Rainfall Intensity - Hydraulics and Hydrology - Forum - Hydraulics and Hydrology - Bentley Communities

# Rainfall Intensity

## Hydraulics and Hydrology

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#### Rainfall Intensity

• I would do modeling for storm system and would use CivilStorm or SewerGEMS, is anybody able to explain me how to generate the Rainfall Intensity (mm/hour) to use it in the model (or generate hyetograph)? the method suggested is "Chicago 100 Yr storm Method" so how can I generate the rainfall intensity for 24 hours and use it in the model??

as I know the formula is

where

i       =     average rainfall intensity (mm/hr or inch/hr)
td
=     storm duration (minutes)
a,b,c      =     constants dependent on the units employed and the return frequency of the storm.

is that what I should use? if yes how can I obtain those parameter? is there any xls file developed to help? any example??

Note: The Rainfall data is attached

The first thing that you have to do is to convert the table of annual maximum (statistical) values of rainfall depth into Intensity Values (mm/hr) just divide the total depth (precipitation) by the rainfall duration (i.e. for 1971 during 1 hr you have a rainfall max. value of 16.3 mm, then intensity for this year and this specific duration will be 16.3 mm/hr).

I am not sure about the negative values in your table. It must be or negligible rainfall depth or measurement data errors.

Once you have a table of annual maximum Intensity values (where the columns will be duration ranges/steps), you can perform a statistical analysis in order to calculate parameters such as the mean and standard deviation for each duration range.

After that, you can perform an analysis using the Gumbel distribution (a valid option but not the only one) that is used to model the distribution of the maximum of a number of samples of various distributions (in your case maximum rainfall intensity values for a sample obtained during 23 years).

I won´t go in depth into Gumbel Distribution parameters, just say that in hydrology the Gumbel distribution is widely used to analyze such variables as monthly and annual maximum values of daily rainfall. If required I will be  able to attach and excel spreadsheet with a numerical example and calculations to obtain manually IDF tables or equations for different return periods (years) that are associated to Frequency parameter (1/T).

At the end, with a table of Duration vs. (statistical) Rainfall Intensity Values for different Return Periods you can use CivilStorm or SewerGEMS (the software will be able to generate graphs with IDF curves or Intensity Equations) and then use it for example to calculate runoff flow (peak) under modified rational method.

I expect this explanation could be useful for you.

Regards from Colombia,

Juan Carlos Gutierrez Araujo.

Bentley – Latin America

Juan Carlos Gutiérrez Araújo I.C. M.Sc.

Regional Sale Engineer - Bentley Latin America

Juan Carlos gives some good info about converting recorded rainfall data into IDF curves, but I think that has already been done and is given in table 2b in the file you attached.

Do you want to use the attached data to create a design storm hyetograph (rainfall versus time)?

In general here are your options to do that in SewerGEMS:

- Add a new 'Time-Depth' or 'Time-Intensity' storm event group, then add a new storm event from a 'Dimensionless Curve'. The dimensionless curve you select should be relevant to your local area, and the total storm depth for, say, a 100 Yr, 24 Hr storm can come from the attached data

- Add the IDF data you attached (from table 2b) to a new user defined IDF Table. Then create a new Time-Depth event 'From IDF'. This will build up a hyetograph using the IDF data as input.

I don't know what the "Chicago 100 Yr Storm Method" is, but it is possible that they are referring to a dimensionless curve from the Chicago area. We have some built in curves from 'Bulletin 71' (www.isws.illinois.edu/.../ISWSB-71.pdf) which may be appropriate.

Regards,

Mal

• Mal,

you are at the point, I have already the IDF in table 2b, and I would go for the second option you suggested, would you please explain and drive me after I Add the IDF data you attached (from table 2b) to a new user defined IDF Table; how to create a new Time-Depth event 'From IDF' and how hyetograph will be built???

for Chicago Method; here is what I mean

http://www.miduss.com/theory-Derivation-Chicago-Storm.htm

• Any suggestion how to do that??? is there any excel sheet could help?

• Hi Juan,

This is quite interesting discussion.