Geomorphology and rehabilitation of erosion-degraded areas using soil bioengineering in the Rio Bacanga basin, São Luís, Maranhão State

Abstract

The research analysed the geomorphological characteristics of the Bacanga basin of São Luís municipality. Basin characteristics were related to highly developed erosion processes. The approach considered the identification of environmental fragility classes, the monitoring of an experimental station and the rehabilitation of a degraded area using soil bioengineering techniques. The adopted methodological procedures included: 1. Cartographic and bibliographic surveys. 2. Mapping of the hypsometry, slope, land use, rainfall index and geomorphology of the Bacanga basin, along with analysis of the morphostructure and morphosculture of the Gulf of Maranhense and environmental fragility mapping. 3. Establishing an experimental station with two replicate erosion plots and measuring the following parameters: vegetation cover index, soil surface changes using erosion pins, soil matric potential, runoff and sediment loss. 4. The rehabilitation of Sacavém gully using soil bioengineering techniques (using geotextiles constructed from palm leaves of the Buriti tree). Mapping showed that identified gullies are located on the plateau edges of the basin and are very fragile environments. The greatest interval of vegetation cover index development was between February (0%) and March (33.35%) (both 2009), whereas the smallest difference was 5.31%, between May (75.88%) and June (both 2009) (81.19%). The difference of erosion/deposition pin data within the bare and vegetated soil plots was significant using the non-parametric Kruskal-Wallis test (P <0.001). The results obtained from tensiometers at 10, 20, 40 and 60 cm depth showed a significant difference (P <0.001) between the bare and geotextile-covered plots. Soil matric potential measurements indicate that geotextile plots had an improved soil water regime. Rainfall during the measuring period (February-June 2009) was 2,067.5 mm. This caused a total of 494.6 L m-2 runoff from the two bare plots and 208.6 L m-2 from the two geotextile plots. There were significant differences in soil loss between the plot treatments, demonstrating the effectiveness of geotextiles plus grass in decreasing erosion rates. The two bare soil plots lost a total of 4,391.0 g m-2 of sediment, while the geotextile plots lost 255.9 g m-2. Rehabilitation work on Sacavém gully showed that soil bioengineering was a very effective soil conservation technique.