Theory

STORM - In MIDUSS this command allows you to define a rainfall hyetograph either of the synthetic, design type or a historic storm.   The derivation of these are discussed in the following links.

• Derivation of the Chicago Storm
• Derivation of the Huff Storm
• Derivation of the User Defined Storm
• Derivation of the Canadian 1-hour storm

CATCHMENT - This command prompts you to define a single sub-catchment and computes the total overland flow hydrograph for the currently defined storm. 

The runoff hydrographs from the pervious and impervious areas are computed separately and summed. 

The roughness, degree of imperviousness and surface slope of both the pervious and impervious fraction are defined in this command. 

The effective rainfall on these two fractions is computed and stored for future use.  The runoff hydrograph from the two fractions is computed separately and added to give the total runoff.

• Calculating Effective Rainfall
  • The SCS Method
  • The Horton Equation
  • The Green and Ampt Method
• Calculating the Runoff
  • Catchment Shape
  • The Idealized Catchment
  • Conceptual Components of Rainfall
  • Processing the Storm Rainfall
    • Rainfall Runoff Models
      • A Rectangular Response Function
      • The SCS Triangular Response
      • The Linear Reservoir Response
      • The SWMM - RUNOFF Algorithm

Large Catchments - The Lag and Route command is useful for modelling the runoff from very large sub-catchments without having to resort to specifying unrealistically long overland flow lengths. 

Lag and Route computes the lag time in minutes of a hypothetical linear channel and linear reservoir through which the runoff hydrograph is routed. Typically this results in a smaller, delayed runoff peak flow.  Click here for a detailed discussion.

This section explains some of the principles of hydraulics which are used in the Design commands available in MIDUSS.  It is not intended to be a general treatment of hydraulics and you should use a standard text on the subject to obtain information not covered in this Help System.

• Pipe design

•  Channel design

•  Flood Routing

• Detention Pond design

•  Exfiltration Trench design

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