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= applies to MIDUSS Version 2 (current release)
M98 = applies to MIDUSS 98
MIDUSS lets 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 a, b, and c
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
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
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.
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.
(c) Copyright 1984-2019 Alan A. Smith Inc.
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