Environmental Implications in Porto Alegre City Metropolitan Area - RS: a geographical study emphasing the Urban Geomorphology

Author: Profa. Dra. Nina Simone V. Moura Fujimoto

Academic filiation: Department of Geography–Institute of Geosciences – Universidade Federal University do Rio Grande do Sul

Elaboration: November 2001

Phone Number: (51)33166343

e-mail: nina.fujimoto@ufrgs.br

 

ABSTRACT

This work consists of the study on the urban environment in the sub-basin of Dilúvio Stream's Hydrographic Basin located in Porto Alegre City Metropolitan Region, state of Rio Grande do Sul, Brazil. It seeks to learn the effects and responses of the environment to the urbanization process in such area. For this, it seeks the comprehensive knowledge of nature and society dynamics and how they have articulated. The environment analysis divided into the following stages: analysis on the region morphostructural characteristics; geomorphologic and superficial covering material characterization; climatological and hydrological conditioners analysis; superficial water quality analysis; characterization of the historical urbanization process; analysis on the partner-economic features from the study on the vegetable covering and soil uses evolution and effect of the juridical aspects on the urban space arrangement. The result of the research carried out generated environmental data revealing the changes on the environment caused by the urbanization process on the mentioned area.

Keywords: environmental dates analysis, urbanization, urban geomorphology, environmental change, metropolitan region

Environmental implications in the Metropolitan Area of Porto Alegre - RS: a geographical study emphasizing Urban Geomorphology

Nina Simone V. Moura Fujimoto

1. Introduction

The urbanization process that characterizes the XX century is marked by the appearance of great urban gatherings. The environmental problems that come from in big cities show the predatory forms of man's relationships to the nature, becoming more and more serious in densely busy areas, as in the metropolitan areas.

In the outlying countries, urban growth did not accompany the economical growth, favoring the appearance of cities with great deficiency of equipment, employment, and services and an accentuated environmental degradation.

The characteristic fast urban growth of those big cities brought several problems as: not having appropriate collective transport, inefficient basic sanitation, slum process, unemployment, climatic alterations, atmospheric pollution, deforestation, floods and other social-environmental problems common to the Brazilian cities. However, the current problems of the accelerated urban growth did not reach everyone with the same intensity, because what determines that differentiation is the socioeconomic position that each individual occupies in the society.

The urban environmental problems have been a very discussed theme, mainly because it is in the cities that it happens the largest population concentration, which has been causing environmental degradation, committing the quality of that population life.

In the cities construction it is introduced a lot of new materials and equipment that accompany its population growth and its economical activities, articulate to the urban space. Among the main alterations introduced in the built space, the most common are: vegetable covering retreat, new relief ways construction, construction increase, superficial drainage increment, surface corrugation, pondered release and particles and gases accumulation in the atmosphere and artificial energy production, modifying natural elements, as the climate, the air, the vegetation, the relief and the water.

The present study shows a urban environmental analysis that seeks the learning of atmosphere effects and answers to the urbanization process in a hydrographic sub-basin belonging to Arroio Dilúvio hydrographic basin located in the Metropolitan Area of Porto Alegre, State of Rio Grande do Sul, precisely between the Porto Alegre and Viamão municipal districts. The area, which belongs to the municipal district of Porto Alegre, inside of the hydrographic basin in study refers basically to the Campus do Vale at Federal University of Rio Grande do Sul. Most of the hydrographic basin is placed in the Viamão municipal district and, that way, very related to the municipal district formation process that interferes in the Metropolitan Area of Porto Alegre formation context (Illustration 01).

The main objective of this work is making a research focusing the environmental analysis, purposing of analyze, in the referred hydrographic basin, the atmosphere alterations come from the urbanization process. For that it searches, in an integrated way, to know the nature and society dynamics and its articulations.

The environmental analysis in a wider context includes many stages, that begins by area report the knowledge, its development, and the nature and society dynamics. To know the nature and society attributes and how it has articulated is a fundamental part to the analysis. The developed approach is based in an integrated analysis proposal where the environment elements and processes are interrelated and dependent on each other. For that it was included in this analysis a historical approach on the relief forms, use of land and geomorphologic processes, as well as a socioeconomic approach to characterize the geomorphologic processes.

 

2. The Metropolitan Area of Porto Alegre and the Social-environmental Problems

In 1973, the the Metropolitan Area of Porto Alegre was defined. That fact reflected the municipal district’s new situation, whose urban planning started counting an entire area where the center is the capital. Porto Alegre surpassed the physical and administrative limits and its interaction to the neighboring municipal districts revealed the need of a united initiative.

According to ALMEIDA (1993), the population concentration reflects the economical concentration, reinforcing the regional unbalances characteristic State occupation history. That process comes from the modern industrialization that settles in the Country, starting from a monopolist economy, having the foreign capital as main lever, as well as the State. The new industries demanded large areas and even promoted more space concentration, because they needed to be near urban centers, capable to supply manpower, easy transportation, infrastructure proximity, and complementary services.

After 1970, the Urban Development Master Plan of Porto Alegre was enlarged to assist the fast growth of the city, receiving viaducts, tunnels, and bus terminals. Following the new planning, the capital was zoned, defining functions. Some areas acquired basically residential, commercial, industrial, leisure, services and administrative characteristics (LOSS & KNIJNIK, 1994).


 

 

 

 

To insert illustration 1

 

 

 

 


The opening of roads (highways, great avenues, freeway, among others) was important in certain areas of the Porto Alegre city development, because it made possible to create strong functional entails to many municipal districts, like Cachoeirinha, Gravataí, Alvorada and Viamão, favoring those municipal districts growth. In this situation it became evident the urban planning role in the expansion of the city. However planning dashed in the relationship between the economical power of the real estate exploration and the public power political connivance. The real estate exploration has been administering the urban mesh expansion, maintaining empty areas waiting for the most opportune moment to its commercialization.

As the metropolitan expansion, linked to the industrial and residential growth, evidenced the growth of needy nuclei. Close to the city original nucleus and in the main avenues margins ponder all the infrastructure services, population density and trade and services activities. The areas of popular housing and the sub-habitation bags were pushed to the periphery of those axes.

According to ALMEIDA (1993), in the capitalist city urban space, especially the Brazilian cities, we can see a differentiated consumption of habitation space. That reflects the conditions of different social segments to pay their value to the real estate capital detainment. This way we have a differentiated occupation of the space, with areas of great interns homogeneity and strong disparity of space quality. The larger purchasing power social classes can choose the best place for their home; however to the lower income classes remain the cheapest lands or homes out of the market of lands, provoking the invasions.

In 1970, the Metropolitan Area showed a total population of 1.531.254 inhabitants, representing 23% of the State total population. In 1985 it had already reached 31,5%. In 1970, Porto Alegre had 61,75% of the urban population, configuring a strong concentration in the Capital.

The following decade, the highest annual demographic rates discharges are verified in Gravataí, Cachoeirinha, and Alvorada municipal districts showing between 7 and 9%, followed by Viamão and Guaíba with 5 to 7%, configuring an accelerated growth of immediate periphery.

In the 1980-1991 period, Porto Alegre population increased 12,1%, with a rate of 1,055% a year; and the Metropolitan Area increased 32,4% with an annual rate of 2,583%. Rio Grande do Sul increased 17,5%, with rate of 1,4% a year (Table 1). The data of the 1991Census show that the immigration to Porto Alegre decreased, moving for the Metropolitan Area.

 

Table 1: Porto Alegre Metropolitan Area Municipal districts Population Annual Growth Rates (%)

Alvorada

4,090

Nova Hartz

9,314

Cachoeirinha

3,079

Novo Hamburgo

3,788

Campo Bom

3,198

Parobé

10,884

Canoas

2,169

Porto Alegre

1,055

Dois Irmãos

4,977

São Leopoldo

4,952

Eldorado do Sul

5,098

Sapiranga

5,407

Estância Velha

6,414

Sapucaia do Sul

2,579

Esteio

2,997

Triunfo

2,370

Glorinha

0,990

Viamão

3,370

Gravataí

5,230

 

 

Guaíba

5,778

RMPA

2,583

Ivoti

5,693

Rio Grande do Sul

1,478

Source: IBGE (1991) apud LOSS & KNIJNIK (1994).

 

That regional urbanization process reveals a territory marked by the space segregation of their habitation areas. The immediate periphery areas of Porto Alegre contain, predominantly, the lower income social segments of the area. The periphery expansion takes place with the systematic contribution of the public politics in the habitation section. The great number of habitation offer units that did not find a buyer in the market, allied to a growing dispute results in the invasions appearance predominantly in the Porto Alegre periphery (PANIZZI, 1993). In Porto Alegre according to Table 2 the growth of the sub-habitation areas can be observed during the 1951 up to 1989period.

It is observed that happened the largest rates of the resident population annual medium growth in sub-habitation in the 1951 to 1965 period and along the 80’s.

According to BONIN (1993), in the first period, the population as a whole possessed a high growth rate, and the main factor of the sub habitation growth was the migration field-city due to the change of Brazilian agricultural model. In the 80’s, the field-city migration factor was overcome by reduction of work opportunities that forced the population that used to live in rented houses and that was placed aside the economical production to seek an alternative in the city’s irregular occupation.

