in stormcad modeling, I wonder how can I design An open surface urban network that contains channels( or conduits) as main elements.In fact I need gutters to carry main flow and not just excess flow.

In general, gutters are only intended to model short runs of gutters between inlets/catch basins. If you are serious about modeling the surface network, you should use conduits than represent open or open channel links.

If this covers a large area such that rational method hydrology is not sufficient or have significant storage in ponds or depressions, then you should consider using a dynamic wave solver such as the solvers in CivilStorm. The peak flow solver in StormCAD does not account for flow attenuation over long distances. How large is the area you are modeling?