Document No:  D11#194387
Revision:  2A
Date amended:  21-May-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 Albert Wong by e-mail or on (08) 9323 4153.

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

Revision Register

Ed/Version Number Clause Number Description of Revision Date
1AllGuideline Developed23-Jan-2003
2AllGuideline Amended.11-Aug-2011
​2AHeader​​Contact person changed.21-May-2019​

Table of Content


Floodways are commonly utilised in rural roadways with relatively low traffic volume and where it is impractical or uneconomical to construct a bridge or culvert.

A typical vertical alignment of a floodway is shown in Figure 1 and consists of a horizontal section of length L, taken as the distance between the intersection points of the adjoining sag curves. 

Typical Floodway Long Section.GIF 

Figure 1 Typical Floodway Long Section (AUSTROADS 1990)

The design of floodways often requires specialised inputs from experienced practitioners and the design engineer should refer to design procedures outlined in the Main Roads Floodway Design Guide


Information on pavement and embankment protection for floodways is detailed in the Floodway Design Guide

Pavement protection should be extended beyond the floodway design length as shown in Figure 2. Embankment batter protection shall be provided to the serviceability level or based on a risk assessment of the project site. 


Pavement and Embankment Protection for Floodways.GIF 

Figure 2 Pavement and Embankment Protection for Floodways


Details of the different types of floodway protection used by Main Roads are in the following drawings:

Drawing No 

Drawing Title

 9831-5499  Floodway Approach and Floodway Type 1 Typical Cross Sections
 9831-5500  Floodway Types 2 & 3 Typical Cross Sections
 9831-5501  Floodway Types 4 & 5 Typical Cross Sections
 9831-5502  Floodway Types 2, 3, 4, & 5 Miscellaneous Details



The design objective is for trafficability depths of 200mm over crown of the floodway. To allow for this the floodway depression should be at least 500mm in depth which allows for 150mm of freeboard and crossfall of about 150mm. The floodway approaches will have to be raised if the depression cannot otherwise be provided and there is no skimping on this unless the design is such that headwater depths much less than 300 mm are used.

Floodway levels should be designed so that the depth of flow over the floodway is consistent over the complete floodway length.


Road embankment on the approaches to floodways are susceptible to scour and their length should be kept as short as possible. The angle of the ramp should therefore be as high as possible consistent with other geometric requirements. The preferred length of the ramp is about 80-90 metres.


Sag vertical curves should be kept short in order not to encroach on the waterway in the floodway depression and not to increase the raised portion of the ramp more than is strictly necessary. A 40m vertical curve is considered satisfactory. For such a vertical curve, the radial acceleration at 130 km/h is not more than 0.039 for ramp angles less than 1%.

The crest vertical curves will need to provide adequate visibility. The desirable sight distance is zero object height so that a washout or scour can be seen on the floodway approach within the Stopping Site Distance (SSD). Ramp grades of 0.8% to 1% are typically used. 

It should be noted that the SSD for 0.2m object height will always be satisfied for ramp angles not greater than 1.1% for speeds up to 130 km/h for any vertical curve length.

There are three possible shapes of the crossing:

  • the normal crown section of the road;
  • one way crossfall against the flow; and
  • one way crossfall with the flow.

Main Roads preferred practice is to use a normal crown section. One way crossfall will be considered subject to approval by Main Roads Senior Waterways Engineer. Refer to the Floodway Design Guide for further information. 



Floodways or floodway approaches should not be located in curved sections of road (i.e. either horizontal curves or plan / superelevation transitions).


Floodway design is such that they remain trafficable for all flows up to the design year and that flows at least up to 1:50 year frequency can be contained within the floodway depression without spilling out elsewhere across the road. The length of the floodway is generally taken to be the length between the intersection points of sag vertical curves, no account being taken of the extra capacity of floodway approaches or loss of capacity due to the sag vertical curves. Where only nominal culverting is provided, no allowance is made for capacity of the relief culvert in determining the length of the floodway.


A relieving culvert is sometimes placed at the lowest point of the natural surface to take the perennial or low average recurrence interval flows. This allows all the water to drain away from the road and avoid having water standing upstream of the floodway (below floodway level), softening the subgrade and leading to maintenance problems. The invert level of the culvert should be such that the culvert will run full before water will flow over the road.