THE HISTORY OF A TIDAL. LAGOON INLET AND ITS IMPROVEMENT (THE CASE OF AVEIRO. PORTUGAL)

The described case history is known since the X^ century, and on a scientific level since the end of XVHI^ . Origin and free evolution of the inlet up to 1800, as well as results obtained by artificial improvement attempts subsequently undertaken, are analysed, in order to investigate the main features of local physiography arxl the way it reacts to human interven tions intended to meet ever-increasing navigation requirements. The remarkable success of the projects undertaken, especially of that being executed, seems to legitimate the inference of some principles of general interest as long as tidal lagoon inletsimprovement is concerned. Stress is laid upon difference from principles valid in estuaries' amelioration.

The lagoon of Aveiro (fig.l) is a large body of water, 32 miles long and 1.8 miles wide, in the central west coast of Portugal, separated from the ocean by a sandy barrier beach, 600 to 8.^00 feet in width.There is an extensive area tributary to the lagoon, but the only important river draining to it is the Vouga, supplying from 40.000 cusecs during maximum floods to none in the dry season.Crest elevation of the dunes in the barrier beach reaches forty feet above datum (lowest possible low-water).Maxi mum range of spring tide in the open sea is twelve feet, with; semi-diurnal tides.The wetted surface of the lagoon is about 17500 acres and tidal flow through the inlet reaches 3 500 000 000 cu.ft. in a spring tide.
Formation of the lagoon is geologically very recent, so that doeu ments are available to follow its evolution since the beginning,' in the X*k century, when a sandy spit started to proceed southwards from Espinho, progressively isolating from the sea the ancient bay, wich extended to the mouth of the Vouga River (see figures 2 atad 3 )• Besides the old charts,many other lands of documents (judicial, economical, legal, historical) support this evidence, as the whole life of Aveiro and the rich and densely popula ted.country round the lagoon has always been, and still is, intimately con neeted with the condition of the vast body of water and its inlet* ~~ In the X*fc century, Ovar was an important sea port, ana the tides circulated in the estuary of the Vouga, whose bed was at a lower eleva^ tion than now.In no one of the documents of this tine is there aigr refe--" rence to the existence of a lagoon.Ovar, Aveiro, Vagos, etc. are all dee» cribed as being on the sea coast.In the XII^ century, the entrance to the recently formed lagoon still was located to the North of Torreira.Three centuries later, the bar reaches the position of Sao Jacinto, and in the Beginning of the XVTfcbcen tury the situation of present artificial inlet is attained by the migrating sandy point.Preliminary warnings of a deteriorating inlet can be traced in the literature by the third quarter of the century.This time is the golden age of Aveiro, and the surrounding coun try.More than 100 commercial ocean ships and 50 caravels for cod fishing in Newfoundland were registered,in the port.City's prosperity was at the peak.
The first serious trouble with the inlet happened in winter 1575>when 1he sandy spit was located to the south of Gafanhas and the channel directed southwestwards: a violent storm caused the bar to be obstructed almost totally.But this first trouble was temporary: three years later, Aveiro still was able to contribute with numerous ships to the Great Heet of King SebastiSo, without disturbing its commercial and fishing activities.
In 1534, the inlet was situated two miles to the south of its present location.By the end of the century it was extremely unstable: three to four times in the year the channel's beacons had to be changed.Notwithstanding, sailing directions in the middle of XVH*fr century mentioned Aveiro as a very safe port.In 164-3, the inlet was located near Va gueira, and since then situation rapidly grew worse.By 1685, with the bar near "Quinta do Ingles", navigation in the entrance channel was very difficult and no more than 14 ships were coming to the port in a year.By this time began the attempts to restore the ancient safe chan nel.The city of Aveiro called for the assistance of two hydraulic engineers from Holland, whose advice was not encouraging: natural inlet should be closed and a new one artificially opened accros the barrier beach near S.Jacinto, the project being hard to undertake, expensive and incertain in its results.The city was disappointed with such an advice, and nothing was done by the time.
In the middle of the XVIII th.century, the inlet reaches the sands of Mira, so completing the formation of the barrier beach and the lagoon, at the end 6f;an evolution that had started about seven hundred years earlier.
During this century, the mean number of ships coming to the port fell to 2.4 per year.It is interesting to remark that, while in many years no ship at all entered the lagoon, in some years a lot of them succeeded to come in: 10 ships in 1761, 36 ships in 1765, for instance.In spite of a systematic tendency to the obstruction of the inlet as it proceeds to ihe south, some alternatenesses occur, sometimes during consecutive years, in wich the situation improves to the point that people believes the troubles Even when it became evident that facts would no longer support this opinion, many people sustained that the correct way to restore port entrance conditions should be to improve, by fixing and deepening, the southern natural inlet, according to the location that resoL ted from the free evolution of littoral processes.This opinion prevailed until the beginning of XIX*^ century, when Reinaldo Oudinot and Luis Gomes de Carvalho choosed the location of the present artificial inlet.Later on, it still was in the base of the criticism exerted against -this project in spite of its successfulness.
Obstruction of the inlet was a tremendous disaster, not only to maritime activities but $» the whole life of the neighbouring country: Lagoon and open sea fisheries became impossible, fertilizers provided by lagoon weeds disappeared or deteriorated, the low lands were no longer drained, salt-making ceased, naval yards stopped, and above all, due to loss of drainage, the lagoon became an immense swamp and the sani tary condition of the country very poor (the city of Aveiro had no more than 5300 inhabitants in 1736, 2J400 in 1767, 3500 in 1797, when the number of deaths became double of that of births).

