A Multi-Scale Conceptual Model of Flood-Tide Delta Morphodynamics in Micro-Tidal Estuaries

Abstract

Wave and tide induced sediment transport pathways and rates govern the morphological evolution of estuarine systems. An understanding of the morphodynamics of these systems is required to maintain their commercial, biological and recreational value. The morphodynamics of Port Stephens estuary, a micro-tidal estuary located on a wave dominated southeast coast of Australia were investigated using bathymetric surveys and current velocity data from several locations over the estuary. This provided detailed insight into the rates and direction of movement for the main sedimentary features of the system, and how these features interact with the processes that drive their evolution. We used these findings to develop a conceptual model for estuarine morphodynamics that accounts for fair weather and storm conditions. Our model explains how sediment eroded from the estuarine beaches is trapped by the adjacent flood-tide delta. The model is applicable to fetch-limited estuaries that do not have offshore sources of sediment, where the tidal currents are weak in relation to the incident ocean waves, and that have a wide, stable entrance through which ocean waves can propagate into the estuary. The model is multi-scale in that it encapsulates both short-term and local process, and large scale evolution of an estuary; therefore, it represents a tool that may be used in developing sustainable estuary management strategies.

Wave direction shift triggered severe erosion of beaches in estuaries and bays with limited post‐storm recovery

“…There has been very little work on the relationship between flood-tide deltas and adjacent beaches. In Port Stephens, a drowned river valley estuary located~120km north of the Pittwater, Austin et al (2018) proposed that the flood tidal delta is an important sink for sediments that have been eroded from adjacent beaches, and found no evidence that this sediment could return to the beaches. However, quantification of these processes remains uncertain partially due to the different volumetric scales; the volume removed during a large erosion in one BEB might not cause significant changes in the volume of a flood tide delta.…”

Decadal shoreline erosion and recovery of beaches in modified and natural estuaries

“…10); suggesting that prograding BEBs are controlled by tidal, wind-wave or other processes. For example, Austin et al (2018) and Vila-Concejo et al (2020) found that wave energy within estuaries can be refracted/diffracted around headlands, along estuary/bay shores and over tidal shoals and deltas, encouraging alongshore transport on BEBs. Overall, we suggest that entrance-adjacent BEBs may be less vulnerable to decadal management issues and more attention may be required for those with less stable behaviours.…”

“…Previous BEB studies have focused largely on timescales of months to a few years (Gallop et al, 2020;Harris et al, 2020;Vila-Concejo et al, 2010) and it is not known if BEBs can recover fully over longer timescales -or if erosion is a one-way process for some BEB settings, leading to progressive shoreline retreat (Harris et al, 2020). For example, sediments eroded from BEBs can be transported to nearshore sediment sinks, such as flood-tide deltas/shoals (Austin et al, 2018;Vila-Concejo et al, 2011;Vila-Concejo et al, 2007). In some cases these sediments can be returned to the beach (Austin et al, 2018;Jackson, 1995), but are sometimes permanently lost to the beach system.…”

“…For example, sediments eroded from BEBs can be transported to nearshore sediment sinks, such as flood-tide deltas/shoals (Austin et al, 2018;Vila-Concejo et al, 2011;Vila-Concejo et al, 2007). In some cases these sediments can be returned to the beach (Austin et al, 2018;Jackson, 1995), but are sometimes permanently lost to the beach system. Beach sediment may be imported through alternate pathways like alongshore transport or offshore sources under certain conditions (Vila-Concejo et al, 2010).…”

“…Moreover, there have been few studies of the influence of the often-extensive anthropogenic modifications on BEBs. The effects of anthropogenic intervention such as dredging, reclamation, seawalls and groynes alter BEB wave climate (Nordstrom, 1992), beach planform and shoreline positions (Lowe and Kennedy, 2016) and sediment source/sink pathways (Austin et al, 2018). For example, Carrasco et al (2012) report decadal change to BEB shorelines in a barrier estuary in the Algarve (Portugal), which eroded up to 0.22 m/year due to the dredging of an adjacent navigation channel.…”