Normally what slope should be kept in storm water drain

Here is a good resource: Storm/Flood engineering - storm pipe minimum slope???

Generally it says you want a 2.5 f/s velocity in the pipes.

1% for corrogated and 0.5% for rigid pipes. Remember to follow local codes.

In my area we pitch our storm water runoff pipes between 1/8 and 1/4" slope to the foot. Cheers. Tom