In SWAN a number of variables are used in input and output. Most of them are related to waves. The definitions of these variables
are mostly conventional.
HSIGN | Significant wave height, denoted as H_{s} in meters, and defined as | |
H_{s} = 4 | ||
where E(,) is the variance density spectrum and is the absolute | ||
radian frequency determined by the Doppler shifted dispersion relation. | ||
However, for ease of computation, H_{s} can be determined as follows: | ||
H_{s} = 4 | ||
HSWELL | Significant wave height associated with the low frequency part of | |
the spectrum, denoted as H_{s,swell} in meters, and defined as | ||
H_{s,swell} = 4 | ||
with = 2f_{swell} and f_{swell} = 0.1 Hz by default (this can be changed | ||
with the command QUANTITY). | ||
TMM10 | Mean absolute wave period (in s) of E(,), defined as | |
T_{m-10} = 2 = 2 | ||
TM01 | Mean absolute wave period (in s) of E(,), defined as | |
T_{m01} = 2 = 2 | ||
TM02 | Mean absolute wave period (in s) of E(,), defined as | |
T_{m02} = 2 = 2 | ||
DIR | Mean wave direction (in ^{o}, Cartesian or Nautical convention), | |
as defined by (see Kuik et al. (1988)): | ||
DIR = arctan | ||
This direction is the direction normal to the wave crests. | ||
PDIR | Peak direction of E() = E(,)d = E(,)d | |
(in ^{o}, Cartesian or Nautical convention). | ||
TDIR | Direction of energy transport (in ^{o}, Cartesian or Nautical convention). | |
Note that if currents are present, TDIR is different from the mean wave | ||
direction DIR. | ||
RTMM10 | Mean relative wave period (in s) of E(,), defined as | |
RT_{m-10} = 2 | ||
This is equal to TMM10 in the absence of currents. | ||
RTM01 | Mean relative wave period (in s) of E(,), defined as | |
RT_{m01} = 2 | ||
This is equal to TM01 in the absence of currents. | ||
RTP | Relative peak period (in s) of E() (equal to absolute peak period | |
in the absence of currents). | ||
Note that this peak period is related to the absolute maximum bin of the | ||
discrete wave spectrum and hence, might not be the 'real' peak period. | ||
TPS | Relative peak period (in s) of E(). | |
This value is obtained as the maximum of a parabolic fitting through the | ||
highest bin and two bins on either side the highest one of the discrete | ||
wave spectrum. This 'non-discrete' or 'smoothed' value is a better | ||
estimate of the 'real' peak period compared to the quantity RTP. | ||
PER | Average absolute period (in s) of E(,), defined as | |
T_{m, p-1, p} = 2 | ||
The power p can be chosen by the user by means of the QUANTITY | ||
command. If p = 1 (the default value) PER is identical to TM01 and | ||
if p = 0, PER = TMM10. | ||
RPER | Average relative period (in s), defined as | |
RT_{m, p-1, p} = 2 | ||
Here, if p = 1, RPER=RTM01 and if p = 0, RPER=RTMM10. | ||
FSPR | The normalized frequency width of the spectrum (frequency spreading), | |
as defined by Battjes and Van Vledder (1984): | ||
FSPR = , for = T_{m02} | ||
DSPR | The one-sided directional width of the spectrum (directional spreading | |
or directional standard deviation,in ^{o}), defined as | ||
DSPR^{2} = (2 sin())^{2}D()d | ||
and computed as conventionally for pitch-and-roll buoy data | ||
(Kuik et al. (1988); this is the standard definition for WAVEC buoys | ||
integrated over all frequencies): | ||
(DSPR)^{2} = 21 - | ||
QP | The peakedness of the wave spectrum, defined as | |
Q_{p} = 2 | ||
This quantity represents the degree of randomness of the waves. | ||
A smaller value of Q_{p} indicates a wider spectrum and thus | ||
increased the degree of randomness (e.g., shorter wave groups), | ||
whereas a larger value indicates a narrower spectrum and a more | ||
organised wave field (e.g., longer wave groups). | ||
MS | As input to SWAN with the commands BOUNDPAR and BOUNDSPEC, | |
the directional distribution | of incident wave energy is given by | |
D() = A(cos)^{m} | for all frequencies. The parameter m | |
is indicated as MS in SWAN and is not necessarily an integer number. | ||
This number is related to the | one-sided directional spread of the waves | |
(DSPR) as follows: |
MS | DSPR (in ^{o}) |
