6 Verification of Longshore
Predictions of the longshore current by NMLong-CW were compared
with several data sets from the laboratory and the field. Comparisons with
the measured wave height and water level variation across the profile were
also included, if such measurements were available. The objectives of the
comparisons were to:
a. Assure that the model functioned properly and displayed robust
behavior, especially with regard to the various routines that were
added to describe the current and wave interactions.
. Determine typical values
of the bottom friction and
coefficients for the extended model (in case these values are different
from those employed in NMLong).
c. Assess sensitivity of the current, mean water level, and wave height
by including complete interaction between currents and waves.
d. Assess sensitivity of the calculated current, mean water level, and
wave height by including momentum transport due to the roller.
e. Evaluate model performance with imposition of an external current.
To accomplish these objectives, calculations were compared to five data
sets on the longshore current originating both from the laboratory and the
field. Kraus and Larson (1991) analyzed two of these data sets to test and
calibrate NMLong, those of Visser (1982) and Kraus and Sasaki (1979;
"K&S"). Additional data sets from Kuriyama and Ozaki (1993; K&O),
Smith, Larson, and Kraus (1993; "Delilah"), and Hamilton and Ebersole
(2001; "CHL-L") were also employed in the present study. In the following,
summaries are of the respective data set and the results of the NMLong-CW
simulations are given with reference to the objectives previously listed. The
laboratory data sets are introduced first (Visser and CHL-L) followed by the
field data sets (K&S, K&O, and Delilah). Calculations were also performed
for hypothetical situations involving an external current that represented a
longshore current that might be associated with the tide.
Visser (1982) carried out precise measurements of the longshore current
generated by breaking waves on a plane beach in a large laboratory wave
basin. The cross-shore distribution of the longshore current was measured at
several transects, together with the wave height and mean water level. Seven
cases involving monochromatic waves were undertaken of which four were
Chapter 6 Verification of Longshore Current Model