## 3 Materials and Methods
### Sediment Texture Analysis from Datasets of Borehole Samples  
Stratigraphic alluvial units were sampled from four boreholes of 3 m depth in the upper channel reach of the Rambla de la Azohia. The drilling sites selected correspond to representative bed forms along this stream stretch: two boreholes were located in a longitudinal alluvial bar and another two in lateral channels (main and secondary). Samples were collected from each sedimentary structure for grain size analysis and the calculation of particle shape (Figure 2). Most of these sediments are unconsolidated materials, whose porosity is closely related to the types of packing, grain shape, arrangement, and particle-size distribution [28]. Finer particles have a more significant effect on porosity than coarse particles [29]. For spherical particles of uniform sizes, the porosity is 47.6% by cubical packing, and 26.0% by rhombohedral packing, which is the most compact packing type [28]. The porosity falls between these two values for other packing classes. However, for porous media in elastic deposits consisting of particles of non-uniform sizes, as in our case, porosity cannot be calculated directly, and other physical properties must be taken into account in addition to the packing effect, such as grain shape, particle size, sorting, and roundness [30]. The primary porosity usually increases as the grain size increases, the sediment is better sorted and more loosely packed, the grains become better rounded, and the clay content decreases [31]. For example, poorly sorted grains generally have closer packing and lower porosity across a wide range of grain sizes, as fine grains tend to fill the void space between large grains. Well-rounded grains with high sphericity should pack with a minimum of pore space [32]. [PERSON] et al. [30] considered four classes of grain packs from different aspect ratios (L/I = 1.0, 1.5, 2.0, 2.5). L/I being defined as the ratio of the length of the longest axis of a grain (L) to that of its shortest axis (I). Its results showed that the spherical grain pack exhibits a higher porosity compared to ellipsoidal grain packs. This effect is also shown in average permeability calculations, where the spherical grain pack exhibits lower permeability. Some typical porosity values of natural sedimentary materials are shown in Table 1.  
Figure 2: Sediment texture observed on the surface ((**a**): active channel, (**b**): transition area between active channel and central alluvial bar, (**c**): central bar platform) and in the depth of the trench sections ((**d**): sub-surface layer 1 m deep in the active channel area, (**e**–**h**): sub-surface layers 1-1.5 m thick from top to bottom in a bar sedimentary sequence), and (**i**): representative texture of the alluvial fan body cut by the Rambla de la Azohia in its upper reach. The median grain size (D\({}_{50}\)) and 84 th percentile (D\({}_{84}\)) were calculated for each sample. As effective grain sizes were used the 10 th (D\({}_{10}\)) and 20 th (D\({}_{20}\)) percentiles, that is to say, the particle diameter (mm) of 10 and 20% of all sediment is the smallest, respectively by weight. Many authors [33; 34; 35; 36] have already regularly used these diameters to calculate hydraulic conductivity (HC) in detrittal deposits. The application D\({}_{10}\) as an effective grain size in this case has been limited to uniform sands [37]. However, in the case of gravels and pebbles, we have found a greater relationship between D\({}_{20}\) and the ERT results, which can probably be maintained with the HC values. A sorting index (\(\sigma\)) was also adopted as a measure of granulometric dispersion, which takes the expression \(\sigma=(\text{D}_{84}/\text{D}_{16})^{0.5}\). The derived metrics, particle-shape indices, maximum projection sphericity index (MPSI), and characterisation of sediment textures lead to improved bedload estimates with reduced uncertainty, as well as improved understanding of the relationships between sediment texture, grain size distribution, and electrical resistivity.