Questions and answers.doc - 1 Questions and Answers(October...

This preview shows 1 out of 7 pages.

Questions and Answers (October 28 th , 2008) supplement to the book Waves in Oceanic and Coastal Waters Leo H. Holthuijsen Delft University of Technology UNESCO-IHE published by Cambridge University Press 2007 ISBN-13 978-0-521-86028-4 ISBN-10 0-521-86028-8 1
Image of page 1

Subscribe to view the full document.

2
Image of page 2
Chapter 1 Introduction This chapter is only a very general introduction and no questions are asked. 3
Image of page 3

Subscribe to view the full document.

Chapter 2 Observation techniques Q2.1: What are the two most common techniques to measure waves at sea? A: The most common techniques to measure waves at sea are based on either a buoy or a wave pole. The buoy measures its own vertical position as a function of time (typically every ½ second) by measuring its own vertical acceleration (to be integrated twice to obtain the vertical position) or by using GPS. The vertical location of the sea surface along a wave pole is measured by measuring the change in electronic characteristics along a vertical wire at the sea surface (e.g., electrical resistance). Q2.2: What are the two most common techniques to obtain directional wave information based on these techniques? A: With a buoy, directional wave information can be obtained by measuring the pitch-and-roll of the buoy (with inclinometers) or the yaw-and-sway of the buoy (with horizontal accelerometers or GPS). With a group of wave poles (at least three), directional wave information can be obtained by measuring surface slopes (in an array with small horizontal dimensions) or phase differences (in an array with larger dimensions). Q2.3: Is it possible to measure the surface profile of individual waves from satellites? A: The instruments that are used to measure waves from satellites are the synthetic aperture radar (SAR) and the radar altimeter. The (synthetic) antenna of the SAR is large enough that it can distinguish individual waves. A SAR image therefore shows an image of individual waves (essentially in grey tone) but the transformation to a surface elevation is highly non-linear and has not yet been achieved. The antenna of an altimeter is much smaller than 4
Image of page 4
that of a SAR and the foot print at the sea surface is of the order of 1 kilometre in diameter, which is too large to distinguish individual waves (only an average roughness of the sea surface in the foot print can be estimated, from which the significant wave height can be inferred). 5
Image of page 5

Subscribe to view the full document.

Chapter 3 Description of ocean waves Q3.1: A WAVERIDER buoy gives the sea surface elevation as a function of time. How would you estimate the significant wave height from such a wave record (of 30 minute duration, say)? A: One should first determine the zero level of the record (and usually also its linear trend in time) and remove this level (and trend) from the record so as to have a record with zero mean. Then one should identify all individual waves (the surface profile between two consecutive zero down- crossings) and then, for each such wave, the wave height (the difference between the lowest and the highest level per wave). Ranking all these wave heights and taking the mean of the top (i.e., largest) 1/3 wave heights gives an estimate
Image of page 6
Image of page 7
You've reached the end of this preview.
  • Fall '17
  • ZAHEER AHMED
  • Water waves, wave height, Wind wave, f Peak

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern