Voçoroca do conjunto habitacional São Sebastião – Rio Preto da Eva: dinâmica evolutiva

Abstract

Soil erosion, intensified when rain falls on deforested slopes, has become a constant threat on the urban fringes of Central Amazonia. In the São Sebastião Housing Complex, in Rio Preto da Eva (AM), this pressure gave rise to a gully — a deep and rapidly expanding incision — which serves as a case study to understand how topography, soil, water, and human use interact and exacerbate environmental degradation. The research aims to address two gaps: the lack of multi-year monitoring series of erosive features in humid tropical environments, and the limited quantification of their environmental and economic costs. The main objective was to monitor the morphological evolution of the gully between 2011 and 2024, identify the factors driving its advancement, and estimate the physical and financial repercussions of the erosion.The methodology combined geomatic techniques, soil laboratory analysis, and socioeconomic information. Six surveys using unmanned aerial vehicles (UAVs) generated orthomosaics and digital terrain models that enabled measurement of area, volume, and linear progression of the incision. In the lab, soil samples collected from ten depths along the gully wall were analyzed for texture, density, porosity, infiltration, and penetration resistance, along with sediments deposited in the Agripino stream. Daily precipitation data (CHIRPS) were correlated with growth phases, and interviews with residents supported the estimation of property damage. All results were interpreted through a systemic theory lens, which considers feedback loops between geomorphological, hydrological, pedological, and human subsystems. The results show uninterrupted progression of the feature. The area increased from 3,611 m² in 2019 to 4,910 m² in 2024; the eroded volume grew from 64,600 m³ to 103,600 m³. The cumulative linear advance at the head reached 31 meters, peaking at 8.5 meters during the 2020–2021 biennium, a period of intense rainfall. Surface layers (0–6 m) showed clay content >55% and porosity >40%, facilitating disaggregation by raindrop impact and surface sealing. Below 10 meters, the profile becomes sandy-loam, denser and less porous, predisposing the soil to block falls and subsurface flow. In the streambed, the Old Deposition Area contains an average of 72% sand, indicating hydraulic reworking, while the Recent Deposition Area retains finer fractions, reflecting recent and minimally reworked deposition. The impacts are evident: stream sedimentation, damming, death of riparian trees, and loss of buildable lots. The estimated property damage from land loss exceeds R$113,000. The projected cost for complete stabilization of the incision surpasses R$1.7 million, revealing that delaying corrective actions increasingly burdens public authorities and the community. The average annual progression of 0.5–1 meter at the head translates not only into lost soil, but also into risks to housing security and deteriorating downstream water quality. It is concluded that the gully results from the convergence of steep topography, friable lithology (Alter do Chão sandstones), and urban practices that concentrate runoff — earthworks, poorly designed stormwater networks, and removal of secondary forest. The adopted systemic approach allowed the pace of degradation to be translated into tangible environmental and financial metrics, highlighting the need to integrate engineering works (energy dissipators, revegetation, proper drainage) with land use regulations that respect terrain limitations. Only with preventive urban planning and proactive environmental management will it be possible to contain the gully's expansion and mitigate its socio-environmental impacts.