RESTORATION OF THE PfflET
It is of interest to mention some attempts made to res tore the inlet since the middle of TVTH^ century, when events became ve ry serious.In 175& the engineer in change tried to re-open the closed in let without getting any success.In the following year, captain Sousa Ribeiro took the same way and, with the help of a big flood, succeeded to widen and deepen an occasional channel, opened by a storm near Vagueira, and the enlarged channel maintained itself for eight years, but, after the unsuccessful efforts to fix the new inlet in its position, it began again to proceed southwards, reaching in 1771 the Mira sands (where the old natural bar had entirely closed in 1757).In 1777» British engineer Elsden was charged of the project and its execution; he tried,laaa&fOOSBsfully, to restore the inlet in the position it now occupies.In 1780, Italian master of Hydraulics Iseppi was contracted to solve the problem: he tried un successfully, during three years, to open and fix an inlet near Vagueira.In 1787, the inlet opened by Sousa Ribeiro totally closed, and it was or, dered to field-marshall Luis Vallere to elaborate a project for restoring the channel, wich he did'nt succeed to.In 1791, E.Cabral, master of Hydraulies, was charged to open and fix, at least, a small outlet able to assure the drainage, so urgently needed: but. the littoral sands did'nt allow even this apparently modest requirement.
Finally, in 1801, engineers R.Oudinot and Luis G. de Oarvalho were charged with the project and works to restore and fix the inlet.In the same year this remarkable project was presented and appro-IMPROVEMENT (THE CASE OF AVEIRO, PORTUGAL) ved, the works starting in the following months, placed under the exclu sive direction of Carvalho sihce 1803» According to the project, the artificial inlet should be located as close as possible to the center of masses of the lagoon wa ters, so that, on the whole lagoon the circulation of tides and drainage of fresh waters should occur in the best hydraulic conditions.Past history of # the inlet confirmed that this would be the only reasonable way to succeed in fixing it, it was stated.
Gutting the barrier beach and opening the new inlet was obtained by means of a long and strong dike, wich, firmly rooted in the vicinity of Gafanha, ran accross the lagoon to the barrier beach, compelling the waters of the northern and most important body of the la goon and the river flood to flow against and to pierce the littoral bar_ rier, at the same time leading to the inactivity and closure of any natural inlet located tc the south (see fig. 4 ).
Construction of the dike was extremely hard, especially the last streches, totally separating the northern and southern bodies of the lagoon.Care had to be taken against a sudden and incomplete cutting of the barrier beach.Works proceeded accross all kind of troubles, including the Drench invasions, untill the 3 rd.April l808,at 7 p.m.* In this historical moment, Mien a big flood raised the water le vel inside the lagoon about seven feet above the outside level, Luis Go mes de Carvalho ordered the demolition of the temporary fascine works that protected the beach in the location choosed for the new inlet and, with the point of his boot, opened the trail through wich the accumulated water rushed down to the sea, for ever restoring the lagoon's inlet.Drainage problems were definitively solved and first step given to restore navigation.
For the first time, human intervention overcame natural agents in modelling the inlet* Indeed, Carvalho's works gave satis faction to all requirements for about one century, as long as maintenan ce cares were provided and with the only exception of some very anomalous meteorological situations.

