Man ' s action and slope erosion . A case study in Tâmega Basin ( 1981 ) *

When a catastrophiclandslide occurredina smallriverbasin on theleft bankofthe Tamega valley, in theNortheast ofPortugal, which involved the sudden sliding of severa! tons of arable land and rocks and caused the destruction of a house and the death of fifteen people, the authors decided to carry out a careful investigation of this geomorphological phenomenon. After having formulated a number ofhypotheses, they did a lot offield workin the areaand carried out measurements to determine, as accurately as possible, both thereal extension and the causes of lhe occurence of such a phenomenon in that particular place and at that particular time of lhe year. A series of studiesenabled theauthors to come the conclusion thatthe naturaland human-induced causeshad been responsible for lhe disaster. Disasters like this don't unfortunate!y constitute isolated incidents. Therefore, the authors think it is very importanl to study lhe local geomorphology carefully before any h ouses or the setting up of any human activity within the territory it is responsible for.


Introduction
The small ri ver basin o f the Aros a brook, whose whole area barely covers one square kilometer, was the scene of one ofthe deadliestnatural catastrophes caused by movements o f land which have occurred in Northem Portugal in recent years.Barroso plateaux.The basin is elongated in shape witha North-South orientatíon andit is easily reached by the masses of maritime air coming from the W est that go up the condensation barrier along the Tãmega valley.
This basin is situated on the left bank slope of the ri ver Tãmega, a fairly important stream which flows in a deep narrow valley between the Alvão and * Trabalho desenvolvido a partir de uma comunicação apresentada ao Symposium Time, Frequency and Dating in Geomorphology (Checoslováquia, 1992).Tradução para inglês da Dra.Margarida Vilela.**Professor Auxiliar.InstitutodeGeografia.FaculdadedeLetras.Universidade do Porto.***Professor do Ensino Secundário.
The high areas that borde r the plateaux of Trásos-Montes ín the West form a mountainous line whlch runs parallel to the Atlantíc coast and contributes strongly to the rising o f the maritime a ir masses, thus causing heavy rainfall in the area.This is the reason why that particular line ís commonly known as "barreira de condensação." Though sítuated in quite different exposure conditions, the two meteorological stations located nearest to the Arosa basin -Cavez and Ribeira de Pena-register yearly values as high as 1400 mm S J I.J (Fig. 1) while the number of days of rainfall is more than 100.The numbers show clearly that this is an are a o f hea vy rainfall.Nevertheless, the distribution of the rainfall along the year is not regular; the wet months are followed by a hot dry summer with mediterranean characteristics.

The event
The mass movement, took place on the evening ofthe27thDecember 198l.AccordingtothePortuguese daily newspaper with the largest circulation in the Fot.l-The buildingdamaged bythelandslide.36 country (Jornal de Notícias) the entire area overlooking the T"amega was experiencing aheavy storm.Suddenly it was as ifthere was an earthquake.What might well be called an avalanche of water, mud, big and small rocks, shrubs and trees rolled down the hill in an overpowering maelstrom, destroying terraces painfully built over the years; it crashed into the wall of a coffee shop (Fot.1), tore it apartas if it were made o f pape r, caused the concrete upper floor to collapse and went on its path of destruction to finally stop when it reached the river Tâmega in the valley below.
At the time there was a large number of predominently young people in the coffee shop, drinking and talking, taking advantage ofthe presence of friends and relatives who had come from abroad to spend Christmas and New Year with their families.A significant portion of the mal e population of the small village of Arosa 1 , situated at the edge of the basin, had gathered there.Trapped by the violence and speed o f the phenomenon, they had no chance at all o f escape.Fifteen people died and fourteen were badly hurt.What had really happened?A considerable mass of land from the middle section of the valley began moving, at first as a rotational slide; this movement, however, soon changed into a more complex one, including solifluctions, until it finally became a typical debrisflow.In its rapid descent along the narrow talweg it destroyed every single man-made thing which opposed it, tearing away all sorts of materiais which only served to make the moving Fot. 2 -T he srnall valley of the Arosa brook and the scar caused by the landslide.mass even thicker.
There was no way this enormous mass of mud and rocks could pass through the narrow channel that was the normal course the brook water took to flow under the 211 Road.When it overflowed, it found in its way a building made of stone and brick which had very unwisely been built directly on the talweg.It simply broke down the walls, left part of the upper floor without support and added to the moving mass, not only the building materiais it had snatched away but also all the furniture and the people inside the building.lt only stopped, as said before, when it reached the foot of the slope, i. e., in the bed of the ri ver Tâmega.
On the slope it left a longish scar in the direction of the maximum slope, with a Iength of 69 meters territorium 1.1994 and a maximum width of 17 meters.In all, the landslide affected ao area of 828 square meters (Fot.2).The scarp had amedianheight of 1.5 meters which allows us to estimate the total volume of the materiais removed at about 1,240 cubic meters.
As the talweg was interrupted by terraces and the stream had been diverted to feed small reservoirs which at the time were full, there was a further increase in the mass of soil constituted by the stones which formed the supporting walls of the terraces and the reservoirs.
The dislocation of all these materiais exclusively according to the laws of gravity allowed the brook to regain violently its natural path in the old talweg which in the meantime had been transformed by man with a total ignorance and disregard forthe principies that rule such phenomena as ruo off, evolution of the slopes and fluvial dynamics.

