  The velocity profiles of weak metal absorption lines can be used to
observationally probe the kinematic state of gas in damped Lyman-alpha systems.
Prochaska and Wolfe have argued that the flat distribution of velocity widths
combined with the asymmetric line profiles indicate that the DLAS are disks
with large rotation velocities. An alternative explanation has been proposed by
Haehnelt, Steinmetz, and Rauch, in which the observed large velocity widths and
asymmetric profiles can be produced by lines of sight passing through two or
more clumps each having relatively small internal velocity dispersions. We
investigate the plausibility of this scenario in the context of semi-analytic
models based on hierarchical merging trees and including simple treatments of
gas dynamics, star formation, supernova feedback, and chemical evolution. We
find that all the observed properties of the metal-line systems including the
distribution of velocities and the asymmetric profiles, can be reproduced by
lines of sight passing through sub-clumps that are bound within larger
virialized dark matter halos. In order to produce enough multiple hits, we find
that the cold gas must be considerably more extended than the optical radius of
the proto-galaxies, perhaps even beyond the tidal radius of the sub-halo. This
could occur due to tidal stripping or supernova-driven outflows.
