Highway surfaces and layered pavements generally have a cross slope of two percent from the highway crown towards the kerb or gu�er in order to facilitate drainage. Addi�onal considera�ons include super-eleva�on transi�ons where travel lanes on the higher, outward side of the radius of curvature must pass from a nega�ve cross slope to a posi�ve cross slope. The transi�on creates a sec�on 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. (Huebner et al, 1986). WFT analysis methods can be used to iden�fy transient and steady-state water 'pooling'. Kinema�c-Wave Modelling

The kinema�c-wave model is one of several approxima�ons of the dynamic-wave model used to describe one dimensional shallow-water waves with gradually varied open channel flow. In the kinema�c-wave approxima�on, some of the terms in the equa�on of mo�on are assumed to be insignificant. This means that an equa�on describing uniform flow can be applied. The kinema�c-wave model is therefore defined by the con�nuity equa�on and a uniform-flow equa�on such as the Manning formula, in addi�on to the usually imposed ini�al and boundary condi�ons.

Road Research Laboratory (RRL) Formula

In 1968, drawing on the kinema�c-wave model, the Road Research Laboratory (RRL) in the UK developed

a formula for calcula�ng WFT which was consulted widely in the design of highways. It allowed for rainfall (or water source) intensity in the es�ma�on of water film depth across a given flow path length. The RRL formula omits the variable considera�on of surface roughness, also known as surface texture depth.

The Gallaway Method

The Gallaway methodology for predic�ng WFT was developed by Gallaway et al in 1979 in coopera�on with the Federal Highway Administra�on. This method goes further than the RRL method in that it provides a way to predict aquaplaning speed based on the es�mated water film depth.

WFT es�ma�ons, using Gallaway's equa�on, 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 considera�on. The method correctly shows a significant drop in water film thickness as surface texture depth is increased.

Control of water on road surfaces is an essen�al 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.

CIVIL DESIGNER So�ware+27 (0)87 405 69330800 298 9655 (UK)

info@civildesigner.comwww.civildesigner.com

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 compu�ng simula�ons 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 es�mated. Finite volume schemes are conserva�ve 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:

= It can be used for complicated road geometry = It can be applied to a sec�on of road (up to 1000m) = WFT is calculated across all points = The longest flow path is calculated= The method is useful in iden�fying ponding areas = Finite volume cells can be graphically represented

WFT Simula�on with CIVIL DESIGNER So�ware

CIVIL DESIGNER's water sheet flow modelling on a road surface implements the Finite Volume Method. In addi�on to the FVM results, the analysis output includes maximum depths using the RRL method (as required by SANRAL for their road surface water depth calcula�ons) 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 op�mum design according to the project at hand.

References

Huebner et al, 1986Criteria for predic�ng hydroplaning poten�al, cited in Highway Drainage at Supereleva�on Transi�ons at h�p://ctr.utexas.edu

Gallaway, B. M., and Rose, J, 1979, The Effects of Rainfall Intensity, Pavement Cross Slope, Surface Texture, and Drainage Length on Pavement Water Depths,cited on www.researchgate.net

Gallaway, B. M, 1979, Pavement and geometric design criteria for minimizing hydroplaningcited at h�ps://archive.org/details/pavementgeometri00gall

G. Flintsch, 2017Guidance to Predict and Mi�gate Dynamic Hydroplaning on Roadwayscited on h�p://www.rpug.org

“After 10 years of using alternative software I am pleased to be back using CIVIL DESIGNER, now in the UK. The support and training is outstanding

and whenever I've had a design question, the CIVIL DESIGNER team have helped me out by sending a

video showing me how to solve the issue, or by setting up a support meeting. I am amazed how

quickly I’ve been able to get back into working with CIVIL DESIGNER and how user-friendly the

software is. “Diane Ochse, Solid Structures, UK

If you would like more information about CIVIL DESIGNER or if you would like to see

demonstration videos relating to your project challenges, please email info@civildesigner.com

CIVIL DESIGNER: Graphical visualisation of WFT on a road surface with a rainfall of 200 mm/hr. A point where the WFT has exceeded

stipulated limits is indicated.

CIVIL DESIGNER: Graphical visualisation of WFT on a road surface showing output analysis results according to RRL, Gallaway and

FVM methodology.