BEHAVIOUR OF THE KESTOKED INLET
It is of interest to consider the behaviour of the lagoon and its restored inlet during the 120 years elapsed till the foJL lowing improvement works.
During about 25 years maintenance cares were reasonably provided and inlet's condition very satisfactory (for instance, statistics give 186 sea-going ships entered in the port in 1835)• Maintenance was subsequently neglected, so that ten years later the jetty was seriously damaged, (figs 5 ar*l£).In 1838, a "big flood destroyed the dike near Gafanha and re -opened the ancient inlet in Vagueira.Erom 1847 to 1856, Carvalho's di ke and jetty were rebuild and at the same time the entrance channel pro_ gressively returned to the convenient direction and depths.
In 1859, Silverio Pereira da Silva, director-engineer of the port, proposed to continue Oarvalho's jetty to the low-water line in the beach and to build a new experimental jetty on the Northern side of the inlet's channel^ Ey this time, the inlet presented one of its very anomalous configurations, running northwards for about 3.000 feet, parallel to the coast-line, then turning to west and southwest in its outerextremity.Silverio's project as mentioned above restored in a few months the convenient westward channel {fig.7).
Another important measure taken by Silverio was the full reintegration Of the southern body in the lagoonar unity, wich he succeeded by promoting the closure of the Vagueira inlet in I863 and subsequently restoring a free and large communication between the southern and northern bodies, achieved in 1877• Automatic closure of the southern inlet was ingeniously obtained by allowing the flood tide to be admitted through it and raising difficulties to the circulation of the ebb tide, by means of a number of sluices provided in Carvalho's trans-lagoonar dam near Forte da Barra.
In 1873-1874* an anomalous inlet configuration was again r£ gistered, after a very long dry season.This time (see fig.B I, a strong sandy spit, 400 feet wide, emerging in high water, advanced from the north to 1.000 feet south of the Carvalho's jetty, parallel to the coast-line and at a distance of about 1.100 feet opposite to the inlet's mouth.Silverio faced the situation by continuing the jetty towards the sandy spit, wich was then cut by the first big flood. Silverio's^project for lagoon and inlet's general improvement is worth to mention for the large scale of the works contemplated (see fig. 2: ).During io years it was carried out by the author, but it still was very far from completion when resources exhausted and Silverio retired in 1886.Notwithstanding, this project remained the basis of all improvement conceptions untill 1927.
In 1908-1909, in similar meteorological conditions, an inlet' s configuration similar to the one of 1873-1874 occured.According to a committee of engineers appointed to report on the x^oblem in 1909, documents existing in the archisresn of the port showed that different configurations and extents of sandy spits advancing southwards, and also northwards, opposite to the inlet's mouth, occured from time to time since the restoring of the inlet in 1808.The 1808 inlet channel, became unable to meet ever -increasing navigation requirements since the middle of the XIX th.century.Thence, projects have been prepared for further improvement works, all of them being based on Silverio's 1374 plan until 1927.

