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Scour Characteristics of Saturated Levees Due to

Floodwall Overtopping

Mazdak Karimpour1, Kyle Heinzl2, Emaline Stendback2, Kevin Galle2, Siavash

Zamiran3, and Abdolreza Osouli4, Ph.D., P.E.

1 Research Assistant, Dept. of Civil Engineering, Southern Illinois University Edwardsville,

Edwardsville, Illinois, 62026. 2 Undergraduate Assistant, Dept. of Civil Engineering, Southern Illinois University

Edwardsville, Edwardsville, Illinois, 62026. 3PhD Candidate, Instructor, Dept. of Civil Engineering, Southern Illinois University

Carbondale, Carbondale, Illinois, USA, Email: zamiran@siu.edu, zamirans@gmail.com,

Website: www.zamiran.net, Phone: +1 (618) 334-4572 4 Corresponding Author, Assistant Professor, Dept. of Civil Engineering, Southern Illinois

University, Edwardsville, Illinois, 62026; aosouli@siue.edu, Phone: +1 (618) 650-2816

Publisher: American Society of Civil Engineers

International Foundation Conference and Equipment Exposition. San Antonio, TX (IFCEE

2015)

Permalink: http://dx.doi.org/10.1061/9780784479087.117

Karimpour, M., Heinzl, K., Stendback, E., Galle, K., Zamiran, S., and Osouli, A. (2015)

Scour Characteristics of Saturated Levees Due to Floodwall Overtopping. IFCEE 2015: pp.

1298-1307.

doi: 10.1061/9780784479087.117

ABSTRACT: One of the major reasons of levee failure is erosion due to

overtopping. The overtopping of levees is occasionally the consequence of a flood or

Hurricane. In these events, the surface layers of levees become saturated in advance

of any overtopping due to rain and storm events. In this investigation, the effect of the

saturation ratio and almost vertical impingement of overtopped water from floodwall

on scour development are studied. For this purpose, laboratory-physical models of a

typical levee on the banks of Mississippi river with a scale of 1:20 were constructed.

A nearly 3 mm thick wooden plate, which was embedded in the crest of the levee

represented the floodwall. Silty soil materials with various saturation ratios were used

to observe scour potential of the soil. In all the tests, the scour development and the

stability of the wall were monitored and analyzed for each test during overtopping.

Hydraulic and geometric parameters including Densimetric Froude number,

compaction level, water content and levee dimensions were recorded to have a

representative comparison between the laboratory models and constructed levees in

practice. In addition, the erodibility of the levee materials was determined using

Erosion Function Apparatus (EFA). The results of EFA tests were compared to

physical model test results to explore and discuss the vulnerability of levee systems to

erosion when the surface soils are saturated and floodwall overtopping is experienced.

Keywords: Levee erosion, floodwall, levee, levee modeling, soil erosion, EFA test,

overtopping

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INTRODUCTION

Among many protection measures against floods, levees are the earthen structures

that are widely used on the bank of rivers and lakes to prevent the flood from causing

destruction. According to recent events of catastrophic floods worldwide including

the 2005 Katrina Hurricane (Briaud et al. 2008), Midwest floods in the U.S. in 2008

(Villarini et al. 2011) and the 2008 Hurricane Ike in the Caribbean Islands and Gulf

Coast (Tirpak 2009), the need for stronger and more stable protection measures

against these natural disasters is highly increasing.

In the overtopping process, water level exceeds top of the levee and causes erosion

in the crest and the body of levee. In some cases, floodwalls are constructed at the

crest of the levee to raise the height without any change in the body or geometry of

levee (Duncan et al. 2008). After the water level exceeds the top of the floodwall and

the overtopping process commences, the water starts to free fall from top of the

floodwall with high velocities. As the water nappe impacts the levee material, it

causes significant amount of erosion. The turbulence created due to this free fall,

increases the generated scour (Xiao, 2009).

Predicting the erosion due to overtopping can help in preventing levee failures.

Briaud et al. (2001) developed an apparatus called Erosion Function Apparatus (EFA)

to estimate the scour generation and erodibility of different types of soils. In this

apparatus, the soil is subjected to horizontal flow of water and is eroded. By using

these tests, the shear stress and the erodibility of the examined soil can be identified.

