Frequently Asked Questions - Supplement to Austroads Guide to Road Design Part 5, 5A, and 5B

The new Road Planning and Design supplements to Austroads Guide to Road Design Parts 5, 5A, and 5B replace the withdrawn Road Drainage Manual, and facilitate the continuing harmonisation of Transport and Main Roads Technical Publications to Austroads guides, while allowing for Queensland specific needs. Furthermore, the new supplements capture recent advancements in the application of more robust hydrology methods in different circumstances recommended by Australian Rainfall and Runoff. 

The following are frequently asked questions that have been compiled for practitioners to provide additional context and information. 

Why Transport and Main Roads is withdrawing the Road Drainage Manual and producing supplements to Austroads Guide to Road Design Parts 5, 5A, and 5B?

Transport and Main Roads is committed to national consistency wherever possible and does not intend to duplicate information already available in the Austroads Guide to Road Design. Transport and Main Roads has produced supplements to guide practitioners to Austroads Guide to Road Design Parts 5, 5A, and 5B where relevant, and address aspects that are unique to Queensland practice. The supplements include advice that may:

• vary from national practice due to local environmental conditions (e.g. soil types, climate)
• vary from national practice due to differing funding practices
• take account of research undertaken by the department or other jurisdiction, superseding what is documented at the time of publication by Austroads
• not feature enough detail in the Austroads guides, and
• satisfy local legislative requirements.

What does this replacement mean in TMR projects?

Since the Road Drainage Manual has been withdrawn, it should not be referred to in design, construction, and/or delivery of the Transport and Main Roads projects. The supplements to Austroads Guide to Road Design Parts 5, 5A, and 5B are the replacement to the Road Drainage Manual and are now the main point of reference for anything relevant to drainage design.

What is the difference between the supplements to Austroads Guide to Road Design Parts 5, 5A and 5B and Road Drainage Manual?

Road Drainage Manual was the Transport and Main Roads standalone manual used for those involved in the planning, design, operation, and maintenance of road drainage infrastructure in Transport and Main Roads projects.

The new supplements to Austroads Guide to Road Design still serve as the key departmental point of reference for anything road drainage related, however are now 3 separate documents that have been harmonised with Austroads Guide to Road Design Parts 5, 5A, and 5B. Therefore, the supplements should always be read in conjunction with mentioned Austroads Guide to Road Design Parts 5, 5A, and 5B. The content in the supplements takes precedence over the contents in the Austroads Guide to Road Design.

What are the key changes in the AGRD Parts 5, 5A and 5B?

Chapter 6 (Hydrology) in Austroads Guide to Road Design Part 5 has been extensively revised. These changes are mainly due to the new approaches taken in flood estimation by the Australian Rainfall and Runoff. The latest edition incorporates the latest flow estimation techniques from Australian Rainfall and Runoff released in 2019. There are minimal changes in Parts 5A, and 5B. Please refer to the change log and webinars published in the Austroads website (below link) for further details.

https://austroads.com.au/publications/road-design/web-agrd05a-23

What are the key highlights of the supplements to AGRD Parts 5, 5A, and 5B?

Whilst the supplements should carefully be read for any applicable section in road drainage projects, below are some key highlights.

• Introduction of a new departure / relaxation process for critical drainage parameters that exceed the Normal Design Domain. (Part 5)
• Considering a range from 0.8m to 2.0m increase for sea level rise and all tidal levels to allow for projection to 2100. (Part 5)
• Accepting freeboard design criteria stated in Queensland Urban Drainage Manual for basins in addition to those mentioned in the Austroads Guide to Road Design. (Part 5)
• Limiting applicability of Rational Method. (Part 5) 
• Setting allowable afflux / impact targets on a project-by-project basis in Transport and Main Roads projects considering various parameters. (Part 5)
• Targeting to achieve minimal / no impact for design flood events from 63% to 1% AEP, and to avoid excessive or severe impacts for design flood events between 1% and 0.05% AEP (or more extreme events if deemed appropriate). (Part 5)
• Discouraging application of scuppers on bridges. (Part 5A)
• Applicability of the flow limit criteria in major storm events to both sag and on-grade pits / locations. (Part 5A)
• Considering 375 mm diameter as the minimum diameter of any pipe for the longitudinal drainage system and 450mm diameter for the transverse drainage system. (Part 5B)
• Considering 375 mm high as the minimum height of the box culvert and 300 mm in constrained situations where all reasonable attempts to fit a 375 mm high box section have failed. (Part 5B)

Is the newly introduced departure / relaxation process similar to the Design Exception / Extended Design Domain process?

No, the Design Exception / Extended Design Domain process does not apply to drainage criteria at this stage. Practitioners should follow the departure / relaxation process mentioned in the supplement (Section 1.6, Part 5) when critical drainage parameters exceed the allowable range. This departure / relaxation process also does not apply to flood criteria such as afflux and flood immunity.

What if aquaplaning exceeds the allowable limit? Should I go through the Design Exception / Extended Design Domain process or relaxation / departure process?

Aquaplaning issues are mainly triggered by the road geometry, therefore the Design Exception / Extended Design Domain process in line with the Road Planning and Design Manual Volume 3, Part 1 should be followed.

What are the Rational Method applicability considerations?

The new supplement (Part 5) now defines applicability of the Rational Method as the primary method for flood estimation based on the time of concentration only and the reference to catchment area is no longer applicable. In other words, application of this method is only supported for small rural catchments, or small developed / artificial catchments in the road corridor with times of concentration of up to 30 minutes.

Despite the above allowance, the Rational Method shall not be used in road catchments where timing, storage, volumes, and/or hydrographs are of importance. Sizing a basin, design a long and continuous drainage network (pipe / channel), and/or where inflow(s) required to be calculated are some examples for these situations.

Application of this method for validation purposes is still supported for longitudinal drainage or for small rural catchments with catchment areas of up to 25 km² and/or corresponding times of concentration of less than two hours.

What are other recommended hydrological design methods by Australian Rainfall and Runoff (ARR) for specific road design applications?

The other recommended hydrological design methods by AAR are:

• Regional Flood Frequency Estimation (RFFE)
• At-site Flood Frequency Analysis
• Runoff routing methods
• Direct rainfall (rainfall-on-grid)

There are limitations in application of each method. Each method should be used in specific circumstances and requires a high level of expertise in hydrology.

Where can I find more details regarding design flood estimation methods?

Please refer to, AGRD Part 5, Chapter 6 or the ARR, Book 1, Chapter 3 for further details regarding the design flood estimation methods.

Where can I go for more information?

For all technical enquiries the relevant departmental contacts have been listed in page iii of the supplements to AGRD Part 5, 5A, and 5B.

For general enquiries or feedback please get in touch with Road Design Unit at roaddesignstandards@tmr.qld.gov.au.