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Frequently asked questions

 

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Storms

 
V2 = applies to MIDUSS Version 2 (current release)
M98 = applies to MIDUSS 98

MIDUSS let’s you use five different types of single event storms. These include the Chicago hyetograph, the four Huff quartiles, the Canadian Atmospheric Environmental Service (AES) storm, a number of storms defined by the mass rainfall distribution curve (*.mrd files) and finally a historic storm.   The Edit Storm tool lets you design your own mass rainfall distribution curve.

The Chicago parameters can be found using the IDF-Curve Fit tool. It allows you to enter either depth or rainfall intensity for a range of time intervals to describe the Intensity-Duration-Frequency of rainfall. The program then automatically optimizes the ‘, ‘, and ‘ parameters to give the best fit between an IDF equation and the observed rainfall.

The r parameter defines the fraction of the storm duration at which the peak rainfall intensity occurs. It depends to some extent on the nature of the meteorology defining the storm. Typically in mid and eastern Canada a value of r between 0.4 and 0.5 is appropriate. On the west coast a value greater than 0.5.may be better.

If you plot the hyetograph from a Chicago storm with different values of r your will see that it shows the same distribution of rainfall intensity but with the peak rainfall skewed to the left or right. If the value of r is greater than .5 it is clear that the peak intensities will fall on ground that is already quite saturated. It follows that the peak of the runoff hydrograph will be larger than for a storm in which r is significantly less than .5. This is particularly true for catchments which have a significant fraction of pervious ground.

Normally a timestep of not less than 5 minutes should be used to define a storm. This is because the characteristics of the storm are based on data that has been recorded at time intervals not less than 5 minutes. For a long storm you may find it convenient to use a larger timestep of 15 minutes or perhaps even 30 minutes if the duration of the storm is much in excess of 24 hours.

In the Chicago storm option of the storm command you can define a multiplier equal to 1, 2, 3 or more which defines the time interval for which the rainfall intensities are calculated. For instance, if the timestep is set to 5 minutes for short times of concentration you can set the multiplier to 2. The rainfall intensities are computed for an interval of 10 minutes. This will significantly reduce the maximum rainfall intensity. You can experiment with this in the Storm command.

This depends to some extent on the duration of the storm and also the accuracy with which the data has been recorded. Typically for a historic storm of 24 hours a timestep of 15 or 30 minutes is not unreasonable. If you are extracting a historic storm from rainfall records you may be limited to 1 hour rainfall measures. This may be rather course if you are modelling catchment areas which have times of concentration significantly less than 1 hour. In that case you could use a timestep of 15 minutes and enter the intensity for each hour as four equal values.

MIDUSS is predominantly a program for designing stormwater management facilities. Continuous simulation is more appropriate for quality modelling and as a screening model to identify periods of rainfall that represent a critical or extreme condition. Also, continuous rainfall records may extend over many years during which land use may vary significantly. In MIDUSS, one of the advantages is that you can easily compare the runoff for pre and post development conditions. For that reason, if you wish to represent historic conditions it is best to extract a finite period of rainfall that represents an extreme event within the long period of record.


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