Water Quality Evaluation Using Hydraulic Dredge Constituent Fate Modeling (HD-CFM), Bayou Rigaud, Louisiana, USA
Type:
Presented during:
WODCON XVII: "Dredging in a Sensitive Environment" - 2004, Hamburg, Germany
Authors:
E. Russo and L. Mathies
Abstract: Water quality modeling is instrumental for identifying feasible hydraulic dredging alternatives, where effluent contamination is a concern for meeting Water Quality Standards (WQS). This approach was employed to support channel maintenance dredge planning at Bayou Rigaud, Louisiana, USA. The waterway has a history of sediment contamination. A possible wetlands creation site at nearby Fifi Island was identified as a candidate to use the dredged materials beneficially, with effluent discharge into adjacent open waters. Effluent discharges are regulated by Sections 401 and 404 of the Clean Water Act and must not violate Federal and/or state WQS. A sediment evaluation conducted in accordance with the Environmental Protection Agency (USEPA)/US Army Corps of Engineers (USACE) Inland Testing Manual indicated ammonia as the contaminant of concern. A case study was conservatively made of the most contaminated channel reach. An existing marsh plot at Fifi Island was modeled as a partially-confined outflow corridor, configured for dredge effluent treatment. Evaluation of effectiveness in outflow corridor treatment involves: (1) contaminant dilution considering tidal fluctuations; and (2) wetland plant consumption of ammonia. Dissipation of ammonia in the effluent plume that would attenuate beyond the wetland corridor outfall was also evaluated in the Mixing Zone (MZ) of the receiving waters. The degrees of effluent polishing were benchmarked to WQS for identification of potential discharge constraints. A Hydraulic Dredge Constituent Fate Model (HD-CFM) was developed and used to evaluate a range of dredge discharge flow rates to characterize the contaminant flux during dredging. Monitoring during construction will be conducted to ensure WQS are met, as well as to verify the modeling effort.
Keywords: Sediment contaminant evaluation; hydraulic dredging; wetlands treatment; water quality standards, effluent water quality modeling.