Large scale embankment breach experiments in flume

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Large Scale Embankment Breach

Experiments in Flume

Gensheng Zhao, Paul Visser, Patrik Peeters

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1 Introduction

Em bankm ents, including D ikes and dams, are o f large benefit to people all over the world. Since the beginning o f hum an civilization thousands o f years ago, em bankm ents have been playing vital role in the developm ent o f hum an being. H istory o f em bankm ents is an epitom e o f the rise and fall o f hum an civilization, especially on the defenses o f floods and irrigations from rivers and lakes. Em bankm ents, blocking the floods in the river channels, sea and lakes, have already been protecting hum an lives and properties from flood disasters; Em bankm ent, im pounding large volum es o f waters, have already been used to flood control, navigation, irrigation, w ater supply, hydro-electric power, recreation and so on. However, em bankm ents could also give hum an lives and properties high risks to some extend for their failures due to overtopping, piping and other factors. Further, em bankm ents generated autom atically by the nature are also providing risks, due to the landslides induced by the earthquakes, storms and so on.

The m agnitude and extent o f the losses depend highly on the rate o f the breaching o f em banks, w hich determ ines the discharge through the breach and the speed and rate o f inundation o f the polder, the areas outside the em bankm ents or downstream. Therefore, m odeling o f breach evolution in em bankm ents, predicting the breach param eters (e.g. depth, width, discharge) and the breach flow rate, is o f significant interest for dam age assessm ent and risk analysis. It is also im portant for the developm ent o f early w arning system for dike and dam failures and evacuation plans o f people at risk.

R alston (1987) gave a good description o f the m echanics o f em bankm ent erosion. For cohesive em bankm ents, breaching takes place due to headcut erosion. A t the beginning, the headcut is typically form ed at the toe o f the em bankm ent and then advances upstream until the crest o f the em bankm ent is reached. In some cases a series o f stair-step headcut form s on the dow nstream face o f the em bankment. The action is sim ilar to that described by D odge (1988) for model testing o f em bankm ent overtopping. The relevant processes are headcut initiation and advance by hydrodynam ic and geotechnical m ass wasting.

Zhu et al. (2004) sum m arized ongoing research efforts o f several entities aim ed at developing new m ethods for protecting em bankm ents from erosion during overtopping flow, and for predicting erosion o f protected and unprotected em bankm ents. All o f the studies indicate that

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substantial, and thus, repetition o f model tests is essential. Fread (1988) developed a breach erosion model (BREA CH ) for an earthen dam to predict the breach size, shape, and tim e o f form ation and breach outflow hydrograph. In the model, erosion is assum ed to occur equally along the bottom and sides o f the breach channel except w hen the sides o f the breach channel collapse and if the valley floor has been reached, further dow nw ards erosion is not allowed and the peak discharge w ould be expected.

In the model o f V isser (1998) for sand-dikes, a relatively small initial breach is assum ed in the top o f the dike that is so large that w ater flows through it starting the breach erosion process. By assum ing a trapezoidal shape o f the initial breach w ith the angle o f repose, five stages can be distinguished in the process o f breach erosion (Fig. 1):

1) Steepening o f inner slope from the initial value up to a critical value.

2) R etrograde erosion o f the inner slope o f the dike in the breach, yielding a decrease o f the w idth o f the crest o f the dike in the breach.

3) Low ering o f the top o f the dike in the breach, w ith constant angle o f the critical breach side slopes, resulting in an increase o f w idth o f the breach.

4) Critical flow stage, in w hich the flow is virtually critical throughout the breach, and the breach continues to grow m ainly laterally.

5) Subcritical flow stage, in w hich the breach continues to grow, m ainly laterally.

L ____

" 7 ff/

/ 7/

¿ .„ — S

Figure 1 Schematic illustration of breach growth in a sand-dike (Visser, 1998)

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In the first three stages the initial breach cuts itse lf into the dike; m ost o f the discharge through the breach happens in stages 4 and 5.

C orresponding to the study o f sand-dike breaching by Visser, Zhu (2006) investigated the breaching process o f clay-dikes. The distinct difference from sand-dike breaching is the headcut erosion that occurs during the breaching process o f clay-dikes. Similarly, by assum ing an initial breach in the top o f the dike that is relatively small and trapezoidal shaped, Zhu (2006) classified the breach erosion process in clay-dikes into the five follow ing stages:

1) Stage I ( t 0 < t < tx ): Floodw ater flow s through the initial breach in the dike crest and erodes soil away from the crest and the inner slope o f the dike. B oth flow shear erosion as well as sm all-scale headcut erosion can occur along the inner slope (see Fig. 2(a)(b)).

