DRAINS includes a flood mapping capability for overflow routes. Following a run of the Full unsteady hydraulic model you can turn flood mapping on/off from the View menu. Note that this function is not possible with the Lite hydraulic model.

The mapping relies on DRAINS knowing the location of invert levels at points along an overflow route. It is common practice to draw overflow routes schematically (e.g. to visually separate them from a pipe directly below them). In that case, the flood mapping would also be distorted if DRAINS assumed the on-screen location defined the invert level of the overflow route. To prevent this distortion, you have the option to specify whether the on-screen location is correct or whether DRAINS should assume a straight line from end to end of the overflow route. Right click on an overflow route and tick or untick the item Treat as straight line for flood mapping.

An overflow route cross section is often not symmetrical (e.g. a half road section - the road pavement to the right of the kerb and gutter is different to the footpath to the left). So the extent of flooding to the right of the invert level is often different to that to the left. Often the same cross section is used for the gutters on each side of the road. This has not mattered in the past, but it does matter now when DRAINS plots the extents of flooding.  There are extra radio buttons in the overflow route property sheet (under Cross Section Data tab) where you can specify whether the cross-section represents the view looking upstream or downstream.

The accuracy of the flood mapping relies on the accuracy of the cross section data. Some older models may need revision if the cross sections are not accurate (e.g using a 7 m wide road section when it should be a 5 m wide road section may show the flows crossing the centerline of the road). Adding additional incremental points while drawing an overflow route may also improve the visual representation of the flood mapping, particularly around a kerb return.

You may find you need to draw extra overflow routes in a model. For example if you have only one overflow route into a sag pit, and the outgoing overflow route is at ninety degrees to the incoming one, you may find that part of the pit surface flooding is missing. Drawing an extra incoming overflow route from another direction (in line with the other incoming overflow route) provides DRAINS with the extra information it needs to plot the flood mapping.

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