Environmental Science & Engineering - www.esemag.com - March 2004
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Responsible infrastructure planning through three dimensional hydrodynamic modeling

By Dr. Ray Dewey and Norm Huggins

Cumulative build-up of Total Phosphorus (TP) above the Provincial Water Quality Objective (PWQO) of 0.02 mg/L, showing number of hours over the PWQO out of 4,330 hours along the Toronto waterfront.
Picture courtesy of Modelling Surface Water Limited - Waterfront Modelling Report for WWFMMP, with permission of City of Toronto WES.

The Golden Horseshoe of Ontario is the fastest growing region along the Great Lakes, so millions of people depend on Lake Ontario for drinking water. These same people populate the lands that generate stormwater runoff and treated municipal and industrial wastewater effluent that discharge to the lake. Offshore water treatment plant intakes and outfalls operate in parallel to service the water supply and wastewater and stormwater demands of a growing population.

Fortunately, the ability to monitor and analyze the impacts of potential interference between intakes and outfalls has improved. Environmental engineers are now required to be more reliable in their predictions of future conditions. Some of the questions that have been raised are: Hydrodynamic and water quality models have been applied to predict water quality behaviour under external influences in the Great Lakes regions for many years. They have been developed, improved and applied to specific projects to determine water quality impacts due to shoreline alterations, new or expanded discharges, or to determine the cause of degraded water quality.

Until recently the most common models were based on the two-dimensional equations of fluid motion and mass transport. The models were easier to use and there were several versions available for selection. Some models were in the public domain, which allowed many different people to apply them and learn from them. They could be calibrated and verified from limited data and would provide some valuable results. On the other hand, three-dimensional models, which offered a more in-depth examination of the impacts, were limited to academic research or very specific studies and mostly remained out of the hands of the private sector.

The models have been used to study bacteria contamination of beaches, effluent plume delineation and circulation changes due to shoreline modifications, just to name a few. One of the major problems in implementing both the two-and three-dimensional models is the required boundary data to establish the flow field and the ambient conditions. Two-dimensional models could be run with one or several time series along each open border, the time series data consisting of recording current meter data. Optionally the models could be just applied as wind driven which required even less data. In their evolution these products performed to the limits of their application.

Professional engineering software three-dimensional models for practical application outside of the academic forum alone are now available. These models provide highly accurate threedimensional simulations of water bodies. These models are ideal for work on the Great Lakes as they can simulate the complex thermal structure of a lake during stratification. Besides the standard ability to predict the movement of water and pollutant transport, they now have sophisticated add-on modules to deal with polluted sediments, nuisance growth of algae, eutrophication and nutrient dynamics. In addition they have the added benefit of nesting.

Why nesting? In order to provide accurate simulations of the thermal structure and complex dynamics, it is necessary to model the entire lake. This has several additional benefits. One is that the initial conditions are easy to set. The water temperature in lakes in Ontario drops during the winter and, after the ice break-up, the temperature is almost isothermal over the depth of the water. There may be some cooler surface water just after the ice melts, but as the sun warms the surface, isothermal conditions are formed. Thus one can set the vertical structure quite simply. Imagine trying to set the temperature of the lake in June or July when the water column has a complex temperature gradient. Many temperature profiles would be needed to establish the pattern across an open offshore water boundary.

Effluent plume delineation
Once the model has been properly calibrated and verified it can be applied to determine the spatial and temporal extent of the discharge plume. Effluent discharges in the Great Lakes have very complex plumes due to the currents’ changing speed as well as direction.

Generally the plume will go in a particular direction, mainly parallel-toshore and then suddenly reverse direction due to changes in the wind patterns. After a while the currents will reverse and then the plume will change direction. The alternating directions mean the plumes can impinge upon the shoreline during the current swing as well as remain stationary as the velocity drops to zero and then speeds up again in the opposite direction. Snapshots of these events will show a typical teardrop shape of the plume. However, integrating the plume over several days or months will show the real extent of the plume. It is the longterm continuous simulation that allows the visualisation of the impact of the plume on the water quality of the area.

Determination of water intake location
New water intakes pose a problem, as there is usually no data on the water quality at the depths proposed. In the past small sample lines have been installed at great cost to monitor the quality over short time periods. With a well-calibrated 3-D model one can simulate the behaviour over several previous seasons to determine the water quality at any number of proposed sites and depths. In addition, if other water discharges are nearby such as wastewater treatment plant outfalls, stream and river outlets, hydroelectric cooling water discharges, etc., their impact on the water quality could be explored. Indeed, if a new wastewater treatment plant discharge was proposed, the impact from the discharge could be modelled before any construction could occur.

Circulation and thermal impacts
Three-dimensional models with a heat balance component are necessary for the simulation of the thermal impacts on the receiving water body. In the past modellers have had to use “typical summer or winter conditions” to show the impact of a cooling water discharge. This is a hazy and inferior method compared to continuous simulations involving real dynamic changes occurring over several seasons. Also the vertical changes in the water column can be determined, rather than inferred from “typical” conditions or from the use of two-dimensional models.

Effluent ammonia limits for C of A
Establishing effluent ammonia limits in lakes has been difficult in the past. Initial dilution estimates from a diffuser model such as CORMIX, coupled with ambient water quality data and “best guesses” of water currents, have provided the best method so far. However, under stratified conditions and variable water currents, the estimates can be too severe and generate difficult and challenging treatment requirements in wastewater treatment plants. Now one can apply a well calibrated model to real time conditions and determine what effluent ammonia levels can achieve the Provincial Water Quality Objective (PWQO) without requiring any mixing zones.

The model has the ability to handle ammonia interactions with the environment to account for assimilations and decay. Several iterations can be performed to fine-tune the effluent ammonia levels to even provide monthly levels through the transition periods of spring and fall when limits change from warm weather conditions to cold weather. We have even performed simulations over several years of different meteorological conditions to ensure that any particular year was not an “easy” scenario. Typically 20 years of data are used to characterize the ambient “conditions”; in our case we examined five different years to cover most typical conditions.

Stormwater management
Stormwater discharges have been the major focus of work with this model for the City of Toronto Wet Weather Flow Master Monitoring Plan (WWFMMP). The models allow the determination of the impact of the stormwater on the water quality of the receiving water body. Beach closures have been problematic and these models have identified the causes and then the effectiveness of the remediation actions. When the model is properly calibrated it can accurately predict the temperature changes and water currents.

Nuisance growth algae
One of the water quality problems that results from wastewater discharges are the nutrients in the effluent that cause nuisance growth of attached algae. With the eutrophication module one can simulate the dynamics of nutrients in the water body and observe the impacts of nutrient enrichment and remediation measures.

Long-term cumulative impacts
Continuous simulations over several seasons allow one to study the impacts of the discharges on the accumulation of waste products and the effectiveness of remediation efforts.

Two-dimensional hydraulic models serve as an acceptable screening tool and have been relied upon until recently to predict the challenges to wastewater treatment plant outfalls and water supply plant intakes. With the advancement of reasonably available three-dimensional modelling and the need to predict effluent potential behaviour compliant with MOE and community standards, the two-dimensional model is at best an acceptable screening device available to the engineer and owner at project conception.
Contact Norm Huggins, CH2M HILL Canada at (416) 499-0090, ext. 364 or
Ray Dewey, Modelling Surface Water Limited, (on leave from CH2M HILL Canada) at (416) 757-1749.

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