WP2-3 WP2-4 Results of 3D stability testsvbn.aau.dk/files/171554/abstractfil.pdfthe instantaneous...

DELOS 2nd year meeting, Santander 16-19 February 2003 1

WP 2.3 – WP 2.4Results of 3D stability tests

• Introduction with description of the “stability concept”

• What kind of information exist?

• Tests at AAU

• Conclusion and discussion. Do we need additional tests?

- Morten Kramer -


Introduction• Wave forces acting on a rubble-mound slope can cause armor unit

movement. This is called hydraulic instability.

• Breakage of armor units is another type of instability which is not discussed here.

• Armor unit movements can be rocking, displacement of units out of the armor layer, sliding of a blanket of armor units, and settlement due to compaction of the armor layer.


Why do we perform laboratory tests?

The complicated flow of waves impacting armor layers makes it impossible to calculate the flow forces acting on armor units. Moreover, the complex shape of units together with their random placement makes calculation of the reaction forces between adjacent armor units impossible. Consequently, deterministic calculations of the instantaneous armor unit stability conditions cannot be performed, which is why stability formulae are based on hydraulic model tests. The response of the armor units in terms of movements are related directly to parameters of the incident waves, while treating the actual forces as a “black box” transfer function.


Parameters influencing armour layer stability

• Sea state parameters– Characteristics of waves– Number of waves– Water depth– Mass density of water

• Structural parameters– Shape of profile– Characteristics of materials

Armour layer Dn50,armour

Core Dn50,core<Dn50,armour


Existing tests on LCS

• 2D– Ahrens 1987– Van der Meer 1990/1996– Loveless and Debski 1997– ...

• 3D– Vidal et al. 1992

• Tests were carried out in the wave basin at NRC, Canada, 1991-1992.


Task for the new 3D stability tests at AAU

Influence of:1) Obliquity of short crested waves2) Wave height and steepness3) Crest width4) Freeboard5) Structure slope

By varying water level (one crest height was tested)

Two crest widths were tested

By trying to compare with NRC tests

Armour Dn50=3.3cm

Core Dn50=1.5cmCore Dn50=1.5cm

Armour Dn50=3.3cm

Narrow cross-section Wide cross-section

30cm

130cm 145cm

10cm 25cm


Layout

Structure

200c

m

Wave generatorE

mpt

y ar

ea

Abs

orbi

ng w

alls

Beach

277cm 1250cm

1785cm

225c

m

Em

pty

area

Abs

orbi

ng w

alls

1195

cm

970c

m

258cm

200


Test sections

Beach

Sloping foreshore

Wave Generator

Wave gauge array

3-gauge system127,5

Horizontal seabed

Horizontal seabed

Trun

k te

st

sect

ion

Rou

ndhe

ad

test

sec

tion

123

4

56

7

8


Building the structure


Picture of layout


Test procedure• Built/rebuilt the structure• Fix water level, wave direction, steepness and spreading• Perform test with 1000 waves with small wave height• Measure damage• Increase significant wave height and run 1000 waves• Measure damage• ...continue to increase the wave height and measure

damage until severe damage was observed


Measurements• Waves were recorded continuous during the tests.

• Wave breaking was described from visual observations.

• Damage in terms of displacement of stones was measured after each test by use of digital photos. Damage was classified in categories. Digital video recordings were taken during a few tests of special interest.


Measurement of damage


Colouring of sections

60°

60°

SH

LH

MH C

SS

LS

60°W

W

W

K

Y

Y

G

B

R

N

N

N

W

B

YR G

B

Y

W


Example• Test no. 1-4, position 2 (roundhead from gap)

– Main wave direction perpendicular to structure– Crest width = 0.1m (narrow crest with)– Freeboard = +0.05m– Wave steepness = 0.02


Stone displacements (more than one Dn50):

Seaward head: 0

Middle head: 3

Leeward head: 4


Databank on CDThe CD with the measurements will be delivered together with the report describing the tests in detail. This report is a part of DELOS delivery D31 to be completed within March 2003.

The report will also contain analysed results.


Test 1-4 for roundheadSeaward head, test no. 1-4

0

5

10

15

20

25

30

0.00 0.50 1.00 1.50 2.00 2.50

Stability number Ns=Hs/DDn50

No.

of d

ispl

aced

sto

nes

Middle head, test no. 1-4

05

101520253035

0.00 0.50 1.00 1.50 2.00 2.50


No.

of d

ispl

aced

sto

nes

Leeward head, test no. 1-4

05

10152025303540

0.00 0.50 1.00 1.50 2.00 2.50


No. o

f dis

plac

ed s

tone

sNumber of displaced stones in section

Total number of armour stones in the section*100

Number of displaced stones in % =

0 3

4


Degree of damage

• ND: No damage• ID: Initiation of damage

Displaced stones >1% ~ 2 stones for narrow roundhead

• IR: Iribarren damageBig holes in outer armour layer

• D: Destruction


Influence of freeboard

Narrow structure roundhead leeward head

y = -0.5863x + 2.0447R2 = 0.9194

0.00

1.00

2.00

3.00

4.00

5.00

-4.00 -3.00 -2.00 -1.00 0.00 1.00 2.00

Normalized freeboard Rc/Dn50

Stab

ility

num

ber N

s s=0.02s=0.04Linear (all)

