**4. Materials and methods**

*Recent Advances in Flood Risk Management*

flow of water.

Volcanic lava tongues are associated with lineaments and ancient valleys. These lava formations follow the tracks of the paleovalleys and the direction of the movement of the water. The presence of volcanic languages and Quaternary sediments plays an important role in locating the population and human activities following the presence of springs and loose and fertile soil. The deltas of coastal *wadis* are affected by agricultural activities along the Red Sea coastal plain. **Figure 4** shows the geomorphological and topographic contexts of the city of Jeddah. This figure shows that the city extends on the outlet of several *wadis*, which in spite of the urbanization take back their old courses during strong and intense precipitations. Jeddah has developed all over the coastal plain and beyond, disrupting the normal

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**Figure 4.**

*Geomorphological and topographic contexts of Jeddah city.*

Remote sensing is a crucial tool for researching submersion areas and for assessing natural hazards. The present study is based on the processing of multisource and multidate satellite data, supplemented by cartographic documents and field observations. These different data were integrated in the same mapping projection system before proceeding to a series of digital treatments: digitalization, color composition, improvement, combined classifications using image texture, and fusion. **Figure 5** shows the different stages of treatment. Rainfall statistics were also used.

#### **Figure 5.**

*Methodology of the study—Different stages of treatment.*

**Figure 6.** *Extract from Landsat ETM+ of February 1, 2011. There are only a few small water bodies (circled in yellow).*

It should be noted, however, that the data used do not allow for immediate monitoring of the catastrophic floods that occurred in 2009 and 2011. The lack of information on the intensity and duration of rainfall data that caused the floods does not allow for a detailed analysis of rainfall conditions. For this reason, we will limit ourselves to a general descriptive analysis of the 1970-2017 rainfall data sets for the Jeddah station. At the level of remote sensing data, we did not find SPOT or Landsat images within 3 days after the floods, whereas beyond this period the traces of flood are no longer visible on the satellite imagery, as shown in **Figure 6**. We note that phenomena are as violent as ephemeral. The only image in our possession that is close to the January 27 flood is a Landsat ETM+ acquired on February 1, 2011 (**Figure 6**). The data at our disposal nevertheless allow the monitoring and mapping of the most vulnerable areas.
