Final Report: Urban Storm and Waste Water Outfall Modeling

EPA Grant Number: R825427C011
Subproject: this is subproject number 011 , established and managed by the Center Director under grant R825427
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: Urban Waste Management and Research Center (University New Orleans)
Center Director: McManis, Kenneth
Title: Urban Storm and Waste Water Outfall Modeling
Investigators: McCorquodale, J. A.
Institution: University of New Orleans
EPA Project Officer:
Project Period: January 15, 1998 through January 15, 2001
RFA: Urban Waste Management & Research Center (1998) RFA Text |  Recipients Lists
Research Category: Targeted Research

Objective:

In 1985, an advisory discouraging swimming and other primary contact recreational activities in Lake Pontchartrain was issued by the Louisiana Department of Health and Hospitals (LDHH). This advisory names fecal coliform (FC) bacteria as the causative pollutant and is still in effect today for the south shore area of the lake as mandated by the LDHH in conjunction with the Louisiana Department of Environmental Quality (LDEQ).

A water quality shoreline study in the area affected by the advisory was initiated in September of 1998 and continued until 2001. Five sites that are or were at one time used for primary and secondary contact recreation were selected for study. Two of the sites represent outlets of urban runoff drainage canals while the others are beach or recreational park areas found near the canals. The parameters measured at these sites are FC, salinity, conductivity, water temperature, nitrogen as ammonia, total nitrogen as nitrite/nitrate, total phosphorus, phosphorus as ortho-phosphate, total suspended solids, volatile suspended solids and pH. This project confirmed that fecal contamination at four out of five sites along the south shore of Lake Pontchartrain is caused by urban runoff discharged to the lake via a drainage system of collection sewers, pumping stations and canals. The flows and ambient conditions of this system result in slowly spreading outfall plumes characterized by shore reattachment and low dilution. The fifth site had a signature that indicated an active source other than storm water runoff.

Modeling of the outfall plumes is needed for delineating shoreline contamination and investigating possible remediation. Integral type numerical models for neutral-density and surface-buoyant discharges have been developed for discharge from wide aspect ratio drainage channels. The models presented were calibrated using laboratory results and verified with field data from the south shore site on Lake Pontchartrain. The outfall model was coded in FORTRAN with a Visual Basic interface. Neutral density and buoyant density models were developed. These models solve the continuity, conservation of momentum, mass conservation and buoyancy conservation equations subject to a user defined canal effluent and ambient lake conditions. The models used the longshore currents from the RMA2 model of Haralampides (2000) as well as drifter data from Lake Pontchartrain Basin Foundation (LBPF).

A flume model study was conducted with canal width:depth (W:D) aspect ratios in the range 12:1 to 6:1. This experimental study used heated water to simulate the buoyant plume from a rectangular canal at 90° to an ambient current. Both the laboratory model and the numerical model show very low dilutions in range of 2:1 to 6:1 over a distance of 10W.

The water quality data also confirm a very low dilution of the effluent from the drainage canals. FC net dilutions (decay and dilution) were in the range of 1.5 to 6. FC counts were observed to be less than the primary contact advisory level (200 FC/100mL) within three days of the end of the pumping discharge which gives a decay rate of the order of 2x10-5 s-1. The field data indicate a low dilution ratio of 2:1 to 6:1.

The primary objective of this project was to develop an outfall model that would complement the CORMIX 2 model by including outfalls with aspect ratios greater than 4. A secondary objective was to document the role of storm and ambient water mixing in the drainage canal in the overall dilution process.

Two shoreline studies were conducted: 1) outfall studies and 2) a bacteria study. The outfall studies consisted of surveys of seven of the urban storm water canals and the associated Lake bathymetry in 1997 and 1998 (St. Charles Line, Bonnabel Canal, Elmwood Canal, London Canal, Suburban Canal, No. 17 Canal and the 17th Street Canal). A more detailed study was made of the London Canal in order to permit dynamic modeling of the mixing of the low salinity storm water with the high salinity water from Lake Pontchartrain.

