measuring waves and currents in littoral areas: are the available instruments adequate enough?

7/25/2019 Measuring waves and currents in littoral areas: are the available instruments adequate enough?

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Measuring waves and currents in littoral areas: are the available instrumentsadequate enough?

Diogo MENDES1

1Instituto Hidrogrfico, Rua das Trinas n49, 1249-093 Lisboa

([email protected])

Keywords: Field measurements, Sediment transport, Extreme events

Abstract: This study aims to challenging the manufactures of marine instruments with threechallenges that came up after two field campaigns. These campaigns were done to measure thenear-shore circulation promoted by the wave-induced currents at the breaking zone. The firstcampaign took place in S. Jacinto and the second in Gelfa, NW Portugal. Both beaches areexposed to a wave regime characterized by a wave direction frequently perpendicular to thecoastline, thereby inducing the formation of rip cells. These cells are very dangerous forswimmers and several drowning had already occurred in S. Jacinto beach. Several instrumentswere deployed to measure waves, wave-induced and tidal currents but some limitations and

challenges had appeared.Currently, it is accepted that infra-gravity (IG) waves can play a major role on beach morphology.The equipments that measure waves should provide not only the gravity spectrum but also the IGspectrum automatically in order to study better this type of waves. Also, some equipment thatmeasured currents become submerged during the field campaigns for 1-2 hours. This can also beprevented if the equipments fixed to steel structures were able to move during the tidal cycle.

Morphological changes induced by external forcing (waves and tidal currents) also play a majorrole in littoral areas. Near inlets, they cause the shoaling and even the closure of the inlet mouthand, some coastal lagoons requires frequent dredging works. Near harbours, where the waterdepths are deep enough to allow navigation, accretion takes place and every year harbour

administrations spend financial resources to increase the water depths. During severe storms,morphological changes at beaches occur drastically and few measurements are currentlyavailable during such extreme events. For that reasons, measuring the sediment transport inlittoral areas and during extreme events are of extreme importance. Are the available instrumentsadequate enough?


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1. Introduction

Littoral areas are zones of great interest for the public in general. They are located at the sea-land interface and their productivity range from biological (larvae and mussels) to financial(tourism and surf) resources. However, external forcing, such as waves and tidal currents, canstrongly modify the morphology of these areas. For example, coastal erosion is now a commonproblem in a variety of coastal trenches (Figure 1a). This erosion occurs during strong storms andwill put in danger the nearby constructions and also destroy the habitats of many sea species.Sediment accretion at an inlet mouth and at harbour entrance puts in risk the water quality anddisallows navigation, respectively (Figure 1b and c). Dredging activities are common but are alsosometimes too expensive for the local authorities. Therefore, current knowledge of near-shoredynamics should be improved.

Figure 1. Coastal erosion (a), sediment interaction at an inlet mouth (b) and sediment accretion at harbour entrance (c).

Besides the sediment transport during storms that is mainly caused by cross-shore currents(Roelvink et al., 2009), morphological changes are frequently induced by long-shore currents.These last currents will move sediments to north or south depending of the incident wavedirection. Southwards sediment transport will stop near breakwaters and sediment accretiontakes place upstream. Downstream, due to the vortex-induced currents erosion will occur. If thisbehaviour persists over a year, it can be said that the breakwater potentiate coastal erosion at the

adjacent downwards beaches. To understand the complex near-shore circulation two fieldcampaigns were developed by Instituto Hidrogrfico in June and September 2015. This studyaims to challenging the instrument manufactures with some limitations and issues that appearedafter these campaigns.

2. Field campaigns

2.1 S. Jacinto beach

S. Jacinto beach is located upstream of Aveiro northern breakwater, NW Portugal (Figure 2). Thisbeach is exposed to a very energetic wave regime during the winter with offshore wave heightsreaching up to 10 m. The wave regime are characterized by large swells (10 16 s) mainlycoming from NW and by local wind-generated waves with smaller periods (4 10 s).


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Morphologically, the coastline orientation and the most frequent wave direction (NW) potentiateseveral rip cells along the beach.

Figure 2. Equipments deployed in S. Jacinto beach: ADCP (red square), ACM (green squares), PT (yellow circles), ADV (redtriangle) and ECM (green triangle). Bathymetry contours (dashed lines).

