Tidal networks 2. Watershed delineation and comparative network morphology

Abstract

Through the new method for automatic extraction of a tidal network from topographic or bathymetric fields described in a companion paper [Fagherazzi et al., this issue], we analyze the morphology of aggregated patterns that we observe in nature in different tidal environments. Specifically, we define, on the basis of a hydrodynamic analysis, a procedure for watershed delineation and for the identification of the 'divides' for every subnetwork and look at the resulting drainage density and its related scaling properties. From the systematic, large-scale plots of drainage density and channel width versus watershed area we address the issue of a possible geomorphic criterion that corresponds to the parts of the tidal landscape that are characterized by river-like features. We also analyze the relationship of total contributing tidal basin area to channel widths and to mainstream lengths (Hack's law). We study comparatively probability distributions of total drainage areas and of 'botanical' mass (the area of the channelized landscape upstream of a given section) for tidal and fluvial patterns and find altered scaling features of tidal landforms that reflect the complex interactions of different mechanisms that shape their geometry. Simple geomorphic relationships of the types observed in the fluvial basin (e.g., power laws in the watershed area versus drainage density, mainstream length, or channel width relationships) do not hold throughout the range of scales investigated and are site-specific. We conclude that tidal networks unlike rivers exhibit great diversity in their geometrical and topological forms. This diversity is suggested to stem from the pronounced spatial gradients of landscape-forming flow rates and from the imprinting of several crossovers from competing dynamic processes.