Field measurements of extreme waves in the intertidal zone, Aran Islands, Ireland

Abstract

Recent research has shown that interaction with bathymetry can lead to wave height amplification of up to a factor
of 12. These extreme waves are important for coastal erosion and very large boulder movement but long-term
field records in such high-energy conditions are challenging to acquire, and very scarce. Over three winters
(2016-2019) we deployed custom-built pressure sensors in the intertidal zone at three locations, varying from a
near vertical cliff to a flat bedrock platform, on the Aran Islands off the west coast of Ireland.
Reconstruction of surface elevation from pressure sensor measurements in shallow water is difficult, and
only very recently have methods been derived that accurately reconstruct heights and shapes of progressing waves
close to the breaking limit in shallow water. Under these conditions, hydrostatic pressure assumptions do not
return accurate wave heights, and non-linearity must be taken into account. The situation becomes even more
non-linear, however, if water depth decreases very rapidly, as is common at steep rocky coasts. In these cases,
the assumption of progressing waves does not hold, and many waves are breaking. This greatly complicates
interpretation of pressure-sensor data from the intertidal zone. In addition, very shallow water (from zero to a few
metres) leads to very large ratios of wave height to water depth, beyond standard analytical wave theories. Waves
may break onto the sensor or be reflected from cliffs, which is not covered by the progressing-waves assumption.
Smoothing or filtering is needed to compensate for spurious oscillations in the higher order corrections, but the
frequency of those oscillations can be similar to actual physical wave crests, and distinguishing between them is
difficult.
Solving these problems is vital for understanding the interplay between high-energy waves and rocky coastlines.
We present attempts to apply surface reconstruction methods to field data and the significant issues/uncertainties
encountered. Results highlight the need for independent validation and alternative measurement techniques.

