Water Quality Modeling of Bonnet Carré Freshwater Flows in the Pontchartrain Estuary Rachel Roblin Alex McCorquodale Ioannis Georgiou

Model Background.1-D tidal, salinity, and water quality model was developed to simulate the response of the Pontchartrain Estuary to Bonnet Carré Flows.The model was calibrated and validated using a 17- year period (1990 – 2007) of water quality data collected by USGS, LADEQ, USACE, LPBF, UNO and others. Water quality was calibrated using the 1997 Bonnet Carré opening.

Model Background and Development Cell-Link model structure for the Pontchartrain Estuary

Mass Balance Model Hydrology (Precipitation, evaporation, runoff) for the interior. Hydraulics of passes, MRGO, ICWW, Rigolets Chef Menteur, IHNC and Pass Manchac. Sediment Loads. Sediment resuspension and deposition. Tides and tidal exchange of sediment and salinity between Mississippi and the interior.

Hydraulics Hydrology Salinity Balance Suspended Sediment McCorquodale and Georgiou, 2007 McCorquodale et al, 2008

Algal Bloom Risk Model Turbidity Temperature Nutrients Salinity

Algal Bloom Risk Model This probability approach was first presented by McCorquodale et al (2004) and was supported by the research of Ismail (1999) Haralampides (2000) Dortch (1999, 2001)

Water quality Model Diagram showing chemical interactions considered in the Pontchartrain Estuary model (Roblin, 2008)

Live Algae as bloom indicator Roblin, 2008 Thomman and Mullen, 1987

Boundary Conditions Hourly input (tides, wind) Daily input (tributary flows, sediment and nutrient loads, including atmospheric deposition of nitrogen based on Wang, 2003 ) Monthly input (precipitation and evapotraspiration)

Calibration (nitrogen)

Calibration (phosphorus)

Validation (Salinity)

Applications Bonnet Carré (1997 and 2008) 2008 opening of the Bonnet Carré Spillway (photos by Georgiou, 2008)

Bonnet Carré Spillway 1997 hydrograph shifted to end of April

Early simulations using 1997 hydrograph Probability of an algal bloom on northeast Lake Pontchartrain with live algae concentrations (April 2008 opening with 1997 flow conditions).

2008 Bonnet Carré Flow (estimate) ~167,000 cfs* Leakage flow estimate * Flow estimates are based on reports by the USACE on number of open bays (linear interpolation between reports); Leakage flow is based on Cruise 2007 weir equation; Head above weir based on measured stage at Bonnet Carré in Mississippi River; Weir elevations obtained by Brantley 2008 (personal communication).

Simulated Salinity for 2008 based on 2008 Bonnet Carré Flow and the 1990 Tributary Flows.

Simulated DIN for 2008 based on 2008 Bonnet Carré Flow and the 1990 Tributary Flows.

Simulated Live Algae Index for 2008 based on 2008 Bonnet Carré Flow and the 1990 Tributary Flows.

Probability that the Ambient Conditions are Ideal for an Algal Bloom for 2008 based on 2008 Bonnet Carré Flow and the 1990 Tributary Flows.

Summary The model predicts that algal blooms will occur in Lake Pontchartrain starting in the 3rd week of May and lasting through June. The predicted algal bloom in northwest Lake Pontchartrain in late May has the highest probability of occurring (for the 1997 event) There simulations indicate that the earliest algal bloom (May) can be expected in the Northwest part of the Lake. The algal bloom in the South is suppressed initially but could occur in late June. It is interesting that the greatest potential algal bloom does not correspond to the highest DIN concentration which occurs in the Southwest. This is consistent with the simulations using the 1997 Spillway hydrograph lagged in time to start when the 2008 opening occurred.

……and future work Extend work to 3D for spatial distribution of plume capture, and better prediction of algae blooms (based on work by Chilmakuri 2005, McCorquodale et al. 2008, and Georgiou et al. 2007), based on the ECOMSED and FVCOM hydrodynamic and sediment transport models. Validate model results with current and future observations of bloom observations (spatial and temporal based on MODIS data and in-situ observations of estuarine recovery.

Thank you Questions…???

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