Sediment (dis)connectivity and contemporary rates of chemical and mechanical denudation in selected Mediterranean drainage basin systems in eastern Spain

Denudation and the relative shares of chemical and mechanical processes and denudation are controlled by a range of environmental drivers and are in most environments and landscapes worldwide significantly affected by anthropogenic activities and disturbances. Anthropogenic pressures can significantly affect the sediment (dis)connectivity in defined drainage areas.

The Quisi, Pou Roig, and Mascarat drainage basins in eastern Spain (Calpe region) are located as neighbouring drainage-basin systems in a Mediterranean, mostly mountainous and anthropogenically modified environment. The drainage basins show significantly different characteristics. Quisi has the highest share of urbanized surface areas and has, at the same time, a comparably high availability of fine-grained sediments. Pou Roig and Mascarat have large shares of terraced surface areas, with Mascarat being the steepest of the three drainage-basin systems.

The three intermittent streams drain directly into the Mediterranean Sea. The selected study areas are characterized by a mild Mediterranean climate with a mean annual air temperature of ca. 18°C and a mean annual precipitation sum around 435 mm (measured slightly above sea level). During the coldest months (January, February) frost and snow can occur in the highest elevations although the mountain ranges are situated close to the coast. In contrast, maximum summer temperatures (July, August) can easily exceed 30°C and south-facing hillslopes and rockwalls are exposed to high solar radiation. The lithology in the area is clearly dominated by marine limestones. Elevation ranges from sea level up to 1126 m a.s.l. Relevant geomorphological processes include chemical and mechanical weathering, rockfalls, debris flows, splash and slope wash, fluvial erosion, and fluvial solute, suspended sediment and bedload transport.

This ongoing GFL-research (since 2018) is focussed on the detection of sediment sources, sediment (dis)connectivity, spatiotemporal variability and rates of contemporary denudational processes and land-to-sea solute and sedimentary fluxes. Our work includes detailed field and remotely sensed geomorphological mapping and computing of morphometric drainage basin parameters, combined with the statistical analysis of high-resolution meteorological and rock-temperature data, and the observation and monitoring of sediment-transfers, runoff and fluvial-transport events. In the field, we are using a combination of different observation, monitoring and sampling techniques, including different tracer techniques and sediment traps in stream channels, remote time-laps cameras, and event-based high-resolution field monitoring combined with frequent water and sediment samplings.

Sediment connectivity is significantly reduced by extended terraced areas within the drainage-basin systems, particularly in Pou Roig and Mascarat. Sediment transfers, the intermittent runoff, and fluvial transport and land-to-ocean fluxes are almost entirely controlled by pluvial events. High runoff during extreme rainfall events forms a relevant hazard particularly in the lower parts of the drainage-basin systems. Mechanical fluvial denudation shows a clearly higher spatiotemporal variability than chemical denudation, with the highest rates of mechanical fluvial denudation being measured in the Quisi drainage basin. Altogether, drainage-basin wide chemical denudation dominates over drainage-basin wide mechanical fluvial denudation which is explained by partly limited sediment availability, sediment deposition and short- to long-term sediment storage at numerous defined locations within the drainage-basin systems, and by the predominant marine limestones found in the drainage-basin areas.