2) Stage II {tx < t < t2)'. The steepened inner slope o f the dike holds the critical slope angle throughout Stage II and acts like a headcut during the erosion process ow ing to its large steepness (see Fig. 2(b)(c)).

3) Stage III ( t2 < t < /, ): The headcut still m aintains the critical slope angle . The breach enlarges rapidly, accordingly also the flow rate through the breach, w hich in turn accelerates the breach erosion process in the dike. A t the end o f this stage, the dike body in the breach has been w ashed away com pletely dow n to the dike foundation or to the toe protection on the dike outer slope (see Fig. 2(c)(d)).

4) Stage IV ( t 3 < t < t4 ): In this stage the flow in the breach is critical. B reach erosion takes place m ainly laterally, w ith flow shear erosion along the side-slopes o f the breach and the resulting discrete side slope instability being the m ain m echanism s for the breach enlargement. Vertical erosion in this stage relies m ainly on the geom etrical and material features o f the dike (see Fig. 2(d)(e)).

5) Stage V ( t 4 < t < ts ): In this stage the breach flow is subcritical. The breach erosion still occurs m ainly laterally and at the end, the velocity o f the breach flow is reduced to such an extent that it can no longer erode aw ay soil m aterial from either the dike body or the dike foundation, hence the breach grow th process stops (see Fig. 2(e)(f)).

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Figure 2 Breach development process in clay dike (Visser, 1998; Zhu, 2006)

O vertopping breaching is the m ost frequent form o f em bankm ent failure. The m agnitude and extent o f the losses depend highly on the rate o f the breaching o f em bankm ents, w hich in turn determ ines the discharge through the breach and the speed and rate o f inundation o f the polder, the areas outside the em bankm ents or dow nstream o f the breach. Therefore, the

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m odeling o f breach evolution in em bankm ents, the predicting o f the breach param eters (e.g. depth, width, discharge) and the breach flow rate, is o f significant interest to dam age assessm ent and risk analysis. It is also im portant for the developm ent o f early w arning systems for dike and dam failures and for evacuation plans o f people at risk.

The em bankm ent breaching process can be divided into several stages according to the researcher’s hypothesis and the prototype surveys, lab experim ents. Researchers have different m ethods to classify the breaching processes because o f the em bankm ent different materials. In order to reveal the physical process o f breach or the breach mechanics, the physical model studies are urgently needed to im prove and push the breach model development. Large scale physical model and/ or prototype tests are the only tools to solve the breach m o del’s bottleneck.

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2. Research Objectives

The m ain research objective is to investigate the m echanism o f the breach grow th in cohesive em bankm ents and model the process o f the breach grow th in the cohesive em bankm ents in the m ethodology o f Energy Theory, H ydraulics, R iver M echanics, Soil M echanics and the A cceptable R isk A nalysis as well. The detailed research objectives in the flum e experim ents are as follows:

• To get insight into the breach developm ent o f cohesive em bankm ent

• To clarify the roles o f headcut erosion (form ation and m igration) in the deepening process o f the breach (Zhao, et al, 2013)

• To clarify the roles o f lateral (helicoidal) erosion in w idening process o f the breach • To study the scour hole developm ent dow nstream the breach

• To study the influence o f the initial trench’s location on the breaching process • To com pare the lab experim ent and field experim ent and m ake clear the scale factors

Physical model is a useful and popular tool to investigate the breaching process in the em bankm ent and has been applied by the form er researchers (Visser, 1998; Zhu, 2006) in the laboratory, however, the scale lim itations increase the uncertainty o f the breaching developm ent and the result distortions have been generated. In the m eantim e, researchers use prototype data analyses and conduct the real em bankm ent breach tests in the field to get insights into the m echanism o f breach developm ent. B ut the data collected from prototype are usually not complete. The m easurem ent accuracy is not high enough w ith the field experim ents and the costs are m uch higher than physical model in the laboratory.

In order to reduce the scale im pacts and defects o f the prototype experim ents, the large scale sedim ent m odels in the flum e are desingned to investigate the em bankm ent breaching process, including surface erosion, headcut erosion and laterial erosion (helicoidal erosion). The breach hydrological process and topography changes are also m easured in these experiments.

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3 Experiment Design and Setup

3.1 In trod u ction o f th e F lu m e

The 5 runs o f experim ents w ere conducted in the flum e o f 60 m x 3 m x 3 m o f Changjiang R iver Scientific R esearch Institute, Changjiang R iver W ater Resources Com m ission, China. The m axim um discharge that can be supplied is 1 m /s. The flum e layouts are shown in Fig. 3, Fig. 4 and Fig. 5. Tw o sidewall o f the flum e w ere m ade up o f forced concrete fram e and forced glass, and the transit channel and outlet channel w ere built w ith bricks. There are 14 glasses in the m ain test reach w ith a total length o f 35 m.

