When the rain hits the road, effective road drainage keeps traffic moving safely, protects pavements and reduces impacts on surrounding communities. Often hidden from view, road drainage is one of the most complex elements of road infrastructure. Here, we break down the key components of road drainage — from general design considerations and aquaplaning to basins, open drains, culverts and floodways.
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What makes for an effective road drainage system?
An effective drainage system for all road infrastructure must:
- minimise flooding impacts to road, environment and community
- enhance road safety for all road users
- protect road assets
- mitigate adverse environmental impacts.
There is also a strong relationship between geometrics and drainage. Most roads cut across natural flow paths of storm water run-off, so road geometry can be designed to:
- convey stormwater across the road reserve without adversely impacting adjacent communities and landscapes
- control water within the road reserve to allow safe and efficient passage of all road users
- prevent damage to the roadway and associated infrastructure.
Austroads Guide to Road Design Part 5 (AGRD) provides guidance on the planning, design, construction, and maintenance of all road drainage, including surface drainage, subsurface drainage, and flood mitigation.
General considerations when designing road drainage
An important aspect of planning and designing road infrastructure is the calculation of design flood discharges, otherwise known as hydrology. Key factors such as safety, accessibility and road classification inform the flood immunity of a road.
The immunity of a road is defined as the probability of the flood event that reaches any portion of the trafficable lanes. As recommended by AGRD Part 5 Appendix E is a great point of reference for determining the most appropriate coefficient for your catchment. For a given AEP event, two inputs determine rainfall intensity: time of concentration (tc), the time taken for water to flow from the most remote point on the catchment to the outlet, and location. There are many sources of rainfall data but we refer to the Intensity–Frequency–Duration (IFD) tables developed by the Bureau of Meteorology.
Before CAD, catchment areas were measured with a planimeter and expressed in square kilometres. However, these days we are blessed with advanced software applications such as 12d which can quickly measure the area of your catchment.
Understanding aquaplaning
A prime example of the intrinsic relationship between road geometry and drainage is a phenomenon known as ‘aquaplaning’. Friction is the force that keep vehicle tyres in contact with our road surfaces. A loss of friction can result in a vehicle either skidding or aquaplaning. This reduces the effectiveness of braking and seriously compromises the safety of our road users.
Aquaplaning occurs when a film of water causes the vehicle’s tyres to separate from the road surface. To avoid aquaplaning occurring on our roads, we consider these influencing factors:
- Pavement texture and porosity.
- Rainfall intensity.
- Water film depth.
- Tyre condition.
- Driver behaviour.
Road geometry and road surfacing are the two key factors that we have most control over. The phenomena is most likely to occur at horizontal tangent points where the super elevation of our roads transition.
Avoiding aquaplaning is an iterative process that requires the determination of water film depth, necessary geometry adjustments, and selection of an appropriate pavement type. Together with Austroads Guide to Road Design Part 5A Section 4, we leverage current road modelling technology such as Autodesk Civil 3d or 12d to verify that aquaplaning does not occur on our projects.
How can we use kerb and piped drainage
Ever wonder where the water goes in a storm? In urban areas, road surface drainage is collected by kerb and channel. Road crossfall directs run-off towards the kerb, where it accumulates within the channel. The kerb redirects run-off downstream, following the longitudinal gradient of the road, until reaching a low point (or sag) and ponds. The volume and width of run-off increases the greater the distance it is to a sag. So, do we like ponds on our roads? Absolutely NOT!
Run-off that encroaches into trafficable areas creates a hazard to road users. A mitigating measure is to introduce drainage discharge points, such as gully pits. Gully pits come in a variety of forms and are one component of an overall urban drainage network which also includes junction pits and pipes. The inlets capture the run-off from the road surface and convey it to the underground piped network for disposal or treatment. Adding to the challenge of designing an urban drainage network is the diminishing supply of concrete pipes. This has required drainage design to be accelerated, enabling pre-ordering of materials to avoid delays on site.
