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Frequently asked questions

 

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Catchments

 
V2 = applies to MIDUSS Version 2 (current release)
M98 = applies to MIDUSS 98
  • [ M98 ][ V2 ]    When I am using the catchment command do I have to specify the pervious and impervious parameters every time I use it?

No. MIDUSS initially uses default values for each of the infiltration models for rainfall loss. You will normally set these the first time you use the catchment command. After that, the values you have entered will become the new defaults and you do not need to re-enter this information unless the characteristics of the catchment change from one location to another.

  • [ M98 ][ V2 ]    When I enter values for the SCS curve number in the pervious fraction several other things change. What have I done wrong?

You probably have not done anything wrong because the parameters you speak of include the SCS curve number, the runoff coefficient, the initial abstraction and the ratio of initial abstraction to storage S. If you change one of these parameters several other parameters will automatically change for the given storm that you are using. If you change the runoff coefficient then the curve number will change and vice versa. A change in curve number will cause a change in storage potential S which in turn will change the ratio Ia/S. If you change the initial abstraction, the ratio of initial abstraction to storage will also change and vice versa. If you change the initial abstraction for a given curve number both the runoff coefficient and the ratio Ia/S will change. This kind of interactive data entry is unique to MIDUSS.

  • [ M98 ][ V2 ]    When I am using the catchment command and displaying the pervious fraction characteristics pressing the [Display] button does not show me the runoff hydrograph. How do I get this?

In the catchment command the tabs labeled Pervious and Impervious define the effective rainfall on these fractions. When you press [Display] in either of these two parts of the form you will see the hyetographs of the storm and the effective rainfall. You can see the actual runoff hydrograph only by changing to the Catchment tab and pressing [Display]. This shows the runoff hydrograph for the pervious, impervious fractions and also the total runoff as the sum of these two hydrographs.

  • [ M98 ][ V2 ]    The peak flow for the total runoff is often less than the peak from the pervious and impervious fractions. Is this an error?

No. The difference is apparent because the time to peak of the pervious and impervious hydrographs is usually quite different. This means that when the two hydrographs are added to give the total runoff, the peaks do not occur at the same time. In general the peak of the sum of two hydrographs will be numerically smaller than the sum of the individual peaks of the two hydrographs.

  • [ M98 ][ V2 ]    After defining the catchment, I try to design a pipe. However the current peak flow shows a value of zero. Ho do I design a pipe for the catchment runoff?

After modelling the catchment runoff you must add the runoff to the inflow hydrograph before you can design any of the elements in the design menu.

Run the file in Automatic mode using the [EDIT] command in the Control Panel.   When the initial large catchment is displayed reduce the area to represent the undeveloped fraction and press [Display] to modify the results and Accept it.   Execute the Hydrograph/Add Runoff which normally follows the Catchment command.   Now in the Control Panel press the [BACK] command twice to return to the original Catchment.  Click [EDIT] to display the parameters.  Now change the area, percent impervious and any other parameters which should be changed to represent the developed fraction.  Press [Display] to get the new result and Accept it.  Now press [EDIT] again to use the Add Runoff command a second time.  The single large catchment has now been replaced by two smaller ones and you can continue downsream.


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