  Weakly nonlinear internal wave-wave interaction is a key mechanism that
cascades energy from large to small scales, leading to ocean turbulence and
mixing. Oceans typically have a non-uniform density stratification profile;
moreover, submarine topography leads to a spatially varying ocean depth ($h$).
Under these conditions and assuming mild-slope bathymetry, we employ
multiple-scale analysis to derive the wave amplitude equations for triadic- and
self-interactions. The waves are assumed to have a slowly (rapidly) varying
amplitude (phase) in space and time. For uniform stratifications, the
horizontal wavenumber ($k$) condition for waves ($1$,$2$,$3$), given by
${k}_{(1,a)}+{k}_{(2,b)}+{k}_{(3,c)}=0$, is unaffected as $h$ is varied, where
$(a,b,c)$ denote the modenumber. Moreover, the nonlinear coupling coefficients
(NLC) are proportional to $1/h^2$, implying that triadic waves grow faster
while travelling up a seamount. For non-uniform stratifications, triads that do
not satisfy the condition $a=b=c$ may not satisfy the horizontal wavenumber
condition as $h$ is varied, and unlike uniform stratification, the NLC may not
decrease (increase) monotonically with increasing (decreasing) $h$. NLC, and
hence wave growth rates for both triads and self-interactions, can also vary
rapidly with $h$. The most unstable daughter wave combination of a triad with a
mode-1 parent wave can also change for relatively small changes in $h$. We also
investigate higher-order self-interactions in the presence of a monochromatic,
small amplitude bathymetry; here the bathymetry behaves as a zero frequency
wave. We derive the amplitude evolution equations and show that higher-order
self-interactions might be a viable mechanism of energy cascade.
