CFD Modeling for Structure Designs in Environmental Impacts Mitigation June 05 Navid Nekouee, Hugo Rodriguez and Steven Davie
Environmental Impact Mitigation Design Savannah Harbor Expansion Project (SHEP) - Design of New Savannah Bluff Lock & Dam (NSBL&D) Fish Passage - Design of the Diversion Structure on the Savannah River at McCoy s Cut
Fish Passage Structure Specifications - An elliptical weir configuration of boulders provides lower velocity and shallower depths near the banks - The combination of 6 full weir pairs, and head losses at the roughened crest, will keep average head loss at about 0.5 ft under worse case flow scenarios (during low flows when total head-loss is greatest) - Varying slope, the downstream end of the channel and they will remain largely submerged during most spring flow conditions. - Rock arches offer turbulent flow patterns and oxygenate channel flow. Pools provide as resting locations for migrating fish. - Channel Meander resemble natural channel and reduce slope and set up secondary helical flows that facilitate sediment transport and may guide migrating fish.
- The dam is currently operated to maintain a normal pool elevation of.7 ft under 3,800 cfs flow condition. Model Setup - The future condition after the spillways are raised to the gates level increase the upstream pool elevation to 4. ft (NAVD-88). - The flows of 8,000 cfs and 3,00 are maximum and minimum flows that would pass through the fish passage under the future operating conditions. Scenario Flow (cfs) Flood year 30,000 Wet year 8,000 Intermediate flow 5,500 Drought year (Nov-Jan) 3,00 Condition Downstream Boundary Condition (ft) NAVD-88 Flood year 4. Wet year 0 Intermediate flow 99 Drought year (Nov-Jan) 96
Model Geometry - Variable computational mesh block sizes - Mesh size varies from 0.4 ft to 5 ft Downstream Boundary (Water Surface Elevation) 6 5 4 3 Upstream Boundary (Flow)
Drought Year Condition (3,00 cfs) 3 A A 3 ft/s Z (FT)
Wet Year Condition (8,000 cfs) 3 A A 3
Flood Condition (30,000 cfs) 3 A A 3 ft/s Z (FT)
Erosion Around the Island Existing River Condition After Construction of Fish Passage
McCoy Cut s Diversion Structure -EFDC hydrodynamic used to determine the structure and mitigation measures impact - Sediment transport and deposition in the Little and Back River areas was analyzed. - EFDC provided a rough idea of the size and shape of the diversion structure
FLOW3D - The computational fluid dynamic model, FLOW-3D, was used as the near field model to determine the velocity field around the structure - Flow-3D determined the pressure field in order to assist in the structural design of its components MSL 4.6 ft 7.7 in MLLW River Bottom Variable from 0 to 0 ft 3 ft Sheet Pile Scour 3 ft Protection
Model Setup - Boundary Conditions Upstream Boundary (Flow) - Variable computational mesh blocks - Mesh size varied from.5 ft to 36 ft Flow Condition Low flow Upstream WSE -6.3 ft (MSL) or Downstream WSE -6.7 ft (MSL) or Back river Outflow WSE -6.5 ft (MSL) or 0 3 6 7 4 5 8 9 Downstream Boundary (Water Surface Elevation) (7,380 cfs) -.6 ft (MLLW) Maximum Tidal flux.8 ft (MSL) or -.0 ft (MLLW).4 ft (MSL) or (39,650 cfs) 7.5 ft (MLLW) 7. ft (MLLW) WSE = Water Surface Elevation -.8 ft (MLLW).7 ft (MSL) or 7.4 ft (MLLW) Downstream Boundary (Water Surface Elevation)
Structure shape Effect of angle theta on diverted flow Angle Upstream flow (cfs) Downstream flow (cfs) Back River flow (cfs) Diverted flow percentage (%) 70 7,380 5,698,68.8 % 00 7,380 5,765,65.8 %
Maximum Tidal Flux Analysis Pressure forces on structure (lbf/sq-ft)
Conclusion (Fish Passage) Fish Passage - Using FLOW-3D, the fish passage hydraulic analysis was performed. - For the wet year flow (8,000 cfs), the average velocities were around 0.5 ft/s in the thalweg and reach up to maximum values of about 4 ft/s around the rocks. - The velocities in the terraces are small which allows the shortnose sturgeons to swim with a speed of.5 ft/s. Water depth in the terraces are above 3 ft and enough for the fish to move. - For the drought year flow (3,00 cfs), water depth in the terraces reaches a minimum of about.5 ft which is enough for the fish to move and the channel does not dry. - The analysis of the velocities around the downstream island after the construction of the fish passage shows that additional erosion effect is insignificant.
Conclusion (McCoy s Cut) McCoy s Cut - Using FLOW-3D near field modeling the angle of the structure to the shoreline was determined to be 70 degrees to maximize the flow diversion to the Back River. - The velocity around the sheet pile for a high tidal flux of 39,650 cfs was 3-4 ft/sec. - The maximum differential pressure acting on the structure was about 0 lb/sq-ft which would be used for structural design of the structure. - CFD models are great tools to assess complex hydraulics and perform detailed analysis and assist designers.
Thank you