Fine-Sediment Transport in the Coastal Ocean: the Amazon and the Atchafalaya Systems

1. 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

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

4. Formation? Resuspension by currents OR Resuspension by waves

8. Foreset Deposits Thanks Geyer and WHOI Graphics

10. 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

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

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

13. Erosion rates of m/yr

14. Localized areas of accretion young marsh mud on sandy beach

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

17. 25 g/l

18. Cold Front Passages

24. onshore west east offshore Velocity, 5 hr averages

25. Discussion • 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. • 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.

26. 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

27. young marsh mud on sandy beach

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

29. 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. • 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.

32. Dean and Dalrymple LFDT, Elgar and Guza

33. Implications • 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.

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

35. 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

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

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

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

40. ~10g/l