Narrow structure roundhead middle head

y = -0.5033x + 2.2271R2 = 0.9652

0.00

1.00

2.00

3.00

4.00

5.00

-4.00 -3.00 -2.00 -1.00 0.00 1.00 2.00


Sta

bilit

y nu

mbe

r Ns s=0.02

s=0.04Linear (all)

Narrow structure roundhead seaward head

y = -0.4789x + 2.3671R2 = 0.9105

0.00

1.00

2.00

3.00

4.00

5.00

-4.00 -3.00 -2.00 -1.00 0.00 1.00 2.00


Sta

bilit

y nu

mbe

r Ns s=0.02

s=0.04Linear (all)

Example: Initiation of damage for roundhead. Preliminary results.

Narrow structure roundhead

0.00

1.00

2.00

3.00

4.00

5.00

-4.00 -2.00 0.00 2.00


Sta

bilit

y nu

mbe

r Ns

Linear (Seaward head)

Linear (Leeward head)

Linear (Middle head)


Influence of crest widthRoundhead, s=0.02, normal incidence waves

0.00

1.00

2.00

3.00

4.00

5.00

-4.00 -3.00 -2.00 -1.00 0.00 1.00 2.00


Stab

ility

num

ber

Ns Narrow

Wide

Trunk crest, s=0.02, normal incidence waves

0.00

1.00

2.00

3.00

4.00

-4.00 -3.00 -2.00 -1.00 0.00 1.00 2.00


Stab

ility

num

ber

Ns NarrowWide

Trunk slopes, s=0.02, normal incidence waves

0.00

1.00

2.00

3.00

4.00

5.00

-4.00 -3.00 -2.00 -1.00 0.00 1.00 2.00


Stab

ility

num

ber

Ns NarrowWide

Example: Initiation of damage. Preliminary results.

Crest widths:

Narrow structure: 3Dn50

Wide structure: 8Dn50


Stability related to obliquity

Trunk, s=0.02, Freeboard Rc/Dn50=1.54

0.00

1.00

2.00

3.00

60 70 80 90 100 110 120

Wave direction [°]

Stab

ility

num

ber N

s CrestSeaward slopeLeeward slope

Roundhead, s=0.02, Freeboard Rc/Dn50=1.54

0.00

1.00

2.00

3.00

60 70 80 90 100 110 120

Wave direction [°]

Sta

bilit

y nu

mbe

r N

s Seaward headMiddle headLeeward head

90°

<90°>90°



Oblique wave attack, test no. 54


Influence of structure slopeRoundhead, normal incidence waves

0.00

1.00

2.00

3.00

4.00

-4.00 -2.00 0.00 2.00 4.00


Stab

ility

num

ber N

s

AAUNRC


The most important differences in the two test series:• Structure slopes were 1:1.5 in NRC tests (1:2 in AAU tests)• No foreshore slope was present in NRC tests• 2D irregular waves were generated in NRC tests (3D in AAU tests)• Damage level description is subjective

Trunk, normal incidence waves

0.00

1.00

2.00

3.00

4.00

-4.00 -2.00 0.00 2.00 4.00


Stab

ility

num

ber

Ns

AAUNRC


Test schedule

0.04-0.10.19032-360.02-0.10.19028-310.04-0.050.19023-270.02-0.050.19018-220.0400.19013-170.0200.1909-120.040.050.1905-80.020.050.1901-4

steepnessboard [m]width [m][°]no.WaveFree-CrestTest

0.02-0.10.259067-690.02-0.050.259064-660.0200.259060-630.020.050.256056-590.020.050.2511052-550.020.050.2510049-510.020.050.258045-480.020.050.257041-440.020.050.259037-40

steepnessboard [m]width [m][°]no.WaveFree-CrestTest

Narrow crest width Wide crest width

Test 1-36: Influence of wave-steepness and freeboard

Test 37-59: Influence of wave obliquity

Test 1-40, 60-69: Influence of crest width


Conclusion & discussion

• Two structures with different crest width was tested in irregular 3D waves. From the results it is directly possible to describe the influence of: – Obliquity of short crested waves– wave height and steepness– Crest width in the tested range– Freeboard

• Limitations of the present data bank / task for supplementary tests:What is the influence of structure permeability on roundhead damage for LCS’s?

How does long crested waves affect the stability compared to short crested waves?

Etc.

WP2-3 WP2-4 Results of 3D stability testsvbn.aau.dk/files/171554/abstractfil.pdfthe instantaneous...

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