Precipitation intensity for highway & bridge drainage design


Drainage projects usually consider a precipitation intensity based on the time of concentration and the return period. However, when designing drainage systems for bridges and highways it is important to verify the compatibility between the precipitation intensity and the driver's safety. Thus, two additional criterion should be considered: Hydroplaning and visibility.

Hydroplaning
Hydroplaning is the situation when when a layer of water is built between the wheels of a vehicle and the road surface. Hydroplaning leads to a loss of traction. The driver  of the vehicle looses control and the vehicle begins to slide. The hydroplaning of a vehicle is a function of the speed, the water depth and the inflation pressure of the wheels.

Figure 1. Hydroplaning concept (Source: Tirebuyer)

The hydroplaning based intensity is a new approach to drainage design. It seeks that rainfall that is just sufficient to cause a water depth of sheet flow at the edge of the traveled way that will cause hydroplaning. The design concept is that removal and control of flooding, caused by rainfall in excess of rainfall that will cause hydroplaning, is overdesign from a vehicle safety standpoint.

The limit speed is defined by the standards of the highways or bridge deck. Then, the speed that initiates hydroplaning is used for defining the water depth D when hydroplaning occurs. The most popular studies about hydroplaning speed are the studies performed by NASA and Gallaway et al., (1979). Then, it is assumed that water flows in sheet with depth D across the surface to the edge of the gutter. Combining the depth D with the rational formula and the Manning equation it is possible to define the hydroplaning design rainfall intensity.

Figure 2. Hydroplaning speed equation reported by Gallawat et al., (1979).

Visibility
The rainfall and the windshield wipers reduce the visibility of the driver. Such visibility reduction also reduces the safe stopping distance.

Figure 3. Driver visibility during rain (Source: Earth magazine)

The visibility design concept is that drainage removal or control of flooding caused by rain in excess of that rain, which will cause driver vision impairment, is overdesign from a vehicle safety standpoint (more drainage capacity than required).

The safe stopping distance is a function of the vehicle speed and the mechanical characteristics of the vehicle (break time and deceleration). Thus, this design criterion uses the rain intensity that allows a visibility equal to the safe stopping distance. Several studies analyzed the visibility considering the effect of different rain intensities and windshield wipers. One of the first methods for estimating the driver's visibility under different rainfall intensities and vehicle speeds is based on the experiments from Ivey et al., (1975).

We can see that there is no single method for estimating the rainfall intensity. Thus, the selection of the highway-bridge rainfall intensity is a multi-criteria process. It is suggested to consider all the criterion; the two criterion from this post and the traditional IDF - rational criterion. Then, engineering criterion should decide the optimum one.

If you require advice for design of highway/bridge drainage or doubts about the rainfall intensity, feel free to contact us.

Figure 4. Driver visibility suggested by Ivey et al., (1975)

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