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The coastal-tract approach to coastal morphodynamics, described in the companion paper (The Coastal-Tract Part 1), provides a framework for aggregation of process and spatial dimensions in modeling low-order coastal change (i.e., evolution of the shoreline, continental shelf and coastal plain on time scales of 10² to 10³ years). Behavior-oriented, coastal-change models encapsulate aggregate dynamics of the coastal tract. We apply these models in a coastal-tract framework to illustrate the use of the concept, and to explore low-order morphological coupling under different environmental settings. These settings are characterized by data-models that we have constructed from four contrasting continental margins (NW Europe, US Pacific, US Atlantic, and SE Australia). The gross kinematics of the coastal tract are constrained and steered by sediment-mass continuity. The rate of coastal advance or retreat is determined quantitatively by the balance between the change in sediment accommodation-space, caused by sea-level movements, and sediment availability. If the lower shoreface is shallower than required for equilibrium (negative accommodation), then sand is transferred to the upper shoreface (NW Europe, US Pacific, and SE Australian cases modelled) so that the shoreline tends to advance seaward. This tendency also occurs when relative sea level is falling (coastal emergence). Coastal retreat occurs when the lower shoreface is too deep for equilibrium (positive shoreface accommodation). This sediment-sharing between the upper and lower shoreface is an internal coupling that governs first-order coastal change. The upper shoreface and backbarrier (lagoon, estuary or mainland) also are coupled in first-order coastal change. Sediment accommodation-space is generated in the backbarrier by sea-level rise (and reduced by sea-level fall), but the amount of space is also moderated by influx of fine sediments from the coast, or sand and mud from fluvial sources. Remaining space can then be occupied by sand transferred from the upper shoreface causing a retreat of the latter (transgressive phases modelled for NW Europe, US Atlantic, and SE Australian cases). [ABSTRACT FROM AUTHOR]

Coastal jetties are commonly used throughout the world to stabilize channels and improve navigation through inlets. These engineered structures form artificial boundaries to littoral cells by reducing wave-driven longshore sediment transport across inlet entrances. Consequently, beaches adjacent to engineered inlets are subject to large gradients in longshore transport rates and are highly sensitive to changes in wave climate. Here, we quantify annual beach and nearshore sediment volume changes over a 9-yr time period along 80 km of wave-dominated coastlines in the U.S. Pacific Northwest. Beach and nearshore monitoring during the study period (2014–2023) reveal spatially coherent, multi-annual patterns of erosion and deposition on opposing sides of two engineered inlets, indicating a regional reversal of longshore-transport direction. A numerical wave model coupled with a longshore transport predictor was calibrated and validated to explore the causes for the observed spatial and temporal patterns of erosion and deposition adjacent to the inlets. The model results indicate that subtle but important changes in wave direction on seasonal to multi-annual time scales were responsible for the reversal in the net longshore sediment transport direction and opposing patterns of morphology change. Changes in longshore transport direction coincided with a reversal in the Pacific Decadal Oscillation (PDO) climate index, suggesting large-scale, multi-decadal climate variability may influence patterns of waves and sediment dynamics at other sites throughout the Pacific basin. • Surveys reveal regionally coherent, multi-annual trends in coastal morphology. • Simple model framework accurately hindcasts longshore transport. • Climate variability controls sediment budget of areas adjacent to engineered inlets. [ABSTRACT FROM AUTHOR]