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Offshore

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Near Shore

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Nearshore location for the wave ray shallow water model

The nearshore selection screen enables one to specify a nearshore location, the set of offshore model points providing the offshore conditions and additional parameters used to transform the offshore wave conditions to nearshore by means of the wave ray shallow water model. Based on (the distance between) the selected nearshore location and the active offshore grid point, the model finds and displays a rectangular frame of offshore grid points to be translated. Next, wave rays perpendicular to the depth contours of the underlying bathymetry map are found. Finally, the model propagates wave spectra along the rays towards the nearshore location, where local growth of waves due to wind (copied from the nearest offshore point) can be added if desired. Wave growth (along wave rays) from the seaward boundary and growth (along wave rays) from land can be switched on and off separately.

• Wave refraction over depth contours (optional)
• Shoaling by sea-bed bathymetry
• Sheltering by islands and capes (optional)
• Dissipation by wave breaking (optional)
• Fetch-limited wave growth due to local wind blowing from the seaward boundary (optional)
• Fetch-limited wave growth due to local wind blowing from the coast (optional)

Local wind is copied from the nearest model point (not necessarily an offshore boundary point). Fetch-limited growth is computed using a growth curve and a JONSWAP spectral shape.

The following wave ray model doucment explains the method used to translate waves to the coast in detail and it provides tips on 'best' model configuration for example the choice of the offshore boundary. In addition, you will find handy rules of thumb to estimate shallow water effects as well as limitations of the wave ray model with reference to more complex nearshore models like SWAN.

• The wave ray model used for nearshore translation (PDF)

The nearshore location can be edited through text fields only. By clicking on the map, the user may define another frame of offshore points. 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 point defining the frame of offshore points. The set of offshore points to be translated is outlined by pink/magenta crosses.

Nearshore parameters are

• water-level and bed-level (m rel. to LAT; water-level minus bed-level gives depth)
• switches to turn on/off refraction, wave breaking, bottom friction and local growth

Set the bed-level to zero (the default) to let the model calculate the bed-level at the nearshore location from bathymetry. If you specify a (negative) bed-level yourself, the model will smoothly adapt the surrounding bathymetry as needed. At each point within the area outlined by the offshore boundary frame, the model finds the total depth as water-level minus bed-level.

Tidal effects can be taken into account by means of the water-level. You migth want to carry out several model runs to estimate the impact of the tide on the nearshore wave conditions. For instance, one run for lowest astonomical tide (LAT), a second run for mean sea level (MSL) and a third run for highest astonomical tide (HAT). You would set the water-level equal to zero (LAT-run), to MSL minus LAT (MSL-run) and to HAT minus LAT (HAT-run).

Wave breaking involves breaking of waves due to steepness and limited depth. Please be aware of the fact that wave breaking due to depth is based on wave height and local depth at the nearshore location. The wave ray model does not know about waves breaking during propagation while travelling from the boundary to the nearshore location. For this reason points with depth shallower than the depth at the nearshore location are marked by red dots in the wave ray plots.

It is your responsibility to check that waves will not break over shallows during propagation. As a rule, waves start to break if significant wave height is about half the water depth. High ocean waves will surely break over offshore reefs at say 1 meter below the surface. For such areas, the wave ray model is not applicable.

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. Clicking on the map re-defines the frame of offshore points to be translated. 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	0	This is the bed-level at the nearshore location. The bed-level is defined as the position of the sea-floor relative to lowest astronomical tide (LAT), given in meters. This should be a negative number (if you want the model to calculate a default bed-level at the nearshore location from bathymetry, set the bed-level field to zero).
Water-level	Editable text field	0	This is the water-level in the area of interest, i.e. the area outlined by the offshore boundary frame. The water-level is defined as the deviation of the sea-surface relative to lowest astronomical tide (LAT) in meters.
Apply wave refraction	Check box	On	Switch to apply wave refraction over depth contours while translating the offshore wave spectra. The wave ray model derives depth contours from the underlying bathymetry map.
Apply wave breaking	Check box	On	Switch to apply breaking of waves because of limited depth and steepness.
Apply growth from the seaward boundary	Check box	Off	Switch to add energy, if needed, to the boundary wave spectra generated by winds blowing from unsheltered directions, i.e. wave growth from the seaward boundary. Wave growth is computed by a simple growth curve using the nearshore depth and the distance (fetch) to the open sea boundary along a straight line in the wind direction. If a boundary wave spectrum already contains a large enough wind-sea component, then wave growth from open sea will not add/change much. An initial condition for the wind-sea is determined from the wave spectra at the boundary. The computed wind-sea spectrum is not simply added to the propagated spectrum but only a fraction, which when added to propagated energy in the same frequency range would produce the amount of wind-sea energy computed by the growth curve. The wind data are copied from the offshore gridpoint nearest to the nearshore location.
Apply growth from the coast	Check box	On	Switch to add energy to the propagated wave spectra generated by local winds blowing from sheltered directions, i.e. from the coast. Wave growth is computed by a simple growth curve using the nearshore depth and the distance (fetch) to the coast along a straight line in the wind direction. The wind data are copied from the offshore gridpoint nearest to the nearshore location.
Click			Re-defines offshore boundary of the wave ray model. Clicking defines a new active offshore point: based on the distance between this offshore grid point clicked on (blue) and the current nearshore point (red), a new offshore boundary (pink) will be computed and displayed. The maximum distance between the nearshore point and the offshore boundary has been set to 3 grid steps. So as long as you click near enough to the nearshore point, the offshore point clicked on will be one of the boundary points that supplies the offshore conditions to the ray model. If chosen too far away, the offshore point will fall beyond the offshore model boundary. To avoid memory overflow, the set of offshore points is thinned if the total number of points exceeds 15: the number of points will be halved i.e. points will be selected alternately.
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 points, the nearshore location and the other inputs, he sends off the final nearshore location selection screen. The wave ray model will then transform the offshore wave conditions to nearshore. Please be aware of the fact that the wave ray translation is quite time-consuming: depending on the number of points to be translated, on the distance between the nearshore location and the offshore frame and on server load, a computation will take 15 to 30 minutes. After pushing the OK-button, offshore model points will be loaded and the status screen of the wave ray model will appear, showing the progress of the job. The status can be updated on request. On completion of the computation, output tables and plots can be generated through the nearshore output screen (just as for the simple shallow water model). For users without access to the nearshore climate functionality, results will be given at a demo location .