In situ observations of wave‐supported fluid‐mud generation and deposition on an active continental margin

Abstract

Wave‐supported fluid muds (WSFM) are a type of gravity flow that can rapidly transport sediment across continental margins. They occur when wave‐induced bed stress maintains suspended‐sediment concentrations (SSC) >10 g L−1, and sediment‐induced stratification near the top of the wave boundary layer limits upward diffusion of sediment. Observations from near‐bed instrumentation are used to evaluate the conditions under which WSFMs form on the continental shelf offshore of the Waipaoa River, NZ. An event in July 2010 featured >130 h of energetic ocean conditions, and water discharge >1900 m3 s−1. A calibrated acoustic backscatter sensor at the midshelf measured near‐bed SSC >50 g L−1, with a strong lutocline occurring >15 cm above the predicted wave‐current boundary layer, resulting in ~5 cm deposition. A velocity anomaly occurred during this time, with offshore‐directed currents faster at 1 m above bed (mab) than at 3.5 mab. Using these observations, we empirically solve a simple buoyancy‐drag force balance to estimate the gravity‐driven velocity of the WSFM, which is always <0.03 m s−1. Extending the force balance across a shelf transect suggests that WSFM‐carried sediment can reach the shelf edge in 50–240 h. Spatial and temporal patterns of deposition predicted by the gradient of modeled sediment flux correlate well with seabed observations on the Waipaoa shelf reported in Walsh et al. (2014). This study highlights the importance of WSFMs for cross‐shelf sediment transport, despite relatively slow gravity‐driven velocities and the infrequency with which they occur.

Wave Generation of Gravity‐Driven Sediment Flows on a Predominantly Sandy Seabed

“…Bottom stresses remained elevated for almost 2 days and reached values well above the threshold of resuspension for silts and fine sands. Following the large stresses, sediment concentration profiles measured by the ABSs showed maximum near‐bottom concentrations of 40 (g/L) and 50 (g/L) at the 12 and 8 m sites (Figures b and c), respectively, which are of similar magnitude to those of previously reported WSGF events (Hale & Ogston, ; Traykovski et al, ). Peak concentrations of 10 (g/L) were reported by the OBS 20 cmab at the 8 m site (Figure b), suggesting that at times the high‐concentration layer was at least 15–20 cm thick.…”

A Turbulence‐Resolving Numerical Investigation of Wave‐Supported Gravity Flows

“…Jaramillo et al () reported 5 cm/s for WSGF events observed at Atchafalaya shelf. Through indirect estimate of the sediment flux budget, Hale and Ogston () obtained lower values of WSGF velocity of 3 cm/s on the continental shelf offshore of the Waipaoa River. However, the shelf slope in these later two field sites is milder ( ) than that reported by Traykovski et al ().…”

“…A key uncertainty of the transport lies in the magnitude of the cross-shelf (downslope) gravity current speed. A literature survey suggests that, although WSGFs have been observed in many continental shelves (e.g., Hale & Ogston, 2015;Jaramillo et al, 2009;Traykovski et al, 2000Traykovski et al, , 2007Traykovski et al, , 2015, the cross-shelf current speed differs by several factors, ranging from a few to tens of cm/s. Moreover, different from typical turbidity current, field data show that WSGF requires persistent wave energy to generate sufficient fluid turbulence in the WBBL, which supports the suspended sediments (Hale & Ogston, 2015;Traykovski et al, 2000).…”

Storm‐driven bottom sediment transport on a high‐energy narrow shelf (NW Iberia) and development of mud depocenters

“…Three primary means, i.e., (a) surface buoyancy plume (hypopycnal) dispersal, (b) dilute suspension dispersal in the bottom‐boundary layer, and (c) gravity‐driven turbidity (hyperpycnal) flows have been suggested to be responsible for transporting fine‐grained sediment (mud to fine sand) across continental shelves [ Walsh and Nittrouer , ]. Whilst the first two means of sediment transport have been documented extensively [e.g., McCave , ; Colby , ; Wright , ; Cacchione et al ., ; Fredsoe and Deigaard , ; Nielsen , ; van Rijn , ; Nittrouer and Wright , ; Bursik , ; Bennett et al ., ; Syvitski and Morehead , ; Hill and McCave , ; Geyer et al ., ; McKee et al ., ; Moriarty et al ., ], mechanisms for triggering and maintaining a gravity‐driven turbidity flow on relatively flat (slope less than 0.012) continental shelves and its quantitative contribution to across‐shelf sediment transport have recently started to attract considerable interest [e.g., Trowbridge and Kineke , ; Kineke et al ., ; Sternberg et al ., ; Ogston et al ., ; Traykovski et al ., ; Wright et al ., ; Scully et al ., ; Friedrichs and Wright , ; Dalrymple and Cummings , ; Harris et al ., ; Hsu et al ., ; Macquaker et al ., ; Ozdemir et al ., ; Corbett et al ., ; Kampf and Myrow , ; Hale and Ogston , ; Hooshmand et al ., ].…”