The shoreline bacteria field study along the south shore of Lake Pontchartrain was performed during the duration of the project. The parameters measured at these sites were fecal coliform (FC) bacteria, salinity, conductivity, water temperature, nitrogen as ammonia, total nitrogen as nitrite and nitrate, total phosphorus, phosphorus as ortho-phosphate, total suspended solids, volatile suspended solids and pH. Rainfall data for the entire sampling period was also collected. Five sampling locations labeled LP#1 through LP#5 were selected for study. All of these sites are potential recreation areas where a swimming advisory is in effect. LP#2 and LP#4 are at outfall while LP#2 and LP#4 are at beaches. LP#1 is at the Inner Harbor Navigation Canal (IHNC).

Two physical models of idealized channel outfalls were constructed and tested. The purpose of these models was to provide additional calibration and verification data for the numerical models. The first and smaller of the models was constructed in the hydraulics laboratory at UNO primarily for visualization of the buoyant plume spreading with a stagnant ambient environment. The second model was built at the University of Windsor in a re-circulating flume. The purpose of these laboratory experiments was to study the behavior of plumes discharged perpendicular to the shoreline into a uniform-density receiving water body with a cross-current flow. In particular, surface-buoyant plumes and neutral-density plumes were investigated for calibration and validation of the integral numerical models presented in this report as well as future 3-D models.

Integral models to predict the behavior of surface jets/plumes discharged to uniform-density water bodies were developed. Differential equations were developed to predict the volumetric flow, velocity, depth, width, density and dilution of the jet at different cross-sections along the jet axis in the near-field region. The models predict the average values of these plume characteristics over the corresponding cross-sections. FORTRAN codes with a Visual Basic interface were developed for both models. These models were calibrated using the data from the physical model and data from the Canada Centre for Inland Waters for the plume of the Hamilton Harbour ship canal neutral density plume. Field data were used to verify the model. A numerical model based on the 3-D formulation of Ye and McCorquodale (1998) was run to determine the general plume behavior. This model has also been applied to the St. Charles Line Canal and the results compared with aerial photographs. This model was compared with solutions using the 2-D FLUENT model. Both models indicated shore-attached plumes with a minimum dilution of approximately 2 within 30 widths along the trajectory.

Summary/Accomplishments (Outputs/Outcomes):

Student's t-tests showed that fecal coliform (FC) is "wet" weather dependent at all sites except LP#1 near the (IHNC). Parameters such as ammonia and nitrate/nitrite showed a definite "wet" weather dependency at sites LP#3 and LP#5, while total phosphorus was not strongly wet weather dependent. FC regression analyses indicated that the "wet" weather effect lasts for two to three days following a significant storm event (> 0.5 inches) at the stations where FC levels were found to be dependent on significant rainfall events, i.e., all sites except near the IHNC. This suggests that there may be an active continuous bacteria source in or near the IHNC. The data suggested that relationships may exist between the presence of FC and the levels of: ammonia (NH3), total nitrogen as nitrite and nitrate (total N as NO2-/NO3-) and ortho-phosphate (ortho-PO43-).

The threshold rainfall (0.5 inches) for elevated FC corresponds to the rainfall to initiate pumping. The study was hampered by drought conditions for this region which reduced the opportunities to collected field data and steadily increased salinity levels were observed in the study area over the monitoring period e.g., from 6.0 to 13 ppt over a 2-year period.

Evidence was found that urban stormwater discharges to this south shore area of the lake are a source of nutrients and solids as well as pathogens (as indicated by FC bacteria). Cross-contamination of the stormwater runoff effluents by sanitary sewer flows is a suspected source of pathogens, nutrients and solids.

The percentage of time that non-compliance was observed during the primary contact recreation season ranged from as low as 25% at the site near Bayou St. John to nearly 85% at the Orleans Canal outlet.

