J. Alex McCorquodale

Salinity, Nutrient, and Sediment Dynamics in the Pontchartrain Estuary

Abstract Algal blooms have been recorded in the Pontchartrain Estuary when elevated nutrient loads combine with other ambient conditions, e.g., low salinity and low turbidity. Nutrient and sediment loads were quantified and incorporated into a dynamic mass balance model for the upper Pontchartrain Estuary to determine the timing of conditions that can lead to algal blooms. General nutrient and sediment loading relationships were developed for the tributaries of the Pontchartrain Estuary where data were available; these relationships were extended to estimate the loading for ungauged areas. The annual water yield for the drainage basin was found to be 500 mm. The annual nutrient loadings from all sources were found to be 21,000 t (13 g m−2 y−1) of total nitrogen and 2700 t (1.6 g m−2 y−1) of phosphorus for Lakes Maurepas, Pontchartrain, and Borgne. The basin sediment yield was calculated to be 42 t km−2. Nutrient and sediment concentrations in the Mississippi River were combined with the estimated leakage through the Bonnet Carré Spillway to obtain nutrient loads related to the river stage. The mass-balance model was applied to assess the occurrence of algal blooms for the period 1990–2008; the model predicted all five observed algal blooms but also indicated a potential for two more that were not documented. The strongest potential for algal blooms was in the northwest quadrant of Lake Pontchartrain although the highest dissolved inorganic nitrogen concentrations were predicted for the southwest quadrant in connection with spillage from the Bonnet Carré Spillway. The lower than expected response of the southwest quadrant may be associated with the higher turbidity due to the Spillway plume. Abstract La afloración de algas ha sido observada en el Estuario Pontchartrain cuando las cargas elevadas de nutrientes se combinan con otras condiciones ambientales (e.g., baja salinidad y baja turbiedad). Las cargas de nutrientes y de sedimentos fueron cuantificadas e incorporadas en un modelo dinámico de balance de masa en la region superior del Estuario Pontchartrain para determinar el momento en que las condiciones son propicias para la afloración de algas. Para los casos donde habían datos disponibles, se establecieron relaciones entre los nutrientes generales y las cargas de sedimentos de los tributarios del Estuario Pontchartrain; en las áreas donde no hubo datos disponibles, estas relaciones fueron utilizadas para estimar las cargas. El rendimiento de agua anual para la cuenca hidrográfica fue de 500 mm. Las cargas anuales de nutrientes de todas las fuentes fue de 21,000 toneladas de nitrógeno total y 2700 toneladas de fósforo. El rendimiento de sedimentos fue calculado en 42 toneladas/km2. Las concentraciones de nutrientes y sedimentos en el Río Mississippi fueron combinadas con las de las estimadas por la fuga de agua por Vertedero Bonnet Carré para obtener cargas de nutrientes relacionadas con el nivel del Río. El modelo de balance de masa fue aplicado para evaluar la ocurrencia de la afloración de algas para el periodo entre 1990 y 2008. Este modelo predijo las 5 afloraciones de alga observadas y también indicó el potencial para 2 afloraciones más que no fueron documentadas. El área con el mayor potencial al desarrollo de afloración de algas fue el cuadrante noroeste del Lago Pontchartrain, a pesar de que las mayores concentraciones de nitrógeno inorgánico disuelto fueron predichas para el cuadrante suroeste en relación con la descarga del Vertedero Bonnet Carré. La respuesta, más baja de la esperada, en el cuadrante sureoeste puede estar asociada con mayores niveles de turbiedad debido a la mancha de la descarga del vertedero.

CLIMATIC EFFECT ON WATER QUALITY EVALUATION

An advisory discouraging swimming and other primary contact recreation in Lake Pontchartrain was issued in 1985 by the Louisiana Department of Health and Hospitals (LDHH). The advisory is still in effect today for the south shore area of the lake and names fecal coliform bacteria as the causative pollutant. The suspected source of the contamination in this area is urban stormwater runoff that is collected and pumped to the lake and may be contaminated by sanitary sewer cross-flows. A water quality shoreline study was initiated in the south shore area of the lake in New Orleans by the Department of Civil & Environmental Engineering at the University of New Orleans (UNO). The objective was to determine if the reduced bacteria levels are a result of decreased pollution or if this is a temporary phenomenon caused by a short-term climatic effect. Five monitoring stations were selected for study on the basis of proximity to drainage canals that discharge the stormwater runoff and current or previous use for primary contact recreation. Fecal coliform concentrations was found to be “wet” weather-dependent at all stations except one. There appears to be an active continuous bacteria source near this site since fecal coliform levels there cannot be directly linked to urban runoff. For the remaining areas a general rule of thumb for recreational use of these south shore water is that the user should assume that the water is unsuitable for primary contact recreation, especially in the near vicinity of urban drainage canals, for at least two to three days following a storm event. Precipitation analysis showed a reduction in mean total annual rainfall during the study period amounting to nearly one-third of the typical mean total annual rainfall for the area. Therefore, lower fecal coliform concentrations observed may be due to uncharacteristic drought conditions rather than decreased pollution.

