Sediment Dynamics, Dredging, and Ecosystem Restoration

PIANC Navigation and the EnvironmentNew Orleans, LAOctober 28, 2009

Dr. Donald F. Hayes, PE (Mississippi), BCEEEndowed Professor of Civil Engineering

Director, Institute for Coastal Ecology and Engineering

Institute for Coastal Ecology and Engineering ● University of Louisiana at Lafayette

The LA Coast is aDynamic Environment

http://www.nola.com/speced/lastchance/multimedia/flash.ssf?flashlandloss1.swf

Louisiana Land Loss (1956 to 2006)

Barras, J.A., Bernier, J.C., and Morton, R.A., 2008, Land area change in coastal Louisiana--A multidecadal perspective (from 1956 to 2006): U.S. Geological Survey Scientific Investigations Map 3019, scale 1:250,000, 14 p. pamphlet.

Institute for Coastal Ecology and Engineering ● University of Louisiana at Lafayette

LA Coastal Land Loss Causes Natural Processes

Edge erosion Subsidence Sea-level Rise

Anthropogenic Channelization (multiple effects) Levees Mineral extraction Reduced sediment load Vegetation loss

LA Land Loss Patterns (1932-1990)

Edge Erosion

Interior C

hannels

Interior Ponding

Interior Ponding,

394,164 acres, 57%

Edge Erosion, 207,927 acres,

30%Interior

Channels, 88,824 acres,

13%

Institute for Coastal Ecology and Engineering ● University of Louisiana at Lafayette

Caused by Navigation? Only indirectly Channels facilitate flow

Deeper depths Fewer bends

Results in increased Storm surge Salt water intrusion Off-shore discharge of storm-induced

sediment loads Not just “navigation” channels

Institute for Coastal Ecology and Engineering ● University of Louisiana at Lafayette

Baptiste Baptiste ColletteCollette

South South PassPass

Southwest Southwest PassPass

Tiger Tiger PassPass

BaratariBaratariaaHoumaHouma

AtchafalayaAtchafalaya

FreshwaterFreshwaterMermentauMermentau

CalcasieuCalcasieuGIWWGIWW

Mississippi Mississippi RiverRiver

FourchonFourchon

GIWWGIWWGIWWGIWW

Major Navigation Channels 60 - 70 MCY/year

Maintenance Sediment

Problem Summary

Historical sediment loads no longer delivered through natural conduits across Deltaic Plain

Natural land loss continues Subsidence, Erosion

Anthropogenic influences increase loss rate

Does a sustainable solution exist?

Developing a Sustainable Plan for Restoring Coastal LouisianaNatural systems, such as coastal

Louisiana, are dynamic by nature.

Static designs will not be sustainable.

Institute for Coastal Ecology and Engineering ● University of Louisiana at Lafayette

Sustainable Solution Requires a comprehensive approach

that mimics the dynamics of the natural system

Effectively use available resources Bifurcated approach required

One-time projects to repair historical damage

Perpetual projects to offset on-going coastal loss

Institute for Coastal Ecology and Engineering ● University of Louisiana at Lafayette

Master Plan

Focused on repairing historical damage

Project-based Ambitious

Available Tools

River Diversions Coastal Protection Sediment retention

e.g. terracing Dredged Sediments

River Diversions

Partially mimic historical condition Provide long-term sediment source Low maintenance cost Concerns

Limited sediment load in upper water column

Downstream transport Channel Clogging

Coastal Protection

“Edge Erosion” represents a 30% of land loss

Barrier islands historically protect the coastline

Structural measures must fit with the ecosystem

Passive Sediment Retention Terracing has been used with some

success in the Chenier Plain Long-term sediment growth Design can be optimized and may be

a useful maintenance measure

Navigational Dredging

Logical long-term sediment source Requires transportation and delivery

system to replace historical system Cost will be an issue

Increased pumping distance Increased coordination Additional management required

Increased costs likely beyond the “Federal Standard”

Institute for Coastal Ecology and Engineering ● University of Louisiana at Lafayette

Sediment Resources

MVN Maintenance Dredging

Total Dredged Sediment 67.7 MCY/yr

Fluff (Considered Unsuitable) 16.0 MCY/yr

Not Available 18.0 MCY/yr

Suitable & Available for BU 33.7 MCY/yr

Current Beneficial Use 16.4 MCY/yr

Other DredgingState ChannelsPrivate Docks

??

Sediment Demand Land loss as equivalent volumetric loss

Land loss = 30 to 40 km2/yr (Barras et al 2008) 40 km2/yr 10,000 ac Area determined by “loss of vegetation,” not

depth of subsidence or erosion Each 1 ft of depth = 12 MCY of volume

Sediment volume required to “backfill” these losses Effectiveness reduced by

Lack of confinement, Consolidation, Dessication, Dewatering

Efficiency factor of 0.5 24 MCY/yr per foot of vertical loss

Tractable Problem? Problem is likely tractable, but will

require efficient use of sediment resources

Much more than a volume issue Must match

Temporal and spatial availability Sediment Characteristics Sediment Quality

With BU Demand

Designing BU Projects

Extensive guidance available Understand sediment pumping and

placement Predict retention requirements and

water quality impacts Long-term consolidation Hydrology requirements All BU projects are not equal

Institute for Coastal Ecology and Engineering ● University of Louisiana at Lafayette

Dredged Sediment Value The value of dredged sediment

depends upon its availability in the quantity and quality necessary to provide a specific beneficial use, the preparation of the site to accept the sediment for the desired beneficial use, and the value of that beneficial use.

Institute for Coastal Ecology and Engineering ● University of Louisiana at Lafayette

Sustainability Paradigm Requires development of coast-wide

restoration plans that will result in a sustainable condition, offsetting on-going land loss

Sediment recognized as a resource Must move from sediment “disposal”

(convenience) to sediment “placement” (purpose)

Sustainability Paradigm Plans must be integrated with

navigational dredging to ensure Spatial and temporal sediment availability Sediment quality

Infrastructure required Long-distance pipelines Sediment retention structures Temporary sediment storage areas Likely beyond the “Federal Standard”

Conclusions Sediments from navigational dredging could

possibly offset LA’s coastal land loss if used correctly Requires an orchestrated approach that

integrates sediment availability and transportation with BU designs that serve the long-term goals

May require innovative sediment management approaches

Properly done, this could potentially offer long-term sediment management solutions for LA navigation channels

Need to characterize beneficial uses based upon value provided

Questions and Discussion