That verification is corroborated by the monthly family analysis revenue accomplished by the Metropolitan and Regional Planning Foundation - METROPLAN, in 1980 and 1990, because of what we can notice that there was a considerable increase of families with smaller monthly revenue than two minimum wages (it overcome from 20% to 27%), with special increment of those that received less than a minimum wage (of 6% for 10%).

 

Table 2: The Sub habitation Growth in Porto Alegre

Year

Villas Number

 

Number

Subhabit.

Sub habitation

%

Population

Subhabit.

Population

Porto Alegre

1951

41

3.965

3,9

16.303

394.151

Rates *

2,2

9,2

-

10,4

3,9

1965

56

13.588

8,1

65.595

778.878

Rates *

10,4

5,0

-

6,1

2,5

1972/73

124

20.152

9,5

105.833

950.142

Rates *

2,0

8,2

-

6,2

2,1

1980/81

145

38.093

15,2

171.419

1.125.951

Rates *

3,4

9,6

-

9,6

2,3

1987

183

72.580

24,7

326.608

1.323.315

Rates *

7,6

10,2

-

8,0

1,4

1989

212

88.184

28,0

380.956

1.360.800

Source: SPM–Porto Alegre apud BONIN (1993)

Rates *: Annual Growth Average Rates (%)

 

The situation of the Metropolitan Area, according to researches accomplished by METROPLAN from 1981 to 1991 revealed that the margin population, excluding Porto Alegre municipal district, increased a rate 9,7% a year, in other words, the population estimated in the slums, except for the existent ones in Porto Alegre, corresponded to 8,6% of the total population; in 1991, that value passed to 13,5%. According to the same study, there are 722 slums in the Metropolitan Area, being 217 in Porto Alegre and 505 in other municipal districts. The total population of those nuclei would correspond to 387.778 inhabitants in the Porto Alegre slums in 1990 and 237.451 inhabitants in other municipal districts in 1991.

In the 1980’s and beginning 1990’s, Porto Alegre has only been seeing its social problems getting worse, growing in the same rhythm that the city was becoming an outlying country metropolis where only a minority make use of the urbanization benefits.

Face to this fast population growth, in its majority without satisfactory home conditions, house became one of the most important social problems in the Metropolitan Area. In the almost absolute non existence of a use of soil and occupation policy, the urban expansion will happen, largely, in inappropriate areas or in an inadequate way, having as consequences countless problems to the physical middle, to the own seated population and the public responsible powers for the infrastructure services in those areas.

In a work accomplished by METROPLAN (ORLANDO FILHO & GUIGNO, 1994) objecting to diagnosing the most important problems related to the Porto Alegre Metropolitan Area physical environment, it was verified that the great majority of problems related to the physical environment is due to the form of how its use has happened, for both inadequate areas occupation and carelessness in certain areas that request special attention their occupation. Among the areas fit us to point out those with discharges steepness or drainage headboards, bottoms of valleys, low land bordering water, slopes highly susceptible to mass movements, among others. The direct intervention of these areas not just provokes environmental problems, but also the generation of residues and of effluents and its consequent need of final disposition.

Among the problems diagnosed by METROPLAN, the ones with larger space inclusion and intensity, erosion, mass movements, flood, obstruction by sediments, hydro pollution resources and landscape degradation due to the mining were characterized.

 

3. Geologic and Geomorphologic characterization

In regional terms, the study area locates at the current Uruguayan Plateau South-Rio-Grandense, represented by hills that form a prolonged strip of NE-SW direction predominantly. Those hills are formed by granite rocks generated during apprenticeships of evolution of a orogenic belt, known as Cinturão Dom Feliciano (FRAGOSO-CESAR, WERNICK and SOLIANI Jr., 1982 a, b), that reflects the Brasiliano Cycle activity in south Brazil. The granite Viamão and the granite Santana are the granite rocks types, generated during the belt formation that sustain the forms of relief from the study area, according to PHILIP (1998).

The Viamão Granite is represented in the study area by a relief in hills patterns with carved valleys. This granite was formed after the second continental collision, during the final evolution of the Cinturão Dom Feliciano, which was marked by the development of great flaws elapse about 650 million years ago. The Viamão Granite presents many characteristics, above all textural and structural, which conditioned the relief formation and characterize this granite as a rock easily percolation of waters and, consequently, favorable to the intemperism processes intensification and fluvial chiseling.

The Santana Granite was formed after the development of great elapse flaws that provided Viamão's Granite formation. That granite was formed when they started to prevail the extensions efforts, originating new flaws and reactivating the old ones and, during that reactivation, they have lodged along the newer granites magmas sutures, as the Santana Granite, about 550 million years ago. That granite body constitutes the pattern in hills in the study area and it possesses a prolonged form in NE-SW direction, being controlled by a zone of breaking ductile of NE-SW direction. Being this newer granite than the Viamão Granite have suffered less all of the tectonic tensions that characterize the orogenic belt formation.

The Uruguayan Plateau South-Rio-Grandense stands out by the tectonic characters and litologic from its formation and for its different dissection degrees. It shows a remarkably morphologic diversity in the hills having varied dimensions. In the study area, the Plateau is represented by Morphologic Units or in Similar Ways Patterns, which are:

A). Pattern of Forms in Hills with Convex Tops: this pattern is formed predominantly by hills of narrow convex tops and slopes with straight segments with medium steepness between 30-40% and concave elements with medium steepness among 20-30%. The altitudes are going on average from 80-100m to the maximum point quoted in 293m. The convex tops present prolongation in the NE-SW way, associated to the geological structural conditioning, with medium steepness predominantly in the 2-5% class. The Pattern in Hills is constituted by sediments coming from the granites, in general altered with small covering of sandy material, transitioning for a typically mixture of clay and sand material. The granulometric analyses indicated the existence of a material constituted predominantly by silt and clay followed by sand fractions.

In the Pattern of Forms in Hills, mainly in the discharges and averages slopes sections, as well as in the top of the hill, we can see scars that show the exposed rock and slope ruptures for cut in the rock. In the sections of low slope with inferior to 100m altitudes we find the plane surfaces essentially created by the urban occupation through the residential use. Those plane surfaces are created by re-managing of superficial materials, limited or not by cut steps, for slope ruptures and embankment ramps. The re-manage material is transported later for other slope units reaching the bottom of valley.

The cut steps made in the units of slopes of hills for the plane surfaces creation have a large dimension. Besides the alterations in the slopes geometry for creating plane surfaces, we can see this areas the impermeabilization current mainly of the superficial material for the by land transportation transportations and homes construction compaction. As a result of the occupation, it happens the small erosive furrows installation the by land transportation after a rainy event, mainly in roads that accompany the incline of the slope (Picture 01).

 

To insert picture 1

 

Picture 1: Slopes in Straight Segments to the bottom and the urban occupation on Slopes with Concave Elements in the Forms of Hills relief. It is observed erosive furrows in the by land transportation.

 

In the Forms in Hills there are physical-chemical intemperism through the sphere flaying reworking the lumps and the exposed rocks. In that compartment it happens a tendency to smallest infiltration in the straight segments, prevailing the diffuse superficial drainage in the discharges and averages slopes, concentrating in the low slope. In the concave elements the tendency is the prevalence of concentrated superficial and sub-superficial drainage associated to the infiltration.

The man alterations on the relief forms provide, in general, a decrease of the diffuse superficial drainage, of sub-superficial drainage and infiltration and, superficial drainage concentrated intensification. The re-manage material for the man alterations is transported, in an intense way, to other units of slopes to reach the bottom of the valley.

2). Pattern of Forms in Hills of Convex Tops and Hills of Plane and Wide Tops: that pattern is formed by a group of hills with convex, plane and wide tops showing very closed valley (in V) with predominant medium altitudes among 50-80m and medium steepness in the 10-20% and 20-30% classes. The units of slopes represented by the hills demonstrate a variety in geometric ways, of altitudes and steepness. The identified units are: straight, concave and convex. Sediments coming from the granites, very altered, represented typically by a mixture of clay and sand material constitutes it. The granulometric analyses indicated the existence of a silt and clay predominantly constituted material followed by the fractions of sand (Picture 2).

 

To insert picture 2

 

Picture 2: Occupation made on Slopes with Concave Elements in amphitheater form in the Forms of Hills Relief

 

In practically the whole Pattern of Forms in Hills we can find plane surfaces created by the urban occupation through homes and transport system. The plane surfaces are limited or not by cut steps, for slope ruptures and for embankments ramps. Besides the alterations in the slopes geometry because of the plane surfaces creation, these areas impermeability is currently observed mainly in superficial material to construction of by land transportation transportations and homes compaction. It happens the small erosive furrows installation in the by land transportation after rainy events, mainly when the by land transportation accompanies the slope of the slope.

In the plane tops the cut steps, the slope ruptures and the embankment ramps possess have small dimensions when compared to the convex tops and other slopes units, because the relief forms in plane tops favor the creation of plane surfaces for the homes and by land transportation installation. However, convex element geometry requests a larger material retreat for the plane surface construction. This way, in slopes with convex elements we find, in general, cut steps and slope ruptures with larger dimensions than found them in the other slope units.

The changes in the slopes geometry use to provoke alterations in the superficial materials disposition, exposing them to the rain impacts. The moved again material is later transported to other slope units reaching the bottom of the valley.

