Case Studies
Auckland, New Zealand - September 2001
Primary M2 tide for New Zealand
Derek Goring is the Principal Scientist of the Coastal Hydrodynamics
Group, based in Christchurch, on the South Island of New Zealand. The
Coastal Hydrodynamics Group is part of the National Institute of Water
and Atmospheric Research LTD (NIWA). It is a government-owned, but fully
autonomous company.
Derek is involved in research of coastal, river and eco-hydraulics – the
mechanics of flowing water. The goal of this research is to understand
how sea level varies around New Zealand's coast.
The plot of the month shows how the primary M2 tide for New Zealand propagates
in a counterclockwise direction around the country (a Kelvin wave), completing
the cycle every 12.4 hours. Red represents high tide, blue represents
low tide, and green represents zero elevation. It shows that at any time
there is a high tide somewhere on the New Zealand coast. This is very
unusual and occurs only one other place – Iceland.
The stretch of water between the North Island and the South Island is
Cook Strait. At times there is high tide (red) on one side and low tide
on the other. This means there is a two meter difference in sea level
across a relatively small distance. This causes a very strong tidal flow,
as seen in the tidal currents animation below.
Tidal Currents in Cook Strait
These results were computed from a finite-element hydrodynamic
model of New Zealand’s Exclusive Economic Zone which extends from 24 to
65 ºS and 156 to 210ºE. The model was driven on the open boundaries by
tide data derived from the TOPEX/Poseidon oceanographic satellite. The
results were verified by accurate measurements of sea level at 20 locations
around the coast.
Figure 3
The model produces amplitude and phase for water surface
displacement, as well as eastward and northward velocity for each tide.
This is accomplished using about 30,000 nodes in an irregular grid, as
shown in Figure 3. A FORTRAN program is used
to synthesise the data at around 40 intervals over a tide cycle (12.42
hours). Each of these intervals is written as a zone using the Tecplot
binary TecIO. The resulting .plt file is
loaded into Tecplot, a contour plot is specified, and then animated over
the 40 intervals (or zones).
Figure 4
Figure 4 shows the way tidal data
are usually portrayed with a cotidal chart. Draughtsmen plotted them by
hand before NIWA began using Tecplot. Amplitude is shown as colored contours,
and phase are displayed as lines. In New Zealand, where the tide is so
complicated, it is difficult to absorb all the information in a cotidal
chart. However, in a Tecplot animation information becomes abundantly
clear.
"When we first animated the tides back in 1995 using a very early version
of Tecplot, we shouted 'Eureka!' We finally understood how the tide worked!
Subsequently, we used that animation to persuade politicians and stakeholders
that we were doing good work that deserved generous, long-term funding.
That strategy worked, because everyone can get an appreciation of how
things work from an animation."
More details and plots are available on the Coastal Hydrodynamics Group's
Web page.
Tecplot Tips:
- Writing binary Tecplot files from FORTRAN using the TecIO facilities
makes loading very fast. Chapter 11 of the Tecplot
Reference Manual explains how to do this in an easy to understand
manner.
- Running animations from Raster Metafiles using Framer is more flexible
than using AVI files, especially for presentations using PowerPoint™,
where the speed of the animation can be preset.
- A flooded contour plot on a black background makes a stunning animation
for PowerPoint presentations.
|
RSS Feeds
System Requirements