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30 00 lí R e s e r v o i r a n d s e d i m e n t a t i o n basin(20m X 10m X 3in) 6 0 m F l u m e

c g > e g )

( g )

@

125 1300 5000 5000 i- 7000 5000 2000 j , j Experiment Reach 1 ^ H o r iz o n ta l L a y o u t . 1300 350 350 350 350 350 350 350 350 350 350 350 350 350 5(300 50p0 2 175 2150 2150 2150 2150 2150 2150 2150 2150 2150 2150 2150 2150 2175 7000 5000 2000 cay (¿y S i d e v i e w L a y o u t 11300 12350 14700 12000 60000

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Figure 4 Flume Overview

Figure 5 Flume Side View(Model 5)

In the flum e system, there are a reservoir and a sedim entation basin to supply the flow and deposit the sedim ent in the basin (see Fig. 6). It is 20m long, 10m w ide and 3m deep. The

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discharge is generated by 7 pum ps (see Fig. 7) and controlled by tw o electrom agnetic flow m eters (see Fig. 8).

Figure 6 Sediment Basin and Reservoir

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Figure 8 Flume Discharge Meter (Electromagnetic Flow Meter)

3.2 E m b a n k m en t M odel D esign

There are 5 runs o f experim ents in the flume. All the model param eters are show in Table 1, Fig. 9, Fig. 10, Fig. 11, Fig. 12and Fig. 13. The em bankm ents are built on erodible flum e bed, 0.8m thick o f clay w ith the same characteristics as the dike. There are four em bankm ents designed o f 1.20 m high and one em bankm ent designed o f 0.6 m high to study the scale influence. All the riverside slopes are the same in 5 models, i.e., 1:1. The landside slopes are designed o f 1:3 and 1:2 to study the influence o f the landside slope. A nd the crest w idth is designed into 0.6 m. The initial trench is set to 0.5 m w ide and 0.2 m deep, w ith a slope o f 1:1.

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Table 1 Breach Scale Model Parameters

Parameters

Model 1

Model 2

Model 3

Model 4

Model 5

Experiment Date 01/02/2013 27/02/2013 07/03/2013 14/03/2013 22/03/2013 Initial Trench Location Side Side Side Side Middle

Dam Length(m) 3 3 3 3 3

Dam Height(m) 0.6 1.2 1.2 1.2 1.2 Dam Crest Width(m) 0.6 0.6 0.6 0.6 0.6 Riverside slope 1 V:1 H 1 V:1 H 1 V:1 H 1 V:1 H 1 V:1 H Landside slope 1 V:2H 1 V:2H 1 V:3H 1 V:3H 1 V:3H Bottom Width (m) 3.6 4.2 4 4 4 Flume bed Length(m) 20 20 20 20 20 Flume bed Thickness(m) 0.5 0.5 0.5 0.5 0.5 Volum e(m 3) 33.78 38.64 40.8 40.8 40.8 600 i~i n § 600 1200 2400

Figure 9 Design of Model 1

5400

I-1800

5400

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Figure 11 Design of Model 3

Figure 12 Design of Model 4 5400 1800 5400 5400 1-'1 ' 1800 5400

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-* 5 —

V-2-2 (Model 1)

3000 ■ i ---§ --- ' i ä ----\— 3000

2-2(Model 2,3,4)

2-2 (Model 5)

3-3

Figure 14 Cross-section (Side Breach; Middle Breach)

In order to study the im pact o f initial breach location, the m odels w ere designed tw o types o f initial breach, side breach (Fig. 14 , Fig. 15 ) and m iddle breach (Fig. 14 , Fig. 16). The initial

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breach channel is the triggering force to the breach process, i.e., in practice, it is the w eakest point in the embankment.

Figure 15 Photo of Model 2

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4 Measurement Instrumentation

In the breach process o f cohesive em bankm ent, the m orphological change depends on the hydraulic param eters; conversely, the m orphology influences the hydraulic param eters. Therefore the hydraulic param eters and m orphological param eters both play an im portant role in the breach process. In the flum e experim ents, the hydraulic param eters, including w ater level and velocity, are m easured w ith the equipment. A nd topography is m easured w ith Three dim ensional Survey instrum ent and video cameras.

