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COASTAL ENGINEERING 2004

Figure 5. Relative offshore transport rate as a function of elapsed dimensionless time.

problem numerically the offshore rip loss is expressed in a slightly different way

(formulated here for the loss *Q*off,L at the left-hand lateral boundary),

Δ*Q*

= -*R * 2

= -*R * 2

(16)

Δ*x *i=1

Δ*x*

Δ*x*

where x is the alongshore grid spacing and *Q*1 is the transport across the lateral

boundary. With an impermeable, long groin located here, the boundary

condition is formulated *Q*1 = 0. An equivalent boundary condition is formulated

for the right-hand boundary. Figure 6 illustrates simulated interrelations

between the offshore losses at each of the two groins (where *Q*off,R is the loss at

the right-hand lateral boundary) for a situation where the incident wave angle

flips instantaneously between 11.5 and 11.5 deg every two months. The

simulation starts out with a negative angle inducing an offshore rip current and

associated offshore losses at the left-hand groin. The offshore transport rate

starts out with a high value but decreases quite rapidly (in about two weeks) to a

considerably lower value that remains fairly stable in time at around 5 m3/h.

Simultaneously, the shoreline adjacent to the left-hand groin progrades towards

the groin tip as it recedes at the right-hand groin. Because the initial shoreline

was located at *x *= 0, the system is somewhat asymmetrical initially while at the

end of the third cycles it seems like symmetry has been reached.

A series of simulations was then run to illustrate the impact of the

formulation of the lateral boundary conditions. In all cases, the wave angle

varied sinusoidally according to Eq. (10) with 1/ set to 1, 2.25, 4, 9, and 36

days, respectively. With constant wave period *T *= 3 sec, breaking wave height

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