  Undesired wave reflections, which occur at domain boundaries in flow
simulations with free-surface waves, can be minimized by applying source terms
in the vicinity of the boundary to damp the waves. Examples of such approaches
are absorbing layers, damping zones, forcing zones, relaxation zones and sponge
layers. A problem with these approaches is that the effectivity of the wave
damping depends on the parameters in the source term functions, which are
case-dependent and must be adjusted to the wave. The present paper presents a
theory which analytically predicts the reflection coefficients and which can be
used to optimally select the source term parameters before running the
simulation. The theory is given in a general form so that it is applicable to
many existing implementations. It is validated against results from
finite-volume-based flow simulations of regular free-surface waves and found to
be of satisfactory accuracy for practical purposes.
