Case 1: Wave propagation over Low Crested Structures (LCS)
General Description
The experimental data presented here concerns wave interaction with
low-crested structures. The laboratory experiments took place in the wave
and current flume of the Coastal Laboratory of the University of Cantabria as
a part of the research carried out for the European Project DELOS
“Environmental DEsign of LOw Crested Coastal Defence Structures”. The
following description is extracted from the corresponding DELOS technical
report by Vidal et al. (2002).
Experimental set–up
The wave and current flume is 24 m long, 0.60 m wide and 0.80 m high.
The piston–type wavemaker has two attached free surface wave gauges
integrated in an Active Wave Absorption System (AWACS ) that allows the
absorption of reflected waves from the model. The wavemaker and the rear
absorbing beach occupy 4 m at one of the ends of the flume; another 4 m are
occupied by a false bottom that can be partially or totally removed to set off a
current in the flume. The reversible pumping system has two pumps with a
combined maximum discharge of 150 l/s. The remaining 16 m of the flume are
available for the testing of models. Bottom and side walls of the testing area
are made of glass, allowing the use of LDA or PIV velocity measurements.
Two rubble mounds, low crested structures with 0.25 m and 1.00 m crest
width, were tested. Crest elevation from the bottom (0.25 m), front and back
slope angles (1V / 2H) and rubble characteristics were maintained constant
for both structures. The models consisted of a two-layer armour of selected
gravel and a gravel core (photo 1). Armour and core characteristics are shown
in Table 1.
Photo 1. Structural layout of the model
Table 1. Characteristics of the gravels used for the model
Layer
W50
(g)
D50
(cm)
Porosity
-
Density
(Kg/m3)
Armour
153
3.94
0.53
2647
Core
4.31
1.18
0.49
2607
The low crested structure model was built over a 3.8-m long stainless
steel horizontal false bottom, 0.10 m over the glass bottom of the flume,
(Figure 1). In the frontal foot of the rubble, a Plexiglas ramp with 1V / 20 H
slope connected the false bottom with the bottom of the flume. In the rear end,
another 8-m 1V / 20H Plexiglas ramp simulated the rear beach. Between the
horizontal and the inclined false bottom, one rectangular aperture, 0.08 m
wide, allowed the water to flow below the beach to the return piping system.
When the waves piled-off water behind the breakwater, the head drove the
returning flow through the piping system to the false bottom in front of the
wavemaker, closing the circuit e.g. the return flow system was only forced by
gravity. The system was disabled in a number of tests to evaluate the impact
of recirculation.
z
Distances in cm
x
Wavemaker
200
476
Recirculation tank
Recirculation pipe
100
25
380
Horizontal steel false bottom
1/20 Plexiglas ramp
800
8
1/20 Plexiglas ramp
Recirculation tank
Bottom aperture
2400
Figure 1. Experimental set-up
Test Cases
In the present database, data sets for three test cases corresponding to
the wide-crested model (1.0 m) without recirculation are provided. The
characteristics of these tests are (Table 2):
Table 2. Characteristics of the test cases
Waves
Test
Freeboard
(cm)
Type
Height
(cm)
Period
(sec)
1
Monochromatic
10
1.6
5
2
Monochromatic
10
1.6
0
3
Monochromatic
5
1.6
-5
Instrumentation and available data
The following instruments and gauges were used inside the flume for
measuring the above data sets:



11 resistive free surface gauges to assess free surface evolution
3 pressure gauges inside the structure rouble
12 Laser Doppler Velocimeter measurement points
The coordinate origin was chosen at the intersection of the frontal ramp
with the bottom of the flume (Figure 1). This origin was located 476 cm from
the wavemaker and 200 cm before the outer edge of the LCS front slope.
Positive x axis direction was opposite the wavemaker. The positive z
coordinate was upwards from the bottom of the flume.
Three free surface gauges were installed in the slope in front of the
breakwater to separate incident and reflected waves. Another two free surface
gauges were located over the front slope of the structure at the same x
locations of two of the LDA measurement profiles. Two free surface gauges
were placed over the structure crest and on the leeside slope. Four more
were placed to measure transmitted waves along the flat bottom behind the
LCS.
Figure 2. Free surface gauges location around and over the structure
(N. Garcia et al., 2004).
The pressure gauges were installed over the bottom, inside the LCS
structure core, to measure the wave transmission inside the porous rubble
(Figure 3). These sensors were attached to the steel frame built inside the
LCS for that purpose.
Figure 3. Pressure gauges location inside the structure (N. Garcia et al., 2004).
LDA measurements of u, w velocities were taken at 12 points in the front
slope of the LCS, see Figure 4.
u
w
Figure 4. LDA measurement points location (N. Garcia et al., 2004).
Download data (“.zip” files)
Each “.zip” file contains two general information files and several data files.
The Excel file “.xls” provides details required to use the data files while the
AutoCAD file “.dwg” a sketch of the experimental set-up. The type of the data
files (“.dat”) is identified with three letters: SUR: data from free surface
gauges, PRE: data from pressure sensors and LDA: data from LDA sensors.
Test 1
Test 2
Test 3
References
Vidal, C., Lomonaco, P., Migoya L., Archetti R., Turchetti M., Sorci Μ.,
Sassi G., 2002. Laboratory experiments on flow around and inside LCS
structures. Description of tests and data base. DELOS Project Technical
Report, 19 pp.
Garcia, N., Lara, J.L., Losada, I.J., 2004. 2-D Numerical Analysis of nearfield flow at low-crested permeable breakwaters. Coastal Engineering, 51,
991-1020.