FJ.g. 8. The Inlet
In April 1927, director-engineer of the port, JoSo von Hafe, presented his i>roject, based on an entirely new conception.Currents from the northern and southern bodies of the lagoon should be harmonized by a triangular system of dikes in the upstream section of the inlet channel, this one should be totally canalized through the barrier beach and the northern jetty would go out on the sea for about 1 000 feet, a substantial dredging should be made in the inlet channel.The author would prefer to build two outer jetties, brought to depths of 30 feet under datum, but for the moment resources were•not available and depths of only 10 feet would be enough for the needs contemplated.
In 1930, a committee of British consulting engineers from "Sir Alexander G-ibb & Partners" reported on the problem.They gave the preference to von Hafe's conception but they thought fitt to introduce some modifications in his project, namely by reducing by 800 feet the length of the northern jetty, by widening the inlet channel from 1 000 to 1 150 feet and modifying the interrelation between the widths of the entrance channels to the northern and southern bodies of the lagoon, as well as by dredging an outer navigation channel 660 feet wide and 1 650 feet long out in the sea.
The Council for Public Works accepted the advice of the British committee, but von Hafe protested against the modifications brought to his project, and especially against the suppression of the outer northern jetty, he thought to be of fundamental importance to the successfulness of the project, as long as a clear inlet ohannel was to be obtained.But the committee's ideas prevailed, unhappily, as future will show.
Works started in 1932 according to von Hafe's modified project and were concluded in 193^.They consisted in a jetty limiting the northern bank of the inlet channel, continued northeastwards by an interior dike, and in a triangular system of dikes for concording the currents from southern and northern bodies of the lagoon (see fig. 11 ), as well as in a substantial dredging in the channel for providing the 1 filling between the dikes.
Surveys made in 1934-35 and in 1949 (see figs. 11 and 12 ) show the results obtained.The entrance channel was sensibly improved, the outer bar was repelled seawards by 1 000 feet, emerging sandy spits opposite the inlet never reappeared, controlling depth on the outer "bar increased from 14 to 16 or 17 feet in mean high-water, tidal range at the entrance of the lagoon increased from 7 to 9 feet in spring tides.But some serious deficiencies were revealed, namely, the instability and subdivision of the outer channel and the strong transverse currents wich occured in it under wave action; moreover, littoral sands entered the inlet channel in big amounts, forming a beach against the inner face of the northern jetty and so narrowing the channel, as forese en by von Hafe when he sustained the integrity of his project, especial ly of the northern jetty's length.This -was very harmful, not only to the admission of tides into the lagoon, but also to navigation, due to the v strong currents in the narrow section of the channel, that prevented ships to cross it at the time required to be on the outer bar at local high water time.
Then, the new channel soon became unfit for the increasing navigation, and a new project had to be studied.

THE FIXING AND IEEEENING 03? THE. 0UT5R CHANNEL
The new project for the improvement of the entrance channel to the port of Aveiro was presented in 1937* It consists in the lengthe ning of the northern breakwater for 2300 feet an in the building of a new southern breakwater, 260G feet long, taking root 1300 feet to the south of the old Carvalho's jetty and slightly convergent with the former (see fig. 12 )• Purposes of the project weres a controlling depth of 24 feet in mean high-water over the outer bar (i.e., situating the'bar's crest at -4*00 m, under datum); a straight entrance channel, running westeast, free from crosscurrents; and an inlet practicable without restric tions other than depths available, wich necessitated the removal of the sandy beach from inside the inlet channel.
It was thought that the designed works, causing the removal of the canal's beach, would improve navigation through it and increase the tidal flow admitted to the lagoon.They would protect the inlet from invasion by sands in the shallow aone and direct the canal to Y/STC, wich is supposed to be the direction of the coastal ebb current.The ou ter bar would be formed in greater depths, thence more exposed to the littoral currents.The entrance channel would be protected, accross the shallow zone, from cross waves currents, highly dangerous to navigation and harmful to the maintenance of depths.canal disappeared, greatly improving the situation, -when the northern break water reached the length sustained by von Haf e. Depths on the outer bar are beyond those foreseen in the project, having increased as it was being repelled seawards.

