21 Jul Water Film Thickness on a Road Surface
Control of water on road surfaces is an essential component of highway design both for safety and service life. Surface water slows traffic and contributes to accidents due to hydroplaning and reduced visibility from splash and spray.
Highway surfaces and layered pavements generally have a cross slope of two percent from the highway crown towards the kerb or gutter in order to facilitate drainage. Additional considerations include super-elevation transitions where travel lanes on the higher, outward side of the radius of curvature must pass from a negative cross slope to a positive cross slope. The transition creates a section of the surface with zero cross slope, at which point the runoff will be more dependent on the longitudinal grade of the road.
Water Film Thickness
Vehicle speed, stormwater runoff, tyre pressure, tyre tread depth, average pavement texture depth, and other factors all contribute to skid resistance and incipient hydroplaning. However, research suggests that water film thickness (WFT), especially where greater than 2mm, is the primary variable. WFT analysis methods can be used to identify transient and steady-state water ‘pooling’.
Kinematic-Wave Modelling
The kinematic-wave model is one of several approximations of the dynamic-wave model used to describe one dimensional shallow-water waves with gradually varied open channel flow. In the kinematic-wave approximation, some of the terms in the equation of motion are assumed to be insignificant. This means that an equation describing uniform flow can be applied. The kinematic-wave model is therefore defined by the continuity equation and a uniform-flow equation such as the Manning formula, in addition to the usually imposed initial and boundary conditions.
Road Research Laboratory (RRL) Formula
In 1968, drawing on the kinematic-wave model, the Road Research Laboratory (RRL) in the UK developed a formula for calculating WFT which was consulted widely in the design of highways. It allowed for rainfall (or water source) intensity in the estimation of water film depth across a given flow path length. The RRL formula omits the variable consideration of surface roughness, also known as surface texture depth.
The Gallaway Method
The Gallaway methodology for predicting WFT was developed by Gallaway et al in 1979 in cooperation with the Federal Highway Administration. This method goes further than the RRL method in that it provides a way to predict aquaplaning speed based on the estimated water film depth.
WFT estimations, using Gallaway’s equation, apply the same parameters of flow path, slope and rainfall intensity as the RRL method, but with different indices applied to each. Most notably, and in contrast to the RRL method, the Gallaway method takes the texture depth of the pavement into consideration. The method correctly shows a significant drop in water film thickness as surface texture depth is increased.
Finite Volume Numeric Method
The Finite Volume Method (FVM) can be used to calculate steady-state water depth at all points on a road surface. Finite volume methods are widely used and highly successful in computing simulations for fluid dynamics. FVM assumes that a surface can be represented as a mesh and that a ‘finite volume’ of fluid surrounding each node point or ‘cell’ on the mesh can be estimated. Finite volume schemes are conservative which means that as cell averages change, one cell’s loss becomes another cell’s gain across proximity edge gradients. Some of the strengths of this method include:
- FVM can be used to analyse more complicated road geometry
- FVM can analyse a specific section of road (up to 1000m)
- The WFT at all points on the analysed section of road are calculated (not just the deepest WFT)
- The longest flow path over complicated road geometry is calculated
- The method is useful in identifying ponding areas and excessive flow path lengths
- Finite volume cells can be effectively represented graphically
Handling the Simulation & Analysis with Civil Designer Software
Civil Designer’s water sheet flow modelling on a road surface implements the Finite Volume Method. In addition to the FVM results, the analysis output includes maximum depths using the RRL method (as required by SANRAL for their road surface water depth calculations) and the Gallaway methods. The comprehensive results output, in tabular and graphical format, ensures that the civil engineer has all the analysis data required to achieve the optimum design according to the project at hand.
While always relying on tried and trusted civil engineering principles, Civil Designer gives the engineer the autonomy to consider results from the relevant analysis methodologies and implement necessary design changes according to the project at hand.
View a video demonstration of Civil Designer’s Water Film Thickness analysis functionality below. The demo includes analysis output showing WFT on a road surface according to the RRL, Gallaway and FVM methodology calculations. You can also find this article on LinkedIn: https://www.linkedin.com/feed/update/urn:li:activity:6827864085451821056