### 3.10.4 Design Discharge for Trials

Where a single barrel culvert is not sufficient, multiple barrels or cells will be required and these configurations constitute a parallel system.

Provided that all barrels of a multi-cell culvert are the same type, size and roughness (equal conveyance), and also have the same invert levels/bed slope, flow will distribute evenly.

Design nomographs as shown in Appendix B and Appendix C, are based on a single barrel installation, therefore divide the design discharge (*Q*) by the number of barrels for the trial culvert.

For boxes, a ratio (*Q/B*) is also required. This ratio is determined by dividing *Q* per cell by the nominal box width (*B*).

Any proposed, multi-cell culvert that does not have all cells the same will require specialist advice as the methodology presented in this Guide does not allow for these type of configurations. These culverts must be referred to specialist hydraulic engineers.