1. Controls on sediment availability on the continental shelf and implications for rates of morphologic evolution Patricia Wiberg University of Virginia with contributions from: Peter Traykovski, WHOI Courtney Harris, VIMS Chris Sherwood and Rich Signell, USGS Chris Jenkins, INSTAAR Tim Milligan, BIO and support from ONR EuroSTRATAFORM program

2. The continental shelf comprises a vast reservoir of unlithified, unvegetated sediment with little relief:  unlimited supply of sediment?  potential for rapid morphological change? Observations and modeling suggest that most transport on the continental shelf is strongly supply limited, with complex controls on the rate of sediment availability.

3. Setting the stage … from Geyer and Traykovski

4. Inputs: morphology from R. Signell

5. Inputs: sediments (properties and supply) from C. Jenkins

6. Inputs: waves and currents Significant wave height (m) from C. Harris

8. Processes: wave-current-driven suspension wave boundary layer current boundary layer 05101520 10 0 1 2 3 Concentration (g/L) Height above the bed (cm) u b /s 100 = 1 2 3

9. Processes: stratification 05101520 10 0 1 2 3 Concentration (g/L) Height above the bed (cm) 3.1 kg/m 2 8.2 kg/m 2 No stratification correction With stratification correction

10. Processes: flocculation 1c m 020 70 30 405060 10 Hour Reynolds Stress

11. Processes: armoring 2 u b /s 100 =1 3 limited by armoring unlimited 0.2 0.3 Grain size Fraction Bed distribution 0.1 0

12. 00.511.5 0 0.5 1 1.5 x 10 -4 Erosion rate (kg/m 2 s) 00.511.5 0 0.05 0.1 0.15 Cum. mass eroded (kg/m 2 ) Time (hrs) 0.01P a 0.08 0.16 0.24 0.32P a Time (hrs) Porosity Critical shear stress E=M(  b -  cr ) time E Cum. mass eroded Processes: consolidation and cohesion

13. 00.511.5 0 0.05 0.1 0.15 Cum. mass eroded (kg/m 2 ) Time (hrs) 00.10.20.30.4 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 Cum. mass eroded (kg/m 2 ) Shear stress (Pa) 00.10.20.30.4 0 0.05 0.1 0.15 0.2 0.25 Depth below surface (cm) Critical shear stress (Pa)

14. 01234 0 0.05 0.1 0.15 0.2 0.25 Time (hrs) Cumulative mass eroded (kg/m 2 ) M=1.5e-3 M=3e-3 M=0.75e-3 Two possible modeling approaches: 1)Time dependent erosion flux E=M(  b -  cr ) Surface active or available layer: M avail or  avail =f(  b,  cr (z))  cr  b 2)Limit suspended mass by the mass in (or thickness of) the layer of available sediment

15. from Traykovski et al. (in press)

18. Implications: Concentrations can reach values sufficient to flow downslope as gravity flows from Traykovski et al. (in press)

19. Implications: Morphologic evolution of the continental margin is sensitive to surface layer history from Harris and Wiberg (2002)

20. Question: What controls the rate of relaxation of the bed surface layer following erosion? Reset every 2 weeks Reset every month Reset every season Reset every year Distance from shore (km) Depth below water surface (m) 5 year simulation of sediment redistribution on the margin by waves and currents

21. Potential mechanisms: bioturbation from Choi et al., 2002

22. Potential mechanisms: bioturbation from Boudreau et al., 2001 Dissipation Time (yr) from Wheatcroft and Drake, 2003

23. Potential mechanisms: wave pumping 050100150200250300350 0 1 2 3 4 5 Significant wave height (m) Day of 1999 from Webster, 2003

24. Conclusions Bathymetric and forcing data are available at high enough spatial and temporal resolution for morphodynamic calculations on the continental shelf. Bathymetric and forcing data are available at high enough spatial and temporal resolution for morphodynamic calculations on the continental shelf. Knowledge of initial bed sediment properties is much poorer. Knowledge of initial bed sediment properties is much poorer. Sediment fluxes are strongly limited by a combination of consolidation, armoring, stratification and flocculation. Sediment fluxes are strongly limited by a combination of consolidation, armoring, stratification and flocculation. Shelf sediment transport models are able to capture the important processes governing sediment fluxes on the shelf Shelf sediment transport models are able to capture the important processes governing sediment fluxes on the shelf We don’t yet understand how the bed is reset after an erosion event, but the rate at which this occurs limits rates of morphologic evolution on the shelf. We don’t yet understand how the bed is reset after an erosion event, but the rate at which this occurs limits rates of morphologic evolution on the shelf.