The Nansha Islands comprise the largest atoll archipelago in the South China Sea, accommodating 15% of global atolls. In contrast to reef flats found elsewhere in the Indo-Pacific region that typically have grown close to modern sea level, a considerable portion of atoll rims there are composed of 10-20-m-deep reef flats. To better understand modern processes, particularly whether these deep reefs are host to modern physical reworking or instead may be relict features abandoned by sea-level rise, we conducted a mooring hydrodynamic observation from January to September on a 12-m-deep southwest-facing reef flat. These measurements show a predominance of seasonally-varying waves and stable, moderate tide-driven currents, similar to short-term observations at three adjacent deep-reef-flats. While the reef flat was protected from the northeast monsoon from January to May, the southwest monsoon from June to September caused prolonged exposure to large waves (mean Hs of 1.3 m; orbital velocity 0.22 m/s) and consistent cross-flat currents (0.08 m/s on average), resulting in near-bed skin-friction shear velocities of 0.02 m/s on average. These wave conditions are capable of forming and mobilizing bed ripples while entraining coarse coral sand (d50 = 1 mm) for over half a year. Estimates of potential sediment flux suggest the capability for combined waves and advective currents to deflate the 12-m-deep reef rim by up to 28 mm in 8 months. As these potential losses are similar to reef accretion rates, our measurements imply that modern processes could play a significant role in the maintenance of deep reef flats.

The Nansha Islands comprise the largest atoll archipelago in the South China Sea that accommodates 15% of global atolls. In contrast to the reef flats of the Indo-Pacific region that commonly have grown close to modern sea level, a considerable portion of the atoll rims here consists of reef flats as deep as 6-20 m. To better understand whether these deep reefs are relict features abandoned by sea-level rise or host to modern physical reworking, we conducted an 8.5-month mooring observation of hydrodynamics on the 12-m-deep southwest-facing reef flat of Tiexian reef. These measurements show a predominance of seasonal-varying strong waves and stable currents, in line with short-term observations at three other deep-reef-flats nearby. While the reef flat was protected from the northeast monsoon from January to May, the southwest monsoon from June to September resulted in prolonged exposure to large waves and more vigorous cross-flat currents (mean Hs of 1.3 m; orbital velocity 0.22 m/s), resulting in consistently large near-bed shear stresses (mean of 0.6 N/m2 on reef sediments and maximum of 18.9 N/m2 on coral reefs) that are capable of entraining sediment for almost the entire monsoon season. Assuming a mere ratio of 1-10% mobile sediment would result in rim height deflation on the order of 0.5-5 cm/y. Such values similar to potential rates of reef accretion imply that modern processes could play a significant role in the maintenance of the deep reef flats, which can be surprisingly active environments with living coral colonies and coral debris coverage.

Coastal communities facing shoreline erosion preserve their beaches both for recreation and for property protection. One approach is nourishment, the placement of externally-sourced sand to increase the beach’s width, forming an ephemeral protrusion that requires periodic re-nourishment. Nourishments add value to beachfront properties, thereby affecting re-nourishment choices for an individual community. However, the shoreline represents an alongshore-connected system, such that morphodynamics in one community are influenced by actions in neighboring communities. Prior research suggests coordinated nourishment decisions between neighbors were economically optimal, though many real-world communities have failed to coordinate, and the geomorphic consequences of which are unknown. Toward understanding this geomorphic-economic relationship, we develop a coupled model representing two neighboring communities and an adjacent non-managed shoreline. Within this framework, we examine scenarios where communities coordinate nourishment choices to maximize their joint net benefit versus scenarios where decision-making is uncoordinated such that communities aim to maximize their independent net benefits. We examine how community-scale property values affect choices produced by each management scheme and the economic importance of coordinating. The geo-economic model produces four behaviors based on nourishment frequency: seaward growth, hold the line, slow retreat, and full retreat. Under current conditions, coordination is strongly beneficial for wealth-asymmetric systems, where less wealthy communities acting alone risk nourishing more than necessary relative to their optimal frequency under coordination. For a future scenario, with increased material costs and background erosion due to sea-level rise, less wealthy communities might be unable to afford nourishing their beach independently and thus lose their beachfront properties.

The lower shoreface, a transitional subaqueous region extending from the seaward limit of the surf zone to beyond the closure depth, often serves as a sediment sink or source in sandy beach environments over annual to millennial time scales. Despite its important role in shoreline dynamics, however, the morphodynamics of the lower shoreface remain poorly understood. This knowledge deficit is partly due to the absence of sediment compositional data across the seabed and to the challenges inherent in measuring subtle bed changes (mm-cm/yr) over historical time scales. It is also unclear how diverse lithologies and long-term changes in wave climate influence shoreface morphodynamics as previous work often considers these steady-state systems in equilibrium. To better understand the controls on shoreface dynamics, we extend an existing energetics-based framework to model sediment transport across theoretical shoreface equilibrium profiles under various physical and geologic disequilibrium conditions. We further incorporate varying shoreline input flux scenarios (i.e., accretion, erosion) to investigate potential coastline inheritance controls on shoreface evolution. Equilibrium profile shapes and disequilibrium sediment transport rates are more sensitive to changes in sediment settling velocity than wave period, indicating that grain size provides a strong geologic control on shoreface morphodynamics. We find that at depths greater than 20 meters, shallow water wave assumptions predict larger sediment transport rates (~1-8 orders of magnitude) than linearly shoaled waves. Furthermore, for linear wave theory, we find an abrupt, discontinuous offshore transition where the bed response to changing wave climates becomes exceptionally slow. Our results provide insight into the sediment dynamics that drive the spatiotemporal evolution of the shoreface, improving our understanding of the interactions between onshore and nearshore processes and geological inheritance.