Kamalzare et al. (2013) performed numerical modeling on the scour generation and

overtopping of levees, associated with experimental tests on the simulated levees to

validate the results derived from the computer modeling. The performed laboratory

tests highlighted the importance of water flow parameters in overtopping process,

amount of erosion due to overtopping and the erodibility of soils while levee slope

does not show significant effect on the erosion. Hughes and Nadal (2009)

investigated scour generation in levees due to both wave overtopping and storm surge

overflow while studying hydraulic characteristics of the water flow on the levee. By

performing laboratory tests on simulated levees, experimental correlations concerning

overtopping discharge and wave volumes were developed. Although the provided

equations can only be used in the similar conditions to laboratory tests, they can

potentially be used to provide reasonable estimation of erodibility of levees. Several

investigations have also been conducted to prevent erosion using protection measures

on the land side of the levees (Li et al. 2012, Rao et al. 2012, and Johnson et al.

2013). Especially, Johnson et al. (2013) provided several experimental results for

protected levee erosion including ripraps, gravel, etc.

There are also some studies that evaluated the effect of change in degree of

saturation due to rain falls and drought on soil properties and the slope stability.

Shaikh et al. (1988) performed experimental tests to study the effect of saturation

degree on erosion rate of Na-montmorillonite clayey soils. Their results showed that

with a rise in saturation ratio, the erosion rate and soil erodibility increases. Vanapalli

et al. (1996) studied the effect of soil suction and saturation ratio on the shear strength

of different types of soils, and in particular high plastic clays. Their study show that

as the saturation ratio increases, the shear strength of the soil decreases.

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Prior to overtopping, the rain will saturate the levee material; therefore, affect the

erodibility characteristics of levee soil. In this investigation several laboratory tests

were performed on levees constructed with silty material and various saturation ratios

to identify the effect of levee material saturation prior to overtopping on erosion and

scour generation. The obtained results were also compared with EFA tests to identify

the compatibility and applicability of the conducted levee tests.

LABORATORY TEST SET UP

In order to investigate the erodibility and scour generation of levees subject to

floodwall overtopping, laboratory models of the levees along Mississippi river with 1

to 20 scale were constructed. A sharp wooden plate was placed on the crest of the

levee representing the embedded floodwall. The constructed levees inside the box

were 0.23 m (9 in) wide, 2.1 m (7 ft) long and the top of the levee was 0.35 m (14 in)

from ground surface (Figure 1). 75 percent of the floodwall was embedded in the

levee and the top of the floodwall was 0.47 m (18.5 in) from the ground surface.

FIG. 1. Cross section and top view of constructed levees in laboratory

Different sets of tests including Sieve Analysis (ASTM D1921), Plasticity Index

(ASTM D4318), Compaction (ASTM D698) and EFA tests were performed on a

variety of material. The soil used for construction of the levees was classified as low

plasticity silty soil (ML) with plasticity index of 7 %. Standard Proctor test

(compaction test) showed the optimum moisture content of 19% and the maximum

dry density of 17 kN/m3.

The levee construction process included several steps. In the first step, the scaled

levee was constructed at 80% of the maximum density based on the

Standard Proctorin several 7.5 cm (3 in) thick layers. Each layer was compacted using

a hand tamper. After compaction of each layer, three samples were taken to verify the

compaction levels. Then, water was sprayed on levee surface to simulate rain and

change in saturation ratio of levee material. After spraying water on the material,

three samples were also taken to check the saturation ratio of the soil. The values of

moisture content and compaction level were tried to keep constant by equal spraying

and compacting frequencies for each 7.5 thick layers, respectively.

0 cm 15 cm 30 cm

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For EFA tests, in addition to low plasticity soil material, specimen with higher

plasticity indices were tested. The higher plasticity samples were prepared using 1 to

1 and 2 to 1 ratio of high plastic clayey to low plastic silty material. EFA tests were

performed on these samples with varied plasticity indices and saturation ratios. The

samples were soaked in water before testing to reach different saturation levels. The

summary of each EFA and levee test is shown in Table 1. The procedure for

monitoring the scour variation during overtopping and providing the results provided

by Johnson et al. (2013) tssts hs rosid si si.sdsst eitre hstesteiorhtoitre,s rdesres is

l.s ornth hs id s t ott ei ls o stliss ros ht o eis ori oitres t idrhss ros l n s.s

rrv n o,s oorohtels irso l s1ss n o ls t ott ei ls i siss o s hre s tes idtss sithts irs

ten sitl i sid s orht tltitsr ste ori oi hsl n ss s hsresn ot itresr sid tos oitn s

o t i os.

