Influence of anthropogenic climate change on soil erosion occurrence: Gully Nacala, Mozambique

Gully is an issue of the climate crisis in most coastal areas of the world. More than 70% of the world's sandy coastlines would be eroded in recent decades. Anthropogenic climate change is one of key factors controlling soil erosion occurrence. Soil erosion control is extremely important for soil conservation work and territorial planning. The municipality of Nacala, province of Nampula, Mozambique, has gully susceptibility as a known problem. The Japan International Cooperation Agency (JICA), concerned about the intense urban erosion in the Nacala region, which causes problems not only for the population living in critical areas but also for the proper functioning of the Nacala Port, has been making efforts to identify the causes and mechanisms of the phenomenon and suggest corrective and preventive measures covering the area of the Mocone basin, Nacala. Thus, we obtained information on the causes and mechanisms of evolution of these phenomena to allow the joint search for the features that determine the natural susceptibility to erosion of the municipal territory. We also identified that the soil type in the Mocone basin is sandy-clayey, with an infiltration capacity of 50mm/hour, erodible, with a drainage network mainly characterized by rills and gullies, with periodic surface runoff, only during heavy rains. Our studies indicated a catchment with a significant area upstream of the erosion process (2.34 km²). The maximum project flows obtained in the our preliminary hydro-meteorological studies, considering intense rains with a return period of 100 years, and with duration equal to the time of concentration of the catchment basin (90 min), also proved to be quite significant, with values above of 25 m³/s. Accelerated erosion in this location seems to occur, preferentially, in sectors where those natural factors have been exacerbated by anthropic factors due to inadequate occupation of the physical environment. This urbanization resulted in waterproofing, an increase in surface runoff volumes and a reduction in the concentration time of this basin. In addition, the rainwater drainage of some streets was released directly into the natural terrain, without the proper hydraulic works to reduce energy. The rapid development of the erosion branch towards the Nacala Port proves this close relationship between inadequate urban drainage and the evolution of the analyzed erosion process. The results obtained also prove, once again, the importance of analysis and mitigation approaches for large scale erosion processes that contemplate the catchment basin in its entirety, where erosion control practices or changes in soil cover are necessary.