During S. Jacinto beach campaign, several instruments were moored and others were deployedat the beach (Figure 2). Offshore wave conditions were obtained by an acoustic Doppler currentprofiler (ADCP) moored at 12 m depth. Wave shoaling and further dissipation was measured by 8pressure transducers (PT) in a cross-shore profile. Finally, long-shore currents were obtained at10 m depth by 2 acoustic current meters (ACM) and, at the inter-tidal zone, by 2 acoustic Dopplervelocimeters (ADV) and 1 electromagnetic current meter (ECM). Wave and currentsmeasurements were performed from 8 am to 8 pm of 18 June, 2015. This interval was sufficientto cover half-tidal cycle. A more detailed analysis can be seen in Mendes et al. (2015).

2.2 Gelfa beach

Gelfa beach is located at Vila Praia de ncora, NW Portugal (Figure 3). The beach configurationis characterized by a small bay limited by rocky outcrops both at north and at south. At thenorthern part of Gelfa beach there is a small harbour that had required frequently dredging works.Due to the extent of the southern rocky outcrops and the wave direction (NW), sediments aredriven to the northern part and accumulate at the harbour mouth. Offshore wave climate is similarto S. Jacinto beach.

Figure 3. Equipments deployed in Gelfa beach: ADCP (red square), ACM (green squares), PT (yellow circles), ADV (red triangle)and ECM (green triangle). Bathymetry contours (dashed lines).


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In S. Jacinto beach we were able to accurately measure the wave shoaling and furtherdissipation. Therefore, in Gelfa beach we decided to measure the long-shore circulation patterns.

Several instruments were also moored while others were deployed at the beach (Figure 3) as inS. Jacinto. Offshore wave conditions were obtained by an ADCP moored at 10 m depth.Horizontal currents over the water column were measured by an ADCP and by an ACM at 5 mdepth. Long-shore barotropic fluctuations were measured by 8 pressure transducers (PT). Finally,long-shore currents were obtained at the beach by 3 ADV and by 3 ECM. Wave and currentmeasurements were performed from 8 am to 8 pm of 30 September, 2015. There was also apersistent rip cell in front of the central ADV (Figure 3). Circulation patterns inside the rip werealso continuously monitored during the experiment.

3. Challenges

3.1 Submerged instruments

ADV were attached to steel structures and deployed at the beach during both campaigns (Figure4). These equipments were able to measure horizontal and also vertical currents. Moreover, theyobtained an estimate of pressure and distance to the bottom level. In order to avoid submergingissues, they were installed approximately 30 cm from the sea bottom. However this distance wasnot enough to avoid being submerged during flood tide by a tidal bar. Increasing the initialdistance to the bottom is not a convenient solution because they will be measuring less time.

Figure 4. Submerged ADV during the field experiment.

For that reason, we decided to propose to the equipment manufacture the first challenge: createa movable ADV. Due to physical characteristics, the ADV is composed by the sensor and threetransducers mounted at the end of a steel arm. One of the possibilities to avoid submergingproblems can be the ability of this arm to move up and down. For that end, the synchronizationwith the sensor that tracks the bottom level will allow this arm to enter inside the ADV plastic box,thereby avoiding submerging issues.

3.2 Measuring IG waves

Infra-gravity waves are waves generated by wave groupiness (Okihiro et al., 1992) or by tidallymoving wave breaking-point (Shemeret et al., 2002). These waves have usually periods between30 and 300 s. Due to their wavelength (~1 km) the velocity at the surface is very similar to thevelocity near the bottom (Svendsen, 2006). Since they are also generated due to nonlinear


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interactions between wind waves, as wind waves increase in amplitude, these IG waves will alsoincrease. They usually have small amplitudes (10-30% Hs) but their currents might be very large,

especially during storm events. The spectrum from a PT and from an ECM shows this behaviour(Figure 5). A considerable part of the cross-shore current energy is located at the IG band.

Figure 5. IG (blue) and gravity wave spectrum (red) obtained from PT (left) and ECM (right).

