Mero Yannah

This paper examines the hydrological dynamics and flooding of U-shaped valleys in urban areas, with projection to some stream channel systems in Yaounde and its environs. So, it relies on analysis of unmodified (or natural) stream channel systems, assuming that the quantity of water of an unmodified stream channel system is directly related to the size it usually carries and that it tends to bring back during high discharges or floods. The drainage basins are usually elongated, with the time of concentration depending on the trunk stream length, e.g., 9.35 hours for the Mefou watershed and 5.21 hours for the Mfoundi sub-watershed. The streams continuously adjust the shape of their channels; for instance, in the Mefou River, the floodplain straight channel segments do occur with sinuous line, thus the sediments are deposited to form bars. Such path and deposition of sediments result in meandering. During extremely high discharges, the water flows in a braided pattern around islands and bars. Although generally characterized by dense, dendritic drainage patterns, the stream channel systems sometimes exhibit a remarkable centripetal drainage pattern associated with flood basins. Since the river discharge usually exceeds the capacity of the channels at U-shaped valley junctions, such as the low-lying areas at Nkolbisson, Midtown, Nguem, Etoa, and Mehandan, floodings are recurrent.

Pawara area is a mining district in the eastern region of Cameroon. Mining in the area is generally artisanal and semi-mechanized, practiced by the local miners and immigrants from neighboring African countries and China. The lack of strict regulations and control of mining activities permits the miners to use illegal substances, especially Hg in gold separation. These expose the area to toxic and heavy metals pollution. This study highlights the source of heavy metals concentration in the Pawara soils and the potential adverse effects of Hg on gold separation to the environment and health. Three mining sites and one control site were investigated, namely Site A, Site B and Site C. The control Site 0 (background) is an area where no mining and agricultural activities have taken place. Soil samples were collected at depth of 20 cm, with six from each site (24 samples). Samples were analyzed for Al, Cd, Cr, Cu, Fe, Hg, Pb, Cd and Zn content using atomic absorption spectrophotometry in a graphite furnace. The metals, except for Fe, show high values for all three sites exceeding the background levels in the soils. Hg shows the highest concentration on Site A with a value of 1590 mg kg⁻¹. Pb is highest on Site B with a concentration of 12,274 mg kg⁻¹. The contamination degree was assessed with the help of contamination indices (Igeo—index of geo-accumulation; PLI—pollution load index; RI—potential ecological risk; Eri—ecological risk; Pi—single pollution index; CF—contamination factor) and all parameters show a high degree of contamination on all three sites compared to the control site. Hg, Pb, Cd, Cr and Cu as single pollutants show the highest ecological risk on Site A and Site B where intense mining is taking place. The absence of industrial and large-scale agricultural activities in the Pawara area, the nonexistence of contaminants on the control site and the presence of contaminants on Site C where farming is high and mining is low jointly show that the discharge of mine wastes onto the soils and stream channels are the main source of contaminants and potential pollutants of the Pawara ecological environment.