Numerical Modelling of Shippagan Bay

Susan  Haigh, John Hughes Clarke and Joël Chassé*



*Bedford Institute of Oceanography

Please note that this web page is currently under development


Introduction:

As part of a joint research project between the Gulf Fisheries Centre (DFO, Moncton) and the Ocean Mapping Group (UNB, Fredericton) an acoustic survey was conducted of Shippagan Bay during July and August 2003.  From it's southern tip to it's northern opening Shippagan Bay is approximately 15 km long.  Shippagan Bay opens into the Bay of Chaleur where the main exchange of water takes place. There is an additional smaller exchange of water between Shippagan Gully to the South and the Gulf of Saint Lawrence.

The results of the above survey have two uses in this numerical study.  First, the bathymetry acquired from the survey was used to generate a grid for the model.  Second, the eventual goal is to run the model for the same time as the survey so that we can make a direct comparison between the model results and the survey data. 






INSERT MAP OF SHIPPAGAN BAY HERE
                                                                
  
The Numerical Model:
                                      
Shippagan Bay is fairly shallow with a mean depth of approximately 4m.  The bar graph to the right shows that most of the depth are in the [-0.9,10m] range with negative values indicating above mean sea level.  Because there are areas in the bay which are wet at high tide and dry at low tide, a model that will allow for this is required.  We are using the finite element model Qu_dry which was developed by David Greenberg at the Bedford Institute of Oceanography.  Qu_dry is based on the three-dimensional finite element shelf circulation model QUODDY4_1.1A.  In addition to the features of QUODDY which is a non-linear, free surface, tide resolving model, Qu_dry allows for the flooding and drying of intertidal areas.

Finite Element Grid:
                                                                                                               
In order to create a finite element mesh, bathymetric data from the 2003 survey of Shippagan Bay was used. The high and low tide water lines were manually digitized from the Canadian Hydrographic Services Chart LC 4913.

A grid has been created which contains 2527 nodes and 4352 elements.  The depth range of the nodes is from -0.9m to 20.87m.  The elements range in size from 1.2237e3m2to 1.0792e5m2.  At this time, we have 10 nodes in the vertical with a  sinusoidal spacing.


Shippagan Bay Mesh
Shippagan Bay Mesh: Bathymetry (m)



Boundary Conditions:

Boundary conditions were supplied by Chassé who has developed a three-dimensional numerical model of the  Gulf of St. Lawrence, Bay of Chaleur and St. Lawrence Estuary.  He has provided us with the elevations predicted by his model at the open boundaries of our mesh for the period of June 30th to July 26th 2000.  His simulation includes wind, heat flux and river inputs as well as the five main tidal components (M2, S2, N2, K1 and 01).

The model of Shippagan Bay has two open boundaries.  The larger of the two is between the model domain and Bay of Chaleur, indicated in green to the right.  A smaller open boundary exists between Shippagan Gully and the Gulf of St. Lawrence and is shown in red on the figure to the right.

Elevation data provided by Chassé

Results:

To date we have two runs of the Shippagan Bay model.  In both cases the model was run in the barotropic mode.  The elevation was ramped up over the first twelve hours of the simulation.  At this time the only external forcing is to impose the elevation at the open boundaries.

The first run closes the open boundary to the South.  A movie of the run can be viewed by clicking here.  The second run opens the boundary to the South between Shippagan Gully and the Gulf of Saint Lawrence.   Click  here to view a movie of results from this run. 

Using the IOS Tidal Package by Mike Foreman, we compare the tidal constituents of our model results with those provided by Chassé for his model and observation at Shippagan Tide Station.  We have analysed the results from our simulations at the node of the finite element grid closest to the Shippagan tide station.  The results of the analysis of the sea surface elevation are given below.   The length of our run was not long enough for the N2 tidal component to be included in the analysis.


M2
amp
 M2
phase
 S2
amp
 S2
phase
 N2
amp
 N2
phase
 K1
amp
 K1
phase
 O1
amp
 O1
phase
Observed
 49.9
 216.9
 13.4
 266.0
 11.2
 196.8
 20.1
 295.2
 18.8
 263.8
Chassé 43.9
 219.0
 11.5
 268.3
 9.7
 201.0
 18.2
 292.1
 18.8
 252.4
Closed to South
 44.5
 258.5
 16.7
 310.6
 n/a
 n/a
 15.2
 299.9
 19.1
 271.9
Opened to South
 42.2
 257.7
 15.7
 309.1
 n/a
 n/a
 15.1
 298.6
 19.0
 270.7

The analysis for the two runs do not give significantly different results as far as tidal analysis is concerned.  The results for the diurnal components (K1 and O1) are better than for the ssemi-diurnal components (M2 and S2).  There is a large error in the phase for the semi-diurnal case.  Ways to reduce this error and improve the overall performance of the model are being examined.

Although tidal analsis is an excellent way of comparing our results with observations, it does not give us an indication of how opening the Southern boundary affects the results.  Comparing the movies of the two runs shows some qualitative differences particularly in Shippagan Gully.

Overall, at least qualitatively speaking, the two flow do not differ significantly.  One exception, is in Shippagan Gully where we observe a difference in the timing of the reversal of the flow at high tide.  To get a better understanding of the differences of the two runs, we have plotted the elevations and vertically averaged velocities at node 925 (the node closest to Shippagan tide station) for both runs.  For viewing ease, we have plotted this information for days 15 through 20 of the simulation only.

We see that there is little change in elevation when the opening between  Shippagan Gully and the Gulf of St. Lawrence is included.   There are two main differences in the velocity.  The first is a reduction in the velocity amplitude.  The second is that there is a delay in the timing of the velocity reversal at high tide.  The velocity reversal at low time occurs at the same time for both cases. 


Another way of comparing the two runs is to average the hourly currents over a period of 15 days.  This yields residual velocity fields in which most of the effects of the tides have been removed.  The two figures below show the velocity averaged over the last 15 days of the simulations for the two run.  The most obvious difference  between the two runs is that there is a net flow out of Shippagan Gully into the Gulf of St Lawrence when the southern boundary is open.
Residual velocity: southern boundary closed


Residual velocity: southern boundary opened



Future Work:

Once we have the barotropic model working the next step is to include temperature and salinity and run the model in the baroclinic mode.  This should be particularly interesting as the water in the Gulf of Saint Lawrence which enters Shippagan Bay through Shippagan Gully has different temperature and salinity signatures than the water in the Bay of Chaleur.  Once the baroclinic model is running to our satisfaction we plan to run a simulation for the time period over which the survey was done.  This will enable us to do a direct comparison between the model's results and observations.

Acknowledgements:

We would like to thank Dave Greenberg of the Bedford Institute of Oceanography for many valuable discussions regarding the running of QUODDY and Qu_dry and grid generations.

last modified by S. Haigh on 9 September2004.