
IDF Curve Fit: Computes
Chicago storm parameters ‘a’, ‘b’ & ‘c’ for observed data.

Time of Concentration:
Estimates the time of concentration at various locations in
the drainage network.

Roughness Height:
Converts roughness element height to a Manning 'n’ value.

Edit Storm: Lets you modify an
existing Mass Rainfall Curves file or create a new one.
IDF Curve Fit
The IDF Curve Fit tool manipulates data describing an
IntensityDurationFrequency relates for a particular
geographical locality and can be used in two modes:
To compute the ‘a’, ‘b’ and ‘c’ parameters of a Chicago
hyetograph that most closely approximates a set of observed
rainfall data.
To compute the IDF curve for usersupplied values of the three
coefficients and compare this
with observed data.
The mode is selected by checking the ‘Optimize’ check box on the
form or clearing it to simply compute the curve for specified
values of ‘a’, ‘b’ and ‘c’.
The above display shows data that has been entered for the first
“Optimize’ mode of operation. In the grid on the left for
Time, Depth and Intensity a column of time intervals is
displayed as shown. These values can be customized if
desired.
For any time interval the rainfall can be defined either as a
total depth of rainfall or as an average intensity over the time
interval. Entering either value automatically calculates and
displays the other. The number of data pairs is automatically
displayed in the top of the form and not every time interval
need be entered.
When the [Optimize] button is clicked several pieces of
information are displayed:
The optimal values of the three parameters
The computed values of Depth and Intensity for each time
interval. These are shown in the right hand grid.
A ‘loglog’ graph of both observed and computed values is
displayed. A typical result is shown below.
Time of Concentration
With few exceptions, peak runoff will occur when the entire
catchment area is contributing to the outflow. Thus the storm
duration should be long enough for the runoff from the most
remote area – in terms of time of travel – to reach the outflow
point. This is commonly referred to as the Time of Concentration
Tc.
The time of concentration is calculated as the sum of up to
three components of travel time. These are:
Flood wave travel time of overland flow
Travel time in relatively small collector channels or gutters
Travel time in a storm conduit such as a circular pipe or a
channel of general trapezoidal crosssection.
For the overland flow you can select either the Friend’s
equation or the Kinematic equation.
Each of the three components requires entry of data to describe
the length, gradient and roughness of the conduit or surface. In
addition, overland flow may also depend on the intensity of the
effective rainfall.
On entry of a finite length, the time is computed for each
component and the total is displayed as the Time of
Concentration. If required, one or two of the flow components
can be ignored by entering a zero length in the appropriate data
field for length.
Roughness Height
MIDUSS design routines use the Manning ‘n’ to describe surface
roughness. Users who prefer to define roughness in terms of the
equivalent roughness height can use the Roughness Height tool to
convert from roughness height to Manning ‘n’. Once calculated,
the computed value can be imported into the next design command
by clicking the [Use for Design] button.
Edit Storm
One of the options in the Storm command is to use a predefined
curve known as a Mass Rainfall Distribution curve. These files are
given the extension *.MRD and define the fraction of rainfall depth
R(t)/ Rtot as a function of the ratio of elapsed time over total storm
duration. Typical examples are the various Huff storm quartiles and
the SCS hyetographs. The Edit Storm Tool lets you edit or create
MRD files.
The values can be edited by graphical manipulation or numerically.
Graphical Edits Position the mouse pointer on one of the vertical
grid lines and either
above or below the red line. Each mouse click causes the numerical
value in the table to increase or decrease by 0.01 and the plotted red
curve shows the change.
Numerical Edits Click on any cell in the grid
with the exception of the 0.0 and 1.0
values and type in the desired value. Any change in either the
graphical or tabular display is reflected in the other. The array of
values must start with zero and end with 1.0 and the intermediate
values must increase monotonically.
