HYDRODYNAMIC AND SEDIMENT TRANSPORT MODELLING COMPONENT STUDY

EXECUTIVE SUMMARY

The Hydrodynamic and Sediment Transport Modelling Component Study (“Modelling Component Study”) for the Petitcodiac River Causeway Environmental Impact Assessment (EIA) was conducted in accordance with the “Final Terms of Reference for the Hydrodynamic and Sediment Transport Modelling Component Study for the Petitcodiac River Causeway Environmental Impact Assessment”, as approved by the Technical Review Committee. The study was designed and conducted in accordance with the earlier recommendations of the Petitcodiac River/Estuary Modelling Workshop held in March 2002.

Numerical modelling was one of the “predictive tools” used by the AMEC Study Team to provide a description of the anticipated physical characteristics of the Petitcodiac River in response to the Status Quo and the Project Options. The other “predictive tools” included the experience of the AMEC Study Team with the Petitcodiac River system, interviews with people familiar with the history and evolution of the river, analyses of patterns and trends that the river has experienced over time and a projection of those trends into the future and empirical relationships, river engineering and hydraulics formulations. The predictions made by the numerical modelling were at a macro level (on a river stretch basis rather than a discrete line or point basis) and were used to complement the other “predictive tools” used to draw the conclusions in the EIA Report. The numerical models were used to provide the resolution required by the EIA and were not intended for design purposes.

Numerical modelling was initiated through conceptual models (mass balance) and preliminary models (0-D and 1-D) to simulate the conditions within the estuary. One-dimensional (1-D) and two-dimensional (2-D) numerical models were used to carry out the numerical modelling exercise. The GEN1D and TELEMAC-2D hydrodynamic models and the CUMBSED sediment transport model were used. The assumptions used in applying these models to the Petitcodiac River estuary, as well as the limitations of the models, are outlined in this report.

An extensive data collection program was undertaken and an analysis of historical data was carried out in association with the Modelling Component Study. The calibration and verification of the hydrodynamic models was carried out based on the following events:
  • Reproducing water levels estimated using published tidal information from the Canadian Hydrographic Service for a 14-day tidal cycle in 1966, before the causeway was constructed (01/02/1966 to 15/02/1966); and
  • Reproducing measured tidal information for a 14-day tidal cycle in 2003 (17/05/2003 to 31/05/2003); and measured salinity/velocity information (2003).

The quality of the calibration/verification of the hydrodynamic models was measured by their ability to predict high water including the height and arrival time. Model predictions for this study of the Petitcodiac River estuary varied between 1% and 6% of observed high water elevations and between 1% and 2% of observed arrival times. These results compare favourably with published studies and numerous other studies carried out by the consultants. For the above comparisons, the high water differences were expressed as a percent of the tidal range (about 13 m at Hopewell Cape and 7 m near Moncton) and the time of arrival differences were expressed as a percent of a tidal cycle of 745 minutes. The calibration/verification of both the 1-D and 2-D hydrodynamic models is considered satisfactory, for the purposes of the EIA (Sections 8.1 and 8.2 of the EIA Report).

Calibration and verification of the sediment transport models were carried out based on the following events:
  • Reproducing the substantial changes that occurred in the estuary after the construction of the causeway from 1966 to 2003, including a comparison of the observed and predicted cumulative tidal volumes in 1991 and 2001;
  • Reproducing the infilling in the headpond that occurred during the 1988 gate openings;
  • Reproducing the seasonal infilling and subsequent erosion that occurs at the Gunningsville Bridge, near Mill Creek and at Hopewell Cape (2003); and
  • Reproducing the substantial changes that occurred in the estuary during the years that the causeway was being constructed.

The relative success of the calibration/verification exercise was measured by the ability of the sediment transport models to predict changes in cross-sectional area and water / sediment volumes upstream of a section. It is important that cross-sectional areas (channel capacity) are in close agreement since they will influence the flow of water and sediment through the river (U.S. Army Corps of Engineers, 1993). Model predictions for this study of the Petitcodiac River estuary were within 20% of observed values. The calibration/verification of both the 1-D and 2-D sediment transport models is considered reasonable for this type of model and, in particular, for their application in predicting the physical changes in the Petitcodiac River estuary associated with modifications to the causeway. The Petitcodiac River estuary experiences some of the highest tides and sediment concentrations found in any estuary in the world.

In addition to the above calibration/verification events, the following scenarios were considered:
  • Evaluation of what will likely happen to the estuary if nothing is done to the estuary, for the period 2005 to 2025; and
  • Evaluation of what would have likely happened to the estuary if the causeway had not been constructed, for the period 1966 to 2003.

The relative success of these two hypothetical scenarios was measured by the ability of the sediment transport models to compare to the general understanding of how the next 20 years would look like if the causeway remained in place or what would have happened in the last 40 years if the causeway had not been built. The model predictions were in reasonable agreement with accepted beliefs.

The Modelling Component has demonstrated that the numerical models were able to reproduce reasonably well observations and measurements before, during and after the construction of the causeway. The model output, together with historical observations and trend analysis can therefore be expected to predict changes into the future to a satisfactory degree, for the purposes of the EIA, should the causeway operation remain unchanged (Status Quo) or one of the Project Options be implemented.

