SMS:Long Wave Input Toolbox

Infragravity (IG) waves are surface gravity waves with frequencies lower than the wind waves (hence the reference to them as long waves). IG waves consist, among others, of long-period oceanic waves generated along continental coastlines by nonlinear wave interactions of storm-forced shoreward-propagating ocean swells. They differ from normal oceanic gravity waves which are created by wind acting on the surface of the sea.

While normal wind waves typically have periods in the order of 20 seconds (or less), these waves can interact with coastlines which filter out the energy and covert some of the energy to subharmonic frequencies (ranging from 50 - 350 seconds). These are the IG waves. IG waves could also refer to phenomena such as tides and oceanic Rossby waves.

Motivation

When modeling harbors, it is possible energy in the IG range could result in resonance in the harbor. Therefore, it may be useful to simulate these terms in a CGWAVE analysis of a harbor.

IG waves may also influence navigation, coastal inlets, and coastal structural design projects.

The Toolbox

Example of the Long Wave Input Toolbox dialog.

To define wave conditions consistent with infra-gravity waves, the interface in SMS includes the Infra-Gravity Wave-input Toolbox (IGWT). This tool utilizes a one dimensional Boussinesq model to compute wave conditions for input to CGWAVE.

Input

The IGWT requests for the following inputs:

• An input wave spectrum (from file or from SMS) – This defines the wave conditions at a deep water point.
  • When coming from a file, this refers to a frequency spectrum or *.spf file. These are directly input to the one dimensional Boussinesq model. The file format is defined by the developer of BOUSS1D and BOUSS2D.
  • When coming from a spectrum, the user chooses a spectra that has been loaded into or created by the SMS spectra generator. SMS creates a series of frequencies and energy densities from this spectrum that are fed into the one dimensional Boussinesq model.
• Offshore water depth – This parameter refers to the depth in meters at the deepwater buoy (or site). If this is deeper than the Boussinesq limit, the depth will be set to the limit.
• Nearshore water depth – This refers to the depth in meters at the predominant breaking location. This value can be approximated as twice the incident significant wave height.
• Minimum/maximum long wave period – This refers to the cutoff periods for the IG wave spectrum. Typical values are from 15 to 600 sec.
• Number of components – using this parameter the user specifies how many wave components will be generated in each specified direction for CGWAVE. This term is referred to as 'N' in the discussion below.
• Maximum oblique angle – The one dimensional Boussineesq model ignores wave direction. In fact, the directional bins from the input spectrum are ignored as they are converted to a frequency spectrum. This value defines a total variation (in degrees) for the directions to be considered. They are centered around the shore normal direction of the CGWAVE mesh.
• Number of angles – This value should be a positive integer. The toolbox will create N different wave components for each of this number of directions. If this number is 1, all the components will be generated in a shore normal direction. If this number is 2, then two sets of components will be generated, each half the oblique angle either side of the shore normal direction.

Process

The IGWT is actually setting up and running a one dimensional Boussinesq model as well as a pair of utilities to compute wave components to be applied on the CGWAVE mesh. Since this process is not always successful, it can be useful for the user to understand the process and in some cases, work through the process manually. As the tool is exercised and modified (feedback is welcome), the need for manual operation should be reduced, but understanding the process is still valuable. The three executables used in this process are distributed with SMS and can be found in the models area of the installation in the BOUSS2D folder. Each can be run in a DOS prompt in interactive mode if needed.

The 1D Boussinesq Run

The IGWT uses the BOUSS-1D model executable bouss1d.exe which is a one dimensional version of the Boussinesq equations. SMS creates a one dimensional profile starting at the specified deep water condition, maintaining that depth for five wavelengths and then transitioning up a gentle slope (1:50) to a depth of zero (0.0). The IGWT uses a grid spacing of 1/20 of the wavelength computed from the provided frequency spectrum.

The one dimensional run creates a number of output files at the specified near shore depth. These including significant wave height, mean water level, runup and a time series of water surface. The IGWT will use the water surface file (*_ts_eta.001) as input to the spectral analysis utility in the next step.

Spectral Analysis

The IGWT uses the utility spec_anal.exe to process the water surface time series at the near shore depth and create a spectrum. This analysis can also be run manually at a DOS prompt after the one dimensional Boussinesq run generates the time series file. The utility will prompt the user for the name of an output file (qspec.001 by default). This output file in used as input in the next utility to generate wave components.

Wave Component Extraction

The IGWT uses the utility export_v1.exe to process the frequency spectrum at the near shore location and extract wave components at regular freqency spacing from the minimum to the maximum defined range. The utility creates a text file (out.txt) that lists these components.

Once the three processes have been run, the IGWT reads the out.txt file and distributes the wave components over the specified directions (centered around shore normal).

Output

The IGWT creates a series of wave components for each of the directions specified in the input. If the number of angles is 0, then there will be N components distributed through the frequency range. In the number of angles is 1, the toolbox will generate 3*N components (one direction offset from shore normal in each direction.

It is possible that zero energy components may be added. The user should verify the generated components.

Approach

The IGWT utilizes a one-dimensional version of the BOUSS-2D model to transform wave spectrum from the "deep-water" limit of the Boussinesq model (H < L/2). A constant 1:50 slope is assumed between the offshore and nearshore water depths. If complex offshore topography exists, use BOUSS-2D to bring the waves to the nearshore.

External Links:

• Infra-Gravity Wave Input Toolbox (IGWT): User’s Guide (May 2007) PDF

Related Topics

• CGWAVE Model Control