Sediment Dynamics, Dredging, and Ecosystem Restoration PIANC Navigation and the Environment New Orleans, LA October 28, 2009 Dr. Donald F. Hayes, PE (Mississippi), BCEE Endowed Professor of Civil Engineering Director, Institute for Coastal Ecology and Engineering
The LA Coast is aDynamic Environment http://www.nola.com/speced/lastchance/multimedia/flash.ssf?flashlandloss1.swf Institute for Coastal Ecology and Engineering ● University of Louisiana at Lafayette
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.
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 Institute for Coastal Ecology and Engineering ● University of Louisiana at Lafayette
Interior Channels Interior Ponding Edge Erosion LA Land Loss Patterns (1932-1990)
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
Major Navigation Channels 60 - 70 MCY/year Maintenance Sediment Baptiste Collette Mississippi River GIWW GIWW Calcasieu Freshwater Mermentau South Pass Barataria Atchafalaya Fourchon Houma Institute for Coastal Ecology and Engineering ● University of Louisiana at Lafayette Tiger Pass Southwest Pass
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 Louisiana Natural systems, such as coastal Louisiana, are dynamic by nature. Static designs will not be sustainable.
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
Focused on repairing historical damage Project-based Ambitious Master Plan Institute for Coastal Ecology and Engineering ● University of Louisiana at Lafayette
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”
Sediment Resources Institute for Coastal Ecology and Engineering ● University of Louisiana at Lafayette
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
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) Institute for Coastal Ecology and Engineering ● University of Louisiana at Lafayette
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