EROSIVE PROCESSES IN URUCUTIUA RIVER BASIN : PAÇO DO LUMIAR TOWN – MA

The modeling of the terrestrial prominence is the result of the endogenous and exogenous agents, denominated as erosion agents, that constitute a set of forces that contribute to the soil erosion development and the prominence modeling that can be a risk to man according to the modifications of the humanized environment. In Brazil, the problem of erosion is old. The states of Parana, Sao Paolo and Rio Grande do Sul are the states that have the best exponents of control for these processes. In the rest of the country, the lack of interest of the government institutions results in absence of reference materials to detail the studies so that the planner have difficulties to understand the situation to prevent it. This study justifies itself because the area in question has been invaded without a compatible plan where we observe environmental problems in different frequencies and magnitudes whose the final product is the elaboration of the map of vulnerability to the erosive processes in Urucutiua river hydrography basin that will subsidize the redefinition of the process of local occupation that is inserted in area of high susceptibility to erosion according to Cirillo (2003, p. 50).


INTRODUCTION
The modeling of the terrestrial prominence is the result of the endogenous and exogenous agents, denominated as erosion agents, that constitute a set of forces that contribute to the soil erosion development and the prominence modeling that can be a risk to man according to the modifications of the humanized environment.
In Brazil, the problem of erosion is old.The states of Parana, Sao Paolo and Rio Grande do Sul are the states that have the best exponents of control for these processes.In the rest of the country, the lack of interest of the government institutions results in absence of reference materials to detail the studies so that the planner have difficulties to understand the situation to prevent it.
This study justifies itself because the area in question has been invaded without a compatible plan where we observe environmental problems in different frequencies and magnitudes whose the final product is the elaboration of the map of vulnerability to the erosive processes in Urucutiua river hydrography basin that will subsidize the redefinition of the process of local occupation that is inserted in area of high susceptibility to erosion according to Cirillo (2003, p. 50).

MATERIALS AND METHODS
Firstly, there was a bibliographical review and reading of publications related to the theme with field activities as the delimitation of field and the definitions of the areas where there were more intensive superficial erosion.An interpretation of the hydrographical basin cartographical material was done and the erosive processes were roused based on Canil's methodology (2001) that consists in geological, geomorphological and pedological data, where some parameters correlated to the erosive processes were suscitated, added with the use and occupation, the cross of the classes of high, medium and low susceptibilities, determined in each attribution.
The materials used to the office and field activities were the data of the IBGE census from 1970 to 2000; aerophoto of Maranhão state done by INPE; scale 1:15.000,strip 1, number 2, of 1999; DSG planialtimetrical maps, in partnership with SUDENE; scale 1: 10.000, sheets 08 and 09, year of 1980; stereoscopy, compass, GPS, camera, PC, and inkjet printer.

Location and geographical situation
Paço do Lumiar town is located in the northeast of the Maranhão island, with an area of 121,4 kilometers square and its limits are: on north Raposa town and Atlantic ocean, south, east and west São José de Ribamar town.The Urucutiua river hydrographical basin, with an area of 2,7 kilometers square, is located in the northwest part of Paço do Lumiar town (map 01), between the geographical coordination 02º 27' 42''S and 44º 10' 47''W (to the north); 02º 28' 44'' S and 44º 11' 08'' W (to the south); 02º 28' 14'' S and 44º 10' 39'' W (to the east); and 02º 28' 17'' S and 44º 12' 01''W (to the west).It is possible to get there by highways MA-203 and MA-204.

Physiology
Two kinds of geological formations encircle the area of study (map 02), the Acuí Formation, consisted of "depósitos aluvionares recentes, formados por cascalhos, areias e argila, dispostos em faixas estreitas ao longo dos principais cursos d'água" (CIRILLO, 2003, p.26), in Urucutiua river basin, these sediments appear in the medium and low course of the channel of the river; and the Barreiras Formation, where, according to Rodrigues et al. (1994, p.73), "[...] é composta de sedimentos areno-argilosos e ocasionalmente conglomeráticos, desorganizados, de cor rósea-avermelhados, ferruginizados, não a pouco consolidados".Map 03 -Soils Map for Urucutiua's river basin.Source - CIRILLO (2004, p. 36) The prominence of the basin is flat, with medium decline of 5%, but near the channel, the decline increases 10% in the medium course because of the form of inserted valley, in the high and low declines varies around 2%.The vegetation removed is predominant on area, with border thicket and mangrove swamp in small areas in the low course.The hydrography is characterized by dendritic whose contributors are most intermittent channels.The climate of the area resembles the climatic type of the island with "temperatura média anual de 27,8ºC com amplitude térmica média de 8,5ºC" (PEREIRA & ARAÚJO, 2001, p. 144), with the predominance of soft winds of the northeast and raininess of 1.800mm/year (SEMA, 1999, p. 10).

Human aspects
According to Cirillo (2003, p.31) "o processo de ocupação do interior da referida ilha e das regiões circunvizinhas seguiu o sentido noroeste-leste, difundindo-se nos interflúvios dos principais rios da ilha".With the enlargement of the road system, the access to the other areas has been facilitated including the human occupation of these areas.

Main erosive processes in the area
Processes of the physical environment The main erosive processes identified in the area are: laminar erosion, furrow, ravines and excavations of the riverside.
The laminar erosion predominates in the basin thanks to the sharing process of the area and because people prioritized the flat area, taking the vegetation off.The ravines are in the main and secondary roads of the area, without appropriate pavement.The excavation of the riverside is the most serious form of erosion in the area.It contributes to the destruction of the Urucutiua River and its tributaries.It offers many problems to people who live in the riverside area as well as to piezometric surface because of the solid residues thrown in the channels as if the river were garbage.

PROPOSITIONS
Located in an area of high susceptibility of erosion, the Urucutiua river hydrography basin had its area mapped, standing out its physical attributions and the soil occupation form, making cross of these items based on the susceptibility to erosion, the preliminary susceptibility map to erosion in the Urucutiua river hydrographical basin were made (map 04), identifying places where erosion could happen thanks to the interference of human action.Map 04 -Erosions Map for Urucutiua's river basin.Source - CIRILLO (2004, p. 39) The low susceptibility on the edge of the high and low course of Urucutiua River predominates, thanks to the presence of vegetation and absence of human actions.The attributions decline, pedology, use and occupation of the soil contribute to the limitation of the susceptibilities in the area, but the geology will predominate in the edges with high decline.
Control action can be taken to a good plan, doing some constructions whose purpose are to avoid or decrease the energy of drainage of the superficial water making parallels construction to streets to the level curve, installation of hydraulic energy dissipater, pavements of streets, gutters, pluvial water galleries, and many more.The creation of a committee of the basin is another solution to the existent problems and in short time the analysis morphometrical and hydrological of the basin to make a plan.