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Design Flood Estimation with Consideration of Structure Blockage

Author(s): W Weeks; Je Ball; M Babister; M Retallick

Linked Author(s): Monique Retallick, Mark Babister

Keywords: Floods; Blockage; Design; Risk; Guidance

Abstract: As part of the development of the revised edition of Australian Rainfall and Runoff (ARR) ,existing approaches presented in ARR have been carefully reviewed considering both international and Australian practice, recent research and on-going data collection. Arising from this review, it has become apparent that the influence of blockage at structures on the design flood levels upstream of the structure and on design flood flows downstream of the structure has been neglected. Review of international practice has shown that the situation is similar elsewhere. Blockage can sometimes have a severe impact on the capacity of drainage systems. Determination of likely blockage levels and mechanisms, when simulating design flows, therefore is an important consideration in quantifying the potential impact of blockage of a particular structure on design flood behaviour. Blockage of drainage structures is a subject where a range of advice has been provided in different guidelines. Many drainage guidelines do not mention blockage at all, so it is ignored in many cases. In other situations, especially where there has been an observed or perceived blockage problem in historical flood events, blockage may be specified for extreme conditions. Other guidelines provide inconclusive advice. In fact, the actual evidence for the impact of blockage on design flood events is very limited and the evidence for any clear quantitative design advice is lacking. Outlined herein is development of a design guideline for consideration of blockage in the planning and design of culverts and small bridges crossing waterways. The guideline described is an attempt to provide an approach that allows a consistent analysis methodology, while not becoming too extreme in either direction since there are risks in either under-or over-estimating the influence of blockage on the design flood.


Year: 2015

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