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Offshore

Location Climate Monthly statistics Histogram Scatter table 2D Directional rose Scatter table 3D Persistency Coverage of windows Number of windows Delay until next window Data of Past Events Timeseries plot Vector map Timeseries download Quick Stats

Near Shore

Location simple model Seabed profile Location ray model Climate Monthly statistics Histogram Scatter table 2D Directional rose Scatter table 3D Persistency Coverage of windows Number of windows Delay until next window Timeseries download

Nearshore location for the simple shallow water model

The nearshore selection screen enables one to specify a nearshore location, an offshore model point for the offshore conditions and additional parameters used to transform the offshore wave conditions to nearshore by means of the simple shallow water model. The simple model assumes coastlines parallel to the coast, i.e. perpendicular to the coast normal. Simple offshore to nearshore translation of wave spectra (wind data is just copied) takes the following phenomenons into account

• Wave refraction over parallel depth contours
• Shoaling by sea-bed bathymetry
• Sheltering by islands and capes
• Dissipation by wave breaking
• Dissipation due to bottom friction

After calibration of offshore wave model spectra with altimeter waveheight, nearshore wave spectra are found by applying refraction, sheltering and shoaling to the offshore wave spectra. Wave breaking is applied to the (unbroken) nearshore waveheight integrated from the resulting nearshore wave spectra. Method and validation of the offshore to nearshore translation are described in a separate Word document.

The current nearshore location is marked by a red cross (drawn inside a red square). Initially, the center of the offshore data area is taken as nearshore location. A blue cross marks the offshore wave model point. The nearshore location can be edited through text fields only. By clicking on the map, the user may define another offshore point.

Nearshore parameters are

• water-level (m rel. to LAT)
• bed-level (m rel. to LAT)
• seaward normal direction (degrees)
• switch to overrule the default unsheltered directional sector(s)
• unsheltered directional sector(s) (overruling the default)
• switch to turn on bottom friction (and to specify a seabed profile)

  Depth is found as water-level minus bed-level. The seaward normal direction is defined as the seaward direction perpendicular to the coastline, relative to the North and positive clockwise. Thus 0 degrees means that the seaward normal points to the North while 90 degrees will let it point to the East. It is drawn as an arrow using a thin dashed line. The user can overrule the default seaward normal direction.

  The default unsheltered directional sectors are computed by waveclimate.com based on (a subset of) the coastlines drawn. Unsheltered sectors are shaded. The set of coastlines used to detect sheltering depends on

  • The size of the plot area
  • The resolution of the map (high resolution is used as soon as the minimum and maximum latitude differ less than 10 degrees)

  Optionally, the user can override the default sheltering by checking the designated checkbox and by entering the boundaries of a directional sector from which the unsheltered waves are supposed to come (according to the user).

  Waves coming from directions which differ 90 degrees or more from the coast normal are blocked by waveclimate.com: the corresponding 180-degree sector is allways sheltered.

  By default, bottom friction is turned off when offshore wave spectra are translated to nearshore wave spectra. Bottom friction should be turned on if waves travel over large areas with relatively shallow water. In most cases, bottom friction can be ignored. If you are in doubt, perform computations with and without bottom friction. An extra input screen will be presented if you switch on bottom friction. In this extra screen you can specify the seabed profile along the seaward normal.

We have chosen to plot square maps in terms of degrees (rather than in terms of kilometers). In order to keep the unsheltered directional sector(s) consistent with the coastlines, the boundaries of the unsheltered directional sectors are plotted (a bit) distorted. This distortion does not exist on the equator and increases towards the poles. As a consequence, the plotted seaward normal direction is (a bit) distorted too. The displayed numerical values of the seaward normal and of the boundaries of the unsheltered directional sector will actually be used by waveclimate.com and should be consistent with the directional distribution presented in scatter tables of waveheight versus wave direction.

Mind that, occasionally, nearshore waves (seem to) come from sheltered directions. This can be seen by thinking of the unsheltered sectors as a mask which is applied to each offshore wave spectrum. If the nearshore location is behind an island and the seaward normal points towards the island and waves are allowed to come along both sides of the island, the resulting nearshore spectrum can be double-peaked and the corresponding mean wave direction could be close to the (sheltered) coast normal direction. In fact the mean wave direction is ambiguous in such a case. Please refer to the validation of the nearshore climate branch for an example.

