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Development of the Neuse Estuary Eutrophication Model: Background and Calibration

Development of the Neuse Estuary Eutrophication Model: Background and Calibration By James D. Bowen UNC Charlotte Neuse River Estuary Model Pamlico Sound Applied Water Quality Modeling Research Neuse Estuary Neuse River Estuary Facts About the Neuse River

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Development of the Neuse Estuary Eutrophication Model: Background and Calibration

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  1. Development of the Neuse Estuary Eutrophication Model: Background and Calibration By James D. Bowen UNC Charlotte

  2. Neuse River Estuary Model Pamlico Sound Applied Water Quality Modeling Research Neuse Estuary

  3. Neuse River Estuary

  4. Facts About the Neuse River • 3rd Largest River Basin in NC (6,234 mi2) • 200 miles long, 3000 stream miles • Estuary in lower 50 miles • 1.5 million people in basin, mostly near headwaters • Nutrient loading has doubled since 70’s

  5. Neuse River Problems: Algal Blooms Blue-GreenAlgae Bloom near New Bern

  6. Neuse River Problems: Low DO 1997 Bottom Water DO Conc.

  7. Low DO and Fish Kills: 94-96 Cherry Point Streets Ferry

  8. Water Quality Research Project MODMON = MODeling and MONitoring • Interdisciplinary Applied Research • Water Quality and Biological Monitoring • Water Quality Modeling to predict w.q. improvement (30% nutr. red.)

  9. Physical Processes Neuse EstuaryEutrophicationModel

  10. Neuse EstuaryEutrophicationModel Water Column Biological Processes

  11. Benthic/Water- Column Interactions Neuse EstuaryEutrophicationModel

  12. Neuse Estuary EutrophicationModel

  13. Special Features of Modeling Unusually challenging system to model • intermittent, weak stratification (wind driven) • no strong tidal forcing • sediments have important effects on nutrient and DO dynamics • blooms of several different phytoplankton groups @ different times and places

  14. Neuse Estuary Eutrophication Model • based upon 2-d laterally averaged model CE-Qual-W2 • Nutrient, phytoplankton, organic matter, DO model • 3 phytoplankton groups (V.3) • summer assemblage, diatoms, dinoflagellates

  15. 1 m /mmax 0 Light, Nutrients W2 Phytoplankton Growth Model 1 T.R.M. 0 Topt Temperature m = mmax * min(m / mmax) * T.R.M.

  16. S1 S1 S2 S2 S3 S3 S4 W2 X-section Representation • trapezoidal cross-sections for each segment Layer 1 Layer 4 Sediment Compartments • quasi-3d sediment/water-column interaction model

  17. W2 Sediment Submodel • simple sediment diagenesis model • 1 constituent: Sediment organic carbon (SOC) • SOC fate processes: • redistribution, decomposition • SOD decomposition rate determines fluxes: • O2 demand, PO4 release, NH3 release • N, P, S, Fe redox reactions not considered • e.g. NH3/NO3, NO3/N2, SO4/H2S • can simulate sediment “clean-up”

  18. 1991 Simulation Description • Time Period: • March 1 - September 27, 1991 • Boundary Data Frequency • Daily Flow and NO3, monthly WQ • Hydrodynamic Calibration Data • hrly. water elevations, salinities, velocities @ 3 estuary stations • WQ Calibration Data • monthly mid-water nutrients, DO, chl-a @ 4 estuary stations

  19. H2O & N Inflows - 1991

  20. Inflow N/P molar ratio - 1991 Redfield Ratio

  21. Other Model Characteristics • 62 horizontal segments, 18 layers • execution time step = 10 min. • 2 branches: Neuse & Trent Rivers • 12 tributaries: 9 creeks, 3 WWTP’s • 16 state variables • Boundary Conditions: Flow @ Streets Ferry, Elevation @ Oriental

  22. Neuse Estuary Model Results Transport Model • Water elevations • time histories • spectral analysis • Salinity distributions • time histories @ one segment • animations

  23. Elevations @ Cherry Point Observed Model March April May

  24. Water Level @ New Bern MAE = 0.1 m Julian Day

  25. Elev. Fluctuations - Power Spectrum Observed @ Cherry Point n = 0.020 Amplitude (m) Model Frequency (Cycles/day)

  26. 0 4 8 12 16 Salinities @ Cherry Point Model: Surface Observed: Top Bottom Salinity (ppth) Model: Bottom May Sep Mar July

  27. Modeled Salinities - September 1991

  28. 1991 Predicted Salinities: May - Sept. animation

  29. Neuse Estuary - 1991 Nitrogen

  30. Neuse Estuary - 1991 Chl-a Conc.’s

  31. WQ Conditions: Summary • Seasonal/Spatial Trends • nutrients decreasing downstream • April mid-estuary phytoplankton bloom • June upper-estuary phytoplankton bloom • several pulses of high NOx conc. @ New Bern • August high-flow event • high nutrients, low chl-a @ New Bern • high Sept. chl-a @ New Bern

  32. 1991 WQ Simulations • Single parameter displays • Nitrate • Phytoplankton • Cumulative chl-a • Multi-parameter display • New Bern time history

  33. Modeled Nitrate - September 1991

  34. 1991 Predicted Nitrates: May - Sept. animation

  35. Modeled DO - September 1991

  36. 1991 Predicted DO: May - Sept. animation

  37. Modeled chl-a - September 1991

  38. 1991 Predicted chl-a: May - Sept. animation

  39. Sal. NOx DO Chl Water Quality Prediction - New Bern 0 Surface Middle 6 .5 0 Surface 10 Middle 4 50 0 May Sep Mar July

  40. Calibration Summary • Transport Model • elevation variations predicted within 0.1 m • salinity variations within 2 ppth • dynamics nicely represented • Water Quality Model • blooms of phytoplankton well represented • seasonal variations also represented • New Bern chl-a shows influence of physical processes

  41. Summary, continued • Water Quality Model • DO dynamics fit expectations based on 1997 monitoring • Overall model performance • consistent with previous modeling efforts • sufficient for water quality improvement predictions

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