4.1 W a ter L evel

W ater level m eters (see Fig. 17) are set on the flum e to m easure the w ater level process in the breach. There are 8 w ater level m eters setup along the flum e from the inlet channel to the tailgate. In the upstream 4 m eters are fixed to m easure the w ater level change process o f the reservoir o f the em bankment. 2 m eters are used to m easure the em bankm ent breach process ju st above the crest o f the em bankm ent and in the initial channel o f the em bankment. In the dow nstream o f the em bankm ent, there are m eters to m easure the w ater level process and control the tailgate w ater level.

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4.2 V elocity

3 Electrom agnetic V elocity M eters (see Fig. 18) w ere fixed upstream o f the breach, in the breach channel and dow nstream o f the breach to m easure the flow velocity process, respectively. Particle trace w as used to indicate the velocity distribution at the flow surface w hile the 3 H igh-speed video cam era systems recorded the breaching process. A ccording to the video records, the surface velocity could be m easured and calculated using the trace particle movements.

Figure 18 Electromagnetic Velocity Meter System

4.3T op ograp h y

Topography Survey Instrum ent is used to m easure the em bankm ent topography variation every 5 minutes. A three D im ensional L aser Scanner (see Fig. 19) is used to m easure the breach geom etry variation. V ideo Cam eras w ere fixed above the breach to record the breach process. The scour hole and breach channel developm ent w ere m easured and recorded w ith topography survey instrum ents and video cam era through the glass wall o f the flume.

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5 Soil Test

5.1 Soil C ollection

The em bankm ent m aterial is the m ixture o f clay and silt collected and delivered from H ouguanhu Lake bank. The excavator (see Fig. 20) w as used to pick up the clay from the site to the trucks.

In total 41 truckloads o f soil w ere delivered to the experim ent haii w hich is m ore than 2,000

2 . 3 • •

m for reproduction. The total am ount o f soil w as m ore than 125 m . The soil (see Fig. 21) w as first transported to the experim ent haii and then paved to a layer o f 10 cm by the workers.

Figure 20 Excavator and Truck in the delivery

III II

MI I

I HI UI I IHI l i l i

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5.2 Soil R ep rod uction

A fter 20 days in the air, the soil w as dry and re-produced to a small diam eter o f block from the natural size (see Fig. 22). The m axim um size o f re-produced soil w as controlled to be less than 5 cm.

Figure 22 Re-produced Soil

In order to build the m odel easily w ith the soil, the dried soil was reproduced again by the grinder (see Fig. 23) into finer size. The controlled diam eter is 1.5 cm and the fine soil was shown in Fig. 24. It is suitable to reproduce into any size used in the model construction.

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Figure 24 Re-produced fine soil

As for the model construction, a suitable w ater content and soil size w ere requested. The blender (see Fig. 25) w as used to adjust the soil w ater content and m ix the clay sample homogenously.

Figure 25 Blender to reproduce the soil sample

5.3 Soil T est in th e lab

Cohesive em bankm ent breaching is a hydraulic phenom enon coupled w ith soil mechanics. So a series o f soil tests in the lab w ere conducted before the flum e experim ents w ere conducted.

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properties. Some soil properties are intrinsic to the com position o f the soil m atrix and are not affected by sample disturbance, w hile other properties depend on the structure o f the soil as well as its com position, and can only be effectively tested on relatively undisturbed samples. Some soil tests m easure direct properties o f the soil, w hile others m easure “index properties” w hich provide useful inform ation about the soil w ithout directly m easuring the property desired.

Soil tests in the laboratory concerned in this breaching experim ent contains w ater content, density (dry density, relative density), particle size analysis, proctor com paction test, A tterberg lim its (shrinkage limit, plastic limit, liquidity limit), direct shear stress, triaxial shear test, perm eability, and com pression.

5.3.1 Density and Water Content

In order to keep the em bankm ent hom ogenous, the m odels w ere m ade layer by layer, each layer having a thickness o f 20 cm. A nd the sam ples w ere collected after every layer had been com pressed. Then the bulk density, dry density and w ater content w ere tested in the soil m echanics laboratory. The bulk density distributions w ere shown in Fig. 26. The bulk density distributions in the 5 m odels had small differences, and in the construction error ranges. The dry densities (Fig. 27) also have well hom ogenous distributions. W ater content tests (Fig. 28) provide the w ater content o f the soil, norm ally expressed as a percentage o f the w eight o f w ater to the dry w eight o f the soil. It can im pact the cohesion o f the m odel material.