Impact of Multiple Freshwater Diversions on the Salinity Distribution in the Pontchartrain Estuary under Tidal Forcing

Abstract Numerical experiments of multiple freshwater diversions into the Pontchartrain Estuary under tidal forcing were conducted to evaluate the impact on salinity and tidal flow distribution. A validated numerical hydrodynamic and transport model was used to assess the impacts on tidal flows, circulation, and salinity as a function of additional freshwater input in the estuary from hypothetical diversions combined with channel modifications in the Mississippi River Gulf Outlet. The cumulative and specific impacts were compared with existing conditions. It was concluded that upper and middle estuarine salinity regimes are coupled, and diversion flows need to be managed in accordance with historic inputs. This study also showed that if total freshwater input is not of an order similar to the existing natural tributary flow, the average salinity in the upper estuary could be reduced by 1.5 ppt (±0.5 ppt), which is approximately 40% of the existing long-term salinity of the upper estuary. The additional flow into the upper estuary will produce changes in the flow through the tidal passes on the order of 5%–6%, will decrease hydraulic detention times in the estuary, and will cause an additional increase in the ebb-dominance of the estuary.

Stratification and Circulation in Lake Pontchartrain

A three-dimensional hydrodynamic model (Princeton Ocean Model, POM) was used to study the dynamic behavior of a saltwater plume originating from a navigation canal and advancing in the lake. The Inner Harbor Navigational Canal (IHNC) is part of the Mississippi River Gulf Outlet, which permits ships to navigate from the Gulf of Mexico to Lake Pontchartrain and the Mississippi River at New Orleans. Lake Pontchartrain is a relatively shallow, brackish estuarine lake with a mean depth of less than 4 m and a mean salinity of 7 ppt. At times, the IHNC brings highly saline water (> 20 ppt) into Lake Pontchartrain. Under certain conditions, this higher density water has been observed to form a thin layer of high salinity water over a large area near the bottom of the lake. Field data showed that the stratified zone was approximately 0.5 m deep and up to 250 km 2 in area. A three-dimensional hydrodynamic model (POM) was developed for the area to study the effect of Lake circulation on the location and stability of the saltwater plume. Field and laboratory data were used for model calibration and verification. The forcing functions for the model include tide from nearby connections to the Gulf of Mexico, wind and river flows from tributaries to the north and west. The model includes 20 sigma levels with surface and bottom refinement in order to capture the momentum transfer from the wind shear and the density current near the bed. The horizontal grid resolution is 600 m.

A Statistical Methodology to Discover Precipitation Microclimates in Southeast Louisiana: Implications for Coastal Watersheds*

Abstract This study quantifies the spatial distribution of precipitation patterns on an annual basis for southeast Louisiana. To compile a long-term record of 24-h rainfall, rainfall reports collected by National Weather Service (NWS) cooperative observers were gathered from National Climatic Data Center (NCDC) archives, private collections of observational data held at regional and local libraries, NWS offices, and local utility providers. The reports were placed into a digital database in which each station’s record was subjected to an extensive quality control process. This process produced a database of daily rainfall reports for 59 south Louisiana stations for the period 1836–2002, with extensive documentation for each site outlining the differences between the study’s data and the data available from the NCDC Web page. A statistical methodology was developed to determine if the four NCDC climate divisions for southeast Louisiana accurately depict average monthly rainfall for the area. This method em...

Discussion of “Tractive Force Design for Sanitary Sewer Self-Cleansing” by LaVere B. Merritt

In the original paper, Merritt presents a detailed summary ofChapter 5 of the 2007 edition of the joint ASCE MOP 60-WaterEnvironment Federation (WEF) FD-5 Manual, Gravity SanitarySewer Design and Construction (ASCE-WEF 2007). He endorsesthe manual’s recommendation to use the unit tractive force (TF)method to calculate the minimum slope of sanitary sewers carryingthe minimum flow at the beginning of the design period. He alsoreproduces the method to calculate such a flow, by using criteriapresented in Chapter 3 (ASCE-WPCF 1970, 1982; ASCE-WEF2007).In Merritt’s literature review, he fails to mention that the TFmethod to attain self-cleansing in sewers carrying settleableparticles was proposed as early as 1954 by Fair and Geyer,who suggested that to have self-cleansing conditions at all flowrates, the unit tractive force must be equal to that generated in asewer running full at a minimum velocity of 0:6m∕s(Fair andGeyer 1954). This minimum velocity was considered for manyyears sufficient to provide self-cleansing in a sanitary sewer run-ning full, without regard to particle size. The unit tractive force,in this case, can be calculated by using the following equation(La Motta 1996):T ¼ 0:571464γn

Fate of Pathogens in Stormwater Plumes

Modern tools for management of recreational waters include field monitoring, laboratory analyses and computer modeling. A case study of a brackish receiving wa…

Urban Rainwater Composition Study

Rainwater samples have been collected in New Orleans from October 1999 to October 2000. The samples were analyzed for water quality parameters: pH, total Kjeld…