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• Erosión
• Geomorfología
• Suelo
• Materiales naturales
• Evaluación

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4.3 Erosion risk evaluation The evaluation of soil erosion risk was obtained applying the Van Zuidam methodology (1986). It is a qualitative approach based on the knowledge of the surveyor to detect and recognize during the field survey, different factors involved in the erosion dynamics: geological, geomorphological, climatological, land cover and land use. Obtained results were analyzed and compared to the field data in order to evaluate, where possible, a correspondence with the actual soil erosion. Four main categories of parameters were used for the qualitative assessment of soil erosion risk:  slope: slope gradient, length, form;  soil/geology: soil depth, texture, surface sealing;  vegetation/land use: vegetation cover, rainstorm frequency, conservation practices;  erosion and mass movement: rating of wind erosion, sheet erosion, rill/gully/ravine erosion, mass movement. The morphology, soil and geology parameters are listed in Table 46, Table 47 and Table 48, the vegetation and land use parameters are listed in Table 49 and the geomorphology processes are shown the Table 50. Each parameter was divided in classes and a rate was given, according to its contribution, to the erosion susceptibility. Regarding the climatologic condition factors, the rainstorm frequency was considered as homogeneous for all the area and it was ascribed to value of 3 that corresponds to the heavy rainstorm frequency of several times during a year. It was not always possible to recognize the structure of the underlying strata and the rock weathering degree, so the missing data were not reported and then rated as zero. As most of the study area is used for agricultural practices, the land cover was divided into the following vegetation density/land use conditions:  agroforestry and forest plantation were rated as 1;  natural vegetation was rated as 2;  the fallow systems was rated as 12;  the other types of agriculture areas like irrigated cultivation, rainfed arable cultivation were rated as 3. The rating classes of erosion risk parameters were obtained combining the spacing and the depth of rill/gully/ravine erosion process. The rating of the vegetation cover was multiplied by the frequency of rainstorm and consequently the result was added to all the other factors ratings; the obtained sum was compared to the erosion susceptibility class as shown in Table 51. Considering the geomorphological aspects of the study area, the original Van Zuidam criteria was slightly modified to better fit the characteristics of the Thiès province. So, as for the texture, the presence of clay material was given a high value, especially in the low areas that are not affected by erosion like the bottom of Tanma Lake. Consequently, the final rating of the texture was affected by the value of the slope gradient: if the slope gradient was lower than 3%, the clay texture was not taken into account and a new value of 1 was given. The areas belonging to the urban, airport, military zones, quarries were excluded from the assessment and were classified as not relevant (NR).

Results and discussion As the study area is mainly characterized by a flat to gently sloping surface, the erosion effect is considered to be very minimal. In fact, as shown in the erosion risk map, (Figure 27) the erosion

susceptibility is considered to be slight along the coastal and fixed dunes area and in the Thiès cuestawith shallow sand coverage. Indeed, in the marginal areas with the forest plantation of Casuarina equisetifolia, the erosion degree decreases to a low class, mainly because the presence of the forest limits the erosion processes, in particular wind erosion, and preserves the soil. For this reason, the area is ascribed to class 1. Meanwhile, along the dissected accumulation glacis and the cuesta outliers, the erosion susceptibility is moderate due to the increase in steepness, where water action is the main geomorphological agent. While in the southern part of the region, the wind erosion is more dominant and the area is affected by moderate erosion susceptibility. In the middle part of the study area, along the plateau with a shallow sand coverage, the erosion susceptibility is considered to be high, due to the presence of dense drainage network and a low vegetation coverage, so the area is easily prone to gully and rill erosion. High values of erosion susceptibility are present along the Thiès scarp, especially in the area near Mont Rolland, where big gully erosion is present as verified during the field survey. Slope gradient (%) Value 0-2 3-7 8-13 14-20 21-55 >56 Table 46 - Rating for slope factors.

Rating 1 2 4 8 24 32

Depth of unconsolidated material (cm) Ratin Description Value g Very shallow 0-24 4 Shallow 25-49 3 Moderately deep 50-99 2 Deep >100 1 Table 47 - Ratings for soil/geology factors. Texture Description (*2) C, SiCL SC, CL, SCL, L, SiL, Si SL, S C, SiCL in flat area

Rating 8 4 1 1

Slope length (m) Value Rating 0-14 1 15-49 2 50-149 4 150-499 6 >500 8

Depth of the first impermeable layer (cm) Ratin Description Value g Shallow 0-49 4 Moderately shallow 50-99 2 Moderately deep 100-149 1 Deep >150 0

Surface sealing Description Slight hard Hard Very hard Extremely hard

Rating 1 2 4 6

(*2) C (Clay), SiCL (Silty clay loam), SC (Sandy clay), CL (Clay loam), SCL (Sandy clay loam), L (Loam), SiL (Silty loam), Si (Silt), SL (Sandy loam), S (Sandy).

Table 48 - Rating for soil factors.

Vegetation cover (%) Value (*1)

Rating

MF, FP NV HE1, AA1, AA4, AA6, AP1, AT1 AA2

1 2 3

Rainstorm frequency Ratin Description g Exceptional 1 Once in a year 2 Several times a 3 year

Conservation practice Rati Description ng Benching -6 Terracing -4 Contouring -4

12

(*1) MF (Agroforestry), FP (Plantation forestry), NV (Natural vegetation), HE1 (Nomadism), AA1 (Shifting cultivation), AA4 (Rainfed arable cultivation), AA6 (Irrigated agriculture), AP1 (Non-irrigated perennial field cropping), AT1 (Non-irrigated tree crop cultivation), AA2 (Fallow system cultivation).

Table 49 - Rating for vegetation/land use factors. Rill/gully/ravine Description Value Slight 1 Moderate 2 Severe 4 Table 50 - Rating for erosion.

Sheet erosion Description Value Slight 1 Moderate 2 Severe 4

Wind erosion Description Value Slight 1 Moderate 2 Severe 4

Description Ratings sum Not or insignificantly susceptible to erosion 0-16 Slightly susceptible to erosion 17-24 Moderately susceptible to erosion 25-29 Highly susceptible to erosion 30-48 Very highly susceptible to erosion 49-64 Extremely susceptible to erosion >65 Table 51 - Rating sum and corresponding class for erosion assessment.

Class 1 2 3 4 5 6