Full text

Field measurements of extreme waves in the intertidal zone,
Aran Islands, Ireland
Pál Schmitt1, Rónadh Cox2, Frederic Dias3, Louise O’Boyle1and Trevor Whittaker1
(1) Queen’s University Belfast, Marine Laboratory,12-13 The Strand, Portaferry, Newtownards BT22 1PF, United Kingdom,
(2) Geosciences Department , Williams College, Williamstown MA 01267 , USA,
(3) School of Mathematics and Statistics, UCD Science North G.03, Belfield, Dublin 4, Ireland
Project
This collaborative international project (2015-present) addresses knowledge deficits in
the study of storm waves and their interaction with steep rocky coasts.
Combining field measurements of boulder creation and boulder movement,
instrumentation in the shallow offshore area and on supratidal platforms,
numerical and analytical modeling [3], and physical wave-tank experiments,
we are working to generate a synthetic understanding of relationships between coastal
wave energy, onshore flow, and mass transport.
Field Work
Field campaigns were undertaken on the Aran Islands off the west coast of Ireland.
Before-and-after photography was used to produce a database of >1100 boulders
Figure 1: Location of pressure sensors (red)
and ADCP (black)
moved by storm waves during winter 2013-2014 [2]. The
largest boulder moved was 620t. Boulders were relo-
cated at elevations up to 26m, and at distances up to
220m inland. New boulders, weighting 10s of tonnes,
were extracted from bedrock. An ADCP deployment
over 3 month in 40m water depth 3km off the Inishmaan
coast yielded wave data in intermediate to shallow wa-
ter depth. Extreme loading and aeration of the water
makes acoustic doppler devices infeasible in shallow wa-
ter. Three custom made pressure sensors were deployed
at three sites on Inishmaan. Profiles vary from a near
vertical cliff to a flat bedrock platform. Two successful
campaigns during the winters of 2016/17 and 2018/19
with 100% data return have already been completed, a third deployment will be re-
trieved when weather permits.
Reconstructing Surface elevation from bottom mounted pressure sensors
– The issue
•Literature focusses on intermediate and deep water situations
•Only very recently extension of methods to higher order up to the breaking limit
for progressing waves [1]
•Pressure traces affected by water motion varying from linear progressing to fully
reflected waves, plunging breakers, swash or bores with varying tidal elevation
Experimental Tank Testing
Experimental tank tests were performed in the wide wave basin in the Marine Lab-
oratory, QUB at 100th scale. The tank bottom slopes from the deep section,
where the Edinburgh Design Ltd. absorbing wavemaker is installed, to an absorbing
beach. Experiments were performed for two water levels (4 and 2cm) and seastates
(Hs=0.06m,Tp=1.2sand Hs=0.12m,Tp=1.2s). A vertical wall was installed
6cm from the shallowest probe position to investigate the effect of a vertical cliff face.
IP68 vented gauge pressure transducers (25Y models by Keller Ltd.) were installed
flush with the tank floor. Resistance wave gauges were placed directly above the
sensors. All sensors recorded with 128Hz.
Figure 2: Experimental setup in the wide wave basin, QUB. 100th scale. The wavemaker (right) creates waves which
transform while progressing over the rising bathymetry towards the beach/cliff (left).
Are we missing significant coastal wave events?
Figure 3: View over a submerged platform below a cliff on the field site on the Aran Islands. Boulder deposits are visible on the cliff top. High aeration levels of the water make the use of Acoustic Doppler equipment unfeasible.
940 945 950 955
−0.02
0
0.02
0.04
Time [s]
Elevation [m]
WP
L
HO
Figure 4: Example experimental time trace. 100th scale. Water depth 2cm,Hs=12cm,Tp=1.2s. Data is shown
for the resistance wave probe (WP), hydrostatic reconstruction (L) and higher order correction (HO).
70 75 80 85 90 95
−1
0
1
2
3
4
5
6
7
Time [s]
Elevation [m]
WP
L
HO
Figure 5: Example time trace of simulation results. Full scale. Probes are located at the platform edge as shown in
Fig. 6 a. Wave period is 6s, still water level 14m, the platform rises 10m
Numerical Tank Testing
A custom numerical wave tank [4] based on interFoam of the OpenFOAM-v1806
toolbox allows for easy variation of bathymetry, water level and access to field variables.
Tests are run for different platform shapes (square and sloped Fig. 6). A wall conditions
simulates a cliff face at the platform end. Three numerical wave probes record surface
elevation across the length of the platform or slope. Corresponding pressure probes at
the bottom record data similar to our setup in the field.
Figure 6: Example bathymetries under investigation in the numerical wave tank. Waves propagate from a low reflection
wavemaker on the left towards a cliff face on the right. Simulations are run using OpenFOAM-v1806 and a custom numerical
wave tank. [4]
Results
•Results indicate 30% higher mean wave height and up to 100% higher extreme
wave events than predicted by hydrostatic reconstruction from pressure sensors
•Non-linear reconstruction [1] somewhat better but often not applicable
•ADCP measurements in those location unfeasible due to aeration and loads
•Urgent need for review and better post-processing of pressure data measurements
in the near shore region
•Alternative measurement techniques are required
References
[1] P. Bonneton, D. Lannes, K. Martins, and H. Michallet. A nonlinear weakly disper-
sive method for recovering the elevation of irrotational surface waves from pressure
measurements. Coastal Engineering, 138:1–8, 2018.
[2] Rónadh Cox. Very large boulders were moved by storm waves on the west coast
of Ireland in winter 2013–2014. Marine Geology, 2018.
[3] James George Herterich and Frédéric Dias. Wave breaking and runup of long waves
approaching a cliff over a variable bathymetry. Procedia IUTAM, 25:18 – 27, 2017.
IUTAM Symposium on Storm Surge Modelling and Forecasting.
[4] Pál Schmitt, Christian Windt, Josh Davidson, John V. Ringwood, and Trevor Whit-
taker. The Efficient Application of an Impulse Source Wavemaker to CFD Simula-
tions. Journal of Marine Science and Engineering, 7(3), 2019.
Acknowledgements
Pál Schmitt was supported by a research grant from the De-
partment for the Economy Northern Ireland under the US-
Ireland R&D partnership programme grant No 801. Ronadh
Cox was supported by National Science Foundation award
number 1529756. Frederic Dias was funded by Science
Foundation Ireland (SFI) under the research project ”Under-
standing Extreme Nearshore Wave Events through Studies
of Coastal Boulder Transport” (14/US/E3111).