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

To define wave conditions consistent with infra-gravity waves, the interface in SMS includes the infra-gravity wave input toolbox. 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 and includes:
  • When coming from a spectrum, the user chooses a loaded spectra. 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 30 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.
• 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.

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.

No check is made to remove zero energy components. The user should verify the generated components are what is wanted.

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:

• May 2007 ERDC/CHL CHETN-I-73 May 2007 Infra-Gravity Wave Input Toolbox (IGWT): User’s Guide [1]