The bottoms of valley can be open or closed (in V) and they occupy the marginal areas to the streams and some nascent, with little lateral and very longitudinally development. The superficial material is predominantly sandy. There is an occurrence of rude material originating from the slopes, whose characteristics identify them as tecnogenic material. They are rude sediments with gravels, glasses, plastics, woven, and erasers presence, among others. The man occupation in the bottoms of valley modifies the geometry of the land starting from the embankments and small cuts. In some spaces it can be observed that homes are on the tecnogenic deposits.

The cuts, the embankments, the waters canalization that come out in urban atmospheres redirect the hydra flows existent and they create new drainage patterns. Those alterations provide, on one side, a decrease of the diffuse superficial drainage, of the sub-superficial drainage and of infiltration and, for other, an intensification of the superficial drainage, because they don't allow the infiltration of water in the soil and they create true pluvial beds in the by land transportation transportations during rainy events.

3). Pattern of Forms in Plane Areas: this pattern gets a series of plane areas perfectly individualized and disposed, predominantly, along the courses of water with predominant medium altitudes among 55m to 65m with very low steepness. The outstanding configuration is to present plane areas in form of alveoli disposed loud in sections, medium and low of courses of water. The alveoli are characterized for presenting prolonged forms, and secondarily possess semi-circular form. It is constituted predominantly by sand-siltic-loamy sediments came from the processes influenced by mass and fluvial movements in its formation. In this Pattern, it can be distinguished four Relief Forms: in Plane Alveoli Colluviums-Aluvionares Forms, in Alveoli with Fluvial Terraces Forms, it Forms in Alveolus of Lacustrine Plain and in Alveoli with Plains in Amphitheaters of Nascent Form.

The alveoli plane is predominantly determinate in fluvial confluence, in general around 50 to 60m altitudes. Those plane alveoli get an enlarged section of the valley, filled up with transported sediments, predominantly by the mass movements and the fluvial action that used to be deposited at those places thanks to the structural conditioners existence that provided the strangle to this water courses to side that the river flows.

The Plane Alveolus is sections that possess a tendency to water infiltration and soil density. However the occupation of these forms happens through small cuts and/or embankments in the original morphology, of the impermeability due to the compaction, of paving surfaces construction, increasing the superficial drainage, because impermeability does not allow the water infiltration to the soil.

Usually the watercourses margins that carve the plane alveoli don’t have vegetable covering and show alterations in its original morphology because of the home building interventions. In that case, erosive processes appear by covering in the courses of water margins because of streams deep chiseling, of sediments, essentially sandy, and of original morphology alterations (Picture 3). Close or not to the erosion events by covering in the streams margins, it is also possible to observe tecnogenic material deposition banks in the lowest areas in the bottom valley and the homes installation on the tecnogenic deposits.

 

To insert picture 3

 

Picture 3: Plane Areas Relief Form. The erosive process is noticed by covering in the stream margins, being contained by tires.

 

Alveoli forms with Fluvial Terraces can be found in the outlets of two streams and they are adjacent to the Tecnogenic Fluvial Plains Forms. As well as the Plane Alveoli Forms, this compartment locates along an enlarged section of the valley, filled out by transported sediments predominantly by the mass movements and the fluvial action. However sediments use to overflow until reaching the adjacent fluvial plain that is in inferior altimeter position, being characterized as a fluvial terrace, generally in an inferior 50m altitude.

As well as the Colluviums-Aluvionares Plane Alveoli, the Alveoli with Fluvial Terraces possess tendency to the water infiltration and soil density; as well as erosive processes happen by covering in some courses of water spaces because of the deep chiseling of streams, of sediments essentially sandy and of original morphology alterations thanks to the urban occupation. Many times we can find the tectogenic material deposition in the lowest areas close to the bottom of the valley. These Forms occupation happen through small cuts and/or embankments in the original morphology, of impermeability due to compaction, of surfaces paving and construction increasing the superficial drainage, because impermeability doesn't allow the water infiltration to the soil.

The Form in Alveolus of Lacustrine Plain is configured in a glide large area with drainage difficulties and among 55 to 60m altitudes. It is an enclosed plane area for slopes varied morphology and up to 80m altitudes, whose steepness can arrive to 30%. According to their morphologic characteristics and by the covering material analysis indicate a deposition typically of lake of bottom. In this relief form, the man interventions are related to the embankments implantation and the canalization constructions and/or ditches, because the area is associated to flood periods and it locates inside of the maximum water level, where the water sheet is practically appearing on the surface. The occupation process alters the level of the land, elevating above the natural floods level, and it modifies the hydra flow through the canalization construction and/or ditches to the accumulated waters drainage.

The Forms in Alveoli with Plains in Amphitheater of East many times are in the beginning of some streams surround lakes nascent. That relief form is configured at amphitheaters surrounded predominantly by slopes with concave elements. Those relief forms possess vegetable covering of ciliary forest and little gram when they happen around the lakes nascent. When it happens in the nascent without a lake occurrence, the urban occupation happens only partially, in the adjacent areas to the slopes with concave elements that surround this compartment. Remainder ciliary forest associate to arboreal cultivations recently introduced covers the remaining of the plane area.

4). Pattern in Forms of Plains: that pattern extends along the stream Mother of water with inferior altitudes to 50m and it forms an extensive area it glides represented by a flood basin along referred him stream, where the barrage Mother Of water was built (Illustration 01). That pattern is characterized originally by an existent morphologic configuration in the area until the construction of the barrage Mother of water, inaugurated in 1962.

After the construction of the referred barrage, two new relief forms, different in your formation, they start to compose the whole pattern practically in plain, they are them: Fluvio-lacustrine Plain Tecnogenic Forms and Fluvial Plain Tecnogenic Forms.

The Pattern in Fluvial Plains Form can be characterized starting from the Topographical Rising and the Survey Profiles accomplished by the National Department of Works and Sanitation (DNOS) for the referred barrage construction in 1957. They also constitute plain original characteristics registrations the old pictures and residents and Federal University of Rio Grande do Sul employees depositions.

The 1957Topographical Rising represents the flood basin that was filled out by the barrage lake. In that work, the Mãe D’agua stream was, practically the whole represented extension, vegetable covering characterized by ciliary forest, confirmed by the Professor José Leite de Souza's deposition: “...very small ciliary vegetation that accompanies the stream two margins. That vegetation was destroyed when the barrage began, it was all cut to avoid the decomposition and to bring inconveniences for the barrage waters rigidity...”

The occupation evolution related to the population increase, predominantly in the 70 and 80 decades, and the several intervention modalities in the hydrographic basin provided a transformation in its original characteristics. This intervention had as consequences the decrease of the natural way quality, the relief modification, morphodynamic new processes appearances and tecnogenics deposits creation.

In the Forms of Plains Pattern the man action alters the dynamics geomorphologic starting from the barrage construction and, later, for the fluvial dynamics intensification associated to the artificial lake lacustrine deposition. The fluvial and lacustrine processes are intensified and they overcome a lot the natural processes. This way two relief forms by man action created and induced come out. They are: Fluvial-lacustrine Tecnogenic Plains Forms and Fluvial Plains Tecnogenic Forms in.

The Lacustrine-lacustrine Plains Tecnogenics occupy a wide glide area around the Mãe D’agua Barrage lake, formed by an intense soil superpose process. The sediments deposition results in a first moment, from the in lagoon form sedimentation, that happens with the drainage speed decrease, and, more recently, from the slopes coming sediments transported by the fluvial processes.

The sediments deposited by in lagoon forms deposition processes are characterized mainly by silts and clays and in smaller part by sands. Those sediments are the about 50cm of Mãe D’agua barrage lake depth. The sediments that recover the in lagoon forms deposits, from its surface to about 0,50m depth are associated to the fluvial processes with slopes material contribution. Those materials are usually related to the most critical rainy episodes, understanding the thickest part materials usually associated or coming from man activities, as gravels and small pebbles, bricks, plastics, papers and glasses, in other words, recognized as tecnogenic deposit. The urban debris found with the land material in depth can be observed covering again all the Fluvio-lacustrine Tecnogenic Plains practically (Picture 4).

 

 

 

To insert picture 4

 

Picture 4: Fluvio-lacustrine Tecnogenic Plain Forms relief formed by tecnogenics deposits. Much of urban debris is noticed on its surface.

 

On this plain a tecnogenic dejection cone is observed. That cone is one close to one of the streams when it comes to an end in the plain compartment, evidencing the great contribution of material originating from slopes.

Man interventions in this compartment are related to the embankments implantation and to the canalization and/or ditches constructions, because the area is associated to flood periods and it locates inside of the water maximum level, where the water sheet is almost coming out. The occupation process alters the level of land, elevating above the natural level of floods and that modifies the hydra flow through the canalization and/or ditches construction for the accumulated waters drainage.

The Tecnogenic Fluvial Plains mean to the plains that extend along the stream where the Mãe D’agua barrage has been built. That plain, starting from the tecnogenic material deposition, suffers an elevation in the topographical level. Deposits made by urban debris in almost all its extension have covered the sediments loamy silt and sandy silt that covered again that compartment.

Those characteristics indicate the fluvial processes performance with slopes material come from the watershed, because of intense rains. That way, the relief formation in that compartment happens in the flood periods, when new slopes materials transported are deposited associated to the fluvial processes.

