What is aquaplaning?
For road users to be able to stop or steer their vehicles, the vehicle tyres must grip the road surface. Grip is achieved by means of friction generated in the areas where the tyres make contact with the uppermost particles of the road surface. If the friction available is insufficient to resist the forces generated by acceleration, braking or cornering manoeuvres, the tyre may slip over the road surface. In dry conditions, surface friction is generally at a level that supports most normal manoeuvres, however the level of friction available decreases when a road surface is wet or flooded.
Many factors exist, including tyre tread and pressure, road surface and geometry, vehicle speed and depth of water over the surface, which influence effective lateral and longitudinal friction between the tyre and road surface. When a loss of grip or traction occurs, the effect on a moving vehicle is either skidding or aquaplaning, both of which represent a major driving hazard. Aquaplaning is the least understood, yet most dangerous condition.
Potential problem areas that need to be checked
Areas of superelevation development can cause problems, particularly where the longitudinal gradient is relatively flat. Approaches to junctions such as a roundabout can also be problem areas, particularly because vehicles are braking from higher speeds.
Another area that can cause problems are on and off ramps, where the extra width of roadway across the through lanes, gore area, and ramps can cause very long flow paths.
Before you start
The result of an Aquaplaning analysis is a delta terrain model and flow lines. The elevations in the delta terrain model are those of the calculated water depth, and these are normally very small - in the order of a few millimetres, or a fraction of an inch, and they will all start at zero. For this reason, it is recommended that you create a new DGN before you use Aquaplaning, and reference in a terrain model of the road surface. If you do this, then the range of elevations for the elements in the DGN will be those of the delta terrain model, and nothing else. This will mean that a thematic display can be used to effectively show the variation in water depths.
If the DGN contains other elements, such as the road surface for example, then the range of elevations will be from zero (for the smallest water depth) to whatever the highest elevation is for the road surface.
The Aquaplaning command prompts you to select a terrain model of a road surface. To avoid unnecessary processing, it is important to restrict this terrain model so that it only covers the road surface - not road edge features such as curbs, sidewalks, verges, etcetera.
This applies equally to the median or central reserve area of a dual carriageway or divided highway. If the median modelled an open channel for example, this could cause long flowlines to be created along its invert, which would have unacceptably high film depths. In this situation, it may be better to create two separate terrain models - one for each side of the road.
Calculations for water film depth.
The tool offers a choice of two formulae to calculate the water film depth.
D is the water film depth above the top of pavement texture (mm).
T is Average pavement text depth (mm) (See Texture Depth section.)
L is the length of drainage path (m) (See Drainage Path section.)
I is the rainfall intensity (mm/h) (See Rainfall Intensity section.)
S is the slope of drainage path (%) (See Drainage Path section.)
Road Research Laboratory
d is depth of flow (mm) at the end of the flow path
lf is the length of flow path (m)
I is rainfall intensity (mm/hr)
Sf is the flow path slope
Note that, once the analysis has been done, it is possible to change the formula used for an individual flow line. To do this, you need to display the 3D model which contains the delta terrain model and the flow lines. In this model, you can select a flow line, and change the formula that it uses in the Properties dialog. Doing this will automatically update the delta terrain model.
The slope used in the calculations for the water film depth can either be the instantaneous slope between each pair of points on the flow line, or an "Equal Area Slope", which is calculated as follows:
- Divide this area by the length of the flow line then multiply by 2.
- Plot this new ordinate (at the highest point of flow path) and join it back to the point of analysis.
- Calculate slope of this line (the equal area slope).
(From Austroads Guide to Road Design Part 5A)
The figure above illustrates the 'equal area slope' over a flow path containing several sub-sections.
A report is available for a selected flow line. To create a report:
- Select the flow line.
- Click the Create Flow Line Report icon on the context sensitive menu. The Report Browser opens.
- Expand Civil Geometry.
- Select Aquaplaning.xsl.
This style sheet shows pertinent information, such as the film depth, slope, distance along the flow line, and the elevation.