Chris - The following report looks at this type of situation with the
cross-pipe in a junction box, presenting experimental data and design
parameters. Not exactly the same as your situation, but you could
probably use the results with some judgement, safety factors, etc.

"Hydraulic Performance of Conflict Junctoin Boxes", S.C. Krane,
F.Romano, K. Cecil, D. Olin, A. Perez, S. Ethier, and E. Droz, Report
No. 710, Dept. of Civil and environmental Engineering, University of
South Florida, Tampa, Fl 33620. Prepared for Florida DOT & US FHA,
available from NTIS. 29 pp.

Ron Kilmartin
ronkilmartin@attbi.com