General Standards
Table of Contents
1. Chapter 1 of 6 GENERAL STANDARDS
1.1 General
There are a number of rainfall-based methods of peak flow estimation which are widely used in Australian practice. They differ fundamentally in the approach adopted in converting the design rainfall to flows. Analysis of rainfall data from daily and pluviography rainfall stations throughout Australia has been carried out by the Bureau of Meteorology and a method of extracting the required information for specific areas has been formulated and presented in Australian Rainfall and Runoff (ARR).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.
1.2 Site Investigation
Detailed site investigation plays a major role when obtaining design flows.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 cross fall).
1.3 Catchment Analysis
Catchments can be defined from either topographical mapping or from stereoscopic aerial photographs.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.
1.4 Flood Information
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.1.5 Documentation
Accompanying any design flows for Main Roads shall be a brief report (the report of design flows shall be incorporated within the design report for the project if applicable, or submitted separately for review) containing as a minimum the following information:- How the design flows were calculated e.g. the Index Flood Method was used.
- Justification for the method used.
- What background information - scale of topographically mapping, contour interval etc.
- Any site observations made that were used to justify reduction / increase in design flows or confirmation of design flows.
- Any assumptions used for the determination of the design flows.
- Plan / copies of aerial photographs showing the catchments, the proposed alignment and road chainage system.
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.