Tidal networks: 2. Watershed delineation and comparative network morphology

Abstract

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. IntroductionIn this paper, the second in a series of three, we quantify various tidal network properties including common power law relationships which have been well documented for terrestrial river systems [Rodriguez-Iturbe and Rinaldo, 1997]. Our goals here are both to explore tidal channel scaling properties and to infer, based on scaling breaks, possible changes in dominant formative process through a network. We anticipate that spatial variation in the dominance of ebb versus flood tides and in erosional resistance associated with vegetation and sediment texture could give rise to limited scaling compared to that found in terrestrial systems (where single power law relationships typically apply across many orders of magnitude). Hence we propose to apply common power law relationships quantified for terrestrial systems to tidal systems and use these analyses to identify possible geomorphic "signatures" of dominant processes. In order to perform morphometric analysis of tidal networks, we need an objective procedure for delineating the drainage area to any link. Here we first introduce a new method, based on flow hydrodynamics, for delineating drainage directions and contributing areas throughout the tidal network...

A stochastic model for the formation of channel networks in tidal marshes

“…As a result, tidal channels systematically dissect marsh platforms, and often create a dendritic network somehow resembling the fluvial network of terrestrial watersheds. Tidal networks have basic geometric properties common to other natural networks [Fagherazzi et al, 1999] but lack of scale invariance characteristics that are peculiar of fluvial patterns [Rinaldo et al, 1999a[Rinaldo et al, , 1999b. The absence of scale invariance can be ascribed to the numerous physical and biological processes that shape the tidal channels and act at the same temporal and spatial scales.…”

“…[3] Recent studies have determined the water directions on the marsh platform and the delineation of the drainage area of each tidal channel [Rinaldo et al, 1999a], thus enabling a theoretical characterization of drainage density in salt marshes [Marani et al, 2003]. Based on the watershed delineation for tidal channels reported by Rinaldo et al [1999a], we have built a numerical model able to simulate the development of tidal networks in saltmarshes.…”

“…Based on the watershed delineation for tidal channels reported by Rinaldo et al [1999a], we have built a numerical model able to simulate the development of tidal networks in saltmarshes. The model lays the foundations for a systematic study of the role of physical and biological processes on network formation and characteristics.…”

Signatures of sea level changes on tidal geomorphology: Experiments on network incision and retreat

“…The use of a controlled laboratory environment provides also the distinct advantage of isolating the effects of RMSL changes on landscape evolution, among those of the other physical and biological processes which, acting over overlapping spatial and temporal scales [e.g., Rinaldo et al, 1999aRinaldo et al, , 1999bFeola et al, 2005;Mudd, 2011], shape the tidal landscape. Our results can be used to benchmark mathematical models which, to various degrees, conceptualize and simplify the actual governing processes.…”

“…The drainage density is addressed here following the approach proposed by Marani et al [2003] that relies on the statistics of the unchanneled flow lengths, ' (i.e., flow-path lengths from any unchanneled site to the nearest channel). Such lengths are determined on the basis of drainage directions defined by time-averaged hydrodynamic gradients [Rinaldo et al, 1999a]. While the classical Hortonian drainage density (total channelized length divided by the watershed area) is a poorly distinctive measure of how the catchment is dissected by the channel network, the drainage density computed as the inverse of the mean flow distance from any unchanneled point to the nearest tidal channel indicates how efficiently the network drains (feeds) its catchment during ebb (flood) [Tucker et al, 2001;Marani et al, 2003].…”