I have long been intrigued by the formation of the sand islands of the Ria Formosa. I show below the history of the artificial sea-access in Tavira, and further translate a study of Culatra Island, giving something of the history of the artificial access for Faro and Olhão.
Natural and Artificial Seashore Developments, and Longshore Drift
The consequence of the shape of the Algarve and its geographical position is that rocks and cliffs are eroded by marine and aeolian action in the Barlavento (the windward) and the ensuing detritus in the form of sand is deposited on the sand islands and beaches the Sotavento (the leeward). The point at which the change from windward to leeward takes place is at Cabo de Stª Maria on the island of Barreta to the south of Faro. This longshore drift seems always to happen in an eastward direction, and this process accounts for the rocky nature of the shoreline in the Barlavento, and the huge expanses of sandy beach in the Sotavento.
A lesson which might be drawn from the construction of artificial works on the shoreline, such as dredging, and the construction of groynes, jetties and artificial openings in the sand islands, is that such developments nearly always attract some countermeasure on the part of the sea. And quite often, the action of the sea and its accompanying storms can be both alarming and unpredicted.
The Artificial Opening on Tavira island
In particular, Tavira Island, the long sand bar at the mouth of the River Gilão, has had an immense effect on the development of Tavira as a town, and its relative decline in importance since the sixteenth century. As the scouring effect of the river diminished, affected I believe by the increasing number of water and tide mills on the River Gilão, so the sand island both widened and grew eastwards. The late nineteenth century saw the occasions when imports to Tavira could no longer be brought by sea into the Gilão. These imports had to be docked in Olhão or perhaps Vila Real de Santo António, and brought overland to Tavira.
After much urging by the Câmara, the government eventually gave permission for a channel to be cut in the island to give access to the sea. In 1918, the Câmara created the Junta Autónoma das Obras da Barra e Porto de Tavira, which devised a plan to open a new channel in the sand island (in Portuguese - uma nova barra artificial) to make a direct access from the bay of Quatro Águas to the sea.
As the bar (the entrance from the sea to the lagoon) at Cacela was also beginning to silt up at that time, reducing further the size of boats which were able to enter the lagoon, it became imperative for work to begin. The contract was put out to tender in May of 1926, and work began on 27 September 1926. When the storm of 1941 struck, the bar had been in use for only fifteen years.
The immense cyclone of 15 February 1941 caused huge devastation in the whole of Portugal and Spain, and of course the whole of the Algarve was also affected. The weather was so bad that the Tavira port captain prevented the fishing boats from going out. One result of the deluge of rain which occurred immediately before 15 February was that trees standing in waterlogged soil were easily uprooted in the high winds. One particular ship from Lisbon wanted to take shelter in Quatro Águas bay, but the channel was silted to the extent that it was not accessible and the ship had to make instead for Vila Real.
The furious sea made a new opening in the sand island, about 500m to the east of the artificial opening. It was given the name of Barra do Cochicho (Whispering Bar). The arraial (depot) of the tuna armação Medo das Cascas was located in between the artificial and new openings in the island. Within a short time, both the depot and the neighbouring police post on the island had succumbed to the waves. The cyclone had the effect of increased silting at Quatro Águas both the artificial opening and the bay itself. And the armação Medo das Cascas decided on 27 April to build a new arraial near the Fortaleza do Rato, which opened for use in January 1945. The new arraial Ferreira Neto is now the hotel Albacora.
Recuperative works to dredge again the artificial opening at Quatro Águas began in November 1961 and occurred again in 1977. Judging from the article below, about Culatra Island, it would not be surprising if the dredging work should prove necessary frequently in the future.
Date 10 June 2021
It is reported today that the Direção Geral of Natural Resources, Safety and Maritime Services has opened a tender process for dredging the Port of Tavira. The cost of the work is estimated at 444,570,55 (plus IVA) and is aimed at the dredging of the bar (the artificial opening in the sand island) and the bay of Quatro Águas. The recovered sediment will be sprayed onto the adjacent sand islands where there is a deficiency of sand.
