Engineering Innovation on the N3 Ashburton
Integrating hydraulic modelling, pavement engineering and automation to improve surface drainage.
Two digital modelling platforms. More than 10 000 modelled pavement slots. A permeable BRAOG surfacing solution.
Naidu Consulting’s Roads and Highways team, including Roads and Highways Specialist Daylin Armugam and Roads and Highways Engineer Koval Woolganathan, undertook a detailed assessment of surface water flow on the N3 at Ashburton. The work focused on locations where runoff traverses wide carriageways with 4-5traffic lanes, increasing water film depths and the associated risk of aquaplaning.
Understanding the surface drainage challenge
Aquaplaning occurs when a layer of water builds up between a vehicle’s tyres and the road surface, causing the tyres to lose contact with the pavement. This results in reduced tyre grip, affecting the driver’s ability to steer, brake and maintain control of the vehicle.
According to the SANRAL Drainage Manual, aquaplaning is generally considered unlikely at speeds below 80 km/h on asphalt surfaces, although it can occur at lower speeds depending on water depth, tyre condition and pavement characteristics. The manual also specifies a maximum surface flow depth of 6 mm on travelled lanes during a 1 in 5 year storm event. With the N3 operating at a posted speed of 120 km/h, effective surface drainage is particularly important to minimise water accumulation and reduce the risk of aquaplaning during rainfall.
At critical locations near horizontal curves and superelevation transitions, the initial analysis indicated surface water film depths of approximately 6 – 12mm. The assessment therefore considered how to improve drainage performance while retaining the required road geometry and pavement configuration.
Figure 1. Initial surface flow assessment at km 3+200 to km 3+400.
Two models supporting the assessment
The team used HEC-RAS 2D with unsteady flow simulations to investigate runoff distribution and areas of ponding or accumulation under the assessed rainfall conditions. Autodesk Civil 3D provided the detailed road surface geometry needed to evaluate the effects of crossfall, longitudinal gradient and proposed drainage interventions.
Option 1: Pavement slots and design automation
One option involved introducing slots into the paved surface to intercept runoff and shorten the distance water travels before reaching a drainage path. Accurately representing these features was important for assessing their influence on the modelled surface flow paths and calculated water film depths.
To avoid manually constructing thousands of features, the team developed a custom macro that generated more than 10 000 individual slots in the geometric model. This improved modelling consistency, substantially reduced repetitive work and allowed a detailed assessment of the slotting alternative.
The slots indicated a reduction in surface flow, however, the analysis showed that water film depths could still reach approximately 6mm, whilst the potential for siltation and blockages within the slots raises concerns for long-term maintenance. Despite these limitations, the slots reduced the high-risk aquaplaning zone from approximately 200m to 50m, representing a 75% reduction and confined the aquaplaning zone from multiple lanes to a single lane.
Figure 2. Surface flow assessment associated with the pavement slotting option at km 3+200 to km 3+400.
Option 2: BRAOG permeable surfacing
An alternative was Bitumen Rubber Asphalt Open Graded (BRAOG), a permeable open-graded asphalt surfacing, containing interconnected voids that allow water to infiltrate the surface layer and drain laterally towards a side drain. Rather than relying solely on surface runoff moving across the pavement, BRAOG allows water through the surfacing layer, reducing the volume of water remaining on the road surface.
The BRAOG option was assessed and the analysis showed it outperformed the slotted option by facilitating water infiltration into interconnected voids and lateral drainage through the surfacing layer.
Figure 3. BRAOG surface flow assessment km 3+200 to km 3+400.
Understanding the difference in drainage mechanisms
The two alternatives fundamentally differ in how they manage surface runoff. The slots provide drainage at discrete locations, requiring water to travel across the road surface before reaching an interception point. In contrast, BRAOG facilitates water infiltration over the surfacing area, reducing the distance the surface runoff travels on the pavement surface, before entering a drainage pathway.
This distinction is particularly significant on multilane carriageways, where extended flow paths can increase the water film depth. The comparison demonstrated the importance of evaluating not only the drainage capacity of individual interventions, but also how effectively they intercept and convey runoff across the entire road surface.
Figure 4. Comparison of surface drainage mechanisms.
Designing for greater rainfall resilience
The surface water film assessment applied the Road Research Laboratory and Gallaway methods, considering rainfall intensity, flow path, surface slope and, where applicable, pavement macrotexture. The assessed SANRAL drainage criterion limits water film depth to 6 mm during a 1-in-5-year rainfall event.
To support more resilient infrastructure, the drainage options were also evaluated under the more severe 1-in-10-year rainfall event. This additional assessment exceeded the standard requirement and informed the comparison and selection of the preferred solution for the new 10-lane freeway. It also aimed to strengthen climate resilience and safeguard road users during heavy rainfall.
The slotting option did not satisfy the 1 in 10-year analysis, as the slots became filled and surface runoff depths exceeded the 6 mm assessment criterion in certain areas. By comparison, the BRAOG permeable surfacing performed significantly better, with most areas remaining within an acceptable range, although isolated locations approached the 5 mm to 6 mm zone.
Under such an extreme rainfall event, vehicle operating speeds would also be expected to reduce substantially, which lowers the aquaplaning risk. In this context, surface runoff depths in the 5 mm to 6 mm range present a comparatively lower risk than the slotting option, where blocked slots resulted in runoff depths exceeding 6 mm. This supported the adoption of BRAOG as the preferred final solution.
Digital engineering delivering practical solutions
This project illustrates how hydraulic analysis, geometric modelling and engineering automation can be combined to test practical pavement drainage options. The final BRAOG surfacing approach demonstrates the importance of considering both the movement of surface runoff and the ability of the pavement layer itself to manage water, while evaluating long term drainage performance and maintainability.
At Naidu Consulting, we continue to apply engineering expertise and digital innovation to deliver safer and more resilient transport infrastructure.
