MRWA Supplement to Austroads Guide to Road Design Part 5

Document No:  D15#370385
Revision:  2B
Date amended:  11-Mar-2019

The information below is intended to reflect the preferred practice of Main Roads Western Australia ("Main Roads"). Main Roads reserves the right to update this information at any time without notice. If you have any questions or comments please contact Zahirul Baten by e-mail or on (08) 9323 4160.

To the extent permitted by law, Main Roads, its employees, agents, authors and contributors are not liable for any loss resulting from any action taken or reliance made by you on the information herein displayed. 

Revision Register

Ed/Version Number​Clause NumberDescription of Revision ​Date
1AllGuideline Developed.​30-Jun-2015
​2​All​Guideline amended.​27-Jun-2017
​2AAll​​​Document hierarchy clarified.​07-Feb-2019
​2B​2.5.1​Drawing 200531-0010 amended.​11-Mar-2019


Table of Content

​MRWA Supplement to Austroads Guide to Road Design

Part 5: Drainage - General and Hydrology Considerations

This Supplement has been developed to be read in conjunction with Austroads Guide to Road Design (GRD) Part 5: Drainage – General and Hydrology Considerations (2013), a copy of which can be purchased via the Austroads website.

In Western Australia, Main Roads' policies, guidelines and standards take precedence over Austroads Guides and Standards Australia Standards. National Guides and Standards take precedence over International Guides and Standards, unless specifically stated otherwise.

This Supplement has the same structure as the equivalent Austroads Guide and only additional requirements, clarifications, or practices different from Austroads appear. Where appropriate, this Supplement may also contain additional sections and figures not covered by Austroads, but the numbering sequence found in the Austroads Guide remains. Figures and tables in this Supplement replace those with the same figure or table number in the equivalent Austroads Guide.




1.2.2 Definitions of Key Terms - Gutter Definition

The term gutter is locally used (and in particular within this supplement) to describe that part of the road cross section immediately adjacent to the kerb where runoff accumulates and flows. It does not imply that a specific concrete gutter is in use.


1.4 Inter-agency Relations

The Department of Water (DoW) is the primary authority providing guidance on the requirements for both state and local drainage systems. It is good practice for designers undertaking drainage works on behalf of road authorities to make early contact with DoW. Other authorities that commonly provide input on drainage designs include, but are not limited to, the Water Corporation and (depending on the project's location) Swan River Trust.


1.8 Use of Software

For review purposes Main Roads' preference is for designs to be undertaken using DRAINS (Watercom) or StormCAD (Bentley Systems) but this does not necessarily exclude the use of other programs.

Where drainage basins form part of the design then Main Roads support the use of PC Sump (Coffey Geotechnics Pty Ltd).




2.5.1 Protection for On-road Users

Main Roads' Standard Drawing 200531-0010 provides details of a traversable end treatment.


2.5.2 Floodways

Designers should also refer to Main Roads Floodway Design Guide.


2.5.4 Cyclists

Main Roads practice is that in general no additional provisions shall be made for cyclists during inclement weather. However in certain cases additional infrastructure may be required. Refer to Main Roads' supplement to Part 5A for further information.


2.6.1 Pedestrians and Cyclists

It is not Main Roads practice to direct path users through multi-use culverts.


2.7.3 Drainage Basins

The requirements for fencing around detention and retention basins should be in line with Main Roads supplement for the Guide to Road Design Part 5A.

The requirements for maximum depths and batter slopes in detention and retention basins should be in line with Main Roads supplement for the Guide to Road Design Part 5A.


2.7.4 Fencing

Particular requirements for fencing around detention and retention basins may be found in Main Roads supplement for the Guide to Road Design Part 5A.




3.2.2 Sea-level Rise

Refer MRWA Guide to Climate Change.

MRWA Guide to Climate Changes states that it is now a requirement that the implications of 300mm sea level rise (450mm for structures) be considered as part of planning, design and construction consideration for all rehabilitation and expansion projects near coastal areas.


3.2.3 Rainfall Pattern

Refer to MRWA Guide to Climate Change

MRWA Guide to Climate Change states that consideration for the impacts of climate change needs to be addressed in MRWA projects. 'A Guide to Flood Estimation' published by ARR (Australian Rainfall and Runoff) in 2016 provides practitioners, designers and decision makers with an approach to address the risks from climate change in projects. CSIRO and Bureau of Meteorology have developed an approach based on temperature scaling using temperature projections for the target years of 2030 and 2090. Further details can be found at the 'Climate Change in Australia' website. In general, CSIRO and BOM suggested that 5% increase in rainfall correlation with every 1.0 degree Celsius increase in temperature (based on RCP scenario of 4.5 for Southern and South-western Flatlands Cluster), however, IFD (Intensity Frequency Duration) rainfall data is recommended to be adjusted for future climate using the method outlined in Book1, Chapter 6 of ARR 'A Guide to Flood Estimation 2016'. 


