A numerical study of the superharmonic instabilities of short-crested
waves on water of finite depth is performed in order to measure their
time scales. It is shown that these superharmonic instabilities can be
significant - unlike the deep-water case - in parts of the parameter
regime. New resonances associated with the standing wave limit are stu
died closely. These instabilities 'contaminate' most of the parameter
space, excluding that near two-dimensional progressive waves; however,
they are significant only near the standing wave limit. The main resu
lt is that very narrow bands of both short-crested waves 'close' to tw
o-dimensional standing waves, and of well developed short-crested wave
s, perturbed by superharmonic instabilities, are unstable for depths s
hallower than approximately a non-dimensional depth d = 1, the study i
s performed down to depth d = 0.5 beyond which the computations do not
converge sufficiently. As a corollary, the present study predicts tha
t these very narrow sub-domains of short-crested wave fields will not
be observable, although most of the short-crested wave fields will be.