InterveningFactors
For the occurence of this landslide there is no simple linear explanation as it is the outcome of various factors , involving nature and human intervention(F.REBELD , 1977REBELD , , 1983)), whicharenow to be analysed.
The maio cause, the one that was really decisive in the onset of the landslide at that precise moment and in that particular place was, no doubt, the heavy and continuous rainfall which had occurred in the days before and on the very day of the catastrophe.

I. 1994
The observation of the records of the daily rainfallregistered in the nearestmeteorological station (Fig. 2) show that in the days before the event ithad rained very heavily for 20 days in Cavez and 18 in Ribeira de Pena.While in Cavez there had been nine days with rainfall less than 10 mm, in Ribeira de Pena it rained more than 10 mm every day.The mentioned records show that the total amount of rainfall in the twenty days preceding the disaster reached397.6 rnm in Cavez and415.8 in Ribeira de Pena.Such values are uncommon for this area; the average rainfall for the month of December in both villages is less than 200 mm (Fig 3).
Another determining factor concems the steepness of the slopes which characterize the whole basin (Fig. 4).Themap shows thatinclines between8Yzand 18l/z are predominant in the basin but they alternate with areas where the slopes are gentler (YoUNG, 1972).The landslide began in one of the places with the steeper incline.
A third cause is to be found in the fact that the whole basin is situated on granite bedrock.In the place where the landslide began this rock is covered by a thick layer of weathered granite.If, on the one hand, the altered granite makes it easier for the rain water to seep into the soil, on the other, the abundance of clay may prevent it from penetrating deeply in to the ground and retain it in the superficiallayers o f the soil (B.MARQUEs, 1990).Besides, almostall the slope in which the small valley is situated is smoothed by slope deposits, resulting from solifluctions; they are fairly thick (2 m) and date back to a cold, humid period (A. PEDROSA, 1993).These deposits, which are heterometric and unconsolidated have a big fine clay matrix (65%) formed mainly by chlorite (75%), montemorillonite(18%), and4% ofkaolinite which contrasts with large-sized blocks -with a diameter superior to 40 em.Consequently, there is in the area an important factor of instability, since the clay gains plasticity and thus creates the ideal conditions to enable the weight of the blocks to exert enough pressure to destroy the existing balance and set off a movement of land.
The topographic conditions played a part in the process as well.The small valley in which the landslide took place can be subdivided into two sections of different incline.Upstream from the place where this landslide occurred there is a small reception basin with a gentler incline, used for agriculture, which contrasts sharply with its lower section which has a much steeper incline.
The existence in the place where the movement began of a layer of vegetation mainly formed by herbaceous plants and small shrubs reinforces the scenario.By making the runoff more difficult, this vegetation cover increased the quantity of water retained in the soil and, consequently, the weight and instabili ty o f the weathered materiais.All these factors helped to create a set of circumstances which were propitious to the setting off of a full-scale landslide: a series of very rainy days, a small reception basin in weathered granite, covered with vegetation and opening on to a narrow deep natural system of drainage with a steep slope.One can' t forget that the movement began exactly in the place where the angle which separates the two mentioned sections changes, i.e., where a gentle slope becomes moderately steep ( Y OUNG, 1972).
Apart from that, there is still the whole process o f incorrect human intervention in the area which is responsible for the extent of damage and loss of life in a phenomenon which is fairly common in humid mountainous regions.
In our opinion the element that most contributed to increasing the intensity o f this natural phenomenon h as something to do with the alteration o f the natural conditions of the runoff caused by a rural path (Fig. 4).It was built to enable the farmers to get to their fields and to the forest.lt winds up the slope, disregarding the small streams it crosses and goes even in to the next basin.Thus, besides concentrating all the runoff, the path itself has become an element of the drainage system, as if it were a ri ver bed.A large part of the runoff, together with the water it collected on its way through the neighbouring ri ver basin, was consequently channelled to a place immediately above the si te where the movement of land originated.
This large quantity of water gushed into the maio small valley through a water pipe 40 em in diameter which allowed its way under the abovementioned path and contributed decisively for the onset of the landslide.
Had the quantity of raio been normal, this temporary brook wouldn't have a large volume of water.Along the talweg anumber of small reservoirs had been built which retained the water to be used later for the irrigation of the fields nearby.Owing to the steep angle of the slope, these fields were on terraces supported by small walls made of loosely piled stones.Much of this material was, as said before, incorporated in the original moving mass, contributing to its increase in weight and volume and causing theevent to have moredramatic consequences.
U nl ike what happens in bigger valleys in which the streams are permanent, in this case there were terraces across the river bed, which cut off the natural courseof the talweg.However, what transforrned a natural event, even if it was of greater magnitude than usual, into a catastrophe was the existence of a building right at the bottom of the valley 2 .ltstood at SLOPE ANGLES LANDSLlDES ... the si de of the road in a recess which had resulted from the correction o fone of the many curves of the road; the builder had erroneously taken for granted that the drainage pipe under the road would defend it from the effects o f the brook water.