TABLE 1. Specifications of the material used for EFA and levee tests

Test

No. EFA Levee Material

Plasticity

Index

Compaction

Level (%)

Saturation

Ratio (%)

1 ML 6.9 85 58.7

2 ML 6.9 85 22.7

3 1:1 Ratio 13.9 85 50.9

4 1:1 Ratio 13.9 85 58.2

5 1:1 Ratio 13.9 85 91.7

6 2:1 Ratio 17.7 85 46.7

7 2:1 Ratio 17.7 85 86.3

8 ML 6.9 80 43.8

9 ML 6.9 80 85.3

10 ML 6.9 80 91.4

After setting up the levee for the desired test, the reservoir (i.e. water-side of the

levee) was filled with water and the water flow overtopped the floodwall. Special

attention was made to avoid wave actions in these series of tests. The generated nappe

due to overtopping process started erosion of the levee material. In order to keep the

water circulating as the test continued, the excess water was collected and pumped to

reservoir. The test was continued until scour depth and width reached equilibrium.

Monitoring hydraulic and geometric specifications of scour was conducted during

each test. One of the most important hydraulic characteristics of flow, when overtopping is in

progress, is Densimetric Froude number (Eq. 1) which allows to compare the flow in

full and lab scaled models (Johnson et al. (2013).

𝐹𝑟𝑑 = √𝑢2 + 𝑤2/√𝑔∗𝑑50 (1)

Where, w is the vertical velocity at the impact point of water to soil, u is the

horizontal velocity component, g* is moderated gravity acceleration and is defined as

𝑔(𝑆𝐺 − 1)(1 − 𝑛), SG is the specific gravity of soil solids, n is the porosity of the

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soil and d50 is the median grain size of soil particles used in construction of the levee.

Vertical velocity can be estimated by free falling equation which is √2𝑔ℎ . It is

noteworthy that h is the falling height which is the height of top of the floodwall to

the depth of the scour. According to Johnson et al. (2013) the range for Densimetric

Froude number in the field is from tens to thousands.

RESULTS AND DISCUSSION

A typical scour profile is shown in Figure 2. The scour depth and scour width are

demonstrated in this figure. The scour depth variation during floodwall overtopping

was monitored and it is shown in Figure 3. The scour profile in the first 90 seconds of

the test is also shown in Figure 4. Test results show that by changing the saturation

ratio, the scour depth’s variation with time is different for levees with different

saturation levels. According to these observations, with greater saturation levels, there

is less erosion happening in the early stages of overtopping. Also, creation of the

scour hole behind the wall is more pronounced in levees with low saturation levels. In

levees with high saturation levels or fully saturated, the erosion profile does not show

a bowl shape scour immediately behind the wall. This might be due to the fact that in

higher saturation levee tests, the overflooded water will start running off the levee

surface rather than penetrating the soil or causing bowl shaped scour. It is noteworthy

that although the results in figures 3 and 4 are presented for the first 300 and 90

seconds, respectively, but duration of the tests extended to more than 1000 seconds.

However, the equilibrium scour depth and length reached approximately at 300th

second.

FIG. 2. The measures of scour depth and length in the experimental study

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FIG. 3. Effect of saturation ratio on scour depth for lab-scaled tests on levees

constructed with silty soils and compaction ratio of 80%

0

2

4

6

8

10

12

14

16

18

0 50 100 150 200 250 300

Sc

ou

r D

ep

th (

cm

)

Time (s)

Test No. 8

Test No. 9

Test No. 10

0

2

4

6

8

10

12

14

16

18

0 100 200 300 400

Sco

ur

Dep

th (

cm)

Time elapsed (s)

Levee-ML-44

Levee-ML-85

Levee-ML-91

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Fig 4. Scour profile for a) Test No. 8., b) Test No. 9., and c) Test No. 10.

Figure 5 shows the Densimetric Froude number for the levee tests. This figure

shows the scaled model levee test covers the lower range of Densimetric Froude

numbers typically observed in the field or full-scaled models. In all the tests, this

parameter increases with time of flooding as the water fall height increases due to

scour development.

The general flow condition during the tests is a vertical flow passes from the

floodwall and generates a thin nappe after overtopping. Accordin to Figure 5, the

Densimetric Froude number of the flow varies from 90 to 150 during the test.

Figure 6 compares the scour rate versus time for same levee material with different

saturation ratios. In this figure the scatters after 200th second show that the soils with

higher saturation ratios have more resistance against the scour while overtopping is in

progress. According to this figure the average of scour rate in different times for drier

soils is higher than more saturated soils.

Figure 7 shows the comparison of EFA and levee test results. It is observed that in

EFA tests for similar material, with a rise in saturation ratio, the erosion rate for

samples with higher plasticity indices will increase significantly while for low

plasticity material (i.e. ML), the erosion rates remain relatively unchanged. It can also

be noted that as the water velocity increases, the amount of scour rate increases. This

figure also shows that an increase in plasticity of material will result in decrease in

the scour rate and erodibility of material.

