Formation and Removal of a Coastal Flood Deposit

Abstract

During floods, small mountainous rivers transport large sediment loads to the coastal ocean. This sediment often deposits near shore, and if conditions allow, various types of gravity flows (including hyperpycnal river plumes) can transport the sediment across the shelf within minutes to months of delivery. The stochastic and energetic nature of small‐mountainous‐river flood dispersal presents challenges to observations of transport processes and flood‐deposit formation during events, however. In this study, the removal of two dams on the Elwha River afforded an opportunity to closely monitor coastal sediment dispersal using boundary‐layer instrument systems during 4 years of intense sediment loading. A flood in March 2014 generated fluvial sediment concentrations of 14–20 g/L, coastal boundary‐layer concentrations of up to 14 g/L, and 23 cm of muddy sand deposition in a few days. Limited evidence exists for a gravity flow lasting <2 hr during slack tide, coinciding with the waning phase of the secondary flood peak. However, rapid sediment deposition over the 4‐day event period was dominated by advection of sediment in the boundary layer and by suspension settling from turbid river water, which never exceeded the well‐established 35–40 g/L required for hyperpycnal river plume formation. Within 3 weeks, the entire sandy flood deposit had been eroded by strong tidal currents. This system highlights the difficulty in forming hyperpycnal river plumes within tidally energetic systems, even from an extremely turbid river, and the conditions that can erase major sediment delivery events from the sedimentary record.

Dispersion, Accumulation, and the Ultimate Fate of Microplastics in Deep-Marine Environments: A Review and Future Directions

“…In such high efficiency transfer zones, the direct connection between terrestrial outflow and submarine canyon would be expected to result in a concentration of microplastics; in particular the larger size fractions but potentially also microfibers. If a river lacks a direct connection to a submarine canyon ( Figures 5B-D), then along-shelf currents and wave action can redistribute and disperse sediments, thus reducing the efficiency of the river to slope connectivity (Mulder et al, 2012;Eidam et al, 2019). These systems may be termed 'buffered, ' with the input conditions being less faithfully recorded in the sink ( Figure 6); an example being the Indus River -Indus Submarine Fan (Romans et al, 2016).…”

Dynamic Response of the Fluid Mud to a Tropical Storm

“…During the calm weather, the thickness of the FM was observed with a tidally averaged mean value of <0.3 m around the Changjiang Estuary and its adjacent coastal regions (Ge et al, 2018; X. Yang et al, 2015). Under some deposition events, much thicker FMs could occur, evidenced by previously recorded studies of the Waipaoa River shelf, the northern California shelf, the Eel River shelf, the Louisiana shelf, and the Elwha River (Eidam et al, 2019; Hale et al, 2014; Ogston et al, 2000; Traykovski et al, 2000, 2015). However, it is a considerable challenge to monitor and simulate the FM features under extreme weather conditions, particularly during a storm event (McAnally et al, 2007; Wan et al, 2014).…”

World’s largest dam removal reverses coastal erosion

“…6d). Additionally, the strong tidal currents of the Elwha River delta setting, which are responsible for export of the majority of sediment off of the submarine delta 21,28–30 , may inhibit sand wave and spit formation by preventing the submarine formation of these features.…”

“…Our initial mass balance showed that only about 5.4 Mt of this sediment could be accounted for in the subaerial, intertidal and subtidal regions of the river delta 21,22 . The remaining ~13 Mt of sediment was transported offshore of the submarine delta and dispersed broadly in the Strait of Juan de Fuca, largely owing to the strong tidal circulation of the region that regularly mobilizes sand and finer sediment 28–30 . In the end, the release of sediment had wide-ranging effects on turbidity, sediment flux, morphodynamics and ecosystem change within the Elwha’s riparian, estuarine and coastal systems 22,31–34 .…”