Kraus-Sasaki (K&S) Data - TR-02-22-RPT

Figure 37. Calculated (with and without roller model) and measured mean

Figure 39. Calculated (with and without roller model) and measured

TR-02-22-RPT_2
Page Navigation
59

Although the shift in the current distribution obtained by the roller model

is satisfactory, the distribution appears to be somewhat narrow, implying that

the lateral mixing is not sufficient. The present formulation of the mixing

coefficient (Equation 35), where ε depends on H and um, has a tendency to

generate insufficient mixing in the surf zone. In this zone, breaking prevails,

and strong turbulence is generated in the water column enhancing both

vertical and lateral mixing. The next section includes some trial simulations

where mixing produced by the roller is parameterized and incorporated.

Kraus-Sasaki (K&S) Data

Kraus and Sasaki (1979) measured the longshore current distribution

along seven transects on a sandy beach facing the Japan Sea, from which an

average velocity distribution was obtained. The incident waves during the

measurements were clean swell with a significant wave height of 1 m, a mean

wave period of 4.1 s, and a mean wave angle of 9 deg at the point of incipient

breaking. The water depth was measured by rod and transit, and the beach

profile had a step-type shape. No measurements were made of the wave

height variation. Kraus and Larson (1991) discussed the data and the basic

conditions for the numerical simulations more extensively.

Figure 39 compares calculations and measurements (the beach profile is

also shown). The peak in the measured current is fairly well predicted,

whereas the mixing is more pronounced for the measurements in the inner

part of the surf zone where the beach slope is small and the profile has a

shelf-type shape. Also, the offshore tail in the current distribution was

calculated to decay with a smaller gradient than what was observed. The

computations were carried out by Monte-Carlo simulation assuming a

Rayleigh distribution in the offshore. There was no tuning of the parameter

values, but the friction coefficient was changed until the results visually fit

the measurements and the mixing coefficient was held constant (Λ = 0.50).

A friction coefficient value of cf = 0.0035 was obtained if the roller model

was employed and cf = 0.0030 if the roller model was switched off.

As seen from Figure 39, the roller model shifts the peak in the current

toward the shore, improving agreement between calculation and

measurements. However, even after introduction of the roller model, there

are larger disagreements between model and data than for the previous

laboratory simulations, both with respect to the measured offshore tail and

the flat distribution in the surf zone. To improve the agreement and evaluate

the sensitivity of the model to some of the parameters, simulations were made

with different values on βD than were recommended by Dally and Brown

(1995), as well as for alternative mixing formulations.

Figure 40 illustrates the result of changing βD on the longshore current

distribution. A smaller value on βD implies a lower dissipation rate in the

roller, which in turn means that the roller keeps its mass and momentum for a

longer distance, thereby shifting the forcing more shoreward. Thus, the peak

in the current will be translated shoreward if βD is decreased, as seen in

Figure 40. Somewhat better agreement between the calculations and the

Chapter 6 Verification of Longshore Current Model

Index

Privacy Statement
Press Release
Contact