*3.2. Topography*

Topographic analysis is a significant factor for flash flood hazard and risk assessment. Satellite-based Digital Elevation Models (DEMs), including SRTM-1arc second (30 m spatial resolution), were downloaded and processed to extract elevations, contours, slopes, aspects, and hydrological analyses shown in Figure 5a–c.

### *3.3. Watershed Delineation*

Hydrology is considered the main effective factor in flash flood intensity and its risk. The ArcGIS-based spatial modeler is used to delineate and map stream orders, flow accumulation, flow direction, and watersheds using the Strahler equation. The drainage basins that have been selected for study contain two large valleys, as shown in Figure 5e.

**Figure 5.** Multi-criteria applied in this study. (**a**) Elevation (m); (**b**) Slopes Degrees; (**c**) Streams; (**d**) Rainfall from 2010 to 2020 in mm; (**e**) Land use/Land cover in 2020; (**f**) Hydrological Soil Group based HYSOGs250m.

### *3.4. Rainfall (mm)*

The climate is one of the most important factors which directly affects the intensity of flash floods and run-off processes. Satellite-based rainfall data of this study area were collected and analyzed from 2015 to 2020 (https://power.larc.nasa.gov/data-access-viewer/ (accessed on 5 February 2023)). Figure 5d shows a spatial surface estimation of the rainfall data using the IDW interpolation method.

### *3.5. Land Use/Land Cover (LU/LC)*

Landsat-8 OLI imagery taken in 2022 was downloaded from the USGS website: https: //earthexplorer.usgs.gov/ (accessed on 28 January 2023). Preprocessing was done, and then unsupervised classification algorithms were used to map the land use/cover classes for the study area. This map is significant for flood hazard and risk assessment.

### *3.6. Hydrological Soil Group (HYSOGs250m)*

The global hydrological soil group data were downloaded from the website: https://daac. ornl.gov/SOILS/guides/Global\_Hydrologic\_Soil\_Group.html (accessed on 7 February 2023) to identify the geographic distribution map of soils inside the study area; three different soil categories were detected in the study area as shown in Figure 5f.

### **4. Results and Discussion**

Eight selected spatial criteria that applied in this study have been reclassified to hazard degrees for each factor separately from (1 to 5 values) (very low to very high), respectively, as shown in Figure 6.

### *4.1. Topographic Hazard Zonation*

Five hazard degrees were assigned for topography; the lowest elevation land considers a higher rate of flash flood than the highest elevated area.

### *4.2. Slopes Hazard Zonation*

The slope degrees are a topographic factor that refers to the flow speed of the rainfallrunoff water. The areas with fewer slope degrees are considered at risk of flood and inundation than the steep slope cliffs.

#### *4.3. Distances from Wadis (Rivers) Zonation*

The surrounding land of the channels is much more prone to effect by floodwater. In this study, the 3rd, 4th, 5th, and 6th stream orders are considered the main channels that are filled by water during floods. A 200 m buffer zone is considered a very high-risk zone, while the land far from 1000 m from the main channels is a safe area.

### *4.4. Drainage Density Hazard Zonation*

It is a hydrological factor that refers to the number of streams in the study area. GIS is capable of calculating the line density of streams in the sq. km. Areas with a higher density are considered more at risk than lower-density areas.

### *4.5. The Permeability Hazard Map*

The study area has different geological formations, and the quaternary deposits are considered permeable land (low-risk area), while the Eocene limestone formations were considered a semi-permeable zone (moderate risk). The carboniferous formation is much older and considered an impermeable zone (high risk).

### *4.6. The Soil Group Hazard Zonation*

Three soil groups were selected. Group (A) is considered the lowest hazard zone because it refers to sand-deep sandy soils with very high intrusion rates. Group (B) is a moderate hazard zone because it has relatively fine grains of soil with moderate intrusion rates. Moreover, group (C) is considered the highest hazard degree zone in this study area because it shows fine grains of soil with low intrusion rates.

### *4.7. Land Use/Land Cover Hazard Map*

The classification of satellite imagery of the study area produced several classes of land cover categories. The classes of dry valleys, Wadi paths, and flood-prone areas are considered the highest-risk areas, while the cultivated land is less risky.

### *4.8. The Precipitation Hazard Map*

Rainfall data density was classified into five hazard zones according to the amount of rainfall (mm). The higher rainfall area is assigned as a higher risk area.

### *4.9. AHP Weighted Overlay Calculation*

Multi-criteria decision-making analysis techniques include using the analytical hierarchy process analysis, which links multi-spatial data on the same scale. To apply these techniques, weighted overly modeling was used to calculate the weight of each factor and link all factors together using mathematical equations. Table 1. shows a pairwise comparison matrix for factor criteria (selected eight factors). In addition, the percentage of importance criteria values calculated in this study is shown in Table 2.


**Table 1.** Pairwise comparison matrix for factor criteria.

El: Elevation; SL: Slopes; Li: Lithology; RF: Rainfall; DoR: Distance from Rivers; DD: Drainage Density; LC: Land Cover; Sg: Soil Groups.



The resulting map from this study is presented in Figure 7. Three different hazard and risk zones were identified using the previous criteria. The higher hazard degree zone was displayed in the red color in the downstream run-off water. The main highway of Elgeish Road and Minia–Ras Ghareb highway, as well as the cultivated land along the fluvial fans, will be seriously affected by the downstream water. The moderate hazard zone covers a larger area surrounded by human activities.

**Figure 7.** Flash flood hazard map based on MCDA.
