Statistical characterization of erosion and sediment transport mechanics in shallow tidal environments. Part 1: erosion dynamics

Abstract

Wave-induced bottom shear stress is one of the leading processes that control sediment erosion dynamics in shallow tidal environments, because it is responsible for sediment resuspension and, jointly with tidal currents, for sediment reworking on tidal flats. Reliable descriptions of erosion events are foundational to effective frameworks relevant to the fate of tidal landscape evolution. However, the absence of long-term, measured time series of bottom shear stress (BSS) prevents a direct analysis of erosion dynamics. Here we adopted a fully-coupled, bi-dimensional numerical model to compute BSS generated by both tidal currents and wind waves in six historical configurations of the Venice Lagoon in the last four centuries. The one-year-long time series of the total BSS were analyzed based on the peak-over-threshold theory to statistically characterize events that exceed a given erosion threshold and investigate the effects of morphological modifications on spatial and temporal erosion patterns. Our analysis suggests that erosion events can be modeled as a marked Poisson process in the intertidal flats for all the considered configurations of the Venice Lagoon, because interarrival times, durations and intensities of the over-threshold exceedances are well described by exponentially distributed random variables. Moreover, while the intensity and duration of over-threshold events are temporally correlated, almost no correlation exists between them and interarrival times. The resulting statistical characterization allows for a straightforward computation of morphological indicators, such as erosion work, and paves the way to a novel synthetic, yet reliable, approach for long-term morphodynamic modeling of tidal environments.

Statistical characterization of erosion and sediment transport mechanics in shallow tidal environments – Part 2: Suspended sediment dynamics

“…The areas of the lagoon where over-thresholds SSC events cannot be modelled as Poisson processes are mostly represented by salt marshes and tidal channels in all configurations (see dark blue areas in Fig. 2), similar to the results for bottom shear stress (BSS) (D'Alpaos et al, 2024). On salt marsh areas, both BSS and SSC thresholds (τ C and C 0 , respectively) are seldom exceeded (Figs.…”

“…In the POT analysis, the threshold value plays a critical role, and its choice deserves careful attention. As already noted for BSS (D'Alpaos et al, 2024), SSC is also locally influenced by many factors, making the threshold nonuniversal and highly site-specific. In the case of erosion dynamics, the identification of the threshold with the critical shear stress for erosion seems to be relatively straightforward, offering the advantage of preserving a physical meaning related to the process.…”

“…when field data are available. Since the hydrodynamic model performance has been reported when considering the erosion dynamics (D'Alpaos et al, 2024), here we summarize the ability of the sediment transport model to reproduce SSC by reporting the standard Nash-Sutcliffe model efficiency (NSE) parameter computed when field data are available and refer the interested reader to the Supplement (Figs. S4 and S5) and the literature (Carniello et al, 2012;Tognin et al, 2022a) for a more detailed comparison.…”

“…All configurations were forced with tidal levels and wind climate measured during the whole year of 2005 because the cumulative distribution frequency of wind velocity measured in 2005 is the closest to that of the whole period from 2000 to 2020 (D'Alpaos et al, 2024) and, consequently, can be considered a representative year for the wind climate (Fig. 1).…”

“…However, a complete spatial and temporal coverage of suspended sediment concentration (SSC) to describe these processes is hardly available, preventing the effective representation of depositional dynamics in long-term modelling approaches. Through aiming to couple erosion and deposition dynamics in a unique synthetic theoretical framework, we here investigate SSC dynamics, following a similar approach to that adopted for erosion (D'Alpaos et al, 2024). The analysis with the peak-over-threshold theory of SSC time series computed using a fully coupled, bi-dimensional model allows us to identify interarrival times, intensities, and durations of over-threshold events and test the hypothesis of modelling SSC dynamics as a Poisson process.…”