Harry Roberts Harry Roberts Coastal Studies InstituteCoastal Studies Institute

School of the Coast and EnvironmentSchool of the Coast and EnvironmentLouisiana State UniversityLouisiana State University

River Diversions:River Diversions:Geological Boundary Conditions Geological Boundary Conditions

and Other Considerationsand Other Considerations

Louisiana’s Coastal Land Loss: A Regional Geology Problem

Mississippi Delta Land Loss and Gain

USGS

• Crustal Downwarping

• Compaction-Dewatering of Young Deposits

• Faulting

• Decreasing Sediment Supply-Increasing Accommodation

Key Geologic Factors in Land Loss

River Diversions: River Diversions: Best Hope for Off-Setting Best Hope for Off-Setting

Coastal Land LossCoastal Land Loss

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

USGS

Sediment Storage in Sediment Storage in the System Before the System Before

Human Intervention Human Intervention

VicksburgVicksburg

Baton RougeBaton Rouge

New MadridNew Madrid

MemphisMemphis

New OrleansNew Orleans

MississippiMississippiincised valleyincised valley

The Lower The Lower Mississippi River Mississippi River

and Deltaand Delta

• Glacial-period Glacial-period braided streams braided streams within incised within incised valleyvalley

• Holocene valley Holocene valley filling and delta filling and delta constructionconstruction

• Valley fill reflects Valley fill reflects interactions interactions between climate between climate and sea-level and sea-level changechange

Post-Glacial Alluvium and Deltaic Sediments = ~60.500 km2

Post-Glacial Alluvium and Deltaic Sediments = ~60.500 km2

Longitudinal Profile of the Longitudinal Profile of the Lower Mississippi Valley and DeltaLower Mississippi Valley and Delta

Tracing Late Pleistocene Braided Streams into the Tracing Late Pleistocene Braided Streams into the Subsurface Using Base of Backswamp DepositsSubsurface Using Base of Backswamp Deposits

Based on 325 USACE boreholes from Blum et al. (2008)

braided stream deposits

Lower Mississippi Valley and DeltaMagnitude of Post-Glacial Deposition

Total storage = 1860-2300 km3 or 2790-3450 BT of sedimentStorage rate = ~230-290 MT/yr over 12,000 yr post-glacial period

sediment isopachs adapted from Kulp (2000)sediment isopachs adapted from Kulp (2000)

Balize Delta > 100 m Thick

Mississippi River Discharges and Mississippi River Discharges and Sediment StorageSediment Storage

Pre-DamPre-DamSediment Load*: ~400 – 500 MT/yrSediment Load*: ~400 – 500 MT/yrMean Sediment Storage**: Mean Sediment Storage**:

230 – 290 MT/yr 230 – 290 MT/yr

1976-20061976-2006Mississippi & Atchafalaya Sediment Mississippi & Atchafalaya Sediment

Load: ~205 MT/yrLoad: ~205 MT/yr

*Suspended Sediment Load Meade et al. (1990); *Suspended Sediment Load Meade et al. (1990); Kesel et al. (1992) **Avg. over 12 kyrsKesel et al. (1992) **Avg. over 12 kyrs

Lower Mississippi River Sediment LoadLower Mississippi River Sediment LoadPre- and Post-Dam LoadsPre- and Post-Dam Loads

Data courtesy of USGS Baton Rouge

Modern post-dam sediment loads are ~65% of the long-Modern post-dam sediment loads are ~65% of the long-term mean storage component alone term mean storage component alone

Data courtesy of USGS Baton Rouge

230-290 MT/yr

Projections of Projections of Sea-Level Rise and Sea-Level Rise and

SubsidenceSubsidence

Increasing AccommodationIncreasing Accommodation

Global Sea-Level RiseGlobal Sea-Level RiseSea-Level Change Data and ProjectionsSea-Level Change Data and Projections

Satelliteobservations

Tide-gaugeobservations

from IPCC 2007

~1mm/yr ~1.7 – 2.0 mm/yr

~3.0 mm/yr

Coastal Plain GPS Vertical Velocities

Coastal Plain Elevations

Projected Submergence: 2000 vs 2100

Baton RougeBaton Rouge

LafayetteLafayette

Projected future land loss of 10,500-13,500 km2

The Louisiana Coast in 2100?

New accommodation ~12-16 km3

(Requires ~18-24 BT to fill accommodation)

New New OrleansOrleans

New New OrleansOrleans

Baton RougeBaton Rouge

LafayetteLafayette

Projected future land loss of 10,500-13,500 km2

1400-1800 km2

with 50% ofsediment

load

The Louisiana Coast in 2100?

Mass balance Mass balance considerations considerations present tough choices present tough choices for diversion for diversion scenariosscenarios

A Goal of River Diversions:A Goal of River Diversions:Maximize Retention of Sediment Maximize Retention of Sediment

Within the Coastal PlainWithin the Coastal Plain

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?

• 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?

Primary Physical Process Data Collection Sites

Delta ADCP

Big Hog Bayou Cross-Channel ADCP Site

Partitioning of Discharge: Wax Lake Delta

Important Positive Impacts of Important Positive Impacts of Hurricanes, Tropical Storms Hurricanes, Tropical Storms

and Cold Frontsand Cold Fronts

Hurricane Isaac

Delta Side-Looking ADCP

Tropical Storm Lee

September 4, 2011

Primary Physical Process Data Collection Sites

Delta ADCP

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

Marsh Accretion Study Site

MM11

MM22

MM33

Marsh Grass Pushed Over by Water from Big Hog Bayou

Sediment Accretion

Inland Marsh Site M1Inland Marsh Site M1 Wax Lake Delta Site 9Wax Lake Delta Site 9

4 cm / 6 months4 cm / 6 months 1.5 cm / 5 months1.5 cm / 5 months

Impact of Cold Fronts:Impact of Cold Fronts:Less Energetic but More Less Energetic but More

Frequent than Tropical StormsFrequent than Tropical Storms

Rising Flood and Cold Fronts Form Synergistic Sediment Delivery Processes

Cold Front Modulation of Sediment Transport

• Prefrontal:

– Onshore Winds

– Water Level Set-Up

– Onshore Sediment Transport

• Prefrontal:

– Onshore Winds

– Water Level Set-Up

– Onshore Sediment Transport

• Postfrontal:

– Offshore Winds

– Water Level Set-Down

– Offshore Sediment Transport

• Postfrontal:

– Offshore Winds

– Water Level Set-Down

– Offshore Sediment Transport

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

Water Level Demeaned and Offset with Wind Speed

What Are Implications of What Are Implications of River Diversion Locations?River Diversion Locations?

Prefrontal Conditions

RESULT: Marshward Transport of Sediment (20-30 times/year)RESULT: Marshward Transport of Sediment (20-30 times/year)

Postfrontal Conditions

RESULT: Sediment Deposition, Marsh Substrate Accretion, Nutrient LoadingRESULT: Sediment Deposition, Marsh Substrate Accretion, Nutrient Loading

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.

Placement of River Diversions in Landward

Parts of Interdistributary Basins Will

Maximize Sediment Retention Through

Landward Suspended Sediment Transport

by Cold-Front-Related Processes.