The following four scenarios were modelled to cover the anticipated range of changes associated with the Status Quo and Project Options as described in Niles (2001) and the final EIA Guidelines (NBDELG, 2002). These scenarios do not correspond directly with the EIA Project Options as these were finalized at a later stage. A linkage between the modelling scenarios and the EIA Project Options is presented in this report. The 2-D sediment transport model was used to support the short-term results of the 1-D model, while the 1-D sediment transport model was used to evaluate the long-term sedimentation and erosion of the Petitcodiac River estuary (20 years) under these scenarios. • Scenario 1: Status Quo (reference); • Scenario 2: 1 gate open during open water season; • Scenario 3: 5 gates open during open water season; and • Scenario 4: Partial Bridge.

The response of the Petitcodiac River system to extreme hydrological events was also modelled to evaluate the risk of flooding along the estuary. The following types of events were considered in this evaluation:
  • Severe winter rainfall/snowmelt event, in conjunction with restricted present day channel dimensions due to winter ice conditions;
  • Extreme winter rainfall/snowmelt event, in conjunction with restricted present day channel dimensions due to winter ice conditions;
  • Extreme fall rainfall event;
  • Extreme fall rainfall event, in conjunction with a storm passage (high winds from the Bay of Fundy and rise in Mean Sea Level in order to account for climate change); and
  • 100-year flow at the causeway in present day ice-free channel conditions, in conjunction with high winds from the Bay of Fundy and rise in Mean Sea level.

The 2-D sediment transport model confirmed the 1-D sediment transport model results in the short-term (1 year) for Scenarios 1 and 4. The long-term 1-D model results echoed the general understating that, under Scenario 1, the river should experience continuous infilling. Although the width of the channel seemed to respond to the fixed width of the control structure at the causeway, the results showed that the cross-sectional area would continue to decrease due to the rise of the bed, in particular around the bend at Moncton. Under these conditions, it is anticipated that more flooding is to be expected. Besides, the development of a high sill at Moncton could have a direct impact on water circulation, hence the quality of the water between the causeway and Moncton.

Under Scenario 4 (Partial Bridge), the long-term results also confirmed the general understating that the channel downstream of Dover should reopen to its pre-causeway conditions. Around the bend at Moncton, the channel widened as the flow cut into the banks at Outhouse Point. This is due to the fact that the tidal prism would extend further up in the estuary under this scenario. However, the existing sill (sediment accumulation) at Moncton might not disappear as easily as the river widens. This would remain a control point, and would, in particular, dampen the tidal bore at Moncton. At the landfill site on the Moncton side, it is anticipated that, in the case of Scenario 4, the wetlands in front of the landfill would not be eroded. During the causeway construction (winter 1967-1968), when a 200 m opening was maintained in the embankment; substantial deposition was already observed on the Moncton side. It is not expected that the river would react differently to an opening of similar size in the causeway.

The predicted response to the extreme hydrological events did not identify any potential for flooding during the winter months, downstream or upstream of the present day location of the causeway under Scenario 3. There exists a risk under Scenario 1. There is also a risk of flooding in the event of a heavy rainfall event and associated high flows. The risk is increased when the fall event is associated with a storm passage to the point that the causeway embankment could be overtopped. This study illustrated that precautionary measures to limit the effect of flooding, such as optimized gate operation strategy and maintenance of headpond level based on storm warning, could improve the resulting water level in the estuary if implemented in a timely manner. Similarly, the implementation of Scenario 3 appeared to be beneficial in that the risk of flooding was, if not alleviated, at least considerably decreased upstream of the causeway.

It should be noted that, in these studies, the model predictions were limited to indicating whether the water could top the riverbanks under a certain event, but that the extent of flooding beyond the banks of the Petitcodiac River was not determined.

The modelling that has been conducted as part of this study is considered acceptable for the purposes of the environmental effects assessment for the EIA. The field data collection and numerical modelling programs were designed to support the EIA, the effects assessment and the development of an implementation strategy for the Project Options. In that sense, they have substantially advanced the understanding of the river hydrodynamics and sediment transport characteristics.

The resolution that was required for the effects assessment has been achieved through the combination of numerical modelling and other predictive tools (i.e., trend analyses, river engineering formulas, historical observation). While the predictions on the river and estuarine characteristics developed from the numerical modelling are accurate from the perspective that they are consistent with information developed from other approaches used, the level of precision attained is that which is acceptable for environmental effects analysis. The modelling outputs should not be used for situations where more precision is required, for example engineering design. Those situations may require additional and more extensive sampling and analysis of site-specific physical information and river and estuarine behaviour or application of a conservative engineering approach in setting design parameters although the numerical models could also be of value to assist with evaluating and predicting general trends associated with changes in the river during the implementation of the project options or with the continuation of the Status Quo.

As noted in this report, the Petitcodiac River system is unique and dynamic (macro tidal, high water velocities, high suspended solids concentrations). A number of assumptions were made to characterize and simulate the river behaviour in the context of the EIA. These assumptions were considered satisfactory and sufficient for the EIA. However, there are areas where research could be undertaken to further define some of the parameters and mechanisms associated with the river’s behaviour and how they can be reflected in the numerical models. Possible areas of research could include:
  • The influence of vertical stratification on the hydrodynamic and sediment transport processes, as well as the influence of turbulence and the role of fluid mud.
  • Micro scale issues such as wind and wave effects.
  • Coupling the slumping of the river banks that can occur due to erosion and undercutting with the sediment erosion processes.

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