Conveyance and sediment management. Is it worth it?

Roca, M. and Sayers, P. and Bast, H. and Barlow, J. and Hensman, L. (2009) Conveyance and sediment management. Is it worth it? In: 44th Defra Flood and Coastal Management Conference, 30th June - 2nd July 2009, Telford, UK.

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Abstract

The Environment Agency is currently developing System Asset Management Plans (SAMPs) for each of its Flood Risk Management Systems (FRMS). Across the Agency, the expenditure on conveyance and sediment management is significant. The benefit achieved for this expenditure is however not known and difficult to determine. Typical approaches to the assessment of flood risk fail to reflect changes in the conveyance and or sediment regime of a channel. As a result, they often suggest that channel maintenance contributes little (or unknown) flood risk management benefit. This is a serious omission and limits the development of effective and efficient management policy and investment planning. This paper presents an innovative broadscale approach to explicitly estimate the benefit of channel maintenance activities (defined as actions taken to manage either the conveyance or sediment within a watercourse). The “benefit” is defined as the change in the economic damage (to residential and commercial properties) expressed as the Expected Annual Damages (EAD) arising through different management options. The method utilises expert rules to translate postulated changes in the approach to management (of both the channel vegetation and river sediment) into changes of conveyance. Credible but broad scale methods based on the Conveyance Estimation System (CES) are then used to estimate the likely impact on these changes in conveyance into changes in river water level for a range of return period storm events. The impact of the change in water level is assessed translated to a change in flood risk through the so-called RASP High Level risk analysis tools and expressed as a change in the EAD. Validation of the method is presented through three pilot sites from different Environment Agency Regions (Anglian, Thames and North West Region) and comparison with more detailed RASP analysis. The validated method is then applied at a national scale to 85 catchments of England and Wales to provide an assessment of the national benefit derived through the “Business as usual” management scenario as well as changed conveyance management activities.

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