Steven Y. J. Lai

Submarine canyons and gullies

Submarine canyons are deep incisions observed along most of the world’s continental margins. Their topographic relief is as dramatic as that of any canyon or river valley on land but is hidden beneath the surface of the ocean. Our knowledge of canyons has therefore come primarily from remote sensing and sampling, and has involved contributions from various oceanographic disciplines. Canyons are a critical link between coastal and shelf waters and abyssal depths; water masses, sediment, nutrients, and even litter and pollutants are carried through them. Advances in technology continue to provide new insights into canyon environments by pushing the frontier of deep marine observations and measurements. In this chapter we describe the main geomorphic features of submarine canyons and what is known about their formation and the processes that shape them. We also consider submarine gullies, which are small valleys commonly found within or alongside submarine canyons on the continental slope and may represent an incipient stage of canyon development.

Asymmetric Effects of Subaerial and Subaqueous Basement Slopes on Self-Similar Morphology of Prograding Deltas

Deltas form over basements of various slope configurations. While the morphodynamics of prograding deltas over single-slope basements have been studied previously, our understanding of delta progradation over segmented basements is still limited. Here we use experimental and analytical approaches to investigate the deltaic morphologies developing over two-slope basements with unequal subaerial and subaqueous slopes. For each case considered, the scaled profiles of the evolving delta collapse to a single profile for constant water and sediment influxes, allowing us to use the analytical self-similar profiles to investigate the individual effects of subaerial/subaqueous slopes. Individually varying the subaerial/subaqueous slopes exerts asymmetric effects on the morphologies. Increasing the subaerial slope advances the entire delta; increasing the subaqueous slope advances the upstream boundary of the topset yet causes the downstream boundary to retreat. The delta front exhibits a first-retreat-then-advance migrating trend with increasing subaqueous slope. A decrease in subaerial topset length is always accompanied by an increase in subaqueous volume fraction, no matter which segment is steepened. Applications are presented for estimating shoreline retreat caused by steepening of basement slopes, and estimating subaqueous volume and delta front using the observed topset length. The results may have implications for real-world delta systems subjected to upstream tectonic uplift and/or downstream subsidence. Both scenarios would exhibit reduced topset lengths, which are indicative of the accompanied increases in subaqueous volume and signal tectonic uplift and/or subsidence that are at play. We highlight herein the importance of geometric controls on partitioning of sediment between subaerial and subaqueous delta components.

Stream power controls the braiding intensity of submarine channels similarly to rivers

We use physical experiments to investigate the response of submarine braided channels driven by saline density currents to increasing inflow discharge and bed slope. We find that, similarly to braided rivers, only a fraction of submarine braided networks have active sediment transport. We then find similar response to imposed change between submarine and fluvial braided systems: (1) both the active and total braiding intensities increase with increasing discharge and slope; (2) the ratio of active braiding intensity to total braiding intensity is 0.5 in submarine braided systems regardless of discharge and slope; and (3) the active braiding intensity scales linearly with dimensionless stream power. Thus, braided submarine channels and braided rivers are similar in some important aspects of their behavior and responses to changes in stream power and bed slope. In light of the scale independence of braided channel planform organization, these results are likely to apply beyond experimental scales.