12 -♦— Model 0102 11 - — Model 2702 10 -A— Model 0703 9 Model 1403 _ 8 E ° 7 0 I T— 1 6 ■*— Model 2203 <u > i re _ l 5 4 3 2 1 0 1.600 1.700 1.800 1.900 2.000 2.100 2.200 2.300 Bulk D en sity (g /c m 3)

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12 Model 0102 11 Model 2702 10 _{Model_0703} M odelyJ 403 Model 2203 9 - 8 E ° 7 T— I 6 <D > i ra _ l 5 4 3 2 1 0 1 1.2 1.4 1.6 1.8 2 2.2 Bulk D en sity (g /c m 3)

Figure 27 Model Material Dry Density Distributions

12 11 Model 0102 10 _{Model 2702} 9 _{Model 0703} Model 1403 Model 2203 <u > i re _ l 5 4 3 2 1 0 5 10 15 20 25 30 M o isu re C o n te n t (%)

Figure 28 Model Material Bulk Density Distributions

5.3.2 Particle Size Analysis

Particle size analysis is done to determ ine the soil gradation. Coarser particles are separated in the sieve analysis portion, and the finer particles are analyzed w ith a hydrometer. The distinction betw een coarse and fine particles is usually m ade at 75 pm. The sieve analysis shakes the sam ple through progressively sm aller m eshes to determ ine its gradation. The hydrom eter analysis uses the rate o f sedim entation to determ ine particle gradation (see Fig. 29). The sand-clay m ixture w as used to build model 4.

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100 Model 1 90 ■ Model 2 80 Model 3 Model 4 70 - - ■ Model 5 60 V O) cu C 50 V o 5 40 Q . 30 20 0.00001 0.0001 0.001 0.01 0.1 1 10 P a r t ic le S iz e ( m m )

Figure 29 Model material grading

5.3.3 Proctor Compaction Test

Com paction is the process by w hich the bulk density o f an aggregate o f m atter is increased by driving out air. F or any soil, for a given am ount o f com pactive effort, the density obtained depends on the m oisture content. A t very high m oisture contents, the m axim um dry density is achieved w hen the soil is com pacted to nearly saturation, w here (alm ost) all the air is driven out. A t low m oisture contents, the soil particles interfere w ith each other; addition o f some m oisture will allow greater bulk densities, w ith a peak density w here this effect begins to be counteracted by the saturation o f the soil.

A ccording to the proctor com paction test process, 4 clay sam ples (w ithout M odel 4) w ere prepared w ith different com paction times. The relationships (see Fig. 30) betw een optim um w ater content and dry density w ere obtained after 5 tim es o f com pactions. The optim um w ater content is 21.3% and the m axim um dry density is 1.61g/m based on the com paction tests.

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o 'S!) I? m ö o Q £? Q Water Content (% )

Figure 30 Relationship between Optimum Water Content and Dry Density

5.3.4 Atterberg Limits

The A tterberg lim its are a basic m easure o f the nature o f a fine-grained soil. D epending on the w ater content o f the soil, it may appear in four states: solid, semi-solid, plastic and liquid. In each state the consistency and behavior o f a soil is different and thus so are its engineering properties. Thus, the boundary betw een each state can be defined based on a change in the soil's behavior. The A tterberg lim its can be used to distinguish betw een silt and clay, and it can distinguish betw een different types o f silts and clays. The A tterberg Lim its test results are shown in Table 2.

(1) Shrinkage lim it

The shrinkage lim it (SL) is the w ater content w here further loss o f m oisture will not result in any m ore volum e reduction. The test to determ ine the shrinkage lim it is A STM International D4943. The shrinkage lim it is m uch less com m only used than the liquid and plastic limits.

(2) Plastic lim it

The plastic lim it is determ ined by rolling out a thread o f the fine portion o f a soil on a flat, non-porous surface. The procedure is defined in A STM Standard D 4318. If the soil is plastic, this thread will retain its shape down to a very narrow diameter. The sam ple can then be rem olded and the test repeated.

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As the m oisture content falls due to evaporation, the thread will begin to break apart at larger diameters. The plastic lim it is defined as the m oisture content w here the thread breaks apart at a diam eter o f 3 mm.

(3) Liquid lim it

The liquid lim it (LL) is the w ater content at w hich a soil changes from plastic to liquid behavior. The original liquid lim it test o f A tterberg’s involved m ixing a part o f clay in a round-bottom ed porcelain bowl o f 10-12cm diameter. A groove w as cut through the pat o f clay w ith a spatula, and the bowl w as then struck m any tim es against the palm o f one hand.

Table 2 Atterberg Limits test results

liquid lim it (LL) liquid lim it (LL) Plastic lim it Plasticity Index Plasticity Index

w L17 w L10 Wp Ipi7 Ipio

%

48.3 38.6 19.6 28.7 19.0

5.3.5 Direct Shear Stress

The direct shear test determ ines the consolidated, drained strength properties o f a sample. A constant strain rate is applied to a single shear plane under a norm al load, and the load response is m easured. If this test is perform ed w ith different norm al loads, the com m on shear strength param eters can be determ ined (see Table 3).

