1. Geological Boundary Conditions and Other Considerations
River Diversions:
Geological Boundary Conditions and Other Considerations
Harry Roberts Coastal Studies Institute School of the Coast and Environment Louisiana State University

2. Louisiana’s Coastal Land Loss: A Regional Geology Problem
Mississippi Delta Land Loss and Gain
USGS

3. Key Geologic Factors in Land Loss
Crustal Downwarping
Compaction-Dewatering of Young Deposits
Faulting
Decreasing Sediment Supply-Increasing Accommodation

4. River Diversions: Best Hope for Off-Setting Coastal Land Loss

5. When the Scale of Coastal Land Loss is Considered — Sediment Availability is a Problem
USGS

6. Sediment Storage in the System Before Human Intervention

7. The Lower Mississippi River and Delta
Vicksburg
Baton Rouge
New Madrid
Memphis
New Orleans
Mississippi
incised valley
The Lower Mississippi River and Delta
Glacial-period braided streams within incised valley
Holocene valley filling and delta construction
Valley fill reflects interactions between climate and sea-level change
Post-Glacial Alluvium and Deltaic Sediments = ~ km2
Post-Glacial Alluvium and Deltaic Sediments = ~ km2

8. Longitudinal Profile of the Lower Mississippi Valley and Delta
Tracing Late Pleistocene Braided Streams into the Subsurface Using Base of Backswamp Deposits
braided stream deposits
Based on 325 USACE boreholes from Blum et al. (2008)

9. Lower Mississippi Valley and Delta
Magnitude of Post-Glacial Deposition
Balize Delta > 100 m Thick
Total storage = km3 or BT of sediment
Storage rate = ~ MT/yr over 12,000 yr post-glacial period
sediment isopachs adapted from Kulp (2000)

10. Mississippi River Discharges and Sediment Storage
Pre-Dam
Sediment Load*: ~400 – 500 MT/yr
Mean Sediment Storage**: – 290 MT/yr
Mississippi & Atchafalaya Sediment Load: ~205 MT/yr
*Suspended Sediment Load Meade et al. (1990); Kesel et al. (1992) **Avg. over 12 kyrs

11. Lower Mississippi River Sediment Load Pre- and Post-Dam Loads
MT/yr
Data courtesy of USGS Baton Rouge
Data courtesy of USGS Baton Rouge
Modern post-dam sediment loads are ~65% of the long-term mean storage component alone

12. Projections of Sea-Level Rise and Subsidence Increasing Accommodation

13. Sea-Level Change Data and Projections
Global Sea-Level Rise
Sea-Level Change Data and Projections
Satellite
observations
Tide-gauge
observations
~3.0 mm/yr
~1mm/yr
~1.7 – 2.0 mm/yr
from IPCC 2007

14. Coastal Plain GPS Vertical Velocities

15. Coastal Plain Elevations

16. Projected Submergence: 2000 vs 2100

17. The Louisiana Coast in 2100? Baton Rouge Lafayette New Orleans
Projected future land loss of 10,500-13,500 km2
New accommodation ~12-16 km3
(Requires ~18-24 BT to fill accommodation)

18. The Louisiana Coast in 2100?
Mass balance considerations present tough choices for diversion scenarios
New
Orleans
Baton Rouge
Lafayette
km2
with 50% of
sediment
load
Projected future land loss of 10,500-13,500 km2

19. A Goal of River Diversions:
Maximize Retention of Sediment Within the Coastal Plain

20. Fundamental Questions: River Diversions
How are the sediments partitioned within the coastal-shelf system?
What is the sediment retention capability in the delta and adjacent marshland?
What are the important processes linked to sediment transport to the delta-marsh-offshore?

21. Primary Physical Process Data Collection Sites
Delta ADCP
Delta ADCP

22. Big Hog Bayou Cross-Channel ADCP Site

23. Partitioning of Discharge: Wax Lake Delta

24. Important Positive Impacts of Hurricanes, Tropical Storms and Cold Fronts

25. Hurricane Isaac

26. Delta
Side-Looking ADCP

27. Tropical Storm Lee
September 4, 2011

28. Primary Physical Process Data Collection Sites
Delta ADCP
Delta ADCP

29. Wax Lake Outlet ADCP, Current Speed (cm/s)

30. Marsh Accretion Study Site

31. Marsh Grass Pushed Over by Water from Big Hog Bayou

32. Sediment Accretion Inland Marsh Site M1 Wax Lake Delta Site 9
4 cm / 6 months
1.5 cm / 5 months

33. Less Energetic but More Frequent than Tropical Storms
Impact of Cold Fronts:
Less Energetic but More Frequent than Tropical Storms

34. Rising Flood and Cold Fronts Form Synergistic Sediment Delivery Processes

35. Cold Front Modulation of Sediment Transport
Prefrontal:
Onshore Winds
Water Level Set-Up
Onshore Sediment Transport
Postfrontal:
Offshore Winds
Water Level Set-Down
Offshore Sediment Transport

36. Big Hog Bayou, Side-Looking ADCP, Current Speed (cm/s)

37. Demeaned and Offset with Wind Speed
Water Level
Demeaned and Offset with Wind Speed

38. What Are Implications of River Diversion Locations?

39. Prefrontal Conditions
RESULT: Marshward Transport of Sediment (20-30 times/year)

40. Postfrontal Conditions
RESULT: Sediment Deposition, Marsh Substrate Accretion, Nutrient Loading

41. Project Results Suggest:
Placement of River Diversions in Landward Parts of Interdistributary Basins Will Maximize Sediment Retention Through Landward Suspended Sediment Transport by Cold-Front-Related Processes.