4.3 Erosion risk evaluation

Erosion assessment was achieved using the Van Zuidam qualitative methodology (1986) which evaluates soil erosion risks based on the knowledge and field survey of different factors that can lead to erosion. The result was compared to field data in order to analyse the correspondence with the actual erosion status and the susceptibility of the soil to erosion risk.

Methodology

The qualitative evaluation takes four main characteristics into account:

• slope: steepness, length, form;
• soil/geology: soil depth, texture, sealing susceptibility, rock weathering degree, structure of underlying strata, depth of the first impermeable layer;
• vegetation/land use: vegetation cover, rainstorm frequency, conservation practice;
• erosion and mass movement: rating of wind erosion, sheet erosion, rill-ravine-gully erosion, mass movement.

Each factor was divided in classes and a rate was given according to the influence in the erosion dynamic (Table 52-Table 55). The rainstorm frequency was considered as homogeneous for all the area and it was ascribed to the class of a few rainstorms every year and rated as "2". It was not always possible to recognise the structure of underlying strata and the rock weathering degree, so the missing data were not considered and were rated as "0". In agricultural relevés, with a steepness less than 5% and with a slope length greater than 100m, the value of the slope length was assigned, by default, to 100m to reduce the factor influence on erosion processes. The rating classes of erosion parameters were obtained combining the spacing and the depth of rill, gully, and ravine erosion processes (Table 56). The same was done for mass movement factors (Table 57). The rating of the vegetation cover was multiplied by the frequency of rainstorm and successively the result was added to all the other factor ratings; the obtained sum was compared to the erosion susceptibility class showed in Table 58. The original Van Zuidam thresholds were slightly modified to better fit with the characteristic of the Ben Slimane province. The areas for land use that are precluded from the assessment (like urban areas, airport, golf court, and quarry) are classified as NR (not relevant).

Results and discussion

It is possible to see that the areas less susceptible to erosion are located in the northern part of the study area, inside the cork oak forest of Ben Slimane and in the central part where the land is flat or almost flat (Figure 41).

The central part, occupied by agricultural fields, presents low erosion susceptibility due to the flat or almost flat nature of the area; the differences are due to the nature of the soils. Vertisols, e.g. in land unit 1, are classified in class 1. Instead a moderately erosion susceptibility (class 2 and 3) is present where the soils are hydromorphic.

Despite the undulating aspect of the northern part of the study area, the presence of the forest limits the erosion process and preserves the soil. For this reason the area is ascribed to class 1. In some marginal areas of the forest the susceptibility class increases to class 2 because of the reduction of the natural vegetation cover. Where natural vegetation is replaced by agriculture, a susceptibility class increase can been observed, like in land unit 5 ascribed to class 3.

Regarding all other areas the slope constitutes the most relevant aspect. The southern hills are generally steep, but the presence of afforestation precludes huge erosion processes; in fact there is an intensification of erosion dynamics inside the agricultural land unit (land unit 21 is ascribed to class 5). Table 59 shows a synoptic view of the erosion assessment.

Steepness (%)		Length (m)		Form
Value	Rating	Value	Rating	Value	Description	Rating
0-2	1	0-14	1	waning	steep upslope element, gentle downslope element		1
3-7	4	15-50	2	waxing	gentle upslope element, steep downslope element		2
8-13	8	50-150	4	maximal	gentle upslope element, gentle downslope element		3
14-20	16	150-500	6
21-55	24	>500	8
56-139	32
>140	24

Table 52 - Ratings for slope factors

Table 53 - Ratings for vegetation/land use factors

Table 54 - Ratings for soil/geology factors and for erosion (wind, sheet, and rill/ravine/gully) and mass movement factors

Table 55 - Ratings for soil/geology factors

Table 56 - Rill/gully/ravine comparing table to assess the final rating

Table 57 - Mass movement comparing table to assess the final rating

Table 58 - Ratings sum and corresponding class for erosion assessment

Table 59 - Results of erosion risk evaluation

Figure 41 - Erosion risk evaluation map (van Zuidam, 1985, modified)

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