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COASTAL ENGINEERING 2004

**1.0**

**0.8**

**0.6**

**0.4**

**0.2**

**0.0**

**0**

**2**

**4**

**6**

**8**

**10**

**Elapsed Dimensionless Time (**ε t/ L2 )

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),

*Q *- *Q*1

Δ*Q*

*Q*

*Qoff *,*L *= - *R*

= -*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

*morphodynamic response factor *ζ on the offshore rip losses following the above

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