Infratech Civil

Product:  Autodesk SSA  |   Subject: Storm and Sanitary Analysis

In this exercise, we will learn about stormwater 

Using SCS TR-55 Time of Concentration

1. Open the file 04.01-SEWERS-Intro.SPF 
2. You will notice that we have our background information imported from Civil 3D.
3. In the Data Tree, double-click on the project options.

4.  In the Project Options window, make sure to choose the following parameters
   1. The right Units for your project file, Metric or US units,
   2. Activate Compute lengths and areas while digitizing
   3. Choose Rational for Hydrology method. It is the main criteria for the whole design.
   4. Next, let’s set the Time of Concentration (TOC) using the SCS TR-55 method.
   5. . For a small or relatively simple catchment, we can define the time of concentration by using a user-defined value, usually between 5 to 10 minutes, depending on the agency and the size of the project.  However, most of the time, we need relatively elaborate methods such as the SCS TR-55 method. For this example, let’s choose the SCS TR-55.  This will avoid some comments from the reviewing agency, as this method is accepted across the board. Using an arbitrary user-defined method may only trigger a request for justification, which is not really worth the time needed to enter the SCS TR-55 or other methods.
   6. Set a Minimum Allowable Time of Concentration of 5 min. We need to specify a minimum time of concentration (TOC) to prevent high runoff values. IDF curves are often provided at 5 minutes increments, so, if we have a small Subbasin with a calculated time of concentration (TOC)  of 2 minutes using the SCS TR-55 method, for example, the software will use 5 minutes, which we can find in the IDF table.
   7. Change the Hydraulic Routing Specifications to Hydrodynamic. We have already explained earlier that the Hydrodynamic Method is the complete routing method because it works even in adverse flow conditions.
   8. Finally, leave all other values to their default and click OK.