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 Minhdu Nguyen by e-mail or on (08) 9323 4541.
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.
The design flood estimation shall be carried out in accordance with Australian Rainfall And Runoff, A Guide to Flood Estimation 2001. Guidance on the use of the probabilistic Rational Method and the Index Flood Method is given in Chapters 4 and 5 respectively.
Catchment characteristics such as size, slope, shape, soil type and vegetation cover should be taken into account when selecting the method for flood estimation. For example, some techniques recommended by the ARR for flood estimation were based on large catchment areas with steep slopes and rocky impervious soil. Use of these methods for long narrow shape and flat slope catchments with pervious soils can result in exceptionally high peak flow figures. Therefore catchment features together with anecdotal evidence and historical flood observations shall be considered and compared with computed design flows.
Information regarding the past flood event can be gathered using evidence such as debris marks on trees and watermarks on permanent structures like walls, retaining walls, bridge piers etc during site visits.
Further information on past events can be gathered from local people. This information should be used carefully because memories are often not clear. People can often provide photos or identify a point on the ground or feature where the flood level peaked.
Misleading flood information concerning the depth over a road sometimes occurs too. People normally describe depths from some feature such as a guidepost located at the edge of the road. However the depth at the centreline from which floodway analysis is undertaken is about 150mm less than at the road edge(depending on road width and crossfall).
Main Roads has a library of mapping and aerial photography. Prior to the purchase of this material for use on Main Roads projects, designers and Project Managers should contact the following persons to identify what information Main Roads currently has: Senior Mapping Surveyor for aerial photography on 9323 4669 Data Manager for mapping information on 9323 4655
Guidance on the purchase and use of aerial photography can be found in the document Aerial Photography.
In some sections of the Western Australia the contour interval on the mapping is large which can make the catchments difficult to accurately define. In these situations it is important that the designer undertake a detailed site investigation as described in Section 1.2 to confirm where possible the magnitude of the design flood.
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.
References Australian Rainfall And Runoff; A Guide to Flood Estimation, Volume 1, 2001. Australian Rainfall And Runoff; A Guide to Flood Estimation, Volume 2, 1987.