Fine-Sediment Transport in the Coastal Ocean: the Amazon and the Atchafalaya Systems Gail C. Kineke Dept of Geology & Geophysics Boston College Acknowledgements: National Science Foundation Dick Sternberg Office of Naval Research Boston College Coastal Processes Lab

Rouse Parameter, P Suspended load: We have equations to calculate velocity and concentration profiles throughout the water column with some direct measurements

Resuspension by currents OR Resuspension by waves Formation?

Foreset Deposits Thanks Geyer and WHOI Graphics

Processes and Products Fluid muds form at the bottom salinity front on the inner shelf, largely due to estuarine trapping Flow downslope when thick and dense enough to overcome baroclinic forcing, and/or when onshore forcing relaxes (weakening of alongshelf wind) Supply submarine delta with sediments Affect tidal wave, limit growth of the boundary layer, buffer between seabed and water column, cross-shelf integrator and remineralizer of organic carbon

Study site: Atchafalaya River and shallow LA shelf Fine sediments discharged directly onto shallow shelfFine sediments discharged directly onto shallow shelf Previous observations of fluid muds (Wells and Kemp, 1986)Previous observations of fluid muds (Wells and Kemp, 1986) Accreting coastline (Roberts et al. 1989)Accreting coastline (Roberts et al. 1989)

Most of Louisiana shoreline is eroding, section west of Marsh Island is accreting Westphal et al. 1991

Erosion rates of m/yr

Localized areas of accretion mud on sandy beach young marsh

Satellite Images ? Walker et al. Roberts, Huh, et al. Prior observations of muddy surface plume and emplacement of muds on beach point to importance of cold front passages Kemp and Wells

25 g/l

Cold Front Passages

Velocity, 5 hr averages eastwest offshore onshore

Discussion Waves combined with currents cause greatest resuspension and mixing with onshore winds.Waves combined with currents cause greatest resuspension and mixing with onshore winds. Once the winds change to northerly direction, waves decrease and stratification becomes re-established and sediments settle rapidly.Once the winds change to northerly direction, waves decrease and stratification becomes re-established and sediments settle rapidly. Net transport for these events will typically be onshore and to the west.Net transport for these events will typically be onshore and to the west. Waves enhancing resuspension has long been recognized, but there is an additional influence.Waves enhancing resuspension has long been recognized, but there is an additional influence.

Waves combined with currents cause greatest resuspension and mixing with onshore winds. Net transport typically onshore and to the west. Once the winds change to northerly direction, waves decrease and stratification becomes re-established and sediments settle rapidly, forming fluid mud (thin layer, nearbed). Northerly winds drive upwelling with onshore currents nearbed, transporting high concentrations of sediment

mud on sandy beach young marsh

Waves enhancing resuspension has long been recognized, but there is an additional influence.

Discussion Observations of cross-shelf wave characteristics from a prior experiment indicate significant onshore attenuation during these events, likely due to interaction with unconsolidated seabed or fluid muds.Observations of cross-shelf wave characteristics from a prior experiment indicate significant onshore attenuation during these events, likely due to interaction with unconsolidated seabed or fluid muds. This creates a positive feedback, preventing wave energy from reaching the coast, increasing the likelihood of coastal accretion.This creates a positive feedback, preventing wave energy from reaching the coast, increasing the likelihood of coastal accretion. Results from Sheremet compare wave conditions over sandy vs muddy seabed.Results from Sheremet compare wave conditions over sandy vs muddy seabed.

Dean and Dalrymple LFDT, Elgar and Guza

Implications The cycle of cold front passages combined with wave attenuation enhances transport to the coast.The cycle of cold front passages combined with wave attenuation enhances transport to the coast. Perhaps unusually energetic winters would lead to greatest accretion, in stark contrast to sandy coasts where successive winter storms can cause greatest erosion.Perhaps unusually energetic winters would lead to greatest accretion, in stark contrast to sandy coasts where successive winter storms can cause greatest erosion.

Work in progress Cross-shelf wave attenuation (Eron Higgins, Masters 2003)Cross-shelf wave attenuation (Eron Higgins, Masters 2003) Nearshore clinoform, consolidation (Dave Velasco, Masters 2003)Nearshore clinoform, consolidation (Dave Velasco, Masters 2003) Coastal change (Amy Draut, Ph.D., WHOI 2003)Coastal change (Amy Draut, Ph.D., WHOI 2003) Dispersal via surface plume and coastal mudstream (Katie Hart, Masters 200?)Dispersal via surface plume and coastal mudstream (Katie Hart, Masters 200?)

Summary Amazon: –Fluid muds found on shelf, importance of convergence, salinity stratification and fronts, thickness O(tidal boundary layer), persistent Atchafalaya: –Fluid muds very close to shore or onshore (mudflats), associated with rapid settling following resuspension events, thin O(wave boundary layer)?, ephemeral Concentration of source waters not as important

Some Thoughts Coastal evolution depends on what’s happening offshore Interdiscplinary issues

Eastern chenier plain inner shelf: Sigmoidal clinoforms Convex cross-shore profile V.E. = 1300x

Central chenier plain inner shelf: Reflectors truncated by sea floor: eroding clinoforms? Concave cross-shore profile V.E. = 540x

~10g/l