Overland flow paths are used to inform Level AI where low points can be in the roading network, if there is no overland flow paths in your model Level AI will assume the site must drain out via the roading network (where it snaps to the earthworks extent).
Add by clicking Drainage -> Overland Flow Path and then drawing out the path making sure to cross the road centreline and the earthworks extent boundary
Overland flow paths must cross the road centreline to activate. If they do not cross it but are snapped nearby, a Level AI validation warning will be generated when you run Level AI Prepare.
Cul-de-sacs
If you do not specify a overland flow path at a cul-de-sac it will drain out via the road connections, this may not be desirable. Adding an overland flow path allows the cul-de-sac to be a low point.
Drainage Paths
Where you allow a road to be low such that in a flood event water can spill out of the site add an overland flow path making sure it crosses the road centreline and earthworks extent.
Connecting Roads
Additionally Overland Flow paths can be used to 'connect' 2x roads, informing Level AI that water can flow between the 2 roads in a flood event.
Step-by-Step Guide
Below is a guide showing process to add overland flow paths to your model.