FSATUBES OF LOCAL HffSIOaEAFnT
Prom the described evolution of the inlet and the measured results of the works undertaken it is possible to derive the broad lines of the local physiography, concerning the behaviour of the coastal beach, the inlet and the lagoon.
In fact, the collected data show: i. that the littoral drift is alternative in direction, accor_ ding to the meteorological season, but the southward drift clearly pre dominates in normal years; ii. in those years, the mean southwards balance of the foreshore littoral drift scarcely reaches 200.000 m3 annualyj iii. in anomalous meteorological epochs, massive accumulations of sands in the beaches and sea bottom adjacent to the inlet can occur, with results that went from total obstruction before the fixing of the inlet, to the formation of strong emerging sand-spits opposite the in let when this was simply fixed, and to the raising and widening of the outer bar after it has been canalized: so has been the case in 1839» 1859, 1874, 1909, 1949-1950; ±y.anomaly consists in the long-lasting prevalence of a given meteorological feature, wich strongly influences the bottom topography; v. the modelling action of the lagoon's ebb current on the outer bottom can be permanently traced only after the canalization of the inlet; vi. the greater the importance of the inlet, the bigger the volume of sand in the outer bar, canalization effect under this point of view being evaluated in half a million cubic meters; vii.changings in the volume of the accumulated sand due to meteorological, variations by far exceed this effect, the measured accumulation in 1949-1950 having attained three to four million cubic meters; viii.reinforcement of the inlet action always causes the qu IMPROVEMENT (THE CASE OF AVEIRO, PORTUGAL) ter "bar to be reppelled seawards and consequently deepened, in spite of the increased volume of sand in the bar platform (fig.22 )j ix. the flow through the inlet is hydraulic, that is, due to the difference of head between the ocean and lagoon water, wich accounts for the steep slope of the surface profile along the inlet canal,required for carrying a substantial amount of water in and out the lagoon in accordance with hydraulic laws; this is clearly shown by the instantaneous profiles and other characteristics of the tidal flow through the inlet canal (see figs.23 and 21+)} x. inside the lagoon, through its different channels and bodies, tidal flow may be either hydraulic or governed by tidal wave propagation, according to the geometrical characteristics of the chan nels and the bodies Df water they connect

RESULTS OF THE 10RKS iniDHRTAKM
Detailed analysis of the results of the performed works will add some knowledge to the foregoing remarks and help to infer so me rules valid in the treatment of inlets' problems.
Variation in the volume of sands in the platform of the outer bar, from IS50 feet north of the northern breakwater to the parallel of the southern breakwater's root, is shown in Table 1.When considering the figures therein, it is necessary to retain, besides the date of the different works, the meteorological characteristics of the surveyed years.Meteorological records show that year 1365 is situated in a very rainy period, hence in a period of southwest winds; year 1914 has been normal; in 1934 southwest winds were exceptionally prevailing; in the period 1943 to 1947 a* 1 exceptional drought was registered, with a strong predominance of northern winds, slightly at-" tenuated in the following year.Superposition of the longitudinal profiles of the entrance channel accross the outer bar, projected on a vertical plane parallel to the southern jetty (the jetty of Gomes de Carvalho), is particularly instructive.In fact, it reveals some permanent effects of the works carried out, such as the displacement of the outer bar seawards with the reinforcement of the lagoon's ebb current, no matter the volume of the sand deposit (see fig. 25 )• At the same time the bar plat for©* is widened and the bar crest is deepened, even when comparing situations like that of I865 (extreme erosion) with that of August 191)9 (extreme deposition, with an accretion of about 4*000.000cubic meters of sand on the bar) .
The same fact can be seen in a curve in wich abscissae re-  present the outer bar crest's., distance to a base-line accross the in let canal and the ordinates represent the envelop to its depths under datum.If this curve can be accepted-as it seems up to now -, it will be possible to predict the minimum depth to be expected in a gi ven position of the outer bar crest (see fig. 26 ).
The characteristics of the tide in different points of the lagoon were also affected by the inlet's improvement, as shown in Table 2.
The reaction of the inlet channel to the works performed is shown in Table 3 and Table 4* The very renarkable improvement in the hydraulic characteristics of the inlet .channel,especially as a result of the project now being executed, is of vital importance to the maintenance of depths in the whole inlet, including the inner and outer bars, as this measures its ability as regards the admission of tide.(figs.27 to 30).As far as the tidal prism admitted to the lagoon is concerned, calculations by Earl Brown or Marzolo's method show that before 1932 it amounted to 50.000.000cm. in a spring tide, increasing to 65.OOO.OOO cm.rafter canalization works and to about 150.000.000cm.nowadays.Maximum tidal capacity of the lagoon is estimated to about 180.000.000.cm.Therefore, it seems that the inlet canal has been correctly dimensioned.
Evolution of the inner bar is also an interesting item when results of the works' undertaken are considered.
Comparison of the longitudinal profiles of the inner chan nel in 1865, 1914and 1934 and after (see fig. Jl ) and after (see fig. Jl ) shows that canalisation of the inlet caused the deepening of the downstream section, the occurrence of great depths near the extremity of the current-guiding dikes, and the westward displacement and a considerable erosion of the inner bar.
As for the project being executed, results are shown in fig 32 concerning variations in the total amount of sand in the inner bar, and in Table 5 and fig.33 concerning the hydraulic characteristics of the inner channel up to S.Jacinto: in spite of the oscil lation due to meteorological features, a remarkable tendency to the improvement of the channel, especially in the downstream canalized strech, can be traced.Hence, the amelioration of the inlet always occasioned the inner bar to "be deepened and reduced, and the neighbouring upstream channel to be improved on a considerable distance, without any dredgirg being undertaken.This clearly implies that littoral sands are by-passing the inlet, or preferably, that they are not retained by the inlet in their way downcoast: the volume of sand expelled by the ebb tide exceeds that brought in by the flood.
The same conclusion is also suggested by the analysis of the bottom contours in some surveys, namely that of July, 1954*

