Using the ODF Method for Culvert Sizing
Get 50 year runoff value from 50 year Peak flow map
Choose an adequate sized culvert from Table (based on projecting inlet and HW/D ratio = 1 and assumes inlet control)
For streambed simulation designs you will usually size to the width of the stream and use the peak flow calculation as a check. The flow capacity of the culvert is usually greater than expected 50 year peak flow.