fine-sediment transport in the coastal ocean: the amazon and the atchafalaya systems

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

Atchafalaya SystemsAtchafalaya Systems

Gail C. KinekeGail C. Kineke

Dept of Geology & GeophysicsDept of Geology & Geophysics

Boston CollegeBoston College

Acknowledgements:Acknowledgements:

National Science FoundationNational Science Foundation

Dick SternbergDick Sternberg

Office of Naval ResearchOffice of Naval Research

Boston College Coastal Processes LabBoston College Coastal Processes Lab

Rouse Parameter, PSuspended load:

dz

8.0*

zzs

s

cuq

u

w

We have equations tocalculate velocity and concentration profiles throughout the water column with some direct measurements

Resuspension by currents

OR

Resuspension by waves

Formation?

Foreset Dep

osits

Thanks Geyer and WHOI Graphics

Processes and ProductsProcesses 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 Study site: Atchafalaya River and shallow LA shelfLA shelf

• Fine sediments discharged directly onto shallow shelfFine sediments discharged directly onto shallow shelf• Previous observations of fluid muds (Wells and Kemp, Previous observations of fluid muds (Wells and Kemp,

1986)1986)• Accreting coastline (Roberts et al. 1989)Accreting coastline (Roberts et al. 1989)

Most of Louisiana shoreline is eroding, section west of Most of Louisiana shoreline is eroding, section west of Marsh Island is accretingMarsh Island is accreting

Westphal et al. 1991Westphal et al. 1991

Erosion rates of m/yrErosion rates of m/yr

Localized areas of accretionLocalized areas of accretion

mud on sandy beachmud on sandy beach

young marshyoung marsh

SatelliteSatellite Images Images

??

Walker et al.

Roberts, Huh, et al.

Prior observations of muddy surface plume and Prior observations of muddy surface plume and emplacement of muds on beach point to importance of emplacement of muds on beach point to importance of cold front passagescold front passages

Kemp and Wells

25 g/l

Cold Front PassagesCold Front Passages

Velocity, 5 hr averages

eastwestoffshore onshore

DiscussionDiscussion• Waves combined with currents cause greatest Waves combined with currents cause greatest

resuspension and mixing with onshore winds.resuspension and mixing with onshore winds.• Once the winds change to northerly direction, Once the winds change to northerly direction,

waves decrease and stratification becomes re-waves decrease and stratification becomes re-established and sediments settle rapidly.established and sediments settle rapidly.

• Net transport for these events will typically be Net transport for these events will typically be onshore and to the west.onshore and to the west.

• Waves enhancing resuspension has long been Waves enhancing resuspension has long been recognized, but there is an additional influence.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 beachmud on sandy beach

young marshyoung marsh

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

DiscussionDiscussion

• Observations of cross-shelf wave Observations of cross-shelf wave characteristics from a prior experiment characteristics from a prior experiment indicate significant onshore attenuation indicate significant onshore attenuation during these events, likely due to interaction during these events, likely due to interaction with unconsolidated seabed or fluid muds.with unconsolidated seabed or fluid muds.

• This creates a positive feedback, preventing This creates a positive feedback, preventing wave energy from reaching the coast, wave energy from reaching the coast, increasing the likelihood of coastal accretion.increasing the likelihood of coastal accretion.

• Results from Sheremet compare wave Results from Sheremet compare wave conditions over sandy vs muddy seabed.conditions over sandy vs muddy seabed.

Dean and DalrympleLFDT, Elgar and Guza

ImplicationsImplications

• The cycle of cold front passages The cycle of cold front passages combined with wave attenuation combined with wave attenuation enhances transport to the coast.enhances transport to the coast.

• Perhaps unusually energetic winters Perhaps unusually energetic winters would lead to greatest accretion, in would lead to greatest accretion, in stark contrast to sandy coasts where stark contrast to sandy coasts where successive winter storms can cause successive winter storms can cause greatest erosion.greatest erosion.

Work in progressWork in progress• Cross-shelf wave attenuation (Eron Higgins, Cross-shelf wave attenuation (Eron Higgins,

Masters 2003)Masters 2003)• Nearshore clinoform, consolidation (Dave Nearshore clinoform, consolidation (Dave

Velasco, Masters 2003)Velasco, Masters 2003)• Coastal change (Amy Draut, Ph.D., WHOI Coastal change (Amy Draut, Ph.D., WHOI

2003)2003)• Dispersal via surface plume and coastal Dispersal via surface plume and coastal

mudstream (Katie Hart, Masters 200?)mudstream (Katie Hart, Masters 200?)

SummarySummary

• 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 ThoughtsSome Thoughts

• Coastal evolution depends on what’s happening offshore

• Interdiscplinary issues

Eastern Eastern chenier plain chenier plain inner shelf:inner shelf:

Sigmoidal clinoforms

Convex cross-shoreprofile

V.E. = 1300x

Central Central chenier chenier

plain inner plain inner shelf:shelf:

Reflectors truncatedby sea floor:eroding clinoforms?

Concave cross-shoreprofile

V.E. = 540x

~10g/l