In the Tecnogenic Fluvial Plains also happens the tecnogenic dejection cone formation. That cone is located close to one of the streams when it comes to an end in the plain compartment, providing an increase in the topographical level. That cone indicates the from slopes material great contribution for fluvial processes performance (Picture 5). Because of dejection cones formation, they appear in its adjacencies, lowered areas, permanently flooded.

 

To insert picture 5

 

Picture 5: Tecnogenic Dejection Cone in Tecnogenic Fluvial Plains Forms Relief. The sandy sediments are noticed with much of the material thickest part with pebbles, glasses, and plastics, among others.

 

The man interventions are related to the embankments implantation and to the canalization constructions and or fence, because the area is inside of the maximum water level. The occupation process alters the level of land, elevating it above the natural floods level, and it modifies the hydra flow through the canalization and/or ditches construction.

3.1. Current Processes Forms or Man Morphology

In the study area several man interventions have been observed on the relief forms. These interventions altered the original morphology, they have destroyed some of its basic characteristics and they generated new morphodynamic processes.

The first intervention level is in the covering vegetable sphere and use of land, through the covering vegetable remove. The second intervention level happens through the new morphology creation, linked to the mining and the urban way. In this phase great cuts and/or embankments are elaborated in the land for the by land transportation transportations system installation and constructions subsequent installation. During the constructions installation, the superficial materials are modified through a new distribution, a new structuring of deposits and a modification in the attachés resistance.

In agreement to PELOGGIA (1998), the human action on the nature has consequences in three levels: in the relief modification, in the geomorphologic dynamics alteration and in the correlative deposits creation comparable to the quaternary ones (the tecnogenic deposits) due to a group of actions denominated tecnogenesis.

The modifications in the relief provide the tecnogenics relief forms appearance current from created or induced by the human processes activity corresponding essentially to the sixth taxon, according to the classification proposed by ROSS (1992). That taxon includes the smaller forms produced by the processes current morphogenetic and almost always induced by the human action as the erosive furrows, the tecnogenic dejection cones and the covering scars; or the relief small forms that grow for man interference along the slopes as the cuts and the embankments. However it is possible to verify the human action consequences in the fourth taxon, or, in the formation in relief forms individualized inside a morphologic unit or pattern in a similar way. This is the case in the Lacustrine-lacustrine Tecnogenic Plains Form and in the Fluvial Tecnogenic Plains Form inserted in the Plains Way Pattern Form.

The relief modification promotes the creation, induction, intensification or behavior modification in geomorphologic processes (PELOGGIA, 1998). In agreement with the typology and the alteration apprenticeship, it can be described some man activities that generate new morphodynamic behavior patterns:

1. The vegetable covering elimination and the modifications through cuts and/or embankments elaborated for the land transportation and homes execution leads to modifying the slopes geometry, increasing the steepness and exposing the material previously protected from climatic agents' direct action.

2. The land transportation, even respecting the topography end up cutting and addressing the hydra flows, generating non existent drainage patterns. The by land transportation transportations become true pluvial beds during the rainy events, channeling and addressing the flows for sections that previously possessed a different drainage system.

3. The impermeability modifies the water flow, as in as in depth. The waterproof surfaces don't allow the water infiltration in the soil, as well as the air and water circulation.

4. The pluvial waters canalization in the homes leads to change the rainwaters or served waters natural flow direction. At the same time, the canalization reduce the superficial drainage diffuse, that way, not transporting part of the located materials below the canalization and redirecting the materials collected above the canalization.

5. The embankments recover the original vegetation and the natural formation superficial covering materials, creating discontinuity areas among heterogeneous materials, besides altimeter elevate the original surface, altering its steepness.

The correlative deposits creation represented by the tecnogenic deposits represent the third level of human action consequences on the natural way. Those deposit “representing an erosion cycle on mass erosive, and whose sediments are deposited representing the hydrological conditions of transport and the area source”(AB'SABER, 1990 in: PELOGGIA, 1998). The tecnogenic deposits are correlative to the processes related to the human relief appropriation forms, and its existence time characterizes a geological time.

According to Oliveira (1994) Quinary or Tecnogen is the period when the human activity becomes qualitatively differentiated of the biological activity in the Biosphere modeling, unchaining (tecnogenic) processes whose intensities overcome a lot the natural processes. For identifying the establishment of the Quinary or Tecnogenic period it is necessary the recognition of the moment when it happens the erosive processes intensification and the correlative tecnogenic deposits formation.

The outstanding fact that identifies the Quinary or Tecnogen establishment in the study area is the population increase through the migratory process in the Porto Alegre metropolitan area, in the 70 and 80decades, provoking urbanization process acceleration. The occupation, many times disordered, breaks the natural geomorphologic dynamics and it starts to intensify the erosive and depositional processes.

 

4. Climatic and Hydrologic Conditioners

4.1. Pluvial Analysis

The city of Porto Alegre is located, according to MONTEIRO (1968), in a evidently subtropical climate area, showing as outstanding characteristic much lower temperatures than the ones generally found in Brazil, strong annual thermal width, rains satiates annual distribution and dry period inexistence.

The Porto Alegre climate is mainly controlled by air masses of marine tropical (MT) and marine polar (mP) origin. The first one comes form the western South Atlantic Ocean border Subtropical Anticyclone semi-lasting and it usually is humid, hot and unstable. Its frequency is superior in the spring-summer, when the heatstroke is more intense in Southern Hemisphere and the atmospheric circulatory systems have gone to the south.

The marine polar mass, geared up on wide oceanic surface that surround the south continent, reaches the area with superior intensity and frequency in the autumn-winter, when Atlantic Ocean South Subtropical Anticyclone is more to the north. The area starts to receive the Hurricanes and Polar Migratory Anticyclones associated to the Polar front instability, that determine the precipitation of front kind. The hurricanes bring in its mat the Migratory Anticyclones, responsible for the abrupt temperature fall and for the atmospheric instability that settles down after its passage through the front system.

In agreement to LIVI (1998), the cold fronts promote the meteorological phenomena that most disturb the Porto Alegre population life. In the summer a cold front most intense performance reaches smaller continent areas than in the winter. In that period, the fronts act predominantly in the coastal strip, they are less frequent and intense and they move quickly to the ocean, where they vanish. Already in winter, the fronts are more intense and frequent, reaching the whole coastal area of the south continent and large part of its interior. In that period it happens the mass of cold air prevalence on the one of hot air in a large part of South America southern area.

Through the local analysis the data have been used about temperatures and precipitations collected in the 8o District of Meteorology, located relatively close to the study area. The data analysis embraces the period from 1970 to 1999, covering annual precipitations values monthly and daily rates.

The pluvial annual medium analysis in the analyzed period is 1.392,8 mm. The analyzed rainiest years have been the 1972, 1987 and 1984 years with pluvial indexes of 1.984,6mm, 1.731,2mm and 1.677,8mm, respectively. The less rainy years have been 1971, 1978 and 1988 years that presented 1.122,7mm, 1.073,2mm and 1.052mm indexes, respectively.

About the medium monthly precipitation in the analyzed period, the rains are well distributed during the year, being the rainiest period from June to September. In the less rainy months, the monthly averages present among 100mm to 110mm indexes and in the rainiest months, they present monthly averages among 130mm to 145mm.

The Graph 1 represents the rain daily events behavior during the analyzed period. It is observed that the 30-40mm and have 40-60mm strips prevail, followed by the 60-80mm strip and, not rare, enrolls a maximum that can surpass the 100mm. The largest number of events concentrates on the May to October months, however the events with larger pluvial indexes concentrate on January to June period.

 

Source: 8o Porto Alegre District of Meteorology

Organization: Nina Simone V. Moura Fujimoto

 

Starting from the analysis of the data above, it can be verified that they are quite common the events capable to generate floods and/or floods in an urban area. These events can happen because of several rainy days with regular pluvial indexes and also through torrential rains picks with high and concentrated pluvial indexes, or still, being more serious, due to the those factors conjunction. It can be verified by the data analysis that such events have the possibility to happen any time of the year.

The pluvial precipitation registered on June 10, 1999, which reached the 95,1mm index, caused many damages to Viamão municipal district. In that episode, the Municipal City hall, through the Clerkship of Planning, registered the happening in damages synthesis report caused by the intense rain occurred on that day. The rain had beginning in the June 9 dawn, persisting until the June 10 morning, doing a 101,3mm total.

In that document they said that the courses of water hollow out densely on a municipal district urban zone full area, located in the hydrographic basin superior segment of stream Feijó. The most critical places were the courses of water marginal areas, irregularly occupied, and the low areas of Cecília and Augusto neighborhoods. The hollow out happened due to the streams bed obstruction by sediments for man debris, doing that the available section for flowing out was reduced, with high base level and, in many points, obstructed by great load (sand) residues, causing the breaking point, gallery, duct and associated embankments.

Ending the rains, the water level lowered quickly, leaving a destruction trace and many material losses. After the event, the contamination risk come out for diseases transmitted by the waters that drain the basin sewer and that arose up to 2,5m above the normal level, reaching lots of residences half of the windows. In an orientation work to avoid the contamination with mud and water in the reached neighborhoods, the Municipal Secretary of Health detected 350 reached residences and esteemed that about 600 families had been affected by the flood.

