A practical approach to uncertainty in conveyance estimation

Water level prediction is essential for the management of flood management risks through strategic planning, risk mapping, scheme design, forecasting and warning, and channel operation and maintenance. Flood level predictions based on existing software provide no indication of the associated uncertainty. This uncertainty arises from many factors including approximations to the fluid mechanics, natural variability in river resistance and the use of judgement or experience in applying models. These all produce differences between assessment of river channel capacity and its “true” value. In 2001, the Environment Agency commissioned the development of a new conveyance estimation system to reduce the uncertainty associated with flood level predictions. A key component of this work was to quantify the uncertainty in water level for a given flow rate, and present it in a manner which can be readily interpreted by the user and enable better, more informed decisions. This paper provides insight int o the selected approach, essentially driven by the identified end-users and their needs, together with a sensitivity analysis of the parameters contributing to the uncertainty.