ANALYSIS OF FLOW CONDITIONS AT THE IHNC-GIWW SECTOR GATE SLFPA-E October 2016
Motivation SLFPA-E has observed high velocities within the opening of the GIWW sector gate at the surge barrier. Concern about impact on safe navigation. The barge gate has been closed for repairs, thus altering the flow conditions. Existing gaging may not adequately inform SLFPA-E of flow conditions in the navigation channels.
Questions: 1. What are the regional and local conditions that generate the high velocities? 2. How often are the high velocities likely to occur? 3. What can the original modeling studies explain? 4. What is the consequence of keeping the barge gate closed? 5. How can gaging network be improved? 6. What additional modeling is needed?
IHNC Structures
IHNC Structures GIWW Sector Gate Barge Gate
Data Inventory 24 water level data locations 11 wind data locations 2 velocity data locations GIWW sector gate at surge barrier Seabrook
ADCP Data at the GIWW Sector Gate Max ebb = 4.35 fps Max flood = -5.19 fps
Data Analysis Extensive review of data quality Some errors and mis-labeling identified Issues with earlier ADCP data Correlations between different gages Analysis of regional scale hydrodynamics What are conditions of concern? What should we monitor to warn of impending high velocities?
Velocity at Seabrook
Correlation of Seabrook and GIWW
Example of Regional Dynamics Wind and stage data during the Nov 2008 event Strong north and northwest wind on Nov 15
Example of Regional Dynamics Results = hydrodynamic gradient across the IHNC barrier (shown in red)
Summary of Data Inventory & Analysis Linear relationship between velocity at Seabrook and GIWW gate. Moderate relationship of velocity and gradient at the GIWW gate, but gage data affected by set-up due to placement. Good data for regional dynamics, but inadequate at the structure. Recommend improvements to existing gages and placement of additional gages.
Inventory of Previous Modeling Several modeling studies performed during design TABS-MD (ERDC TR-10-9) ADH (ERDC TR-10-10 and TR-10-12) ADCIRC (Arcadis look at surge reduction for USACE-HPO) RMA2 (AECOM look at hydraulics for GIWW Gate Design) HEC-RAS (Gerwick to examine post-storm re-opening and scour potential) HEC-RAS (Haskoning and Arcadis look at System-Wide operational guidance) Plus additional Seabrook modeling.
Inventory of Previous Modeling Differences in modeling approaches include: Surrounding marshes Guide walls Gate dimensions and IHNC footprint Spatial extent and regional dynamics Range of conditions (storm vs tides) Extent of met forcing
Comparison of Model Validation Model: ERDC TR-10-10 ERDC-10-12 AECOM ADH ADH RMA2 Calibration Period Previous study Previous study December 1, 2008 to January 17, 2008 Water Level Gages -- -- 8 Velocity Gages -- -- 3 ADCP Transects -- -- 2 sets, 4-6 in vicinity of GIWW gate locations Verification/Validation Period March 2006, July and August 2008 January through October 2008 June through October 2008 Water Level Gages 1 9 -- Velocity Gages -- -- -- ADCP Transects 2 sets, 3 locations in vicinity of GIWW gate location 2 sets, 7 locations in vicinity of GIWW gate location, 1 set, 6 locations vicinity of Seabrook 4 sets, 7-13 locations in vicinity of GIWW gate location, 1 set, 7 locations vicinity of Seabrook
Regional Dynamics and Model Results hydrodynamic gradient across the IHNC barrier (shown in red) Model results Nov 15 barge open = peak ebb velocity 4.4 fps barge closed = peak ebb velocity almost 7 fps, velocity > 4 fps for 12 hours
Conclusion from Model Review Previous modeling confirms potential for high velocities. Barge Open Percent TR-10-10 AECOM AECOM TR-10-12 TR-10-12 Annual March November March September 1 4.66 4.20 4.20 3.50 3.50 50 2.20 1.90 1.80 2.00 1.90 90 0.59 0.40 0.45 0.50 0.50 Barge Closed Percent TR-10-10 AECOM AECOM Data Annual March November July 1 6.52 6.80 6.70 4.40 50 3.08 3.30 3.00 2.32 90 0.83 0.80 0.75 0.50 Velocity is higher at GIWW sector gate when barge gate is closed. Velocity is lower at Seabrook. ADH reveals eddying details, but neglects adjacent marsh and many regional scale effects (wind & waves). Need updated modeling study.
ADCIRC/SWAN Update As part of this work, we compiled the latest available bathymetric and structure data. Performed update to ADCIRC SL16 mesh within the IHNC corridor including: Resolution increased from 40m to 15m, Mesh carefully aligned to shore parallel contours, Re-interpolation of most recent bathy survey, Implementation of Seabrook and GIWW structures and MRGO closure. Original Updated
ADCIRC/SWAN Update Previous modeling used single (inland) wind data point and did not account for wave setup across Lake Borgne. To overcome limitations of previous modeling efforts: Fully coupled with SWAN wave model. Wave setup can increase hydraulic gradient for westerly winds. Developed a methodology to extract spatially and temporally variable NAM wind fields (North American Mesoscale Forecast System) Updated model and wind methodology tested and stable. NAM Wind Water Currents
Recommended Next Steps Phase 2: Use the updated ADCIRC/SWAN/NAM system Explore regional scale dynamics for variety of scenarios, Identify locations for remote monitoring/gaging to warn of potentially dangerous conditions, Provide boundary conditions for Phase 3. Phase 3: Develop fully three dimensional Flow3D model, Use Flow3D to explore eddying and guide walls, Identify optimal locations for ADCP and local gaging.
Thank You! 28 October 2016 22