A Study of Hydrodynamics, Salinity and Waves in the Acadiana Bay System

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A Study of Hydrodynamics, Salinity and Waves in
the Acadiana Bay System

• William Miller
• Taylor Engineering, Inc.

Study funded by the Louisiana Department of
Natural Resources
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Overview

• Area History Characteristics
• Project Background
• Modeling Overview
• Salinity Model Findings
• Wave Model Findings
• Historical Modeling
• Conclusions

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Acadiana Bays
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Acadiana Bays
Vermilion Bay
West Cote Blanche Bay
East Cote Blanche Bay
Wax Lake Outlet
Atchafalaya Bay
L. Atchafalaya Delta
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Area Characteristics History

• Micro-tidal area lt 1.5 ft tides, primarily
  diurnal tides
• 6 7 ft depths with muddy, shifting bottom
• Subject to strong SE/SW winds
• Pre-1940
• One of the largest concentrations of oyster reefs
  in the U.S.
• Brackish estuarine environment, typical salinity
  10 20 ppt
• Today
• Oyster reefs dredged and mined for shell (1940
  1970s)
• Mississippi River diversion to Atchafalaya River
  1942 construction of Wax Lake Outlet increased
  freshwater to Atchafalaya Bay
• Waters nearly fresh, typical salinity lt 2 ppt
• Adverse impacts on fishing, shrimping, and
  crabbing industries

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Project Background

• Re-establish brackish environment to restore
  fishing, shrimping, and crabbing industries
• Long held opinions attribute freshening to the
  reef dredging
• State of Louisiana proposed to re-establish reefs
  to restore brackish environment
• Taylor Engineering was contracted to determine
  effectiveness of re-establishing reefs through
  numerical modeling

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Modeling Approach

• Public Models
• RMA2 2D hydrodynamic model to establish
  circulation conditions and drive the RMA4 model
• RMA4 water quality model to determine effect of
  reefs on salinity
• STWAVE model to determine effect of reefs on
  waves
• Procedure
• Establish baseline conditions
• Circulation, salinity, and waves
• Modeled multiple reef alternatives
• Identify reef alternative which best meets
  project objective
• Increase salinity in the western portion of
  Acadiana Bays
• Historical Flows Effect
• Model pre-1940s bay-river configuration

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Proposed Reefs
Jaws
GIWW
West Cote Blanche Bay
Vermilion Bay
East Cote Blanche Bay
Wax Lake Outlet
B3,B2
L. Atchafalaya
Atchafalaya Bay
A3,A2
All Reef Crests at MHW A3,A2 Reef S45ºW and
extending 12.5 mi from Pt. Chevreuil B3,B2
Reef S90ºW from Pt. Chevreuil to Marsh Island (14
mi) C3 Reef South and extending 12.5 mi
from Pt. Chevreuil
C3
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Modeling Methodology

• Calibrate models
• LDNR LSU established stations to measure water
  level, current, salinity, and waves
• Model representative seasonal conditions to
  examine annual variations
• Model low frequency storm conditions on waves
• Compare with-reef to no-reef conditions

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Model Mesh
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Calibration Sites
STA 622
Vermilion Bay
GIWW
West Cote BlancheBay
STA 623
East Cote BlancheBay
WG-66
Marsh Island
CSI-14
CSI-3
Atchafalaya Bay
WG-68
Gulf of Mexico
WIS 1065
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Stream Flow Boundaries
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USGS Average Daily Stream Flows
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Model Mesh Boundary ConditionsStream Flow
Boundaries (cfs)
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Offshore Tide BoundaryMean Stream Flow Period
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Wave Model Domain Calibration Sites
28.3 miles
WG-66
28.3 miles
WG-68
WIS 1065
Offshore boundary
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Existing Conditions Salinity Model
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Wind Effects (1)
High Flow (310k) No Wind
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Wind Effects (2)
High Flow (310k) SE Wind
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Reef A2 Salinity Model
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Reef B2 Salinity Model
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Reef A3 Salinity Model
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Reef B3 Salinity Model
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Reef C3 Salinity Model
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Salinity Sampling Locations
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Salinity Changes
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Overall Findings

• Reefs effects on salinity
• Submerged reefs had negligible effects
• Emergent, continuous reefs at western bays
• A2 raised average salinities by 1 to 2 ppt in
  western bays
• B2 raised average salinities by about 1 ppt in
  western bays
• Emergent, segmented reefs at western bays
• A3 raised salinities but less than A2
• B3 raised salinities but less than B2
• C3 raised salinities slightly more than A3
• Reefs did not re-establish the pre-1940 brackish
  environment

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Findings on Waves

• Normally wave heights are small in the bays
• Gulf-swell dissipated by muddy bottom
• Local winds control local wave heights
• Reefs only block waves for a small distance
  downwind
• Zone of influence 5 to 10 miles
• During low frequency storms
• Wave heights are high
• Larger water depths reduce mud influence

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Simulation of Historic Salinity
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Pre-1940 Model Mesh
Vermilion River at Perry
Lower Atchafalaya River
"Jaws"
Bayou Boeuf
Vermilion Bay
Wax Lake
Bayou Black
West Cote Bay
East Cote Bay
Atchafalaya Bay
Terrebonne Bay
Four League Bay
Gulf of Mexico
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USGS Average Monthly Flows
Present Stream Flow (Wax Lake Outlet L.
Atchafalaya)
Pre-1940 Stream Flow (L. Atchafalaya)
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Historical Model Conditions

• Model Mesh
• Pre-GIWW
• Pre- Wax Lake Outlet at Calumet
• Present water depths
• Equivalent Low/Summer Stream Flow
• 50,000 cfs through the Lower Atchafalaya River

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Equilibrated Salinity
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Conclusions

• Reefs did not re-establish brackish environment
  with existing freshwater inflow levels
• Historical model showed significant increase in
  salinity with reduced freshwater inflow (pre-1940
  levels) without Wax Lake Outlet