All stormwater collection and conveyance facilities (pipes, swales,
and structures) shall be designed for a 100-year design storm event,
unless the runoff would naturally drain overland to a stormwater detention
facility, in which case a twenty-five-year design storm event may
be used. All drainage facilities shall be designed such that the hydraulic
gradeline must be a minimum of two feet below the finished grade.
Swales and channels shall provide at least one foot of freeboard above
the energy gradeline. Backwater effects of pipes discharging under
surcharge conditions shall be included.