A steepness index unit volume flood hydrograph approach for sub-daily flow disaggregation

The estimation of sub-daily flows from daily flood flows is important for many hydrological and hydraulic applications. Flows during flood events often vary significantly within sub-daily time-scales, and failure to capture the sub-daily flood characteristic can result in an underestimation of the instantaneous flood peaks, with possible risk of design failure. It is more common to find a longer record of daily flow series (observed or modelled using daily rainfall series) than sub-daily flow data. This paper describes a novel approach, known as the steepness index unit volume flood hydrograph approach, for disaggregating daily flood flows into sub-daily flows that takes advantage of the strong relationship between the standardized instantaneous flood peak and the standardized daily flood hydrograph rising-limb steepness index. The strength of this relationship, which is considerably stronger than the relationship between the standardized flood peak and the event flood volume, is shown using data from six rivers flowing into the Gippsland Lakes in southeast Australia. The results indicate that the steepness index unit volume flood hydrograph approach can be used to disaggregate modelled daily flood flows satisfactorily, but its reliability is dependent on a model's ability to simulate the standardized daily flood hydrograph rising-limb steepness index and the event flood volume. Copyright © 2007 John Wiley & Sons, Ltd.