This work is necessary for the maintenance of fishing and tourist activities related to the sea. It will also reinforce the safety measures for boats, their crews and passengers in the canal serving Tavira, Cabanas and Santa Luzia.
In March, 2021 fishers of these ports asked that dredging take place urgently, since the bar had silted up, putting the safety of boats and their crews at risk. The Câmara of Tavira supported these demands.
Leonardo Diogo, President of APTAV (Associação de Armadores e Pescadores de Tavira) said that fishers had witnessed a degradation of the artificial opening at Tavira, and that the recent south west gales affecting the Algarve coast had caused the bar completely to silt up. At low tide, there was less than half a metre depth in the bar, which put at risk the safety of boats traversing the bar. The problem was worse on the western side.
In the recent past, in 2018, work was done to rebuild the eastern mole of the bar. At that time, three years ago, dredging operations were carried out in the places which now require further work.
The Recent Evolution of Culatra Island
A paper published in LITTORAL in September 2002.
Authors: Tiago Garcia; Óscar Ferreira; Ana Matias; João Alveirinho Dias
Abstract
The evolution of Culatra Island between 1940 and 2001 was deduced by the analysis of vertical aerial photographs. The results obtained for shoreline variations, washover evolution and island growth showed two different evolutionary periods. Before 1985, the shoreline experienced strong erosion in the west and washovers were reduced as a result of shoreline retreat. In the east the dominant process was island growth. After 1985, shoreline stabilisation was achieved both by natural and anthropic processes. At present, the vulnerability of the front dune is generally low, but there are places which would benefit from recovery interventions.
Introduction
Culatra Island is one of five islands forming a part of the Ria Formosa barrier island system off the southern coast of the Algarve. The islands vary in their morphology and in their exposure to incident waves. At Culatra, there is a maximum tidal range of about 3.9m, and a moderately low wave energy regime. The longshore drift in this area is eastward, but there is no agreement on the volume of sediment displaced annually. Estimates vary between 90 and 250 thousand cubic metres.
There are two inlets which limit the island: the Armona Inlet to the east, and the Faro / Olhão Inlet to the west. The Faro / Olhão Inlet is artificial, and has been stabilised by jetties. Its permeability to longshore sediment transport is not clear.
Culatra Island is about 7km long, and its morphodynamic characteristics give three different sectors.
Zone 1: the west end is partly artificial, supported as it is by the Faro / Olhão Inlet jetties built between 1927 and 1955, and a seawall / groin system built in the early 1980s to protect human habitation. The beach is relatively narrow, and the dune bluff transition to the dune system shows that, in the shadow of the jetties, sediment is not supplied to this area. During the 1980s, human occupation eastward of the seawall / groyne system increased rapidly onto the ancient washover beach.
Zone 2: the central sector was the historical location of the ancient inlet, which, like other inlets in the system, used to migrate towards the east. The morphology of this sector shows the different ages of the bodies of sand which form alternating dune ridges and ancient tidal channels and washover breaches. In 1987, the Ria Formosa Natural Park installed sand fences on these breaches, which trapped sediment; a second set of sand fences was erected on the trapped sediment, which is serving to trap further infill. As this work provides some protection from the sea, permanent human occupation occurs in this sector on the inner, northern, margin of the island.
Zone 3: the eastern sector is unoccupied, and the location of the inlet is constantly evolving, with the development of curved sandy spits, separated by active tidal channels on the inner part of the island, and overwash on the ocean side. In this sector, extensive overwash can occur under combined storm surge, spring tide and storm conditions.
Method
Researchers compared vertical aerial photographs of about 1940 with those of 2001. There was a major storm in the whole of Iberia on 15 February, 1941, and the 1940 photographs show evidence of strong erosion, and the photos therefore probably come from after that date.
