OC4213 Lab 3: Surf Prediction Model

In this lab you will be using the matlab program surfer.m to calculate several statistics (percentage of waves broken, wave heights, longshore current, and modified surf index) for swell that begins in relatively deep water and approaches shore over a beach of uniform slope. This program is an adaptation of fortran code developed by the Commander Naval Surface Force, incorporating concepts discussed in the Joint Surf Manual. It conveniently computes predictions of many of the parameters necessary to describe surf and hydrographic conditions. These parameters may be useful in the planning and execution of an amphibious operation, or in better understanding the relationships between waves and bottom topography.

Before starting matlab, you will need to create a link to the programs that perform these calculations. In the directory you have created for oc4213, type the following command:

    ln    -s   ~orzech/oc4213/lab3/*   .

After establishing this link, start matlab and type surfer to initiate the surf prediction program. The program will ask you for several initial values describing the swell and the (constant) beach slope. The values on the right of each box are the defaults, used when you enter nothing. Slope can be entered as a fraction, e.g., 1/120. (Note that, each time you click COMPUTE, your figures will be overwritten.)

After you have entered the values, the program will calculate and plot the cross- shore evolution of breaking waves, significant wave height, and longshore current over the plane beach profile you have selected. It will also return values for maximum wave height and longshore current with their locations, as well as breaker angle and modified surf index.

Objective #1 : To assess the effect of varying the wave height and wave angle on the longshore current velocity. Run the model for three wave heights (e.g., H = 0.75 m, 1.5 m, 3.0 m) holding all other parameters constant, and for three different wave angles from the same quadrant (e.g., alpha= a _o , 2 a _o , 4 a _o ).

Questions:

1. How sensitive is the longshore current to wave height versus wave angle?

2. Make a table (include at least 9 sets of values):

Swell H, dir

Max brkr height

Breaker period

Breaker type

Breaker angle

Max current

Mod surf index

etc......

Objective #2 : To use the modified surf index limits to determine landing capability for various amphibious craft in different surf conditions. For this portion of the lab, use the surfer program in combination with the listing of Modified Surf Limits For Landing Craft (required) from the Joint Surf Manual. To see how surfer calculates the modified surf index, you may also want to look at the Modified Surf Index Calculation Sheet (not required).

Questions :

1. 13-second swell is approaching a 1/100 plane beach at an angle of 30 degrees. What is the maximum significant wave height (measured at 30 m depth) for which it is safe to attempt a landing with an LCU? (Give answer to two significant figures.)

2. You are responsible for planning an amphibious landing along the west coast of Erewhon, a small island nation in the South Pacific. Significant wave height is determined to be about 1.5 m in 25 m depth. The wave approach angle at this depth is consistently 20 degrees, and the period is 10 sec.

a. You are first told that you will only be able to land on North Beach, which has a slope of 1/120. Which of the landing craft (in "Modified Surf Limits for Landing Craft" attachment) can you plan on using?

b. It is then announced that South Beach has also become available for amphibious landings. If South Beach has a 1/200 slope, which additional craft will now be available to carry out this mission?

c. Years later, South Beach is chosen as a training site for First Time Drivers who are practicing their amphibious landings. Assuming all the other wave conditions remain the same as those above, what is the maximum offshore significant wave height for which these drivers can train? (Again, 2 significant figures.)