One can set the nearshore location by entering a latitude-longitude pair. Setting zoom in/out allows zooming into the new location or zooming out of the current location. At any time, one can reset to the (initial) nearshore location screen.

User Controls

Name	Type	Default	Description
Longitude & latitude	Editabletext fields for degrees minutes and seconds. Pull down list for selection of the hemisphere	Center of the offshore data area	The co-ordinate pair of the current nearshore location. Please enter longitude as degrees from [0,180> and minutes/seconds from [0,60>. Additionally, select the Eastern or Western hemisphere. Enter latitude as degrees from [0,90> and minutes/seconds from [0,60> and choose North or South. Re-draw the map around the new location by pushing the "Update map" button.
Zoom in/out	Pull down list	Zoom in	Zoom into the new location or zoom out of the current location. The system uses a zoom factor of 2. This zoom factor defines the size of the new plot area to be drawn next, relative to the size of the current plot area. Opting "Zoom in" will result in a plot area twice as small. Using "Unchanged" will keep the size of the plot area unchanged (you might want to shift the nearshore location without changing the plot size). The system may adjust the zoom factor in order to prevent parts of the plot to fall outside the [90S,90N] latitude range. Longitudes are cyclic. Upon reaching the maximum or minimum plot size, the zoom factor will be overridden and set to "Unchanged".
Update map	Push button		Re-draws the plot using the values of the controls described on this page.
Bed-level	Editable text field	-10	The bed-level is defined as the position of the sea-floor relative to lowest astronomical tide (LAT), given in meters. It should be a negative number.
Water-level	Editable text field	0	The water-level is defined as the deviation of the sea-surface relative to lowest astronomical tide (LAT) in meters.
Seaward normal direction	Editable text field	Value derived from the (nearest) coastline	The seaward normal direction is defined as the direction perpendicular to the coastline, pointing seawards, "going to" relative to the North and positive clockwise. Thus 0 degrees means that the seaward normal points to the North while 90 degrees will let it point to the East. It is drawn as an arrow using a thin dashed line. The system presents a default seaward normal direction based on the coastlines in the vicinity of the selected nearshore location. A new seaward normal direction will be computed and presented as long as the nearshore location changes. The user can overrule the presented default by entering some preferred value in the designated field. A seaward normal direction specified by the user in this way will only be used if the nearshore location is left unchanged. As we have chosen to plot square maps in terms of longitude and latitude, the seaward normal is plotted a bit distorted. This distortion is zero on the equator and gets larger towards the poles.
Allow unsheltered directions	Check box	Off	Switch to overrule the default unsheltered directional sector(s). If this switch is off, the default unsheltered sectors will be plotted and used for sheltering.
from .. to ..	Editable text fields	Blank	Begin and end of the unsheltered directional sector according to the user. Directions are "coming from", positive clockwise and relative to the north. Thus an unsheltered sector between 355 (or -5) and 5 degrees means that offshore wave energy traveling in north-south direction will be translated to the nearshore location and will contribute to the nearshore wave spectrum according to the laws of refraction and shoaling. This unsheltered sector will be used for sheltering only if the corresponding checkbox to the left is checked. As we have chosen to plot square maps in terms of longitude and latitude, the boundaries of an unsheltered sector are plotted a bit distorted. This distortion is zero on the equator and gets larger towards the poles. Directions which are 90 degrees or more away from the coast normal will always be sheltered by waveclimate.com.
Apply bottom friction	Check box	Off	Switch to apply bottom friction in the translation of offshore wave spectra to nearshore wave spectra. Bottom friction plays a role when waves travel over large areas of relatively shallow water. If you are unsure about whether ot not bottom friction influences the nearshore wave climate, compare results obtained with and without bottom friction.
Click			Clicking re-defines the offshore grid point used by the simple nearshore model.
Reset	Push button		Re-draws the initial offshore and nearshore location and re-installs the defaults.
OK	Push button		If the user is certain about the selected offshore and nearshore location and the current nearshore parameters, he sends off the final nearshore location selection screen. If bottom friction is switched off, the system will then transform the offshore wave parameters to nearshore. Upon successfull translation, the output selection screen will appear. If bottom friction is switched on, the offshore to nearshore translation will be delayed: in the next screen the user will have to specify the seabed profile. The translation is then done after sending off this extra input screen. For users without access to the nearshore climate functionality, results will be given at a demo location .