Tuesday, 4 June 2013

post-storm 3D fieldwork

On March 13th  2013, we went again to Barreta Island to undertake a topographic survey. We were predicting overwash on Barreta because on March 7th, during the morning high-tide, there were waves exceeding 3 m.

Our observations point to the occurrence of overwash even during the high-tide of the nigth before the survey. We could still see the difference between wet sand (from overwash) and dry sand on top of the barrier.

Overwash intrusion can be identified by the difference between wet and dry sand.

Overwash maximum intrusion can be identified by the difference between wet and dry sand, and also by some debris at the edge.
On this site, only a subtle change is noticed from a distance, but looking closely a centimetre difference in elevation corresponds to a thin layers of overwash deposition, which masks the overwash intrusion.

Often, we use this wook trunk for reference and location.

Getting in and out of the cold water was tough.

We do allow people to enter the barrier island, in spite of the signs...

Thursday, 23 May 2013

2D Fieldwork, second try

Waves were expected to exceed 3.0 m height  and high tides were more than 1m above MSL, for 22 and 23 February 2013. Yupi!!! We're going to measure overwash.
Lots of stuff to do: charging equipment batteries (only for the GPS there are 4 different batteries to charge), lanterns batteries, check equipment and tools lists, packing the van, buying food, programming equipment, renting the boat and skipper.
OK, we're ready.
We went 1 day ahead to put everything in place to start deploying the instruments at 4:30 in the morning.

Base camp on top of the dunes.
All stations were mounted successfully during the early morning, during the low tide.
Below are some photos of the stations.

GPS base.

Station 3.

Station 4.

Station 5.

Stations 7, 6, 5 and 4 (from left to right).

 But, unfortunately, there were only a few small overwash events.

One of the few overwash events.

Not good enough. We tried during 2 days, and then the waves just dropped. The problem was that we didn't had enough tide level. Maybe next time...

Data were not good but we kept a friendly spirit.

Happy to be doing fieldwork.

The cold and wind was not so funny.

Cleaning everything at the end was also a major task.

Cleaning the equipment and materials after fieldwork. Five people during almost 9 hours.

Thursday, 18 April 2013

Variable overwash intrusion

Just after the Carnival holidays, on February 13th, we made another topographic survey on Barreta Island. This time we expect to measure overwash related to a high tide level due to the equinoctial spring tides.
Overwash intrusion was relatively small at west:

Overwash of the barrier berm, but not of the barrier crest.

Overwash intrusion was longer at East:
Overwash of the barrier until the vegetation on the backbarrier.

The low-tide terrace has significant alongshore variations, even in areas that are not connected to the Ancão Inlet ebb delta.

Low-tide terrace in two sites of the study area, Eastwards of the Ancão ebb delta.

Friday, 8 March 2013

3D post-overwash fieldwork

Conversely to the previous fieldwork, on January 30 overwash maximum intrusion on the barrier top was very easy to follow.

Overwash maximum intrusion visible by colour contrast.

Overwash maximum intrusion visible by colour contrast and shell debris.

At the end of this fieldwork we computed the distance that we make holding the GPS antenna and discovered that in a 2 km coastal stretch we walk 14 km!!!!

We had to wait until February 4th to do the bathymetric survey. Our experienced skipper advised us to postpone the survey because of the waves.

This set of data is very good as there is a high confidence on the overwash intrusion and also there is a good match between the lowest points on the topographic survey and the highest points on the bathymetric survey.

Fieldwork without clear overwash evidences

On January 19th, there was a red alert for waves on the Portuguese coast. In the Algarve, Hs reached 4.5 m with maximum wave height exceeding 8 m. We went to Albufeira groin to undertake a fieldwork campaign within another national project (SPACE). An electromagnetic current meter and 5 pressure transducers were deployed, and cool videos were recorded during the overtopping of the coastal structure.
On January 21st, we made a post-storm survey on Barreta Island, expecting serious overwash all over the island.

Top of the barrier, looking westwards.

 However, we found almost no evidences of overwash. What we did find was significant aeolian rework of the barrier surface.
Aeolian ripples on the bare sand surface.
Now the questions are: Did overwash occur? Did sand transport by wind cover overwash evidences afterwards? We think it is difficult to assume that this area was not overwashed under such energetic waves, but we have no way to prove it. The lesson is to try our best to do fieldwork within the next 24 hours after overwash is supposed to occur.

Meanwhile there was a fishing contest in Barreta Island.

Thursday, 24 January 2013

December fieldwork

Because we've been busy with the project dissemination, during December we've only made one topographic survey. On December 28th we've repeated the 24 topographic profiles to have a reference situation in case storm waves occur.

No overwash occurred during the last high-tides, but probably during December there were overwash events in some parts of Barreta Island.
Shell debris alignments consistent with overwash.

Aeolian rework of the barrier surface is visible in many areas, either by ripples, lag surfaces or depositions behind obstacles on the surface. Some vegetation is covered by this process.
Aeolian ripples and aeolian sand accumulation.

The lagoon margin is varied along Barreta Island. In places, a tidal flat exists, while in others there is a sandy beach adjacent to the tidal channel.

Tidal flat at the backbarrier.

During this survey, barrier erosion was noticed on the lagoon beach, which is likely related to changes in the inner channel currents.
Erosion bluff on top of the lagoon beach.

Dissemination of the project

First results of RUSH project were presented on the 7th Symposium on the Atlantic Iberian Margin (MIA 2012), in Lisbon.

For more information about the conference click here.

The communication details are:
Matias, A., Carrasco, A.R., Pacheco, A., Loureiro, C., Nunes, M., Ferreira, Ó., Armaroli, C., Masselink, G. (2012). Do espraio ao galgamento: uma perspetiva integrada. 7th Simposium on the Atlantic Iberian Margin, Lisbon, Portugal, pp. 30.

Another abstract was accepted for the 8th IAG International Conference on Geomorphology, August 27th to 31st, 2013, in Paris. 

The communication detais are:
Matias, A., Carrasco, A.R., Pacheco, A., Loureiro, C., Nunes, M., Ferreira, Ó., Armaroli, C., Masselink, G. (accepted). Longshore variation of overwash occurrence in a barrier Island (Portugal). 8th IAG International Conference on Geomorphology. Paris, France.