5.3.6 Triaxial Shear Test

This is a type o f test that is used to determ ine the shear strength properties o f a soil. It can sim ulate the confining pressure a soil w ould see deep into the ground. It can also simulate drained and undrained conditions.

The unconsolidated undrained tests w ere used to test Zhuankou clay sample. In the test, the sam ple is not allow ed to drain. The shear characteristics are m easured under undrained conditions and the sam ple is assum ed to be unsaturated. Figure 28 shows the relationships betw een stress and strain under the pressure o f 50 kPa, 100 kpa and 150 kPa. Based on the M o hr’s strain circle m ethod, the triaxial shear test result are shown in Table 3.

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Table 3 Model Sample Characteristics

NO. Sample Soil Type

Sample Characteristics Moisture

content

Bulk

density dry density Degree of saturation

w _P Pd Sr

% g/cm3 g/cm3 %

1 Model 1 silty clay _28.8 1.86 1.47 92.6

2 Model 2 silty clay loam _28.7 1.84 1.49 94.8

3 Model 3 silty clay ₂₆ 1.88 1.51 86.5

4 Model 4 loam sand _16.1 2.11 1.84 93.2

5 Model 5 silty clay _26.4 1.90 1.54 93.7

6 Model Layer silty clay loam _29.5 2.01 1.48 90

Table 3 Model Sample Characteristics (continued)

NO.

Mechanic Indicator

Permeability test Compression test Direct shear stress Triaxial shear test(UU)

avi-2 ESl_2 c q <Pq (Pu K-20

M P a1 MPa kPa o kPa O cm/s

1 0.322 6 ₁₄ 3.6 12.7 3.0 8.02E-06 2 0.327 5 _20.2 1.6 8.1 2.0 7.90E-06 3 0.313 7 ₁₉ 4.8 16.9 0 50E-06 4 0.082 18.2 _8.2 33.6 4.4 32.1 1.36E-04 5 0.254 7.0 _16.0 4.2 14.6 6.0 1.75E-05 6 0.312 6.1 _16.0 1.9 11.9 1.9 71E-06 5.3.7 Permeability Test

C onstant head perm eability tests are used to calculate seepage potential through earthen dams and em bankm ents such as dikes. The testing uses a specialized device referred to as a constant head perm eam eter. In the test, the perm eam eter is filled w ith test soil and w ater run through the sample until the soil is saturated. The am ount o f w ater that is discharged from the soil and w ater m ixture in a m easured length o f tim e is used as an input to a form ula used to determ ine the soil perm eability. The length o f tim e used in the test can vary, but should be consistent during all tests perform ed for a location. The perm eability rate w as listed in Table 3 for 5 runs o f experim ents as well as the clayey layer o f the model.

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5.3.8 Compression Test

A com m on m ethod o f conducting the test, as described in several published standard test methods, is to com press a box at a constant rate o f 12.5 mm per m inute betw een tw o rigid platens. The platens can be fixed so that they rem ain parallel or one can be pivoted or “floating” . The test can be conducted on em pty or filled boxes, w ith or w ithout a box closure.

A fter the com pression test o f Zhuankou clay, the com pression factor avi_2 is 0.265 m P a'1, and

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6 Model Tests

There are 5 runs o f experim ents conducted in the flum e ( Fig. 2) and the dike m odels were built according to the designs (see Table 1). M odel 1 w as conducted February 1, 2013, later M odel 2, M odel 3, M odel 4 and M odel 5 w ere conducted on February 27, M arch 7, M arch 14 and M arch 22, respectively.

6.1 B ou n d a ry conditions

The upstream boundary condition for each run o f the experim ents w ere controlled by the discharge and w ater level. The front w ater level o f the em bankm ent w as kept in the overtopping condition, i.e., w ater levels w ere controlled at 2.00 m for M odel 2, M odel 3, M odel 4 and M odel 5, and as for M odel 1, the w ater level w as controlled at 1.40 m. In order to control the w ater sem i-constant, the discharge (Fig. 31) w as adjusted by the electrom agnetic discharge meter.

