NEARSHORE
OCEANOGRAPHY
SO422, Spring 2009
LAB:
Onshore-offshore sediment transport
The
assignment will be placed in the assignment dropbox on Blackboard by 1330 on
Tuesday 3 February. Insure that the names of all members of the
group (max of 3) are on the file name and in the report itself.
This lab will consist of group effort to collect data and to prepare the
report. Two or three person groups
will run the computer model during the lab period and share printouts.
The computer will take some minimal time for each run, but can run
unattended; your group can run multiple computers at once.
Download the WAVES program, and
place it in a directory under c:\temp\your_name.
Computer simulations can be used for many purposes in science.
Before they can be used, however, the results of the simulation must be
verified. For this lab we will use a
computer simulation of onshore-offshore
sand transport to verify that the computer model behaves properly.
The model was developed by Martinez (1987) in FORTRAN for a Mac.
It was converted to Pascal by Guth for the MS-DOS machine, and now for
Windows. You are to evaluate the
model to verify that it behaves as predicted theoretically.
Determine the results that the model simulates for the following cases,
and state how this corresponds with what you expect from your knowledge of
nearshore oceanography and why this is the case.
Except as indicated, you can see the results with about 5000 iterations
of the model, which will take only a few minutes.
Do not change the values of the parameters beyond those in the default settings,
or you might get complete garbage.
-
Changing mean wave height Ho;
-
Changing wave period T;
-
Changing the initial beach slope (you may want to
run this one longer to be sure of your results, and also blow up the
relevant portion of the diagram);
-
Changing the size of the beach sediment (change
both the large and fine sediment size, and changes of 0.01 cm are
sufficient; you may want to run this one longer to be sure of your
results, and also blow up the relevant portion of the diagram);
-
Adding a tide to the model
(this run will definitely take longer).
-
Comparing a Rayleigh wave distribution to all waves
being "average" (this run will definitely take longer).
Prepare
a Word report on your results, clearly indicating what about the
wave parameters causes the effects you see.
You should have figure for each of the 6 cases, correctly label each
according to the department style manual, and refer in the text to each
figure.
Notes:
The
Rayleigh distribution is a statistical model for the sizes of the waves that
will approach a shore. See Fig.5.6b
on p.144 of the Komar text. When you
have this option selected, you will see a histogram showing the heights of the
waves coming in.
Each
computer run can model three different beaches.
Unless you are a masochist, do not try to vary more than one parameter at
a time (i.e. to test wave height, keep all the other variables the same).
Some
parameters affect all the models running at the same time (e.g. Rayleigh
distribution or tide model), so you may have to do some comparisons between two different
computer runs.
The
lower, offshore end of the profile is fixed.
If you get waves that want to move sand at that point, there is an
infinite amount of sediment available, and your results will not be valid.
Try waves that will do their work closer to shore.
In
changing parameters, keep the values "reasonable"; the initial values
are all reasonable. You do not need monstrous differences to see the changes.
You
need run only enough iterations so that you can see the effect and confidently
describe it.
Remember that the model simulates only onshore-offshore transport,
shoreward of the breaker zone. It
will have no longshore currents, no wave reflection , no reforming of the waves,
and no seaward transport from inside the breaker zone.
REFERENCE
Martinez,
P.A., 1987, WAVE: program for simulating onshore-offshore transport in two
dimensions using the Macintosh computer, Computers & Geosciences, vol.13,
no.5, p.513-540.