4.2. Urbanization Impacts on the Hydrological Processes Analysis

The inundations in urban areas are predominantly consequences of two processes that can happen separately or integrated, according to TUCCI (1995). The first is about the riverine areas inundations; and the second process is due to the urbanization. Besides other inundations and/or floods because of located points, such as river section strangle due to embankments, bridges, highways, obstruction by sediments and garbage; waters speed decrease when it’s close to the main river, to a lake or a reservoir and also due to execution and drainage project mistakes.

The riverine inundations areas happen through the natural process when river occupies its larger bed, according to the extreme rainy events. The impacts on the population are caused, mainly, by urban space inadequate occupation. That occupation usually elapses from the following actions: legislation inefficiency or inexistence that restrict occupation in risk to the flood areas; riverine areas invasion that belong to the public power, by the low-income population and the occupation of medium risk areas that, when they are filled, suffer significant damages.

The inundations due to the urbanization have as cause the urban development intensification and, consequently, the soil impermeability due to the construction of buildings, the by land transportation transportations pavement, sidewalks, parking, among others. That way, the portion of water that uses to infiltrate, starts to drain for the conduits, increasing the superficial drainage. The volume that drained slowly for the soil surface and was retained by the plants, through urbanization, initiates to drain in the channel, demanding larger drainage capacity. The urbanization main effects are the maximum flowing out increase, the flood pick anticipation and the drainage volume increase. The alterations processed by the urbanization provoke big alterations, doing that the rainwaters stay a longer in the surface, causing inundations and floods.

Other urbanization current impacts, besides the drainage one, are those owed to the sediments production increase and the quality of water degradation by the pluvial and/or cloacae sewers and to the aquifers contamination.

During the urban development there is a sediments increase produced by the hydrographic basin due to the constructions, cleaning of lands for new divisions into lots, by land transportation transportations construction, avenues and highways, among other causes. That increase in the sediments production can provide, in the urban areas, the drainage obstruction by sediments, with the drainage capacity reduction and the pollutant substance transport joined to the sediments. The urban aquifers are polluted, mainly, for the sanitary embankments and for the contaminated pluvial waters infiltration by the garbage transport, sediments and wash of by land transportation transportations.

There are many works that try to quantify the alterations provoked by the urbanization in Arroio Dilúvio Hydrographic Basin area, standing out the works developed at the Institute of Hydraulic Researches of UFRGS through the use of mathematical models and/or simulators for the urban floods control.

Among the developed works it stands out the FERNANDEZ & ORDOÑEZ (1979one), that drives their studies to the simulation models and its use in the discharge-pick synthesis in urban basins because of the urbanization. Among other analyses, the work allowed to verify that, for a rain with 10 years and 30 minutes period of duration return in a rural area that suffered an up to 27% increment of impermeable area, the rain volume drained tended to ponder in a short time period. That use to cause very high picks of rains, originating floods and floods that originally didn't happen in occasion of same precipitation. Starting from this evidence, it can be concluded that, how higher the urbanization degree, higher will be the generated superficial drainage gradient, increasing the floods probability occurrence before events with smaller time to return.

To evaluate the urbanization effects in the study area drainage amount, the relationship was made starting from the use of hydro grams of hydrologic station located in Arroio Dilúvio Hydrographic Basin, where it is located the study area, in the 1973-1974, 1983-1984 and 1991-1992 years. The accomplished analysis has been used by MARTINS (2000), having as base the methodology proposed by LAMBERT (1996) apud MARTINS (2000), which relates annual total precipitation, seasonal rhythm and the interannual variety with hydrometrics data.

The analysis allowed establishing a direct relationship between the precipitation picks and flowing out, in that the picks of floods tend to be larger and larger along time. With the urbanization intensification, the superficial drainage gradient has being slowly increased, reflecting in larger and faster flood picks before pluvial events every time smaller.

4.3. Urbanization Impacts in the Quality of Waters Analysis

The pollution generated by the superficial drainage is called diffuse pollution, according to PORT (1995), because it comes from activities that deposit pollutant, in a scattered way, on the hydrographic basin contribution area. The main sources of diffuse loads are: atmospheric deposition, paving consume, vehicles, vegetation remains, garbage and dust, remains and animals dejections, spills and erosion. The principal pollutant that have been carted are sediments, organic matter, bacteria, metals as the copper, zinc, manganese, iron and lead, coming petroleum hydrocarbon, toxicant as the pesticides and the air pollutant that are deposited on the surfaces. The secret connections of sewer, septic sewages effluents, fuel leaks, oil remains, and inks and other toxicant products spilled in gutters contribute to the increase of pollutant loads transported by the urban drainage net to the bodies of water.

To evaluate the pollution for pollutant loads generated by the urban superficial drainage in the study area, the concentration was measured of pollutant thrown by the urban drainage on the receiving body. The quality of water parameters used to evaluate the pollutant concentration was: dissolved solids, solids in suspension and fecal coliform.

The picture 5 shows the quality of water parameters values in some located points in the study area hydrographic basin. The points 1, 2, 3 and 4 are located in the principal streams outlets that come to an end in the Mãe D’agua Barrage; and the point 5 is located in the medium course of one of the streams.

Starting from the analyzed parameters values, a discharge contamination can be verified by current fecal coliform of the release of cesspit and pluvial sewers, as well as the loaded garbage for the urban drainage even courses of water. Visual observations and scents can be verified at the collection places, indicating great contaminations in the streams of study area.

 

Table 5: Quality of Water Parameter Values in the Area of Study with the most important Arroios in 13/03/00

Parameter

Point 1

Point 2

Point 3

Point 4

Point 5

Dissolved solids (mg/L)

233

687

394

601

208

Solids Suspended (mg/L)

85

43

16

26

10

Fecal Coliforme (col/100ml)

85x104

104x104

73x104

120x103

95x103

Flowing out (l/s)

14

44

24

5

3

Source: Analyses made by the Laboratory of Physical and Chemical Analyses of the Center of Ecology of Federal University of Rio Grande do Sul

Organization: Nina Simone V. Moura Fujimoto

 

5. Vegetable Covering and Use of Land Analysis

The occupation evolution related to the population increase and the several intervention modalities in the hydrographic basin provided a its characteristics transformation along the years. In agreement to the IBGE data, the 70 and 80 years were the decades of larger population increment, through the migratory process, in the Viamão municipal districts and other Porto Alegre metropolitan area municipal district. This way, the vegetable covering analysis and use of land of 1973 and 1991 years could demonstrate the those alterations intensity and the interventions modalities happened in the area during that period.

In 1973, the areas with smaller lineal interventions (by land transportation system) and by constructions are in the compartment of hills and in the Federal University of Rio Grande do Sul property area. Those sections are practically absent the constructions, except the division into lots Vila Nossa Senhora Aparecida, approved in 1957 for the City hall of Viamão and some few buildings of the Campus Vale and of the Institute of Hydraulic Researches.

However significant alterations have been observed in the original characteristics of the compartment of hills and inside of the University property. The mined areas found at the hills represent a feature of great alteration in the original morphology, altering the slopes morphodynamic processes. The Mãe D’agua barrage construction constitutes a deep modification in the area original characteristics, altering the fluvial dynamics and, consequently, the hydrographic basin whole dynamics.

The most important modifications are in the hills compartments through the lineal interventions happened because of the by land transportation system, for the homes constructions and, although in smaller proportion, for the agricultural use (horticultures). That intervention group is basically concerning a series of approved divisions into lots, majority, from 1955 to 1960 for the Municipal City hall of Viamão, according to MEUCCI (1987).

The divisions into lots collect mainly the area represented by the Hills Forms and they show scattered homes that occupy the discharges and this compartment averages slopes predominantly mound. The pattern of by land transportation demonstrates a widespread adaptation to the level curves, favoring the slopes stability. No morphologic evidence of morphodynamic-accelerated processes has been observed. The areas without homes inside the urban lots possess vegetable covering of little gram predominantly. The ciliary forests accompany the courses of water and they are significantly preserved.

The compartments in Form of Plain in Plane Areas, together with the bottoms of the you are worth they don't integrate the areas of the divisions into lots. In that sense, they are sections that are with your morphology preserved together with the vegetation represented by the ciliary forests.

The forests and ciliary forests conservation, as well as the evolution of the constructions and the by land transportation system can be observed along 10 years of 1960, 1963 and 1973photographic registrations. Significant forests preservation is noticed practically along this period and a stable occupation pattern.

It is still possible to observe, among the years from 1963 to 1973, evidences of barrage soil superpose, more precisely in a small section to amount lake. In 1963 the lake area extension was in very similar situation to the one of the area in 1973 that, according to measures accomplished in the aerial picture, it was with 48.512m2. In a 10 years period, it must have happened the soil superpose not more than 5% lake of the barrage area.

The change in vegetable covering and the use of land among the 1973 and 1991years, is easily identified the fast suffered transformation for the hydrographic basin during that period, confirming the demographic data compatibility for the study area. The built areas expansion and the superposed soil areas increase in the barrage are the most outstanding characteristics.

In 1991, the most preserved area is still the same as 1973, that is, the compartment of hills and the University property. However, it stands out in that period a significant occupation growth in the existent divisions into lots on the form of hills, the Campus built area increase at the valley and a new approved division into lots appearance in 1985 for the City hall of Viamão.

