Overland flow paths are used to inform Level AI where water can drain from the site. There are 3 types of OLFP:
Normal: These 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 crosses the site earthworks extent boundary). The existing site levels where the overland flow path intersects the earthworks extent boundary are important as this sets the minimum level of the road low point. Normal OLFP's do not necessarily activate if water can flow out another way.
Ditch: These inform Level AI where water can be directed away from your site. Ditches have extra fields on the feature to define the dimensions of the ditch. Ditches are always activated and have feature lines added to final surface.
Saw-tooth inlets: These tell Level AI to saw-tooth the profile along the path so the low points sit on the stormwater inlets, instead of the path falling continuously to one end. Most often used across carparks and other paved areas drained by a line of inlets. See Saw-tooth Inlets below.
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
Normal overland flow paths must cross the road centerline to activate and cross either another road centerline or the earthworks extent boundary. If they do not cross it but are snapped nearby, a Level AI validation warning will be generated when you run Level AI Prepare.
Ditch overland flow paths must cross the earthworks extent.
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. The point where the OLFP crosses the earthworks extent is important as this is the elevation Level AI considers when setting elevation of the road where OLFP crosses it.
Draining Carparks
Overland flow paths can be used to indicate where an impervious polygon (ie a carpark) can drain to the boundary. Using impervious layers convert your hatch/polygon to an impervious polygon, optionally set the type to CARPARK. Draw your overland flow path such that it connects the impervious polygon to either the earthworks extent, another OLFP/Ditch or another impervious polygon.
Where the carpark is drained by its own inlets rather than to the boundary, use a saw-tooth inlets flow path instead - see below.
Saw-tooth Inlets
A normal overland flow path falls continuously towards one end. A saw-tooth inlets path instead has Level AI place a low point on each stormwater inlet along the path, with a high point between them, so runoff is collected at the inlets rather than carried to the site boundary. It is the surface counterpart of saw-tooth roads, and is most often used across carparks and other large paved areas served by a line of inlets.
To use it, draw the overland flow path along the line water should follow, passing over the inlets it should tie to, then set the feature's Type to SAWTOOTH INLETS.
Level AI returns the saw-toothed profile together with its high and low turning points, and the enforced profile is added to the finished surface as breaklines.
The behaviour is tuned in Project Settings under Services (Storm), in the Overland Flow Paths group:
Saw-tooth overland flow paths on: Master toggle. Turn it off to have Level AI ignore saw-toothing without having to change the flow path features themselves.
Saw-tooth flow path minimum drainage grade (%): The minimum grade along the flow path. Level AI works the tooth lengths out from this, so it effectively sets how long each tooth runs. It applies only where the flow path does not carry its own minimum grade; a steeper value set on the feature is kept.
Maximum highpoint to highpoint length: The furthest apart two high points may be, which caps how widely the teeth can be spaced.
Minimum highpoint fall depth: The least fall required away from a high point. Level AI divides this by the maximum highpoint to highpoint length to get the flattest grade a saw-toothed path may sit at, so raising it leaves less room for a tooth.
Maximum ponding depth: The deepest ponding allowed when Level AI checks the flow path drains.
The four tuning settings only take effect while Saw-tooth overland flow paths on is enabled.
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.
Near Ponds
If you have an overland flow path near a pond you can choose to run it through the pond in which case the road will always be higher than the pond lip level. It still needs to run out of the site to activate.
Alternatively you can run the OLFP directly out the site avoiding the pond, this tells level ai the road needs to be higher than the ground at existing ground by the overland flow path.
Step-by-Step Guide
Below is a guide showing process to add overland flow paths to your model.