!/) CD O) ■-CO - C o w 700 -, 600 500 400 300 -200 -100 -T e st 1 T e st 2 T e st 3

J

T e st 4 T e st 5 , 1 1 , 1 1 1 1 1 1----10:00 12:00 14:00 16:00 18:00 20:00 tim e

Figure 31 Discharge process for 5 runs of experiments

In each run o f the experim ents, the w ater tem peratures and sedim ent concentrations w ere m easured ( see Table 4). All the w ater tem peratures w ere above 10 °C and the sedim ent concentrations increased from 3.739 g/1 in the first run o f the experim ent to 5.000 g/1 in the

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Table 4 W ater Tem perature and Sedim ent Concentration

RUN W ater Tem perature (°C) Sedim ent Concentration (g/1)

EXP_0102 10 3.739 EXP_2702 15 3.884 EXP_0703 13 4.556 EXP_1403 10 4.862 EXP_2203 13 5.000 6.2 W a ter L evel

In the 5 runs o f breach experim ents, the w ater levels w ere m easured and recorded upstream and dow nstream o f the model em bankment. WM1 m easured the controlling w ater level o f the flume. W M 4 and W M 5 m easured the upstream w ater level close to the em bankm ent crest. The w ater level processes in the breaching experim ents are shown in Fig. 32, Fig. 33, Fig. 34, Fig. 35, Fig. 36.

WM1

WM2

WM3

WM4

WM5

WM6

140 -120

-E

o CD > CD _l I — a> H—'

I

100 80 -0 5000 10000 15000 20000 25000 30000 T(s)

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W a te r L e v e l( c m ) 0) > (D B 200 -W M1 W M 2 W M 3 W M 4 W M 5 W M 7 1 5 0 -100 -50 0 0 10000 15000 20000 2 5000

T(s)

Figure 33 Water Level Process of M2

W M 1 W M 2 W M 3 W M 4 W M 5 2 0 0 180 1 6 0 1 4 0 -1 2 0 -100 0 5 0 0 0 10000 15000 20000 2 5 0 0 0 T (s )

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W at er L e v e l( c m ) Water L e v e !( c m ) 2 0 0 180 -160 ■ 1 4 0 -1 2 0 -100 -80 1 1 1 1 1 1 1 1---2000 4000 6000 8000 10000 T(S)

WM1 WM 2 WM 3 WM4 WM 5 WM 7 WM 8 200 180 160 140 -120 -100 - 0 T(s)

— i--- 1---1--- 1--- 1--- 1--- 1--- 1--- 1 2000 4000 6000 8000 10000 WM1 W M2 W M3 W M4 W M 5 W M7 W M8

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6.3 M orp h ological P rocess in th e B reach

M orphological process plays ab im portant role in the breaching process o f cohesive em bankments. The change in m orphology results from the erosion triggered by the breaching flow. In the study, the m orphological process w as m easured by the 3D scanner every 5 minutes.

Before the experim ent started, the total topography o f the flum e model w as scanned (Fig. 37). M odel 4 had a side initial breach channel and w hen the flow cam e from the upstream and w ent through the initial channel, the breaching process started via erosion. The surface erosion (Fig. 38) happened due to the flow generated by the high w ater pressure in the reservoir. The flow firstly broke the em bankm ent surface and w ashed away the model material by blocks, not by particles.

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Figure 38 Surface Erosion at the initial phrase of breach

As the developm ent o f the breach, the cascade headcut erosion (Fig. 39) started to develop from the toe o f the em bankm ent after the fully com pletion o f the surface erosion on the model surface. The blocks o f the clay w ere w ashed out by the high velocity breaching flow. The initial breach channel (Fig. 40 ) increased to 1.020 m stim ulated by the breaching flow and the em bankm ent toe w as fully eroded by the headcut erosion.

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Figure 40 Topography Change during the test (Run 4)

As the developm ent o f the headcut erosion processed, the lateral erosion started to play an im portant role in the breaching process. D ue to the helicoidal flow (secondary flow ) in the breach channel (Fig. 41), the under m ining process triggered the erosion at the side toe o f the em bankment. The helicoidal erosion at the side toe broke the balance o f the em bankm ent and the material blocks collapsed due to the unbalanced situation o f the em bankment. The helicoidal erosion stim ulated the lateral developm ent o f the breach channel and m ade the breach w idth increase directly. D ue to the cohesion o f the material, the lateral breach slope (Fig. 42) w as generate very steep by the breach flow. The underm ining process at the side toe o f breach channel usually m ade the breach slope into negative one.

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Figure 41 Lateral erosion of the breach

Figure 42 Steep Breach Side Slope

A fter the initial surface erosion, the em bankm ent breaching process w as driven by headcut erosion and helicoidal erosion. D ue to the high velocity o f the flow through the breach, a

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scour hole developed in the breach channel and at the toe o f em bankm ent (Fig. 43). The em bankm ent toe scour hole started to form in the early stage o f the breaching process, but the eroded material from the em bankm ent covered the scour hole during the follow ing phrases in the breaching. The sour hole form ed at the bottom o f the em bankm ent in the last phase o f the breach and the eroded m aterial w as w ashed away to the downstream.

