  In finite-volume-based flow-simulations with free-surface waves, wave
reflections at the domain boundaries can cause substantial errors in the
results and must therefore be minimized. This can be achieved via `implicit
relaxation zones', but only if the relaxation zone's case-dependent parameters
are optimized. This work proposes an analytical approach for optimizing these
parameters. The analytical predictions are compared against results from
2D-flow simulations for different water depths, flow solvers, and relaxation
functions, and against results from 3D-flow simulations with strongly
wave-reflecting bodies subjected to nonlinear free-surface waves. The present
results demonstrate that the proposed approach satisfactorily predicts both the
optimum parameter settings and the upper-limit for the corresponding reflection
coefficients $C_{\mathrm{R}}$. Simulation results for $C_{\mathrm{R}}$ were
mostly below or equal to the analytical predictions, but never more than
$3.4\%$ larger. Therefore, the proposed approach can be recommended for
engineering practice. Furthermore, it is shown that implicit relaxation zones
can be considered as a special-case of forcing zones, a family of approaches
which includes among others absorbing layers, damping zones and sponge layers.
The commonalities and differences between these approaches are discussed,
including to what extend the present findings are applicable to these other
approaches and vice versa.
