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OC4213 NEARSHORE AND WAVE PROCESSES

LAB #3 Wave Shoaling Observations

February 19, 2009

I. INTRODUCTION/BACKGROUND

In this weeks lab students will be examining some near shore wave data from the

Duck94 experiment. The purpose of this lab is to examine how the energy spectra

of waves change as the wave propagates from offshore across the surf zone. Thislab will also serve to familiarize the students with some of the basic techniques of

spectral calculations and analysis.

II. DATA

The data for this lab consist of bottom pressure data collected across the surf zone

during the DUCK94 experiment. These data were collected in depths ranging from

several cm to approximately 8 m. Data were collected at a sample rate of 2 hz. For

this lab we will be examining a single hour of data (Oct. 18 from 0100-0200 EST)from each of four sites across the surf zone. The wave conditions during this time

were characterized by a moderately energetic directionally narrow swell.

Photographs of the surf zone conditions during this time are shown in Figure 1.The sensor locations and cross-shore bathymetry are shown in figure 2. For this

lab we will be using the data from sites: 2,15, 17, and19.

For each of the four sites to be examined there is a binary file of pressure data

which contains the raw bottom pressure data for that site. These data can be

accessed through the course web site:

www.oc.nps.navy.mil/wavelab/courses/oc4213. Follow this link to lab2_data.Data file names are 10180100.pXX,where XX is the site designation. The data

can be loaded into the matlab environment using the matlab function

load_spuv.m which is described below

III. LAB TASKS/PROCEDURES

Each student is responsible for producing the following for this lab:For eachof the four sites to be examined:

1)Calculate the mean water depth from the bottom pressure data,

2)Calculate the bottom pressure spectra,3)Calculate the surface height spectra from the bottom pressure

spectra

4)Create an overlay plot of the four different surface height spectra and

answer the following questions:

Describe how the surface height spectra evolves across the surfzone. Between which sites does the greatest dissipation ofenergy occur?

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What is the frequency of the peak energy at each site? What are the frequencies of other energy peaks and what do

they represent?

SPECIFIC PROCEDURES

The specific procedures needed to accomplish these tasks can be broken down asfollows. It is suggested that you write a matlab mfile to accomplish steps 2-end:

1)Copy the appropriate data files from the course web site into your workingdirectory.

2)Copy the needed matlab files from the course web site into your working

directory.

3)To load the pressure data into the matlab environment use the matlab function

load_spuv.m. To load the data use the command [tme,prs]=load_spuv;. Thefunction will prompt the user for the name of the data file and return the time

(tme) and pressure (prs).Time will be in matlabs datenum format.

4)Calculate the bottom pressure spectral density. The command:

[bps,f]=psd(prs,npts,hz,'mean') returns the bottom pressure spectral density

(bps)and the frequencies at which the spectrum is estimated (f)given the rawbottom pressure time series (prs), the number of points per segment (npts), and

the sampling frequency (hz). The extra parameter 'mean' indicates that the segment

is to be detrended by removing the mean before performing the spectralcalculations. For this data use 256 points/segment.

5)Estimate the surface height spectral density. This can be estimated as the bottompressure spectral density times cosh2(kh) where k is the wave number and h is the

water depth. The wavenumber array can be calculated using the m-file function

wavenum2.This function returns an array of wave numbers (k)when suppliedwith an array of frequencies (f)and the water depth (in meters)(h):

k=wavenum2(f,h)

Consider water depth to be the average pressure for each time series (Make sure

and convert from cm to meters for the wave number calculation!) You are

now ready to calculate the surface height spectrum using the following:

sfcspc=bps.*cosh(k*h).^2

(The .* and .^ are scaler operators and must be used).

6)Plot the surface height spectra on a semilogy plot. The commonly used

estimation of surface height spectral density is only valid up to a frequency of

about 0.4 Hz for these data so set your x-axis limits accordingly.

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Other info:

Needed standard matlab routines:

psd psemilogy mean

Needed special matlab m-files available on the course web site:

wavenum2.m load_spuv.m

Other matlab hints:

An overlay plot can be created using the commandsemilogy(x1,y1,x2,y2,x3,y3,x4,y4) where x1,y1, etc. are the x and yvariables for each of your data sets.

Use the legend command to identify the site associated with each data set To change x-axis limits use the command: set(gca,xlim,[0 .4]);

Report

Turn in plots, answers/discussions to the questions, and matlab code used to make

the calculations.

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Figure 1 Photograph of the surf zone conditions during the time of datacollection for this lab.

Figure 2 Cross shore bathymetry and sensor locations for data of this lab.