Figure 43 Scour hole of breach

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Figure 45 Model Overview from the Downstream

The breach channel (Fig. 46) w as 2.5 m w ide and the side breach slope w as very steep w ith negative value at some location due to the underm ining process o f helicoidal erosion. The material o f the front em bankm ent w as eroded by the upstream convergent flow. The upstream breach w idth is larger than the dow nstream ones, e.g., 0.21m in M odel 4.

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7 Conclusions and Recommendations

In this project, 5 runs o f breach experim ent w ere conducted in a relative large flume. The experim ents gave a strong support to the hypothesis that the cohesive em bankm ent breach is a hydrodynam ic process coupled w ith soil mechanics. The breaching starts w ith the initial erosion o f the em bankm ent surface and w ashes away the em bankm ent surface. D ue to the surface erosion at the toe o f em bankm ent, the headcut erosion is stim ulated on the em bankm ent slope. The headcut erosion can also develop into cascade headcut m igration due to the non-hom ogenous characteristics o f the em bankm ent material. W hile headcut m igration stim ulates the breach to develop in longitudinal direction, the helicoidal erosion triggers the breach to w iden in lateral direction. Three types o f erosions (surface erosion, headcut erosion and helicoidal erosion) contribute to the erosion process o f the breaching in em bankm ent, however, the breaching flow is the driving force for the erosion. Sedim ent deposition in the breaching process is also o f im portance, generally ignored in the em bankm ent breaching studies.

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References

Dodge, R.A., 1988. Overtopping flow on low embankment dams - summary report of model tests. REC-ERC- 88-3, U.S. Bureau of Reclamation, Denver, USA.

Fread D.L., 1988. BREACH: an erosion model for earthen dam failures. National Weather Service (NWS) Report, NOAA, Silver Spring, Maryland, USA.

Ralston, D.C., 1987. Mechanics of embankment erosion during overflow. Proceedings of the 1987 ASCE National Conference on Hydraulic Engineering, Williamsburg, USA, 733-738.

Visser P.J., 1998. Breach growth in sand-dikes. PhD thesis, Delft University of Technology, Delft, the Netherlands.

Zhao G., Visser P.J., Peeters, P., Vrijling J.K., 2013. Headcut Migration Prediction of the Cohesive Embankment Breach. Engineering Geology, Volumel64, 2013, Pages 18-25.

Zhu Y.H., 2006. Breach growth in clay-dikes. PhD thesis, Delft University of Technology, Delft, the Netherlands.

Zhu Y.H., Visser P.J., Vrijling J.K., 2004. Review on embankment dam breach modeling, New Developments in Dam Engineering, Taylor & Gracis Group, London, UK.

Appendix list

B .l. Velocity Database

B. 1.1. V elocity database for R un 1 (1 A D V , 3 electrom agnetic velocity m eters ) B. 1.2. V elocity database for R un 2 (1 A D V , 3 electrom agnetic velocity m eters ) B. 1.3. V elocity database for R un 3 (1 A D V , 3 electrom agnetic velocity m eters ) B. 1.4. V elocity database for R un 4 (1 A D V , 3 electrom agnetic velocity m eters ) B. 1.5. V elocity database for R un 2 (1 A DV , 3 electrom agnetic velocity m eters )

B.2. Water Level Database

B.2.1. W ater level database for R un 1 (9 w ater level m eters ) B .2.2. W ater level for R un 2 (9 w ater level m eters)

B .2.3. W ater level for R un 3 (9 w ater level m eters) B .2.4. W ater level for R un 4 (9 w ater level m eters) B .2.5. W ater level for R un 2 (9 w ater level m eters)

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B.3. Topography Database

B .3.1 3D Topography per 5 m inutes for R un 1 (3D Scanner ) B .3.2. 3D Topography per 5 m inutes for R un 2 (3D Scanner ) B .3.3. 3D Topography per 5 m inutes for R un 3 (3D Scanner ) B .3.4. 3D Topography per 5 m inutes for R un 4 (3D Scanner ) B .3.5. 3D Topography per 5 m inutes for R un 5 (3D Scanner )

B.4. Film Footage and Photo Database

B.4.1. Film Footage and Photo for Run 1 (2 high-speed video cam eras above the flum e and 3 high-speed video cam eras side to flum e, Photos )

B .4.2. Film Footage and Photo for Run 1 (2 high-speed video cam eras above the flum e and 3 high-speed video cam eras side to flum e, Photos )

B.5. Soil Mechanics Test Result Database B.5.1. Soil test for M odel 1

B .5.2. Soil test for M odel 2 B .5.3. Soil test for M odel 3 B .5.4. Soil test for M odel 4 B .5.5. Soil test for M odel 5