  We examine the moduli dynamics of a specific class of supergravity-inspired
BPS braneworlds, clarifying the role of bulk scalar fields in brane collisions.
The model contains as a special case the Randall-Sundrum model both with and
without a free, massless bulk scalar field. Its low-energy effective theory is
derived with a moduli space approximation (MSA) and agrees with the
corresponding results derived elsewhere. Rather than stabilising the radion, we
look at cosmological evolution of the system stimulated by breaking the BPS
condition on the branes. We examine in detail the range of validity of the MSA
in both the RS and BPS case, paying particular attention to the divergences
that can arise during a collision of the branes. In the absence of
perturbations such an event is finite in the RS model, and accurately described
by the low-energy effective theory. We demonstrate, however, that a collision
is divergent in the BPS case even with an exact FRW geometry
