Technical Note No. 98/9 - More Wave Statistics

Technical Note No. 98/9 - More Wave Statistics

If OFFLOAD.DAT contains Task 4/0 (Wave Gauge - Wave Statistics) or Task 4/1 (Wave Gauge - Wave Statistics plus Hydrostatic Depth Spectrum) data, then the PEDP can be used to recompute the "basic" wave statistics and to compute an "extended" set of wave statistics.

  • Core statistics include mean water depth, standard deviation of the pressure signal, mean spectral period and spectral width. DOBIE computes these internally and they are written to OFFLOAD.DAT when data are downloaded. Everything that follows can be computed from these core statistics.
  • Basic statistics include significant orbital speed at the bed and significant wave height. They are calculated internally by DOBIE using linear theory operating on the core statistics and they are written to OFFLOAD.DAT when data are downloaded.
  • Extended statistics include wavelength, cumulative cube of the orbital speed at the bed and critical orbital speed for sediment entrainment, which are not calculated by DOBIE and are therefore not included in OFFLOAD.DAT.
When PEDP is run, the file OFFLOAD.DAT is opened. If OFFLOAD.DAT contains Task 4/0 data, the user is presented with the following choice (if OFFLOAD.DAT contains Task 4/1 data, the user must first choose to "process the wave statistics" before being presented with the choice):
  • By choosing "99" the file OFFLOAD.DAT is simply written to DOBIE.DAT in a tabular format in order to make for easy plotting and opening in a spreadsheet application. In this case, the core statistics and the basic statistics are simply reported exactly as they were in OFFLOAD.DAT, which is also exactly as they were computed by DOBIE. For most users in most circumstances, this is all that will need to be done.
  • By choosing "10", PEDP will compute the set of extended statistics AND recompute the basic statistics to take account of any changes in the "parameter settings" (see below). Note:
  • If you don't want the basic statistics to be changed but you do want the extended statistics to be calculated, leave all the parameter settings at their default values.
  • The extended statistics always take account of the parameter settings, be they default or changed values.
  • The extended statistics are added in extra columns to the right of the core and basic statistics in DOBIE.DAT.
The user is given choices for certain parameter settings:
  • By choosing "1" the sensor noise level can be changed. The default value is the value entered into DOBIE during setup (this value is written to OFFLOAD.DAT, which is how PEDP knows what it was).
  • By choosing "2" the user can change the upper and lower limits on wave period.
  • By choosing "3" the user can bypass the wave-steepness reality check. The default is to apply the check, which is what DOBIE does internally.
  • By choosing "4" the user can bypass the wave-breaking reality check. The default is to apply the check, which is what DOBIE does internally.
  • By choosing "5" the user can enter a speed in m/s for the critical orbital speed for entrainment of bed sediment. The default is 0 m/s. By entering "99", the critical orbital speed will be predicted burst-by-burst using Komar and Miller's (Journal of Sedimentary Petrology, Volume 45, pages 362-367, 1975) equation. In that case, the user enters a bed-sediment grainsize (in mm). The critical orbital speed now is a function of wave period, thus it is computed burst-by-burst as the wave period becomes known. The critical orbital speed - and several related parameters - are reported in the extended dataset.
  • (Advanced Users) By choosing "6" the user can recompute the standard deviation of the pressure signal by integrating the spectrum and in so doing apply limits to the integration as desired in order to isolate different components of the wavetrain or to perhaps refine the estimate of the sensor noise. This option can only be exercised if OFFLOAD.DAT contains Task 4/1 data (wave statistics plus wave spectrum) AND the PEDP has been run previously to extract the burst spectra from OFFLOAD.DAT and write each burst spectrum to a separate datafile. If the burst spectrum datafiles cannot be found by PEDP then the standard deviation will not be changed.
The extended statistics include:
  • Local wavelength L based on mean spectral period and mean water depth. Computed using linear wave theory.
  • Deepwater wavelength L0 based on mean spectral period. Computed using linear wave theory.
  • Ratio of water depth to deepwater wavelength, h/L0. Indicative of deepwater (>0.25), intermediate or shallow-water (<0.05) wave.
  • Ratio of significant wave height to water depth, Hsig/h.
  • Sensor noise level. Included in the tabular output for plotting on top of standard deviation of the pressure signal to see how they compare.
  • The running total of the cube of the orbital speed. Rate of wave-energy dissipation on the bed by friction is proportional to the cube of the orbital speed at the bed.
  • Critical orbital speed for entrainment of bed sediment. This will have been set by the user (see above), in which case it will be constant, or it will have been predicted using Komar and Miller's model (see above), in which case it will almost certainly be variable since it depends on wave period in that case.
  • The running total of the cube of [orbital speed at bed in excess of critical orbital speed for entrainment of bed sediment]. If critical orbital speed is never exceeded, this will be zero. Thus, this is an indicator of energy expended when sediments are in motion.
  • Percent of the total time critical speed for entrainment of bed sediment was exceeded.
  • Percent of the total time the depth exceeded zero (i.e. inundation time). A useful parameter for intertidal deployments.

MOG
December, 1998