A River of Mud
The Rambla de Valdeinfierno, located in the arid Tabernas Desert of Almería, is an ephemeral stream with a dramatic secret. These dry riverbeds, common throughout eastern Spain, are known locally as 'ramblas'. They remain inactive for long periods, only to be transformed by sudden, intense rainfall. During these flash flood events, the Rambla de Valdeinfierno doesn't just carry water; it transports a dense mixture of water and sediment known as a hyperconcentrated flow.
This phenomenon occurs when the volume of sediment in the water reaches between 20% and 60%. In the most extreme events in the Rambla de Valdeinfierno, sediment concentrations can approach 70% by volume. At these densities, the fluid's physical properties like viscosity and density are altered. The mixture is no longer a simple, turbulent stream. Instead, it becomes a non-Newtonian fluid, where the high concentration of silt, sand, and clay dampens turbulence and allows the flow to carry much larger particles, including gravel up to 45 mm in diameter, over long distances. This behavior is intermediate between a typical river flood and a debris flow, which is an even denser, slurry-like landslide.
The geology of the Tabernas Basin contributes to these events. The terrain is dominated by soft, easily erodible sedimentary rocks, primarily marls and sandstones, laid down in a marine environment around eight million years ago during the Miocene epoch. When intense, short-lived storms hit the region, the runoff quickly strips away large volumes of this loose material from the sparsely vegetated slopes, feeding it into the rambla network.
The Physics of a Hyperconcentrated Flow
A normal streamflow transports sediment primarily along its bed. A hyperconcentrated flow, however, can suspend large quantities of sand and silt throughout the entire water column. The mixture's density can increase from water's 1.0 g/cm³ to between 1.4 and 1.9 g/cm³. This increased density and viscosity fundamentally change the flow's mechanics. The sediment is carried by the water; it is a part of the flow itself, dictating its behavior.
This type of flow has immense erosive power. It can carve out channels several meters deep during a single event. Researchers studying similar ephemeral channels in southeastern Spain have recorded scour holes up to one meter deep forming after a single flood. The sediment flux in these systems is significantly higher than in perennial rivers, demonstrating their powerful, albeit infrequent, geomorphic impact.
Unlike debris flows, hyperconcentrated flows are still fluid-dominated and turbulent, meaning they don't form the distinctive levees or boulder-rich lobes associated with landslides. Instead, they leave behind massive, poorly sorted sandy deposits with little internal structure, a tell-tale sign for geologists studying the rock record. The study of these flows is critical for understanding flash flood hazards in arid regions, as their destructive potential and behavior differ significantly from clear-water floods.