### Approaches to flood estimation

In most general terms, there are two alternative ways to obtain design flood estimates and to relate them to some probability (frequency) of occurrence:

• statistical analysis of observed stream flow data, and
• rainfall-runoff modelling.

Statistical analysis of observed flows is the basic approach for estimation of flood-frequency relationship when adequate data on observed flows are available. If only a short record of streamflow is available, it is sometimes possible to establish a regional relationship with flows from neighbouring stations and to use such relationship to extend the existing short record in order to perform statistical analysis. Application of flood frequency analysis is limited to gauged catchments, i.e. to gauged stream profiles. When flow rates of certain probability of occurrence are required at ungauged sites or along a stream (e.g. for flood zone mapping), other methods are needed for flood estimation.

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.

## Choice of an approach

The choice of an approach and a method for flood analysis and estimation depends on several factors, such as available data, importance of the project, and available resources. A particular method may be good for one case and quite unsuitable for another.

Flood frequency analysis is considered unreliable if the flood-frequency relationship should be extrapolated beyond the range of observations, and especially if it is based on very short flow record. Lack of detailed information on land cover or geology within a catchment would prevent use of more complex rainfall-runoff models. More important projects generally require more reliable flood estimates; it is generally believed that more sophisticated and comprehensive methods improve the reliability of results, but the models involved in these methods are very difficult to calibrate and may be very sensitive to changes in input and parameters. Financial resources, available time and available expertise are also factors affecting the choice of the method for flood estimation.

Finally, it should be noted that all flood estimates are burdened with some amount of uncertainty, regardless of the method of their estimation.