  Using 3D numerical simulations we study the evolution of the H$\alpha$
intensity and velocity dispersion for single and multiple supenova (SN)
explosions. We find that the $I_{\rm H\alpha}-\sigma$ diagram obtained for
simulated gas flows is similar in shape to that observed in dwarf galaxies. We
conclude that colliding SN shells with significant difference in age are
resposible for high velocity dispersion that reaches values high as $\simgt
100$kms$^{-1}$. Such a high velocity dispersion could be hardly got for a
single SN remnant. Peaks of velocity dispersion on the $I_{\rm H\alpha}-\sigma$
diagram may correspond to several stand-alone or merged SN remnants with
moderately different ages. The procedure of the spatial resolution degrading in
the H$\alpha$ intensity and velocity dispersion maps makes the simulated
$I_{\rm H\alpha}-\sigma$ diagrams close to those observed in dwarf galaxies not
only in shape, but also quantitatively.
