Extreme rainfall events

This service focuses on extreme rainfall events and is particularly relevant for flood risk assessment and peak overflow rates.

Single extreme rainfall events (storms) are used for design and evaluation of sewer systems and other hydrological components in the urban environment. In event-based simulation, either an idealized design storm or (less commonly) a historically observed storm is used as input to simulation of sewer system response with focus on peak discharge and surface surcharge.

In the case of design storms, the mean storm intensity is often obtained from an Intensity-Duration-Frequency (IDF) curve, which is a statistical characterization of observed short-term rainfall extremes.

IDF-curves and design storms

In the SUDPLAN tool, extreme value analyses of high-resolution RCM results will be performed to estimate the future changes of short-term rainfall extremes. To reduce statistical variability and obtain a robust result, the change will be calculated for ranges of both durations and return periods and finally approximated by a linear function. The user may retrieve the estimated change for a specific combination of duration and return period, or for selected ranges of durations or/and return periods, in both tabular and graphical format.

Users may further upload local design storms, representing idealized rainfall event time series used for design and evaluation of sewer systems. Based on the results of the IDF-analysis, the design storm may be re-scaled to reflect the expected future change (Figure 1).

Urban downscaling of intense rainfall
Figure 1. Example of an existing (reference REF) and a future, re-scaled (end of century EoC) design storm to be used in sewer system simulations. Enlarge Image

Dynamic design storm generator

It is well known that speed and direction of a passing storm has a strong influence on the resulting runoff in an urban catchment. A storm moving in downstream direction may generate a much higher peak runoff than a storm moving upstream. This fact is however seldom taken into account in urban hydrological engineering, but the storm is assumed to be static, i.e. remain stationary over the catchment during the entire event. This may result in too low estimates of flood risks and other problems.

In the SUDPLAN tool, besides the above traditional, static design storm functionality, users will be able to simulate dynamic, moving design storms and generate consistent rainfall time series in selected locations (Figure 2). Storm direction is selected by the used; storm speed may be selected but also a default value is provided.

Figure 2. The principles for the dynamic design storm generator, illustrated by a storm coming in from the east. The user specifies direction and peak intensity for present conditions. SUDPLAN will be able to simulate a future peak intensity based on a climate scenario.