The generation and processes of wadi flash floods are very complex and are not well understood. In this paper, we investigate the relationship between variations in geomorphometric and rainfall characteristics and the responses of wadi flash floods. An integrated approach was developed based on geomorphometric analysis and hydrological modeling. The Wadi Qena, which is located in the Eastern Desert of Egypt, was selected to validate the developed approach and was divided into 14 sub-basins with areas ranging from 315 to 1488 km2. The distributed Hydrological River Basin Environment Assessment Model (Hydro-BEAM) was used to obtain a good representation of the spatial variability of the rainfall and geomorphology in the basin. Thirty-eight geomorphometric parameters representing the topographic, scale, shape and drainage characteristics of the basins were considered and extracted using geographic information system (GIS) techniques. A series of flash flood events from 1994, 2010, 2013, and 2014, in addition to synthetic virtual storms with different durations and intensities, were selected for the application of this study. The results exhibit strong correlations between scale and topographic parameters and the hydrological indices of the wadi flash floods, while the shape and drainage network metrics have smaller impacts. The total rainfall amount and duration significantly impact the relationship between the hydrologic response of the wadi and its geomorphometry. For most of the parameters, we found that the impact of the wadi geomorphometry on the hydrologic response increases with increasing rainfall intensity.

Flash flood forecasting is indispensable to construct efficient warning and mitigation of the increasing threat of flash floods in wadi systems. Flash floods modeling in arid region is hindered by the lack of appropriate hydrological models and data availability. Rainfall-Runoff-Inundation (RRI) model is applied in Wadi Samail as a case study for the most extreme flash flood events in Oman (1,000 year return period). Detailed sensitivity analysis were conducted for RRI model parameters indicating that the channel and hillslope roughness coefficients are the most significant parameters followed by the soil characteristics parameters such as soil depth, porosity and hydraulic conductivity. The most severe tropical cyclone in the recent history in Oman, Gonu-2007 and Phet-2010 were used to calibrate and validate the hydrological model. The results and the statistical analysis indicating that RRI model could efficiently simulate the extreme flash flood events in the arid wadi system. Where for model calibration, Nash-Sutcliffe efficiency (NSE) and Percent bias (PBIAS) indices equal to 0.93 and -14.3, respectively, and for model validation NSE and PBIAS equal to 0.86 and -12.0, respectively. Further improvements in RRI model is recommended to include the transmission losses and groundwater processes for better representation of the wadi system.

Sustainable management of wadi flash flood (WFF) risks is desperately needed to secure development in wadi systems. Due to rapid flow generation with sudden high flood peaks, spatiotemporal variability of rainfall occurrence, and poorly sited rapid development, most Middle East and North Africa (MENA) region have no comprehensive proper protection from WFFs. In arid regions, single mitigation measures, including storage dams, recharge dams, artificial lakes and embankments, are implemented, although soft mitigation measures are not dominant, such as early warning systems. The single management strategy under climate change impacts is not adequate to reduce flash flood risks; an integrated strategy is required. The objective of the international symposium on flash floods (ISFF) project has been to develop scientific understanding of WFFs in wadi systems; monitor, model, and mitigate; issue warnings; and plan urban development by discussing and networking the strategies in the MENA region. To achieve this goal, the project defines priorities for future research challenges and potential projects for WFFs. This chapter provides a state-of-the-art scientific basis in terms of integrated flash flood management. Further, priorities are defined for the main research gaps, and the emerging research methodologies can contribute to guide the management of WFFs in such regions.

Wadis, an Arabic term referring to a wadi, in the Eastern Desert of Egypt have undergone rapid unsustainable development in areas vulnerable to flash flooding and water scarcity. To reduce the risk of damage and loss of life from flash floods to a wadi’s new residents, the priority is to develop mitigation strategies with distributed (watershed scale) or concentrated (localized) mitigation structures to promote sustainable development. The focus of this study is to develop a new approach that will help in assessing various flood mitigation scenarios in Wadi Abadi in the Eastern Desert of Egypt. The proposed approach considers the limited data availability in the wadi system and utilizes spatial analysis and an in-house developed distributed hydrological model, Hydrological River Basin Environmental Assessment Model (Hydro-BEAM), upgraded with a reservoir routing module. Sensitivity analysis of the key Hydro-BEAM model parameters indicated that the most significant parameters controlling the wadi flood peaks are soil thickness and porosity, runoff coefficient, subsurface layer outlet coefficient, and channel roughness. Digital Elevation Model (DEM) data and satellite imagery were utilized to propose the locations and derive design characteristics of the mitigation structures. The mitigation strategies evaluated in this study resulted in a peak flood reduction percentage of 90% and 86% for the distributed and concentrated dam scenarios, respectively. The results show that a group of distributed dams could outperform a single concentrated dam when flood mitigation and water resources management aspects are considered in the wadi region, where the distributed dams scenario has 600% more protected area and 21% more reservoir volume than the concentrated scenario (i.e., use of one dam). However, the concentrated dam scenario may have advantages due to the cost of construction and operations. The proposed approach can assess the flood risk reduction due to different mitigation measures and provide strategies for development and planning in wadi regions.

The behaviors and impacts of flash floods (FF) are different based on the climatic regions. To understand such difference, two case studies were selected for the analysis: Wadi Uday, Oman and Sume Basin, Paraiba, Brazil. The rainfall-runoff inundation model (RRI) was used to simulate the discharge and flood inundation of the recent flood events to understand the severity and frequency of flash floods to better assess the current mitigation measures. The current FF situations in arid and semiarid basins were analyzed, and the hazards associated with flood phenomenon were assessed for various calculated rainfall return periods using RRI model. To this end, a flash flood index (average water depth per total basin area) was calculated as a basis to understand the impact of flash floods. A coupling of this index with the FF histories was included to provide a comprehensive overview of the FF vulnerability of arid and semiarid basins. We concluded that FFs tend to be more severe and extreme in arid regions than in semiarid regions, despite the lower frequency of FFs and the water scarcity in arid regions. Distributed dams also proved to be more effective in preventing FFs in arid regions than in semiarid regions.

This study examined the spatial temporal impacts of climate variability on maize yield in Kenya. The maize yield data were obtained from the Kenya Maize Yield Database while climatic variable data were obtained from the Climatic Research Unit gridded Time Series (CRU TS) with a spatial resolution of 0.5° × 0.5°. The non-parametric Mann–Kendall and Sen’s slope tests showed no trend in the data for maximum temperature, minimum temperature and precipitation. The spatial maps patterns highlight the rampancy of wetter areas in the Lake Victoria basin and Highlands East of Rift Valley compared to other regions. Additionally, there is a decreasing trend in the spatial distribution of precipitation in wetter areas and an increasing trend in maximum temperature in dry areas, albeit not statistically significant. Spearman’s rank correlation test showed a strong positive correlation between maize yield and the climatic parameters for the Lake Victoria basin, Highlands East of Rift Valley, Coastal Strip and North Western Regions. The findings suggest that climate variability has a significant impact on maize yield for four out of six climatological zones. We recommend adoption of policies and frameworks that will augment adaptive capacity and build resilience to climatic changes.