The barrage soil superpose process is surprising, could arrive the about 50% from its completion in 1963. The busy area for the barrage lake in 1991 was 25.920m2, while in 1973 it was 48.521m2. Among 1963 and 1973, there was the lake soil superpose about 5%, while in the corresponding period to the 1973 to 1991 it happened the barrage soil superpose of 47%. That demonstrates the alterations intensity happened in the 70 and 80decades.

The Forms of Hills compartment occupied by the divisions into lots it suffers a significant increment in your occupation. In 1973 about 10% of area were busy, in 1991 raisin for 90% the occupation for constructions (system by land transportation and homes). Inside of that evolution in the occupation for homes, the busy areas are enlarged; becoming included the plains, the plane areas and the bottoms of valley. Many of those occupations come in irregular situation, because they are public property areas.

The ciliary forests suffered about 60% decrease related to 1973. The busy spaces for little gram are substituted by constructions. Inside the urban lots they can be found green areas that was preserved introduced in the homes during that period.

The increase of constructions provokes a deeper alteration through cuts and/or embankments in the original morphology. Exposed surface areas are significant. They happen in by land transportation and in the own lots in considerable proportions. Inside of the built units it happens a mixture of habitation units, back yards exposed surfaces and by land transportation and vegetable covering of little gram and green areas (vegetation arboreal and/or in shrub form preserved introduced).

In the identified areas as residential of standard bass meet the irregular occupations. Those sections refer mainly to the bottoms of valley and they present typical occupation characteristics. They are small houses with exposed surfaces and a great amount of small dimension cuts and great incision associated to embankments, distributed in a chaotic way.

The Foundation of Metropolitan and Regional Planning - METROPLAN, have identified the villas in metropolitan area irregular situation in 1991. That work had as objective to elaborate an inventory of all the irregular villas and to compare it with the inventory accomplished in 1981 by the same planning organ.

In the Viamão municipal district have been identified 72 villas or areas occupied irregularly, being five villas located inside of the hydrographic basin in study. The villas in irregular situation placed in the study area and its principal characteristic are described in the Table 6.

Most of the metropolitan area irregular villas occupy municipal property areas, as it demonstrates the Table 2 that shows the study area irregular villas. That occupation kind is a very common form that already portrays a practice identified for the 1981 Inventory. That fact reflects the municipalities not operationally to treat those areas, as in equipments endowment as in its maintenance and inspection, associated to the means generated by the available services around.

In the case in study, the irregular areas are in its totality located in public areas whose are about, in its majority, located in areas close to the courses of water. Those areas residents convert the courses of water in receivers of every kind of residues, committing to waters quality. That occupation results in vegetation of margins destruction, causing the erosive processes acceleration and the consequent course of water obstruction by sediments. When levels of water elevation happen, the lands become residues in natura receivers, causing risks to population the health.

 

Table 6: Municipal district of Viamão Irregular Villas Evolution, Located in the Area of Study–1981/1991

Villa Code

Denomination

Proprietor

Time of Existence

Number of Houses 1981

Number of Houses 1991

Growth 81/91 (%)

01

V. in Jardim Universitário

Municipal public

>20 years

50

237

374,0

02

V. in Vila Medianeira

Municipal public

16-20 years

10

55

450,0

03

V. in  Vila Schonmald

Municipal public

16-20 years

65

119

83,1

04

Villa in Vila USBEE

Municipal public

11-15 to the

150

175

16,7

05

V. in Vilas Diamantina and Santa Isabel

Municipal public

16-20 years

100

214

114,0

Source: METROPLAN (1991)

Organization: Nina Simone V. Moura Fujimoto

 

The growth of villas in the study area takes place mainly for the old nuclei amplification, evidenced by the number of houses increase. In its majority, the infrastructure conditions are extremely precarious, mainly in concerning to the sanitary exhaustion. The most common solutions found are the ditches to open sky, latrines and sewage-drain. The most critical and very usual situation is directly the dejections release in the closer courses of water. Such solution doesn't only is about the residences in irregular situation, but most of the study area homes, because they don't have sewerage system.

 

6. Social-environmental alterations in the Metropolitan Area and its Reflexes in the Area of Study

Starting from cartographic, photographic registrations, field observations and interviews, the original morphologic situation can be rescued, as well as the drainage net, the waters quality and the human intervention level in basin hydrographic. It possible to observe that the hydrographic basin is composed by Similar Forms Patterns represented by the: hills, plane areas and plains. Those form patterns were in a considerable preservation situation, because the human interventions were related to lineal elements and the scattered homes with punctual interventions. The natural forest vegetable covering in the hills, the ciliary forest and the little gram covered vast extensions. The superficial waters conditions, rescued starting from the depositions with the local population, they were very satisfactory, with crystalline aspect.

In a certain moment the intervention process begins through the human activities that provoke significant alterations in the original characteristics. The larger width intervention have been the Mãe D’agua barrage construction, inaugurated in 1963, altering the original drainage net and modifying its hydra regime and, later, to the population increase through the intense migratory process to Porto Alegre Metropolitan Area, in the 70 and 80decades, provoking an urbanization process acceleration in whole Big Porto Alegre, as well as in the study area (LOSS and KNIJNIK, 1994). This urbanization process modifies the original morphologic picture, altering the existent dynamics and inserting other characteristics or rhythms to the atmosphere.

The urbanization process principal characteristics in outlying economy countries, as the case of Brazilian cities, refer to an urban growth not accompanied by an economical growth, leading to the great urban gatherings appearance with deficiencies in several services, equipments, employments and one accentuated environmental degradation. That process is  characteristic of corporate urbanization as detaches SANTOS(1994), it is recognized by the big companies economical expansion and investments to their interests, in detriment of social order attendance investments. The modern industrialization that is settled at the Country starting from a monopolist economy, having the foreign capital and State as support, it reinforces the regional characteristic unbalances of the National State occupation history. This way, the cities equipments and norms for the big firms articulation grow up, in detriment of smaller companies and population as all. Consequently we have, on a side, a generation of wealth in a concentrated way and, to the other, and an enormous poverty production.

Concomitant to the industrial and urban growth, as it emphasizes SANTOS (1994), the lacking nuclei growth is evidenced in the big urban periphery axes, due to differentiated habitation space consumption. The larger income social classes can choose their home place, however to the low income classes remains the cheapest lands or out of market homes.

In Porto Alegre specific case, in 1954 it becomes manifest the 1233/54 Law (SALENGUE and MARK, 1993) that regulates the urban divisions into lots to that city, which starts to demand urban lots with infrastructure conditions that elevate their value in the market. This way, it guides to the division into lots of low-income displacement to the bordering areas of adjacent municipal districts. That juridical determination promotes the divisions into lots of low-income appearance, there is not infrastructure services and difficult access to the urban centers most equipped. With difficulty access to the home at market price, associated to the inefficiency or inexistence public politics that restricts the occupation in inadequate areas, the low-income population occupies areas that transgress the effective legislation, whose specific characteristics result in quite peculiar environmental alterations. Under those specific conditions of home, they are the divisions into lots represented in the study area.

On the other hand, the environment that suffers the current human interventions from a specific urbanization process possesses its own characteristics, answering singularly to all the suffered interventions. Such alterations will be described following.

 

7. Main Effects and Environment Answers to the Urbanization Processes

To analyze the most important urbanization effects and answers in the study area we have tried to show the acquired environmental and produced by the area analysis data, the modifications have happened in the urbanized space and the answers to those modifications. The main urban environmental alterations identified in the area have been:

1) Erosive Processes Occurrence and Mass Movements Susceptibility.

In this case, it can be observed the mass movements or  erosive processes deep marks nonappearance. The most significant erosive processes concern to the laminate erosion and lineal erosion, represented by furrows and, rarely, ravines. This way, the area has been classified as presenting high, average and lower susceptibility to laminate and lineal erosion to furrows and ravines processes.

It is verified that this classification corresponds to the slopes units natural potentiality because of their geometry, steepness, litology, geological structures and covering material characteristic. Starting from the original morphology alterations, susceptible atmospheres come into view to the erosive processes and mass movements occurrence. That is because the original morphology modifications come from the vegetable covering retreat, the introduction of ways of current processes created by the human activities, which accentuate the erosive processes, because they promote superficial materials disposition alterations, exposing them to the rain impacts.

The geotechnical parameters (consistence index) and rehearsals about the penetration analyses indicated that the superficial covering material found in the units of slope possess high values, checking high consistence to the soils and high resistance to the erosive processes. That can be credited the mass movements and deep erosions absence as in the Hills Pattern as in the knolls Pattern. However other factor to be considered in that slopes stability condition is the urban plan fact through the by land transportation system defined by the divisions into lots to respect, widespread, the local topography.

 

 

To analyze the different susceptibility degrees to the erosive processes and mass movements areas it has been necessary:

-    to analyze the information about the characteristics morphometric (altitude and steepness), the units of slope geometry, the litology, the geological structures and the superficial covering material characteristics;

-    to identify the forms of current processes created by the human activities, which induce to the slopes instability processes, which are: cuts, ramps of embankments, among others;

-    to identify the forms of current processes induced by human and indicative slopes instability activities, which are: furrows and ravines and

-    to identify morphodynamic processes mainly related to laminate erosion.

This way, the study area following classification has been defined, because of the erosive processes occurrence susceptibility and mass movements.

