Design rainfall theory and developing IDF curves

Step-by-step guide

Designing hydraulic drainage systems based on rainfall theory involves working with historical data that tracks the intensity, duration, and frequency of rainfall in a given area.

IDF curves: graphical tools that describe likelihood of a range of extreme rainfall events.

A typical IDF curve for single design storm shows intensity over time:

Similar to real rain events, an IDF curve starts from 0, builds to maximum, then fades to 0.

Three elements of a storm definition

Return is how often the type of storm occurs.

• Example is 1-in-30-year storm: on average, occurs once every 30 years.
• Average means that two 30-year storms can occur within same year.
• With climate change, severe storms are becoming more common. Storms that happened once every 30 years are now happening maybe once every 10 years.

Duration is how long the storm lasted—here, 60 minutes long.

Shape of the IDF curve is determined by the rainfall theory applied.

• This graph uses obsolete theory in the UK, Flood Studies Report Winter Profile, but its shape is a typical bell-shaped curve.

Next example shows three storms of three different durations, all representing the same return—a 1-in-30-year storm.

• Shows that shorter storms are generally more intense than longer storms, whereas longer storms have greater volume.
• Here, 24 mm in 30 minutes equates to an average intensity of almost 50 mm/hr.
• For longer duration, 37 mm falls in two hours. Greater volume of rainfall over longer period means lower average intensity of 18.61 mm/hr.

When running design analysis, important to represent variety of storms, in case drainage systems are susceptible to short, intense storms or longer, lower-intensity storms.

IDF table

Used to represent variety in rainfall duration and intensity.

Example: the 60-minute storm from the IDF curve has a depth of 31 mm in one hour. In an IDF table, that same storm represents one point on the curve at 31 mm and 60 minutes for average intensity of 31 mm/hr.

Adding storms adds more points that build up to a complete curve.

For this return of 30 years, as the duration gets longer, the intensity gets lower:

Several rainfall theories are built into InfoDrainage that can be applied.

If you are working in a world region not covered by one of these theories, an IDF curve can be developed by inputting several return periods.

For example, these three IDF curves represent return periods of 2, 30, and 100 years.

Note that a higher return period is not a multiple of a lower period. Example: a 10-year storm is not double the intensity of 5-year storm.

IMPORTANT: with increased return, storms do increase, but they also retain same shape. This translates into series of IDF curves, each for a particular return:

Rainfall theories

The shape of a rainfall event is defined by the applied rainfall theory.

Theories built into InfoDrainage that impact drainage design include: the Desbordes, NOAA, SCS or Conservation Service, and Chinese design.

Note: the Chinese design has constant intensity throughout storm; therefore, it has no curved shape.

Theories are represented using a dimensionless profile—graphed as the dimensionless duration against the dimensionless cumulative depth.

Allows representation of storms of any return and duration, yet they retain same shape in the curve.

Example below used in Germany—could be applied to countries in vicinity, such as Poland.

Alternatively, you can develop your own curve, such as this one, generated from the first bell-shaped curve shown previously. Essentially, the return and duration are not included:

In summary, to represent storms outside of available existing theories, two elements are needed:

• IDF curve, which defines return and duration.
• Cumulative depth percentage against time percentage, which is dimensionless and defines the shape of events.