Methodology
Rainfall-runoff model
In case when observed flow data is not available or is inadequate, the alternative is to obtain flood estimates using a rainfall-runoff model. In general, rainfall-runoff models provide flood runoff estimates for a given rainfall input. Depending on the type of rainfall input, there are two possible approaches involving rainfall-runoff modelling:
Design storm approach is the approach in which a design storm represents the input for rainfall-runoff model, and the design flow is the model output. Design storms themselves are obtained by statistical analysis of observed rainfall. The key issue in this approach is the assumption that the frequency of the design flow is equal to the frequency of the design storm. However, having in mind that the rainfall-runoff relationship is neither linear nor straightforward, such an assumption is not always very realistic.
In the observed storms approach rainfall-runoff model is applied to observed rainfall data in order to obtain corresponding simulated flows. The simulated flows are then treated as a substitute for observed flows, and they are statistically analyzed in order to obtain flood-frequency relationship. This approach can be event-based, when separate storms are used as input to runoff models, or continuous, when the complete rainfall record is used for continuous runoff modelling. The disadvantage of the event-based rainfall-runoff modelling is that it requires estimation of initial conditions at the beginning of each event, while continuous rainfall-runoff modelling is quite demanding concerning data and model calibration.
