There are a number of books available.
http://www.amazon.com/gp/aw/s/ref=is_box_?k=Sewage+plant+design
Qadim's book seems to be pretty comprehensive - although we are using it for only a select portion of its content.
As for software, http://www.hydromantis.com/ may be worth a look.
Actually a Thel-Mar weir would fit in a pipe and is a compound weir. You can find more at http://thel-mar.com/
Thel-mar weirs are available from the likes of Pollard Water, USABlueBook, etc.
Keep in mind, though, that you would have to either visually read the level / flow rate or use the...
What TexasChe is looking for is a compound weir:
http://www.openchannelflow.com/blog/article/compound-weirs
That works well so long as you have enough area in which to form a proper weir pool.
http://www.openchannelflow.com/blog/article/foolproof-weir-installations-in-9-easy-steps
After...
Stainless steel or fiberglass is what you need. You need a material that will hold and edge without the need for continual maintenance.
http://www.openchannelflow.com/blog/article/effects-of-crest-rounding-on-sharp-crested-weirs...