3.3 Fauna Passage/Crossings

The technical requirements for fauna underpasses and fish passage can vary considerably. Designers should liaise with the Department of Environment Regulation and Department of Water for specific information relevant to the project.


3.4.3 Spill Management

Main Roads require that oil spill traps:

  • Incorporate a baffle or pipe overflow system to trap up to 3m³ of floating pollutants at each outlet during rainfall up to a 10 year ARI; and
  • Be capable of being maintained.


3.4.4 Typical Steps for Pollution Control and Treatment

Local practice for the control and treatment of pollutants should be in line with guidance from the Department of Water and Department of Environmental Regulation. Refer to Section 3.5.6 for references.


3.7.1 Backwater

Where not stipulated in the project scope, backwater limits based on design implications should be agreed with MRWA Waterways Section on a case by case basis.




4.5.2 Trafficability

Further to the requirement to limit total head at any point across the carriageway to 300mm, Main Roads practice is to limit the static head component to 200mm.


4.6 Selection of ARI

Main Roads typically design table drains and other associated open drains for urban freeways and highways to a 10 year ARI. In rural settings this design event is generally reduced to a 5 year ARI.

For design ARI's applicable to drainage basins Main Roads practice is as specified in AGRD 5A Section 7.2.7 (refer to page 144).


4.7.3 Drainage Basins

Drainage basins utilising the road formation should be designed such that there is no wetting of the pavement during the 20 year ARI event. There is no requirement for freeboard in this particular case (i.e. x = 0 in Figure 4.1[C] of AGRD Part 5).

Drainage basins utilising levee banks should have freeboard of 300mm between the design event overflow invert and the major event overflow (spillway) invert (i.e. x = 300mm in Figure 4.1[D] of AGRD Part 5).


4.7.4 Vertical Controls - Flood and Groundwater Levels

Where the flood immunity of a road is to be based on storms greater than the 20 year ARI, then the only design consideration is for freeboard of 300mm to be taken to the edge of shoulder.

Where the flood immunity is based on storms up to the 20 year ARI, then in addition to freeboard of 300 mm being applied to the edge of shoulder; zero freeboard should be applied to the subgrade surface for the 20 year ARI.

Where a road is built over an area subject to ponding, Main Roads typically applies a 300mm minimum freeboard from the bottom of the pavement layer.


4.7.6 Culvert Design

For large culverts (i.e.waterways area >3m2) and bridges, the required freeboard will typically project specific but in any case  should not be less than 300mm to the locations specified in Figures 4.1(J) and 4.1(K) respectively..

For culverts, Main Roads typically apply a minimum 300mm freeboard to the edge of shoulder during the applicable design event.


4.8.1 Drainage Construction Materials

Materials approved by Main Roads for use in culvert / piped network construction are as detailed in Main Roads Specification 404 - Culverts.


4.8.2 Recycled Materials

Main Roads do not typically support the use of recycled materials in drainage works.


4.8.4 Groundwater

The use of drainage infrastructure to artificially lower groundwater levels will only be considered where consultation with the Department of Water and Department of Environment Regulation has been undertaken.


4.8.5 Self-cleansing Sections

Main Roads endorse the self-cleansing velocities contained in AGRD 5A Section 6.5.4.




5.1 Maintenance Access and Location

For reasons of structural robustness and integration with the road embankment, Main Roads practice is to use in-situ concrete end treatments for culverts and piped network outfalls as per Standard Drawings.

Pre-cast end treatments can be used provided these conform to the general dimension (wing wall slopes, wing wall flare, apron length etc.) as shown on Main Roads standard drawings of culverts. Pre-cast end treatments shall be placed on a concrete bed of class N20 concrete.




6.2 Rainfall - Run-off Relationship

Main Roads Waterways Branch and Regional Offices have very comprehensive flood information which has been collected from past major flood events at major waterways around the state. This historical information is very useful as a comparison with derived values and may identify the need for further investigation to resolve any anomalies.


6.5 Catchments

Main Roads has a library of mapping and aerial photography. Designers should contact Main Roads' Survey and Mapping Branch to identify what information may be available.

Guidance on the purchase and use of aerial photography can be found in the document Aerial Photography.


6.6.1 Rational Method

For detailed information relating to use of the Probabilistic Rational Method applicable to rural Western Australian conditions refer to AR&R Book IV Section 1.4.7.


6.7.3 Run-off Coefficient

MRWA advise reference to Argue John R.; Storm Drainage Design in Small Urban Catchments, A Hand Book for Australian Practice, Australian Road Research Board, Special Report No.34, 1987.





Main Roads' supports the use of PC Sump software for the sizing of infiltration basins in WA.