The FC bacteria, ammonia and total nitrogen as nitrite/nitrate data collected in the shoreline monitoring program confirmed that the near field dilution (up to 30 channel widths) of the drainage canal plumes is very low, of the order of 2:1 to 5:1.

The integral and physical model studies agreed well with plume behavior observed in the field. Both physical and numerical models demonstrated that there is very low vertical mixing when the freshwater plume enters a brackish water body with a densimetric Froude Number similar to the discharges at the Orleans drainage canals. This study also showed that there is a strong probability of two-way stratified flow in the drainage canals with a freshwater layer being discharged to the lake while a brackish layer intrudes into the canal. Dilution results for two of the tests indicated that the near field mixing is very poor for both low and high densimetric conditions. The integral model should not be used outside of the range (width:depth ratio) of 3 < w/d < 20 and for x/w > 30.

Filed data included: salinity, temperature, nutrients and FC. These analyses confirm that the near field dilution (up to 30 channel widths) of the drainage canal plumes is very low, of the order of 2:1 to 5:1. Storm runoff produces FC counts in the range of 800 to 160,000 [MPN/100 mL] compared to the primary contact level of 200.

This project clearly established a statistical relationship between the occurrence of rainfalls in excess of 0.5 inches and fecal coliform (FC) contamination of the shoreline near the urban stormwater outfalls at four out of five sites that were studied.

The duration of elevated FC levels of approximately 2 to 3 days was determined. A regression model based on rainfall in the previous 3 days gave the best R2. The impairment at one of the shoreline sites, near the Inner Harbor Navigation Canal (IHNC), was not correlated with rainfall which indicates that there is another source for the high FC counts.

The behavior of wide plane cross-flowing jets and plumes discharging into a shallow channel has been studied in a physical model. The model had aspect ratios (width to depth) in the range of 5:1 to 12:1 with jet to ambient velocity ratios of 0.5:1 to 4:1. Neutral, buoyant and sinking plumes were considered. Minimum near field dilutions were in the range of 1.5:1 to 4:1.

A product of this project is a predictive model to aid design engineers and environmental managers in assessing the near and intermediate field impacts of existing and proposed outfalls with aspect ratio > 3 which is not available in CORMIX. The integral version of the outfall model has a VB6 user interface. This software and a User's Guide for the integral models have been included with this report.

The in-canal data on salinity and currents showed that there is stratification in the drainage canals during runoff events. The mixing of brackish, relatively clean lake water with the storm water effluent resulted in an in-canal dilution of up to 2:1.


Journal Articles on this Report : 1 Displayed | Download in RIS Format

Other subproject views: All 15 publications 7 publications in selected types All 1 journal articles
Other center views: All 55 publications 13 publications in selected types All 7 journal articles
Type Citation Sub Project Document Sources
Journal Article Barbe DE, Carnelos S, McCorquodale JA. Climatic effect on water quality evaluation. Journal of Environmental Science and Health Part A 2001;36(10):1919-1933. R825427C011 (Final)
  • Abstract from PubMed
  • Abstract: Taylor&Francis-Abstract
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  • Supplemental Keywords:

    human health risk, pathogen indicator, modeling, hydrodynamics, primary contact recreation, surface-buoyant, neutral-density, dilution, stormwater, outfall, cross-current., Scientific Discipline, Geographic Area, Waste, Municipal, Environmental Chemistry, State, Analytical Chemistry, Ecological Risk Assessment, Ecology and Ecosystems, wastewater outfall modeling, waste minimization, urban runoff, municipal waste, groundwater quality, New Orleans (NO), waste management, technology transfer, outreach, urban waste, storm drainage systems

    Progress and Final Reports:

    Original Abstract
  • 1998
  • 1999

  • Main Center Abstract and Reports:

    R825427    Urban Waste Management and Research Center (University New Orleans)

    Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
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