All the equipments deployed were not able to measure directly the IG wave motions. Furtherprocessing of data is therefore required. Here, we proposed the second challenge: the ability todirectly provide the IG waves. Besides ECM, which can be a difficult task to provide these waves,we are convicted that the ADCP could provide them in a simple way. Both ADCP provided thewave spectrum every 20 min. This time interval was chosen to assure stationary and statisticalsignificance of the wave conditions. To provide the IG spectrum, it is just needed to store theinformation during 40 min based on the IG analysis performed by Herbers et al. (1995). Then,every 40 min this equipment could also give the IG wave spectrum. Especially in small bays as inGelfa beach these low-frequency motions (IG waves) can play a major role due to the resonancephenomenon.

3.3 Sediment transport during extreme events

Until now, sediment transport is measure in the field trough indirect measurements using opticalbackscatter sensors (Miles et al., 2015) or by sediment tracers (Miller and Warrick, 2012). Thesemethods are not accurate and are influenced by sea-state conditions. It is require new equipmentthat provides the sediment transport flux from the sea bottom (bedload) to the sea surface(suspended).

Figure 6. Beach photo before (top) and after hurricane Sandy (bottom) in Mantoloking, New Jersey.


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One of the possibilities can pass through the use of an ADCP that can estimate the sedimentconcentration based on the orbital velocities. This corresponds to the third challenge: accurately

measure sediment transport in littoral areas. However, this objective needs to be even morechallenging. The sediment transport is more intense if the external forcing (waves and tidalcurrents) increases. This increase usually occurs during extreme events which are characterizedby strong wind and low-pressure conditions. These conditions when combined with spring hightides cause dune overwash and consequently, dune breaching. It is required an instrumentresistant enough to handle extreme conditions and reliable enough to ubiquitous measure thesediment transport.

4. Resume

Are the available instruments adequate enough to answer the three proposed challenges: (1) tohave a movable arm to avoid submerging problems; (2) to measure automatically the IG wavespectrum; and (3) to measure the sediment transport during extreme events.

Acknowledgements

Thanks are due to all the people that contributed and participated in the field campaign. This research is acontribution to project RAIA.CO(0520\_RAIA\_CO\_1\_E), Observatrio Marinho da Margem Ibrica eLitoral, funded by the European Fund for Regional Development (EFDR) through the ProgramaOperacional de Cooperao Transfonteiria Espanha-Portugal(POCTEC).

References

Herbers, T. H. C., Elgar, S., & Guza, R. T. (1995). Generation and propagation of infragravity waves.

Journal of Geophysical Research: Oceans (1978

2012), 100(C12), 24863-24872.Mendes, D., Pinto, J. P. & Jorge da Silva, A. (2015). Application of an operational model to forecast near-

shore circulation: the case study of S. Jacinto beach. Conferncia Nacional de Cartografia e Geodesia(CNCG15)Ordem dos Engenheiros. Academia Militar, 29 e 30 de Outubro 2015, Lisboa.

Miles, T., Seroka, G., Kohut, J., Schofield, O., & Glenn, S. (2015). Glider observations and modeling ofsediment transport in Hurricane Sandy. Journal of Geophysical Research: Oceans, 120(3), 1771-1791.

Miller, I. M., & Warrick, J. A. (2012). Measuring sediment transport and bed disturbance with tracers on amixed beach. Marine Geology, 299, 1-17.

Okihiro, M., Guza, R. T., & Seymour, R. J. (1992). Bound infragravity waves. Journal of GeophysicalResearch: Oceans (19782012), 97(C7), 11453-11469.

Roelvink, D., Reniers, A., van Dongeren, A. P., de Vries, J. V. T., McCall, R., & Lescinski, J. (2009).Modelling storm impacts on beaches, dunes and barrier islands. Coastal Engineering.56(11), 1133-1152.

Sheremet, A., Guza, R. T., Elgar, S., & Herbers, T. H. C. (2002). Observations of nearshore infragravitywaves: Seaward and shoreward propagating components. Journal of Geophysical Research: Oceans(19782012), 107(C8), 10-1.

Svendsen, I. A. (2006). Introduction to nearshore hydrodynamics(Vol. 24). World Scientific.

measuring waves and currents in littoral areas: are the available instruments adequate enough?

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