On funneling of tidal channels

Tidal channels dissect the tidal landscape and exert a crucial control on the morphodynamic evolution of these landscapes. Improving our understanding of channel equilibrium morphology is therefore an important issue for both theoretical and practical reasons. We analyze the case of a tidal channel dissecting a relatively short, unvegetated tidal flat characterized by microtidal conditions and a negligible external sediment supply. The three-dimensional equilibrium configuration of the channel is determined on the basis of a hydrodynamic model, describing the cross-sectional distribution of the longitudinal bed shear stresses, coupled with a morphodynamic model retaining the description of the main physical processes shaping the channel and the adjacent intertidal platform. Both channel bed and width are allowed to adapt to the flow field so that an equilibrium altimetric and planimetric configuration is eventually obtained, when erosion becomes negligibly small, and asymptotically constant elevations are reached everywhere within the domain. Model results reproduce several observed channel characteristics of geomorphic relevance, such as the relationship between channel cross-sectional area and the flowing tidal prism, the scaling of the width-to-depth ratio with channel width, and the longitudinal distributions of bed elevations and channel widths. In analogy with empirical evidence from estuaries, tidal channel funneling is usually assumed to be described by an exponential trend. Our theoretical analyses, modeling results, and observational evidence suggest that a linear relationship also provides a good approximation to describe longitudinal variations in channel width for short tidal channels. Longitudinal bed profiles characterized by a strong planform funneling tend to attain an upward concavity, whereas a low degree of convergence implies an almost linear profile. Finally, the model allows one to analyze the influence of environmental conditions (sediment characteristics, basin size, tidal amplitude, etc.) on the geomorphological features of tidal channels (equilibrium cross-sectional area and bottom profile, width-to-depth ratios, and planform shape). Wider and deeper channels develop as the width of the domain increases, as the tidal amplitude increases, or as the mean platform elevation decreases. Conversely, narrower and shallower channels result from an increase in the critical shear stress for erosion or a decrease in the flow conductance. We thus believe that this model provides a useful tool for quantitative analyses of long-term morphodynamics of tidal landscapes.