Near Field Effects of Tidal Power Extraction on Extreme Events and Coastline Integrity in the Bay of Fundy

Report

Title: Near Field Effects of Tidal Power Extraction on Extreme Events and Coastline Integrity in the Bay of Fundy
Authors: Watanabe, R.
Publication Date:
March 01, 2011
Document Number: TR-11-01 Rev 01
Pages: 72
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Citation

Watanabe, R. (2011). Near Field Effects of Tidal Power Extraction on Extreme Events and Coastline Integrity in the Bay of Fundy. Report by Martec Ltd and Offshore Energy Research Association of Nova Scotia (OERA). pp 72.
Abstract: 

The objective of this project is to quantify the near-field effects of large scale tidal power extraction from the Bay of Fundy by the use of tidal energy extraction devices on the resulting effects of extreme storm events and coastline integrity by implementing a spectral wave model to numerically simulate wave transformation for operational (with turbines) and nonoperational (without turbines) tidal conditions. The predicted current fields for the operational and non-operational conditions were obtained directly from the hydrodynamic modeling of the Bay of Fundy produced by Fisheries and Oceans Canada (Dr. David Greenberg). Time-dependent current predictions for the near and far-field models were input directly into the wave spectral model to assess the effects of power extraction on wave transformation. The wave-current modelling analysis was organized into three phases.

  • Phase I Seasonal (Non-Storm) Wave Conditions
  • Phase II Storm Wave Conditions
  • Phase III Extreme Wave Conditions

A spectral wave model (one-dimensional and two-dimensional) was integrated with the existing circulation models for tidal currents in the Bay of Fundy to simulate the wave-current interaction. The overall objective of the work is to assess the changes in wave conditions caused by the extraction of energy from tidal currents, which will indicate the effect of the tidal energy turbines on shoreline erosion and coastline integrity.

 

The results of the analysis carried out using the SwanOne wave model (a one-dimensional model) show that changes in the current affect the spectral shape of the waves and redistribute wave energy to different frequency bands. As expected, changes in the current produce a net change in the characteristics of energy propagating through the model. More detailed analysis of the transformations occurring, particularly the redistribution of energy to the various frequency bands, was carried out using two-dimensional models.

 

The modelling of the wave-current interaction using the 2-dimensional STWAVE model with the BIO tidal current model is shown to be a powerful tool to quantify expected changes in wave energy throughout the model domain. The modelling and refinement of the wave-current model was carried out to help define areas of interest in regards to shoreline response to changing conditions. Some of the modelling results from the on-going BIO/Acadia research were integrated into our analysis to incorporate any expected changes in the flow field with the introduction of tidal energy extraction devices. In particular, the extreme case of an intense concentration of 225 turbines installed across Minas Passage had shown a decrease of up to 20% in current magnitude in close proximity to the turbines. Using this value of change in the current field in a portion of Minas Passage and combining with a series of wave conditions, nearshore effects produced only small localized changes in wave energy levels at the adjacent shoreline. Based on the analysis carried out on potential changes in wave patterns as a result of current changes and caused by a set of 225 tidal turbines positioned offshore from Cape Split across a portion of Minas Passage, the resulting nearshore effects observed in the model were small decreases in energy levels at the adjacent shoreline. Therefore, it can be expected that under the conditions analyzed, the energy extraction could cause increased deposition of suspended sediment in some near-field regions of the Minas Basin channel and Passage. Far field effects of tidal changes were not considered as part of this study.

 

Small changes shown in the analysis carried out could still represent more significant changes in sedimentation or erosion as a cumulative effect over the longer term. More detailed information on flow and sediment properties are required. Details of the flow field surrounding the individual turbines and the resultant details of flow around an array require further characterization. Sediment properties in the regions of flow changes should be better quantified for predictive effects of deposition or scour due to flow changes.

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