Effects of Relative Mean Sea Level Variations on Tidal Networks Generated on Experimental Setting

We present the results of laboratory experiments carried out in a large experimental apparatus aimed at reproducing a typical lagoonal environment subject to tidal forcings. The experimental apparatus consists of two adjoining basins reproducing the sea and the lagoon. The tide is generated at the sea by a vertical steel sharp-edge weir, oscillating vertically. The weir is driven by an ad hoc developed software which continuously corrects the weir motion on the basis of water levels measured at the sea, allowing us to generate a sinusoidal tide of fixed amplitude and period, oscillating around mean water level. The bottom of the lagoon is covered by a layer of cohesionless plastic grains, with a density of 1041 kg/m3. The cohesionless plastic grains are characterized by a nearly uniform grain size distribution, with a median grain size of 0.8 mm. The lack of external sediment supply, the absence of vegetation, and the prevalence of bedload transport prevent any deposition processes and lateral surface accretion, attributing a purely erosive character to the experimental lagoon. As a consequence, in this experimental lagoon the main morphodynamic process responsible for tidal network initiation and development is the differential erosion between the channels and the adjacent surface. The experiments were designed in order to analyze the effects of mean sea level variations on channel network dynamics, focusing on the changes of the relevant geomorphic characteristics of the experimental networks, such as e.g. drainage density, based on the probability distribution of unchanneled lengths, and flowing tidal prism. Our results suggest that a decrease in the tidal prism leads to network retreat and contraction of channel cross sections. Conversely, an increase in the tidal prism promotes network re-incision and re-expansion of channel cross sections. In general, contractions and expansions tend to occur within the same planar blueprint and the network re-expands cutting over the vestiges of old channels. However, in some cases, network re-extension through headward growth and initiation of new tributaries follows paths which do not overlap the old ones. A tendency to develop a linear relationship between tidal prism and drainage density is observed which suggests a cyclic response of network structure in terms of efficiency in draining the landscape. The described cyclicity in the contraction-expansion process should reasonably be interpreted as a statistical tendency rather than a pointwise equivalence. Our results show that changes in tidal prism rapidly influence network efficiency in draining the landscape and the related transport processes. This highlight the importance of these experiments for quantitative predictions of the long-term ecomorphological changes of the tidal landscape to relative mean sea level variations.