Geomorphic hazards in south-west Saudi Arabia: The human-environmental nexus

Despite limited attention to geomorphic hazards in Saudi Arabia, the mountainous region in the south-west portion of the Kingdom has experienced a series of devastating debris flows, landslides, rockfalls, and debris floods during the past several decades. While undisturbed hillslopes are relatively stable, many rock slides, rockfalls and landslides are associated with cut and fill slopes of mountain roads and other disturbances in this steep terrain. These human-induced geomorphic hazards act synergistically to exacerbate downslope and downstream disasters, such as debris flows and debris floods. The progressive sediment loading in low-gradient channels (wadis) together with large convective rainfall events, have caused episodic evacuation of these ephemeral channels. As such, hydrogeomorphic linkages between hillslope and channel mass wasting processes may be temporally separated as wadis accumulate sediment, posing challenges for prediction of the timing of damaging debris flows. Furthermore, there is little appreciation of the extent to which human-related disturbances (e.g., mountain roads, construction sites, overgrazing) contribute to these downstream disasters or the vulnerability of passengers, residents, or herders to these anthropogenically induced hazards. Such knowledge and information is needed to develop effective hazard mitigation and real-time warning systems for this and similar dry mountain regions that experience episodic storms. Here we demonstrate the need for improved road location, construction practices, and land management in this steep terrain as well as better appreciation of the hillslope and in-channel hazard nexus to reduce downstream impacts of geomorphic disasters.