From the continent to the coast: the bedload transport across the lower sector of the Guadiana River Mouth (Spain-Portugal)

Abstract

The estuary of the Guadiana River is located in the southwestern Iberian Peninsula and serves as border between Spain and Portugal for more than 40 km. This estuary consists of a narrow rock bounded channel with meandering morphology imposed by the geologic materials. Its mouth is affected by a mesotidal regime, with tidal ranges comprised between 3 and 1 m (spring vs neap tides). This work analyzes the sediment transport patterns across a section located in the low marine sector of the estuary. The transversal profile is composed by a deep channel and a lateral bar. The bed of the deeper area is constituted by well sorted medium-to-coarse sand, displaying large ebb oriented sand waves (mean wavelength of 12 m), in consequence can be deduced that serves as a bypassing channel. The bed of the lateral bar is composed of cohesive bad-sorted sediment with an important population of clayey silts including a variable amount of fine-to-medium sand. The most frequent bottom feature is the plane bed, but sand patches with bedforms of a mean wavelength of 2.5 m can be observed. The fieldwork was developed in July 2010 during two opposite (neap and spring) mean tides. During the surveys, near bottom water velocity was measured using an Acoustic Doppler Current Profiler (ADCP). The speed data were used to calculate values of potential bedload transport (Qb) according with the equations suggested by Bagnold (1963). The values of bed density and rugosity were determined by using of a Laser Difractometer in sediment samples taken with a Van Veen grab sampler. The final values of Qb were compared to the real bedload transport (Sb) obtained from the amount of sand gathered in a Poliakoff sediment trap. A preliminary analysis of the results of Qb shows as the transport in the contrary senses (ebb and flood) is nearly balanced. The values of Qb oscillate between the 69.8 gr/cm during a neap tidal cycle and 329.1 gr/cm during a spring tidal cycle, whereas Sb varies between 14.6 and 72.1 gr/cm during the same tides. We can conclude that the volumes of real transport are always minor than those of potential transport, being in an approximate relation of 1/5.