## 4 Results
### Flood Risk Assessment
In the present study, the geometry indexes were extracted to aid in explaining the classification of the catchments and sub-catchments. Linear indexes that refer to the properties of the drainage network were used widely to evaluate the flood hazards in many regions [40; 54; 55]. Ref. [36] calculated the linear indexes to assess the flood hazards in southwest Sinai in Egypt. In our study, several linear indexes were extracted, including stream orders, total stream lengths, and the bifurcation ratio. Particularly, stream numbers and order are very effective indexes in hydrological studies. The majority of stream orders were recorded for the fourth-orders in five catchments. The high leveling of branching in our drainage system indicates high possibilities of water flows. The results of the Bifurcation ratio index (\(\rm{R_{b}}\)) indicate that all of the investigated catchments reflect low values (1.53-3.74), and their frequencies (F) produce values between 0.491 (SC1) and 0.702 (SC2) and this tends to give suitable conditions for flash flood possibilities. Morphometric drainage density (\(\rm{D_{d}}\)) reveals important catchment characteristics, including land infiltration capacity, climatic conditions, and vegetation cover [56; 57]. Author in ref. [39] stated that the average drainage density index is 0.93 km/km\({}^{2}\), which reaches 1.24 km/km\({}^{2}\) in mountain areas that have high rates of precipitation. Catchments of high drainage density show high rate of flood flows with low amount of groundwater storage, while the low drainage density values indicate suitable conditions for infiltration rate and decreasing rate of runoff potential in addition to erosion-resistant fractured hard rock of the investigated zone [36; 58]. The drainage densities of the study zone reflect low relief, dense vegetation, and a low amount of groundwater for the majority of the study catchments and sub-catchments. The \(\rm{D_{d}}\) index results indicated that, the contribution of local rains to the groundwater is low as expected. Consequently, high values of stream frequency (>0.5) provide high possibilities for water runoff collection that reflect impermeable sub-surface beds, high relief, and minimum infiltration rates [8; 36; 59]. Regarding the density values of the study catchments and sub-catchments, the calculations give moderate values between 0.816 (SC1) and 0.945 (SC4). The only high drainage density refers to SC3 with 1.016 and this value can be considered as a D\({}_{\text{d}}\) anomaly. The previous results reflect moderate relief, moderate level of impermeable subsurface beds, and moderate conditions for groundwater, particularly in the basement rocks area. According to the general morphology of the study zone, the safest zone against the flash flooding risk is the inland coastal zone. This is because of the high infiltration covering and the gentle slope plain strip [49; 53]. In contrast, the impervious basement rocks of the western Red Sea mountain hills reflect high conditions of runoff possibilities [36]. The relief of landforms depends mainly on the topographic and tectonic developments [21; 40]. In the current study, several topographic maps illustrated the topographic evolution of the study area (Figure 5). Refs. [21; 60] categorized landscape slopes into nine groups: from flat running to very steep zones. In this study, the gradient slopes of the study catchments and sub-catchments give the steepest values for the western linear zone. Elevation, contouring, and aspect maps illustrated the topographic signals of the study zone. Relief indexes including relief, relief ratio, and ruggedness number, are important factors that aid in understanding the structural complexity of the topography of any region [21]. The curve number (CN) is a simple example of a moderate flood evaluation factor. It is applied in flood management projects and engineering designs. Low CN curve values dominantly cover the catchments that have beds with a high degree of infiltration losses.