*Soil Erosion Risk Analysis of a Small Watershed DOI: http://dx.doi.org/10.5772/intechopen.111424*

#### **Figure 3.**

*SENTINEL2-A image of the Ramla valley mouth, sandy beach, and adjacent coastal waters (Source: COPERNICUS).*

average elevation of each sub-catchment basin together with the Strahler stream number and their lengths [13].

Detailed morphometric analysis shows that the largest sub-watershed basin is around 110 ha (SWB 3, **Figure 2c**). The mapping of the 4th and 5th stream order categories found in the Ramla watershed area is shown in **Figure 2b**. According to **Table 2**, the longest water channels belong to stream order 2 followed by stream order 1, with a total of 2760 and 1427, respectively. The drainage pattern is tightly linked to a landform, where rocks are flat-lying and preferential zones of structural weakness are minimal.

This integrated analysis points to a sub-watershed structure that is the direct result of an underlying lithology with a hydrology that is a direct function of the geomorphology and topography, including that which is artificially induced. Results show that stream runoff in the Ramla valley catchment is mainly dominated by SWB 34, 31, 33, and 10. Therefore, these sub-watersheds are deemed to be important from a soil erodibility management point of view.

#### **3.2 Degree of soil erodibility from the Ramla watershed**

Soil erodibility within the valley (**Figure 4b**) varies from a low (blue color shading) to a high rate of more than 30 ton ha�<sup>1</sup> yr.�<sup>1</sup> (yellow color shading). The RUSLE estimation showed that the area around the in-Nuffara Hill is likely to be most affected by high erosion rates. This is because its slopes consist of fallow fields which lack proper maintenance of rubble walls together with the occurrence of motorcycle off-roading practice, which together are contributing to higher soil erosion. The high slopes along Xaghra (west) and Nadur (east) headlands, which consist of blue clay and also silty soil, also show a high degree of soil erodibility (**Figure 4b**).

The RUSLE estimation of the Ramla watershed shows that its soil erosivity can be generally considered as tolerable (<10 ton ha�<sup>1</sup> yr.�<sup>1</sup> ). The type of land-use land

*Soil Erosion Risk Analysis of a Small Watershed DOI: http://dx.doi.org/10.5772/intechopen.111424*

#### **Figure 4.**

*DEM (a), and RUSLE, showing 3rd-, 4th-, and 5th-order channels derived by this study, leading to the mouth of the valley and into the coastal waters (b).*

cover present within the tolerable range is generally characterized by seasonal crops and by a gentle slope gradient that tends to cause reduced runoff velocity. However, at higher slope gradients, one can observe elevated risks of soil erosion (>30 ton ha�<sup>1</sup> yr.�<sup>1</sup> ) associated with the onsite presence of abandoned and/or neglected field terraces having poor vegetation cover. Site investigations also point to a significant collapse of protective barriers within these areas (**Figure 5**).

The terrain ruggedness index (TRI), which is based on the sum change in elevation between a grid cell and its neighboring grid cells of 1 m<sup>2</sup> ground resolution, reveals the distribution of the terrain heterogeneity. The resultant TRI map shows high ruggedness within certain parts of the valley system that are also prone to soil erosivity (**Figure 6b**). The TRI map shows high values to the left of the valley mouth, which incidentally is also associated with a significant degree of erodibility (**Figure 4b**, with areas having a soil erodibility of around 10–15 ton ha�<sup>1</sup> yr.�<sup>1</sup> ) incorporating in it a number of stream channels that head directly toward the coast and therefore, contributing to sediment movement toward this direction. High index values are also

#### **Figure 5.**

*High risk of erosion located along the in-Nuffara Hill area (shown in red), which is primarily associated with SWB 34 (Figure 3c).*

#### **Figure 6.**

*Terrain ruggedness index map derived for the Ramla watershed (a). Note the high index value in the inset (b) to the left of the valley mouth associated with significant degree of erodibility (Figure 4b, with areas having around 10–15 ton ha*�*<sup>1</sup> yr.*�*<sup>1</sup> ) and associated stream channels heading toward the coast.*

found within the southern parts of the Ramla valley associated with the high degree of erosivity surrounding the in-Nuffara Hill (**Figure 5**).