1. | 37.5 |
2. | 31.5 |
3. | 27.6 |
4. | 24.9 |
5. | 22.9 |
6. | 21.2 |
7. | 19.9 |
8. | 18.8 |
9. | 17.9 |
10. | 17.1 |
15. | 14.2 |
20. | 12.4 |
30. | 10.2 |
40. | 8.9 |
50. | 8.0 |
60. | 7.3 |
70. | 6.8 |
80. | 6.4 |
90. | 6.0 |
100. | 5.7 |
200. | 4.0 |
400. | 2.9 |
800. | 2.0 |
PROPAGAT | Energy propagation per unit time in -, - and -space | |
(in W/m^{2} or m^{2}/s, depending on the command SET). | ||
GENERAT | Energy generation per unit time due to the wind input | |
(in W/m^{2} or m^{2}/s, depending on the command SET). | ||
REDIST | Energy redistribution per unit time due to the sum of quadruplets | |
and triads (in W/m^{2} or m^{2}/s, depending on the command SET). | ||
DISSIP | Energy dissipation per unit time due to the sum of bottom friction, | |
whitecapping and depth-induced | surf breaking (in W/m^{2} or m^{2}/s, | |
depending on the command SET). | ||
RADSTR | Work done by the radiation stress per unit time, defined as | |
^{2}^{}| S_{tot} - - ^{ . }[( + )E] - ^{ . }(E)| dd | ||
(in W/m^{2} or m^{2}/s, depending on the command SET). | ||
WLEN | The mean wavelength, defined as | |
WLEN = 2 | ||
As default, p = 1 (see command QUANTITY). | ||
STEEPNESS | Wave steepness computed as HSIG/WLEN. | |
BFI | The Benjamin-Feir index or the steepness-over-randomness ratio, | |
defined as | ||
BFI = x STEEPNESS x QP | ||
This index can be used to quantify the probability of freak waves. | ||
QB | Fraction of breakers in expression of Battjes and Janssen (1978). | |
TRANSP | Energy transport with components P_{x} = gc_{x}E(,)dd and | |
P_{y} = gc_{y}E(,)dd | with x and y the problem coordinate system, | |
except in the case of output with BLOCK command in combination | ||
with command FRAME, where x and y relate to the x -axis and y -axis | ||
of the output frame. | ||
VEL | Current velocity components in x - and y -direction of the problem | |
coordinate system, | except in the case of output with BLOCK command in | |
combination with command FRAME, where x and y relate to the x -axis | ||
and y -axis of the output frame. | ||
WIND | Wind velocity components in x - and y -direction of the problem coordinate | |
sytem, except in the case of output with BLOCK command in | combination | |
with command FRAME, where x and y relate to the x -axis and y -axis of | ||
the output frame. | ||
FORCE | Wave-induced force per unit surface area (gradient of radiation stresses) | |
with x and y the problem coordinate system, except in the case of output | ||
with BLOCK command in | combination with command FRAME, | |
where x and y relate to the x -axis and y -axis of the output frame. | ||
F_{x} = - - | ||
F_{y} = - - | ||
where S is the radiation stress tensor as given by | ||
S_{xx} = gn cos^{2} + n - Edd | ||
S_{xy} = S_{yx} = gn sincosEdd | ||
S_{yy} = gn sin^{2} + n - Edd | ||
and n is the group velocity over the phase velocity. | ||
UBOT | Root-mean-square value (in m/s) of the maxima of the orbital motion | |
near the bottom U_{bot} = U_{rms}. | ||
URMS | Root-mean-square value (in m/s) of the orbital motion near the bottom. | |
U_{rms} = | ||
TMBOT | Near bottom wave period (in s) defined as the ratio of the bottom excursion | |
amplitude to the root-mean-square velocity T_{b} = a_{b}/U_{rms} with | ||
a_{b} = | ||
LEAK | Numerical loss of energy equal to c_{}E(,) across boundaries =[dir1] | |
and =[dir2] of a directional sector (see command CGRID). | ||
TIME | Full date-time string. | |
TSEC | Time in seconds with respect to a reference time (see command QUANTITY). | |
SETUP | The elevation of mean water level (relative to still water level) induced by | |
the gradient of the radiation stresses of the waves. | ||
Cartesian convention | The direction is the angle between the vector and the positive x -axis, | |
measured counterclockwise. In other words: the direction where the | ||
waves are going to or where the wind is blowing to. | ||
Nautical convention | The direction of the vector from geographic North measured | |
clockwise. In other words: the direction where the waves are coming | ||
from or where the wind is blowing from. |