-    Laminate and lineal erosion (furrows and ravines) high susceptibility to developmental processes , however deep erosions and mass movements less susceptible.

That section is in Forms of Hills Pattern units of slope, which present the areas largest steepness and altitudes, being, because of that, attributed larger susceptibility to the superficial erosion. This section is constituted by superficial covering material, formed predominantly of silt and clay, followed by the sand fractions, with high to very high resistance to the penetration, checking little probability to deep erosions and mass movements. The steps of cuts accomplished by the plane surfaces creation needed to the homes and the by land transportation construction are extremely spatially representative being possible to represent it in the ways of current processes map. They are also represented in the adopted scale the mining scars. We identify, this way a significant modification in the superficial materials, leaving them more exposed to the precipitations impacts and, consequently, to the removal probability through the laminate lineal erosion processes. Such processes have been observed along the units of slopes in Hills, mainly in the by land transportation and in the mining scars.

-    Medium susceptibility to the laminate and lineal (furrows and ravines) processes development. With no tendency to mass movements.

That section is represented by the units of slopes in Hills Form, whose steepness and altitudes are more moderate than the one found in the units of slopes of hills. The section is constituted by superficial covering material more developed that in hills, material predominantly formed by silt and clay, followed by the sand fractions, with high to very high resistance to the penetration. Those properties of material, associated to the morphometric characteristics, check those lands medium susceptibility to the superficial erosion processes, with no tendencies to mass movements processes. The forms of current processes created by the human activities come in a widespread way along the units of slopes of hills, however its cartographic representation is scarce and punctual. That is owed to the cut steps dimension, the plane surfaces, and the embankments ramps, among, that don't correspond to a  presentation dimension in the analysis scale. Although the Human morphology is practically total or widespread, there are not in ways of current processes evidences that suggest slopes instability, related to the deep erosions or to mass movements. They are only identified laminate and lineal erosion processes, represented by furrows and ravines, mainly along the by land transportation.

-    Low susceptibility to laminate erosion processes development. With no tendency to mass movements processes.

That section is represented by the convex tops of in Hills Pattern and by the in knoll Plane and Convex Tops Pattern. They are compartments with low steepness, whose lineal erosive processes become practically inexistent. The cuts and/or accomplished embankments have a small dimension, and the superficial material, when it is exposed, possesses laminate erosion evidences. A good number of tops is found in the knolls is waterproof, because in those sections they locate the principal access roads, and the urban intervention is very accentuated. In that case, a morphodynamic stability, face to the relief surface high impermeability.

2) Floods Occurrence susceptibility

The floods in the study area are related to the plane areas with depressed topography, located close to the fluvial channels or near the lakes and dam, or directly related to the fluvial channels that drain the hydrographic basin.

So, the Plains and Plane Areas Patterns are the larger potentiality morphologic units to the flood events occurrence. And it has been in those compartments that the residents indicated the problems occurrence during rainy periods. The bottoms of valley, when busy, they also present flood problems, however that situation will be separately identified, it means ,areas with environmental quality high compromised.

Starting from the analyzed pluvial data, it can be observed that the events capable to generate floods in some urban area points are quite common. Those events can be associated to several rainy days with regular pluvial indexes and also through torrential rains picks with high and concentrated pluvial indexes, or still, to these factors conjunction. It is also observed that those events can happen in any time of the year.

To analyze the flood events potential and/or released factors they have been considered:

-    the hydrographic basin Geomorphologic Conditioners;

-    the superficial covering material characteristics;

-    the man Conditioners related to the use and occupation of land and to the interventions through the barrage construction that cause difficulties to the drainage;

-    the climatic Conditioners;

-    the residents' depositions about the situation of the area during the rainy events.

The plane areas and the plains are relatively in topographical situation inferior to the units of slope. This way, they receive the superficial and subsuperfitial water flow from the areas in altimetry superior situation. They are plane areas, whose drainage speed is smaller, providing a concentration of water for precipitation events occasion, although the superficial covering material can, some times, to be predominantly sandy, what checks a larger infiltration. Besides, the modifications caused by the “human” morphology provide a superficial drainage increase due to the surface impermeability by the built/paving or by the soil compaction. Another factor that favors the flood occurrence is the contribution of material increase and/or urban debris transported by the slopes until the bottom of the valley, provoking a fluvial channels level of base decrease and, consequently, of the fluvial drainage capacity. They are, therefore, natural and man factors that act conjugate in the areas subject to flood events.

This way, it has been obtained the following classification of the study area, because of the floods occurrence susceptibility, starting from the factors above-mentioned and some each identified area specific ones.

-    High susceptibility to flood occurrence. When under human intervention (morphology Man), area shows high-risk possibility to home.

They are relatively plane areas with low altitudes associated to a superficial covering material constituted predominantly by clay and silt, whose permeability is very low, determining low drainage and infiltration conditions. The water sheet level is very high, per times appearing on the surface. When referring to the Pattern in Plane Areas, the areas come in depressed topographical situation with drainage difficulties and enclosed for altitude slopes and steepness elevated. In the areas of fluvial-lacustrine plains, where the whole drainage of the basin is addressed, it happens process of intense soil superpose and topographical elevation in reason of the great material contribution transported by the slopes, provoking the elevation of the base level and, in consequence, the elevation of the natural level of the floods. The resident population in those places tells locomotion difficulties for the vehicles or for the pedestrians during the rains strong lingering and/or. Although rarely, some residents declared that leave of your residences they were flooded on those occasions.

-    Medium susceptibility to the flood occurrence. When under human intervention (Man morphology), the area comes with average risk possibility to the home.

They are relatively plane areas with low altitudes, where the flood is related to the basin principal fluvial channels and to the areas formed by the larger beds of  channels (flood plains). Those areas  are constituted by superficial covering material, predominantly sandy, checking a good infiltration capacity and drainage. They still correspond, in its majority, to the sections close to the fluvial outlets of the hydrographic basin principal channels, which naturally happen great material accumulation transported by fluvial processes. Being added the material contribution transported by the urban drainage to the superficial drainage increase, caused by the man interventions, an increment is had in the amount of transported material. That material leads to the channels obstruction by sediments, hindering the fluvial flow drainage for rainy events occasion. In those outlet points they also happen artificial strangles of the fluvial bed caused by the drainage ducts under the bridges built in the system by land transportation, hindering the drainage ditch of the drainage net. Frequently, these ducts retain great part of the transported material, leading to the obstruction of the same ones. The resident residents in the bridges proximities use to say  that, per times, depending on the cleaning of ducts conditions, it happens the overflow during the rains and their residences are reached.

-    Low susceptibility to the flood occurrence. When under human intervention (Man morphology), the area shows low risk possibility to home.

They are plane areas with relatively high altitudes located in the fluvial channels confluences and adjacencies that present, in its majority, in form of V valley, providing a fluvial flow drainage good capacity. Because of its morphology it glides and positioning inferior altimetry related to the units of located slopes to its circuit, they are areas of  water flows concentration. With the superficial drainage increase and the amount of material transported by the slopes, they become susceptible to flood events, because there is fluvial flow capacity decrease because of the obstruction by sediments of the channels. Those areas gather conditions to  present flood events, although in less important proportion, when compared to the areas classified with discharge and average susceptibility to the flood occurrence. When some residents of those places or of its proximities were consulted regarding the conditions of the area during the rains, it was not mentioned any critical episode as the ones that they we have seen previously.

3) Human Intervention level  Classified According to the Morphology

The urban interventions recognition starting from the morphology is a contribution of LIMA (1990), which suggests that the original morphology or pre-urban identification is accomplished in a first stage with compatible particularity to the adopted scales. Later, the urban interventions identification and these interventions consequences supply the typology and the urban intervention degree.

The man intervention degree classification starting from the preserved original morphology identification, semi-preserved original morphology and Man morphology suggested by LIMA (1990) was used in the present work with adaptations to the found reality. That classification has been elaborated through the geomorphologic map starting from the forms originated by natural processes represented by the in Similar Ways and for the Forms of Processes Current Patterns or induced by the human activities. They have been also used the vegetable covering maps and use of land that could help to apprehend the no identified man interventions as forms of current processes in consequences of the adopted scale. The spaces built identified in the use of land mapping possess forms of current processes that, due to their dimension, they are not compatible to the cartographic adopted in this work scale. However they represent important categories in the intervention degree evaluation and, this way, in the definition of the areas that represent the semi-preserved morphology and the Man morphology. The vegetable covering has been also considered in the human intervention degree definition, having as appraiser element the preservation level and/or vegetable covering alteration.

·                   Preserved Original morphology. That morphology is determined by natural factors, originated by past climatic and nowadays processes that can be maintained or possess slow evolution when the vegetation conditions are maintained (TRICART, 1977; AB'SABER, 1966 apud LIMA, 1990). Those surfaces are the one that possess forms very similar to the originals, whose morphologic interventions don’t have direct character. Even if the vegetable covering is not original and that, because of this, might have suffered modifications in its forms, these are just perceptible for instruments to long observation period and not for photo interpretation (LIMA,1990). They are areas of original morphology where extends a vegetable covering of forest and little gram with high preservation level.

