  We have studied numerically the dynamics of a directed elastic string in a
two-dimensional array of quenched random impurities. The string is driven by a
constant transverse force and thermal fluctuations are neglected. There is a
transition from pinned to unpinned behavior at a critical value $F_T$ of the
driving force. At the transition the average string velocity scales with the
driving force. The scaling is equally well described by a power law $v_d\sim
(F-F_T)^\zeta$, with $\zeta=0.24\pm0.1$, or by a logarithm,
$v_d\sim1/\ln(F-F_T)$. The divergence of the velocity-velocity correlation
length at threshold is characterized by an exponent $\nu=1.05\pm0.1$.