Scour Length (cm)

101520253035 0

20

25

30

35

5

Scour Length (cm)

Sco

ur D

epth

(cm)

1015202530350

15

20

25

30

35

5

Scour Length (cm)

Sco

ur D

epth

(cm)

1015202530350

15

20

25

30

35

5

Sco

ur D

epth

(cm)

a)b)

c)

Legend

60 S Scour Line

90 S Scour Line

45 S Scour Line

30 S Scour Line

15 S Scour Line

10 S Scour Line

5 S Scour Line

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FIG. 5. Densimetric Froude Number versus time for silty materials with 80 %

compaction ratio and different saturation ratios

80

90

100

110

120

130

140

150

160

170

0 50 100 150 200 250 300

Den

sim

etr

ic F

rou

d N

um

be

r

Time (s)

Test No. 8

Test No. 9

Test No. 10

80

90

100

110

120

130

140

150

160

170

0 50 100 150 200 250 300

Den

sim

etri

c F

rou

d N

um

ber

Time (sec)

Levee-ML-44

Levee-ML-85

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FIG. 6. Scour rate versus time for the scaled levee tests

According to Figure 7, while the water flow velocity for levee simulated tests are

lower than EFA, their scour rates are within the same range as for EFA tests. This

indicates that more scour and erosion is developed in levee simulated tests than EFA

tests for the same material. The reason for the difference in erodibility of same

material used for both levee and EFA tests is the erosion mechanism in each test. In

EFA test the horizontal water flow erodes the compacted layered soil, while in lab-

scaled levee tests, free water fall overtopped from floodwall impinges the levee

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 50 100 150 200 250 300

Sc

ou

r ra

te (

mm

/ho

ur)

Time (s)

Test No. 8 Test No. 9 Test No. 10

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 50 100 150 200 250 300

Sco

ur

rate

(m

m/h

ou

r)

Time (second)

Levee-ML-44 Levee-ML-85 Levee-ML-91

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surface and erodes the material. Because of the impact of the water jet to the soil

layer and the turbulence created due to this phenomenon, the erodibility of levee

material is more severe. Generally, individual EFA tests can provide a rough

estimation of the erodibility of levees during the overtopping. However, as mentioned

before, EFA can just provide the erodibility potential of soils based on horizontal

flow which is not applied in levee overtopping. Therefore, for more accurate

investigation, scaled levee tests are required to generate free water fall during the

overtopping.

The comparison between EFA and levee results confirms that for low plastic silts the

saturation level variation does not significantly affect the scour rate in longer duration

of floods. However, a slight increase in scour potential was observed in levee tests

with less saturation levels. The reason for this occurrence might be the fact that, when

silty material is exposed to moisture, it will experience more cohesiveness and can

bind the particles more than when it is drier. Consequently, the levee material shows

higher resistance against induced shear stresses and erosion due to overtopping.

CONCLUSION

Erosion due to floodwall overtopping can result in levee breach. In order to study

the effect of saturation level of levee material on erosion rate, several lab-scaled tests

on levees and EFA tests were performed. Levee materials for all these tests were the

same but different saturation ratios representing rain duration in advance of

overtopping were considered. The results of these tests show that the scour rate for

low plasticity silty materials with different saturation ratios is almost the same in EFA

tests and show slight differences in levee simulated tests. However, due to difference

in mechanism of scour in simulated levee and EFA tests, the erodibility potential of

material in simulated levees is greater. The comparison of these results shows that

because of higher induced shear stresses due to free fall of water in overtopping

process, the erodibility of levee material is significantly higher than the erodibility of

soil samples in EFA tests. Based on the EFA test results, silty soil material had the most

erosion rate. Accordingly, 1:1 ratio soil material had more erosion rate than 2:1 ratio soil

material. This means that based on EFA tests, the erosion rate decreases with increase on

plasticity index.

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FIG. 7. Scour rate versus impact velocity for laboratory levee and EFA tests

1

10

100

1000

10000

100000

0 1 10

Ero

sio

n r

ate

(m

m/h

r)

Velocity (m/s)

Test No. 1 Test No. 2 Test No. 3 Test No. 4 Test No. 5

Test No. 6 Test No. 7 Test No. 8 Test No. 9 Test No. 10

1

10

100

1000

10000

100000

0 1 2 3 4 5

Ero

sion

rate

(m

m/h

r)

Velocity (m/s)

EFA-ML-58 EFA-ML-22 EFA-1:1-51 EFA-1:1-58

EFA-1:1-92 EFA-2:1-47 EFA-2:1-86 Levee-ML-44

Levee-ML-85 Levee-ML-91

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