Populated areas in dryland regions have experienced great difficulties in better managing inhabited lands, which are usually under severe hydrological stress (Arzani 2010; El Bastawesy et al. 2013a, b). According to the hydrological systems, surface water supplies to urban areas can be a significant factor limiting the degree of land use (El-Baz et al. 2000; Rubin 1991; Tooth 2000). However, economic expansion has led to the creation of important urban and industrial centers in many dryland areas where there is an adequate supply of surface water (Hutchinson and Herrmann 2008).
Urban areas' growth and their infrastructure networks take place to the detriment of the surrounding landforms which differ in their geological and hydrological properties. The most suitable locations for urban development are the low relief areas of alluvial fans, wadi beds, and piedmonts (Dunne 1991). Notwithstanding, these areas in drainage basin outlets usually contain potential aggregate and row deposit supplies for building materials as well as groundwater resources fed by periodic rainstorms and flash floods (Blair and McPherson 1994).
Consequently, flash floods’ impact on urban areas may vary from catchment to catchment and even within the same catchment themselves depending on sediment loads (El Bastawesy et al. 2009). Besides, the encroachment of urban features on alluvium beds increases surface runoff coefficients due to laying down on the surface of impermeable parcels and pavements (El Bastawesy et al. 2013a, b).
Flash floods are one of Egypt’s principal natural disasters that cause cutting roads, sweeping away homes, and damaging power lines. Flash floods occur mostly in the Eastern Desert and Sinai regions, where high terrain and incised wadis are present (Yousif and Hussien 2020; Mahmoud et al. 2020).
Recently, in urbanized areas within dryland countries, the creation of a perched water table, wastewater seepage, and wastewater ponds has become common (El Bastawesy et al. 2012; Bahabri 2011). Indeed, dealing with wastewater, which is released at a rate faster than its proper disposal, is a major challenge for most urban areas in the dryland (Abuzaid and Fadl 2016, 2018) and it is very harmful to the groundwater quality (Farrag et al. 2019; Megahed and Farrag 2019).
Therefore, the importance of understanding the hydrological processes of these drainage basins is to avert the negative impact of flash floods, prone areas to wastewater seepage and pond (Chin and Gregory 2001; Cooke et al. 1982). Unfortunately, dryland hydrological measurements remain insufficient to establish a detailed description of these systems. Moreover, the hydrological procedures are not fully understood and the hydrological models are uncalibrated (El Bastawesy et al. 2009). Nevertheless, most of these data are also limited and obtained by individuals or organizations during pilot projects and case studies which cannot reflect the diversity of dryland processes (El Hames and Richards 1998; Robinson 1994). This is why sudden flash floods often lead to devastation, and the wastewater impact is gradually increasing (White 1995).
Remote sensing provides a fast data collection tool using multiplied spectrum wavelengths, allowing for accurate diagnosis at different time intervals of the different land use and land cover. Hydrological data indices include water ponds, waterlogged areas bodies, and active channels within a specific catchment which can be achieved using the multi-temporal sets of satellite images. (Subyani et al. 2009; El Bastawesy et al. 2008). Data sets have been widely obtained using GIS due to its ability to handle remote sensing data and thematic maps (Foody et al. 2004; Maidment 1993; Robinson 1994).
This research uses remote sensing and GIS with field measurements to study the effects of recent flash floods that affected several urban areas in Greater Cairo. Wadi Degla was included as the chosen catchment for this research, representing various types of flash floods in terms of wadi system distribution and response.