Current Dynamics of Hydric Erosion in the Kingouari, Mfilou and Djoué Watersheds in the Southwestern Part of Brazzaville City (Congo)

Abstract

The objective of this study is to quantify soil losses in the Kinsoundi 16A district, located in the Kingouari, Mfilou and Djoue watersheds. The aim is to highlight the current dynamic of water erosion in this southwestern part of Brazzaville's urban area and to evaluate the effectiveness of the control measures used. The methodological approach adopted was based on: (i) the measurement of the evolution of the soil surface, using profilometers placed on the 12 main roads in the district where 68 measurement points have been installed, (ii) the three-dimensional measurement (volume) of 22 ravines and gullies that incise the district, (iii) the measurement of slopes along arteries using a clinometer, (iv) the inventory of facilities used to control soil erosion, (v) interviews with populations to understand the history of the phenomenon and the control methods used. The soil losses by surface stripping, measured on 12 roads from 2007 to 2010, are 78.1 t/ha. A destruction of 4.7 ha of land, or 2% of the Kinsoundi 16A territory and 94,247 t/ha of land, was removed from these watersheds in 20 years by the 22 ravines. The aggressive nature of this dynamic is explained by the combination of the following natural and anthropogenic factors: frequent rains (≥ 15 days/month) very erosive with a Rusa index of 287 to 866 points, sandy soils with more than 90%, without cohesion and fragile, densification of the habitat on the ground (2 houses on average per plot of 400 m2), resulting in increased waterproofing of surfaces and responsible for the current erosive runoff, the absence of channels to canalize and control runoff, hence roads transformed into storm water collectors. The control measures used by the populations are inadequate and therefore not very effective (bags of soil that can be degraded in three months, used tires, solid waste dumped at the head of the ravines). Those financed by the State are expensive gabions but of limited effectiveness around the treated ravine. It is therefore imperative to treat the entire district as a risk watershed, by strengthening its vulnerable sectors and by building and developing a coherent system to control rainwater runoff. These curative measures must be underpinned by preventive actions that are more effective and less expensive.