·                   Semi-preserved Original morphology. That morphology corresponds to the surfaces that suffered human interventions mainly lineal (by land transportation) and/or interventions for scattered punctual elements, on which it is still possible to obtain important information of the original morphology. Those surfaces present vegetable covering of ciliary forest and little gram little preserved and green areas introduced inside the urban lots.

·                   Man Morphology. That morphology corresponds to the surfaces that suffered man interventions in total or widespread way, becoming difficult to obtain important information on the original morphology. Those surfaces present scattered vegetable covering of ciliary forest very little preserved and green areas introduced inside the urban lots. They make part of that category of surfaces with forms of current processes created by the human activities as the cut steps, plane surfaces and ramps of embankments of representation significant areola and the surfaces topographically increased or created by the tecnogenics materials.

4) Modifications in the Morphology  made by  Human Activities

The modifications made in the morphology are related to the forms of processes current maids or built by the human activities, which were susceptible to cartographic representation in the adopted scale in this research. They are represented by areola elements as the plane surfaces, the cut steps, the embankment ramps and for lineal elements as the by land transportation.

5) Man Morphology and/or Human Activities consequences

The consequences of the modifications made in morphology by the human activities or due to the human activities are related to two categories: elaboration in new ways and superficial waters quality.

The new relief forms are consequence of the human activities induction. They were classified in forms of small dimension and of great dimension, all represented in the adopted scale. The forms of small dimension are the furrows formed by exit material processes; to the tecnogenic dejection cones and the lowered flooded areas, all current of deposition processes. The plains formed by the tecnogenic material deposition represent the forms of great dimension. Such deposition has caused a topographical increment and intense soils superpose process, being represented by the Fluvial Tecnogenic Plain and for the Fluvio-lacustrine Tecnogenic Plain.

The second consequence is about the quality of the superficial waters. To evaluate the pollution for pollutant loads generated by the urban superficial drainage in the study area, the pollutant concentration has been measured thrown by the urban drainage on the receiving body through the following parameters: dissolved solids, solids in suspension and fecal coliforms. The obtained results indicated that the waters have discharges concentrations of pollutant in all the analyzed points. This way, it can be considered that, along the whole fluvial channels analyzed, waters have been very polluted.

The growth of occupation in the study area has not been accompanied of an infrastructure conditions, improvement mainly in what on the sanitary exhaustion. The most common solutions found are ditches to open sky, latrines and sewage-drain. The most critical and very usual situation is release the dejections directly in the closer courses of water. In a lot of occasions, residents have transformed the courses of water in receiver of every kind of residues, committing the quality of waters.

When it happens the levels of water elevation for occasion of rainy events, the lands become receivers of residues in natura, generating risks to the population health. That situation is still more serious when the lands are subject to the flood events occurrence. In that case, the polluted waters can reach homes, and, consequently, population can have volunteer or involuntary contact with the polluted waters.

 

6 Transgress ways to use the Municipal Legislation – Areas Highly Compromising of Environmental Quality

The occupations in irregular situation are in municipal public areas destined to urban equipments or free spaces, to public use. Such areas were mainly busy by the population whose financial conditions disabled to acquire homes inside of market conditions. They locate mainly along the courses of water and/or in drainage headboards, also occupying the bottom of the valley; in places with high steepness on mining scars. They are few distributed houses in a chaotic way with exposed surfaces and a great amount of cuts of small dimension and great incision associated to embankments.

-    Residential in Irregular use Situation Located in the Bottom of Valley and along Courses of water and/or Headboards of Drainage

The areas of residential in irregular use situation in the bottom of valley and along courses of water and/or headboards initially provided the vegetation of ciliary forest destruction, favoring the erosive processes appearance and the consequent obstruction by sediments of courses of water. Later, the human interventions accentuated the manifestation or acceleration of obstruction by sediments and covering processes observed in the fluvial channels. That happens for the volume and speed of superficial drainage increase that transfers for the bottoms valleys great amount of material from the slopes originating due to the human activities in half urbanized.

Covering scars can probably be observed in several spaces of  courses of water by the volume and of speed of fluvial flow and the transported material amount increase. The population who has their home installed in the fluvial channels margins try to contain the covering process through the placement of heaps of stones and mainly tires, to protect of destruction when level of water elevation happens for occasion of flood events.

Another way, it is observed that a lot of homes are partially on tecnogenic deposits formed in the bottoms of valleys. Those deposits have an extremely heterogeneous constitution in their composition and in their granulometry because of the kind of superficial covering material that originated it and the amount and of current urban debris types. According to PELOGGIA (1998, p.77), that material presents a rigid behavior when it is dry and, in that case, its resistance is relatively high. However ”that behavior is drastically modified with the humidity tenor increase, when the material shows softening and assumes plastic behavior or even viscous fluid in case of saturation.” This way, a compromising exists in these surfaces stability and, consequently, to homes that settle on the tecnogenic deposits. That occupation seems to reflect the exhaustion of areas for the installation homes and the presence of new areas to live demand, resulting recently in the occupation of surfaces formed by the obstruction by sediments of  fluvial channels through the tecnogenic deposits.

In those places, there are flood risks when the level of waters elevation takes place. Although the Municipal City hall of Viamão doesn't have any official registration of assisted occurrence, reports of some technicians of the municipal district and residents confirm that kind of situation. However the flood events seem to reach homes partially, in other words, they are not characterized as a calamity situation. Probably that is associated to the great longitudinal development of fluvial channels that provides a condition of satisfactory flow of water, disabling the most severe floods appearance.

Knowing about the concentration of pollutant discharges rates of for fecal coliforms in the superficial waters, it is possible to foresee that the resident population in the lands reached by the elevation of the waters take great risk of contamination for diseases transmitted by the waters that drain the hydrographic basin sewer. This way, they were suitable critical points with discharges rates of pollutant concentration, considered a risk to the public health.

-    Residences in Irregular Use Situation in Areas of High Steepness

The residences in irregular use situation in areas of high steepness is in municipal lands and in its majority with inferior 30% Steepness. Although practically there is not transgression of laws that they discipline the parceling of soil related to the steepness limits, those areas are in lands destined to municipal urban equipments and they occupy the strips that should be reserved on each side along the courses of water, according to the effective legislation.

The medium steepness predominant in those sections is in 20-30% class and they occupy the slopes with straight segments in the in Forms of Hills Pattern. Those units of slopes have high susceptibility to the development of laminate and lineal erosion processes. Although with little susceptibility to deep erosions and mass movement, the use and occupation for homes of standard bass can indicate situation of geological nature potential risk, geomorphologic and hydrologic. However in those sections there are not evidences of mass movements, or of deep erosive processes, or still, attendance registrations for the City hall of the Municipal of cases of risk associated to the slopes instability. That situation should be attributed to the superficial covering materials discharge consistence and resistance, previously mentioned.

However the countless and small houses with exposed surfaces and cut of little dimension and great incision associated to the embankments provide acceleration of erosive processes (laminate and lineal) starting from the superficial covering material remanaging.

The precarious urban infrastructure that’s the residences are, however, created an area with great environmental problems that commit the population quality of life. A lot of Man features can be observed and they indicate a probable risk situation associated to the instability of slopes, at the same time that evidence the environmental quality compromising in area. Those features are represented by great cut steps, ramps of embankments without technical precision, superficial release of served waters or drain, ducts leak, garbage accumulation, deficiencies technique in the homes construction, among other (MOORISH-FUJIMOTO, 1999).

-    Residences in Irregular Use Situation in Areas with High Steepness on Scars of Mining

The areas in irregular situation with high steepness on mining scars are previously in the same unit of slopes ever told. Inside of the municipal public area occupied irregularly in the in Hills Pattern, a mining scar existed. That scar has been totally busy for residences of standard bass, hardly being possible to identify in that period the mined area. Because being an added difficulty related to the possibility of occurrence of a risk situation associated to the stability of slopes and also for the situation peculiarity, this one has been separately identified.

The occupation of mined or loans areas, with no recovery, is relatively frequent and it makes possible the occurrence of risk situations in many areas of Brazilian territory (GEOLOGICAL INSTITUTE, 1996). The mineral activity is a specific form of use of land, that acts intensely in the original morphology transformation (through cuts) and it modifies the geomorphologic dynamics (favoring the erosion and, consequently, the obstruction by sediments of courses of water), becoming potential the erosive processes occurrence, mass movements and flood.

As previously mentioned, scars of mass movements or deep erosions in those sections are not observed, however they are areas with a kind of Man morphodynamic quite specific.

 

8. Final considerations

Based in the developed work we understand that the conceived environmental analysis allowed learning the principal effects and answers of the urbanization process current environment in the study area. The action of man has been altering the natural dynamics of the study area, contributing in the processes of slopes the fluvial ones, pondered drainage and  obstruction by sediments intensification.

This way, we can affirm that, in general lines, the environmental alterations in the study area correspond basically to very significant modifications in the original morphology and in the geomorphologic processes dynamics, intensifying their natural potentialities. However it has not brought to that hydrographic basin severe environmental degradation associated to risks of geological nature, geomorphologic and hydrologic that could risk the population life. However the erosion and deposition processes intensification has leaded  to the creation in new relief ways associated to the tecnogenic deposits, as well as an accentuated degradation level in the quality of superficial waters. Those facts, associated to the atmospheres of inadequate occupation, promote the appearance of areas with high compromising of  environmental quality.

 

9. Bibliographical references