If you're asking about storm sewer design, try the Federal Highway Administration (FHWA) web site. Download HEC-22 Urban Drainage Design Manual and Hydraulic Design Series No. 4 - Introduction to Highway Hydraulics. These two publications should get you started.
Another ref is Stormwater Collection Systems Design Handbook, Mays, McGraw Hill. It covers all aspects. I picked up a used copy online for $12 plus shipping. Well worth it. HEC22 as Maury stated above it a good source. It can be dl's in pdf format I beleive. Here is FHWA Link