CONCLUSIONS
The treatment of estuaries has long been discussed in BngjL neering literature and a firm and well developed technics is available on the subject.Situation is radically different when lagoon inlets are considered and, at least to our knowledge, no systematic analysis has been published since the very.remarkablearticle by Earl Brown on "Inlets on Sandy coasts" (in 1928) and that by Francesco Marzolo on "Bacini a marea e foci lagunari" (in 1935).This is possibly why, from time to time, instances are found were inlets'problems are dicussed or dealt with according to the principles valid for estuaries' treatment, with the result that, in spite of tough attempts and heavy expenses, success is not obtained, as it always happen when nature is forgotten.
In fact, the improvement of an inlet on a sandy coast cannot depend either on the upstream current or on the propagation of the tidal wave,' wich are essential factors in the improvement of estuaries.The former must essentially depend on the increase in the relation of the tidal volume circulated through the inlet to the volume of sands carried by the littoral drift, wich, in turn, is not necessarily a requirement in estuaries' amelioration.
In order to assure the increase of the tidal flow through an inlet, improvement of the hydraulic characteristics of the inlet cha nnel is needed.For this purpose, from what has been reported about the case of Aveiro it seems advisable: i. to fix the inlet as close as possible to the center of masses of the waters in the lagoon; ii. to canalize the inlet channel accross the barrier beach, and to harmonize the currents from the different lagoon bodies, guiding them to the inlet channel in the best way to avoid the ocurren ce of eddies and of any losses of energy; iii. to situate the outer, "bar as far out in the sea as required for obtaining the depths wanted, wich must be done by means of jetties, these being in many cases, preferably, slightly convergent jetties.
In every case, natural conditions of the specific problem nave to be carefully investigated and taken in account, no general rules being able to suppress the need for such a peculiar investigation.
Success obviously calls for littoral drift transposing the inlet,that is that the sand brought into it by tae flood tide be expelled by the ebb, if necessary with the help of dredging.Other wise, the inner bar would shoal very fast, the inlet would deteriora te, erosion downcoast would proceed to a large scale favouring the' opening of a new inlet by some storm, wich in some instances has been enough to cause the closure of the preceding one and in any case would be very harmful to its maintenance.
Les essais en vue de rouvrir ot fixer I 1 embouchure lagunaire ont tbus failli jusgu ! a ce q.ue, en 1808, on ait reussi a couper le cordon littoral et a localiser definitivement 1'embouchure a son emplacement actuel.Les problemes sanitaires et de drainage etaient r£solus pour toujours, et c»etait le premier pas vers le retablissement de la navigation.

Fig
Fig. 1* The lagoon of Aveiro.
and the port to return, by itself to the ancient prosperi

Fig
Fig. 6.Configuration of the inlet in 1843.

Fig. 9 .
Fig. 9. General Silverio's project for the improvement of the lagoon and its inlet* Fig* 15.Hydrographio surrey of the inlet in June 1951.

Fig. 22 .Fig. 25 .
Fig. 22. Variation in the volume of the sand deposits over the outer bar (curve 1) and in the controlling depths over the same (curve 2), from 1865 to 1954.wafts iistBufamos ffiree o rrsia os to:m sui f a rown

Fig. 26 *CPflfilFig. 27 .
Fig. 26* Controlling depths on the outer bar (ordinates) as a function of its distance to a base-line (abscissae).Envelope-ourve of the minimum depths corresponding to each position of the bar-crest* Fig* S0»Areas of oross sections.

Table 1
CHANGES IN THE VOLUME OF SAND ON THE OUTER BAR OF AVSIRO