The eastward expansion of Culatra Island has been given as the reason for the narrowing of the Armona Inlet. Between 1945 and 1958, the eastern tip of Culatra grew at an average of 32m per year, but after the conclusion of the extension of the Faro / Olhão jetties, until 1976, the rate of growth was 62m per year. Since that time the rate of growth has slackened, and in the years 1985 - 1996, the expansion was at the rate of 13m per year.
The jetties constitute an obstacle to littoral drift, and also create a shadow zone on the downdrift beach. Between 1958 and 1976 the island experienced major erosion, and some of the frontal dune ridge was lost, especially in the west zone. It seems that the completion of the jetties resulted in shoreline retreat in the western zone, and an accelerated accretion at the eastern tip.
During the period 1976 - 1985, the groyne was built at Farol beach, and the seawall was extended, and these works stabilised the shoreline at the western end of the island, but promoted erosion at the eastern end of the island. The effect of the groyne on shoreline evolution is not clear, but its placement in the shadow of the east jetty probably diminished its influence.
Between 1985 and 1996, shoreline changes on the entire island were small. Colonisation of a wide berm by pioneer vegetation aided the formation of dunes, which in turn diminished the possibility of further overwashes.
In the period 1996 - 2001, the western zone underwent a small shoreline retreat, and the dune area was strongly degraded by [human] trampling and aeolian deflation. Storms damaged the seawall / groyne structures. The rest of the island was stable, except that the eastern end continued to grow.
Washovers
Two types of washover were detected. In the eastern half of the island, the predominant characteristics are large breaches separated by small dune ridges, where the adjacent washover fans coalesce. The second type consists of small intrusion washover fans, usually individual.
Previous to 1985, overwashed areas diminished continuously on the entire shoreline, but since the western zone was experiencing strong erosion, the shoreline had retreated to the back of the existing breaches, and washover reduced; a similar pattern occurred on the rest of the island as the breaches were subjected to natural recovery. The sand fences placed in 1987 were effective in reducing washover through strong sand retention in these structures, and at the eastern end a similar sand retention was influenced not by sand fences but by colonies of pioneer vegetation.
By 2001, there were few active washover breaches, and the continuing dune recovery process along the shoreline is responsible for this change. During the entire analysed period, the area where overwashes created large washover structures migrated to the east in association with the island growth.
Comparing island growth and shoreline evolution, it can be seen that the erosion area and accretion peak migrated to the east as the island expanded, moving at a rate of about 62.5m per year. As the island elongated, and the accretion area moved to the east, erosion dominated the western zone.
Whenever a new spit was projected into the mouth of the Armona Inlet, it was immediately incorporated into the island main body by the formation of vegetated dune ridges. These ridges are almost perpendicular to the shoreline, so that overwashes can occupy the valleys between.
Conclusions
The results noted here show that between 1945 and 1985, the island had two distinct behaviour areas: one in erosion, in the west; the other in accretion, in the east. The boundary between the two migrated eastwards. At the same time, shoreline retreat at the western end of Culatra reduced the incidence of washovers.
Washovers in the central and eastern zones were reduced by the use of sand fences in the central sector and naturally occurring pioneer vegetation in the eastern sector.
During the 1990s there were no significant changes in the island. Low evolution rates were maintained and the islands eastward growth continued with moderate annual rates. The correspondence between island growth and island changes was again evident; as the elongation of the island decreased, so too the changes in its shoreline tended also to decrease.
In general there is no great need for recovery interventions, but there are places where the dunes have a high vulnerability to overwashes. The area immediately to the east of the Farol beach groyne shows the combined action of: the lack of sediment; trampling; and aeolian depletion.
The sand fences work well at trapping sediment and dune creation, but rely for effectiveness on a sufficiently wide dry beach to give a greater aeolian fetch. In consequence, any recovery programme aiming at effective and environmentally acceptable results should consider artificial nourishment as a means to supply the lack of sediment induced by the jetties and the groyne at the western end of Culatra island.
W: algarvehistoryassociation.com