Conclusion
The emotional impact this phenomenon had on a local and even national scale was mainly dueto the loss ofhuman lives; it was that fact that transformed it in to a catastrophe.However, such phenomena are much more usual than the few that make big headlines (L.LOURENÇO, 1988;A. PEDROSA, 1992).They are parto f a geomorphological process that is permanently running its course and that man cannot or does not always know how to forecast.
Consequently, it is extremely important to bear in mind geomorphological factors whenever acting upon a geographical space.However, it cannot be understood as a mere topographic aspect.It is more and more criticai not to disregard the dynamics of the morphogenetic processes (J.-C.FLAGEOLLET, 1989;F. REBELO, 1991).The study and understanding of such matters can help to determine the probabili ty o f occurrence of natural hazards mankind is subjected to.In this way, man' s intervention in the land will be more conscientious; h e will avoid conflicts with the environrnent and will not settle in places which are unsuitable or do not offer a minimum of safety.
This problem has more than scientific and techinal aspects.There is also the pressing need to make the people involved and the public in general awareof it beca use they either ignore it or simply do not give it the attention it deserves.Quite often people even show a great disrespect for such matters.Clear proof of this is the fact that the present owner of the building severely damaged in Arosa in 1981 has been insistently trying to get official permission to rebuild the house and only the stubbom opposition of the local authorities has stopped him so far.
Forecasts carried out by experts o f the possibl.eoccurence of natural phenomena must be borne in mind whenever man decides to do anything that transforms even a tiny part of the surface of our planet.This is in ouropinion theright way to achieve a correct land planning.

I
The vil! age of Arosa, which belongs to the "freguesia" (parish) o f Cavez and the "concelho" (municipality) o f Cabeceiras de Basto, issituated nearthe bridge o ver lhe ri ver Tâmega, on the road 206, which connects Guimarães and Vila Pouca de Aguiar.

Fig. 4 -
Fig. 4 -Slope angles and site of the landslides in the Arosa brook basin.