Road drainage basins
You may not know when you have seen them, but drainage basins are everywhere and serve an important purpose in any storm water network. Basins can be used in a drainage system to:
- control the speed of storm water flows (detention)
- store a volume of runoff (retention)
- remove sediment and/or pollutants (water quality).
They can create cost savings for projects together with benefits to public safety and reduced risk of scour and erosion. They can be purpose-built structures, or public open spaces such as lakes, wetlands and sporting fields. The key features of basin design include:
- Peak outflows and depths
- Storage volume
- Duration of ponding
- Attenuation & impacts up and down stream.
When it comes to basin design, we find the most challenging aspect to be determining the most suitable location within the road corridor. Typically, our road corridors are constrained and available land comes with a premium, meaning the size and shape of basins are never the same.
Open drains and channels
Ever followed where the rain falls once it hits the ground? Rainfall tends to follow the path of least resistance until it reaches a low point and forms a pond.
As roads are linear in nature, often large flows need to be managed within our drainage systems. Open channels are a cost-effective measure, if space is not restricted, that minimises materials when designing for such flows.
As open channels capture and convey flows to achieve the road formations immunity target, the following considerations are key to determining an optimum solution:
- Capacity (Flow rate (Q) = Velocity x Area).
- Erosion (landscaping, environment and soil conditions).
- Maintenance (access and frequency).
We love designing open channels as it gives us the best chance of achieving a nature-based solution that minimises impact to the environment (sometimes they can even improve conditions with water quality treatments) and still meet the desired technical requirements of the road authority.
Culverts
How can the water get to the other side of the road? Depending on your location, culverts can be a very cost-effective way to convey water flow under a road.
Culverts come in all kinds of shapes, sizes and materials. The most common culvert types are Reinforced Concrete Pipes (RCPs) and Reinforced Concrete Box Culverts (RCBCs). According to Austroads AGRD Part 5B, selecting the most appropriate culvert type depends on:
- Costs
- Site conditions
- Hydraulic performance
- Afflux
- Constructability
- Structural requirements
- Blockage potential.
While there are a plethora of GIS tools (such as Google Earth and Nearmap) available to us these days, we tend to prefer undertaking a site visit to verify these factors before jumping into calculations. This saves us time by avoiding multiple design iterations.
Discharge is a term used to describe the volume of water that can flow through a culvert. It is typically equal to the catchment run-off but depends on the serviceability of the road on top of the culvert and the adjacent environment.
The minimum design discharge for culverts under major roads is a 1% Annual Recurrence Interval (ARI). This means the major road must not be inundated during a storm with a probability of occurring once in every 100 years! Key inputs for meeting this requirement are:
- Outlet velocity
- Siltation and blockage
- Allowable headwater.
However, the most important consideration in culvert hydraulic performance is whether the flow is subject to inlet or outlet control. This also influences how we treat potential erosion & sedimentation, landscaping, fauna crossings and/or fish passages.
Floodways
We have three options for designing roads that cross floodways – by bridge, culverts or floodway crossings. Floodway crossings consist of a length of pavement on a level grade, designed to be overtopped by frequent rainfall (or high AEP) events.
The following elements are considered when we design a floodway crossing:
- Hydraulic design (discharge capacity and velocity).
- Pavement design.
- Upstream and downstream embankment protection.
- Road signage.
Another design consideration is learning from floodway failures. Our Director, James Gleeson has first-hand experience from the flood damage caused to the John Muntz Causeway following an extreme rainfall event in 2017.
"The original rock protection at the abutments and approach embankments had been completely washed away, severely damaging the road embankment and relieving slab. The embankment protection was replaced with rock armour, a solution more suited to wharf structures, in an effort to combat the frequent, high intensity storm events".
As the effects of climate change increase frequency and intensity of rainfall events, we see floodway crossings playing a vital role in our road network, especially post disaster recovery, as a value for money solution.
As you can see, we are passionate about designing roads that tick all the boxes for road drainage solutions. If we can help you on your next project, please reach out.
Read more about what we can do in drainage design here.