the flow characteristics when the dam breaks, which is a limitation of the physical model

2.4 Chapter 2 Conclusion

On the basis of the same theory, modeling, and experimental planning, the quantities that mainly affect the flow through the rupture and the initial characteristics of the interrupt wave are determined including the fracture size, the upstream and downstream water level and the width of the downstream conductor

The thesis uses a research method that combines experimental model and digital model (Flow-3D) The experimental model was built to ensure the same conditions (prototype of the Son La dam model, l=125scale) The system of equipment simulating the rupture on the physical model allows to approximate the actual simulation of the dam rupture phenomenon, generating intermittent waves at the upstream reservoir, downstream conductor and is granted a patent for a useful solution by the National Office of Intellectual Property

The numerical model is set up and developed by Flow-3D software for the computational domain similar to the experimental model, using the RNG k- model, calibrated, tested by a set of experimental data on the 3D physical model, ensuring accuracy and reliability (with an error of <6%) for implementation, set

up combinations of calculation scenarios for the next research steps