ALBERT

Description: Waves erode sea cliffs by various mechanisms, but the influence of wave power on bedrock coastal erosion has not been well quantified, making it difficult to predict how rocky coasts evolve in different environments. Volcanic ocean islands offer a unique opportunity to examine the influence of waves on bedrock coastal erosion because many islands have relatively homogeneous bedrock, well-constrained initial topography, and considerable differences in wave power between shorelines that face different directions and wave regimes. We used lava-flow ages and the morphology of coastal profiles on Maui, Kahoʻolawe, and the Big Island of Hawaiʻi (USA) to estimate sea-cliff retreat rates at 11 sites that experience nearly eightfold differences in incident wave power. Using a range of possible sea-level histories that incorporate different trends of subsidence due to volcanic loading, we modeled the evolution of each coastal profile since its formation (12 ka to 1.4 Ma) to find the regionally consistent relative sea-level history and the site-specific sea-cliff retreat rates that best reproduce observed coastal profiles. We found a best-fit relative sea-level history prescribed by an effective elastic lithosphere thickness of 30 km, consistent with estimates from observations of total deflection beneath the Hawaiian Ridge. This suggests that coastal profiles may retain a decipherable record of sea-level change. Comparing the best-fit sea-cliff retreat rates to mean annual wave power at each site, which we calculated from 30 yr hindcast wave data, we found a positive relationship between wave power and sea-cliff erosion, consistent with theoretical predictions and measurements on unlithified coastal bluffs. These comparisons provide field evidence that bedrock coastal erosion scales with wave power, offering a basis for modeling rocky coast evolution in different wave climates.

Description: Wave-generated ripples are macroscopic roughness elements that influence fluid flow and sediment transport. For a major group of ripples (orbital ripples), morphology (height and wavelength) is set by the wave conditions. In natural conditions, where wave forcing is highly variable, ripple morphology is frequently changing. We investigate the rate of morphological change after changes in wave conditions (i.e., wave height and period), using laboratory experiments with a step-change in wave forcing. The adjustment time of ripple morphology to new conditions –– the hysteresis time scale (hereafter referred to simply as hysteresis) –– is proportional to changes in wave orbital diameter, and the coefficient of proportionality differs for decreasing and increasing orbital diameter. When the Shields parameter is lower than a threshold (0.043), there is no change in ripple morphology (or changes are extremely slow). In addition, we find the presence of defects (irregularities from straight parallel crestlines) reduces the hysteresis time scale. The dependence of hysteresis on defect density implies that a larger density of defects results in faster ripple adjustment, confirming previous theoretical and numerical model results.

Description: Symmetric sand ripples formed by water waves are common features on modern coasts and in sedimentary rocks. The size and spacing of wave ripples generally scale with water depth and wave conditions, and are widely used to reconstruct coastal environments of the geologic past. Interpretations based on average ripple dimensions and assumed constant wave conditions are informative, but many rippled beds contain striking patterns involving defects—deviations from straight, evenly spaced ripple crests—that suggest more dynamic flow regimes. We report a set of laboratory experiments that reveal how these patterns form in rippled beds adjusting to a change in wave conditions. As the ripples in our experiments evolved toward a new spacing, they developed defects that are widely observed in modern environments and in the rock record. The dominant defect type depends on the sign and magnitude of the adjustment in ripple spacing and the number of wave periods since the change in wave conditions. A regime diagram summarizing these associations quantitatively links ripple defects to transient flow conditions. Our experiments reveal the origin of previously unexplained ripple patterns and add a new dimension to paleoenvironmental interpretations.