University of Central Florida
Civil and Environmental Engineering
CWR4101 Hydrology Laboratory #6C
Time Step Variations and Hydrograph Generation

PURPOSE

  1. To determine the relationship between time step and time of concentration in hydrograph generation using the SCS method
  2. To investigate the differences between combined and separate routing of impervious watershed areas.

    3. EQUIPMENT

    1. SMADA Computer Program
    2. Spreadsheet

      PROCEDURE

      1. Using SMADA create a watershed with the following characteristics
        • Total Area = 100 acres
        • Impervious Area = 45 acres
        • Percent Directly Connected = 30 %
        • SCS Curve Number for Pervious = 60
        • Time of Concentration = 1 hour
        • Unlimited Infiltration Capacity
        • No Additional Abstractions Occur
        save this hydrograph to your disk.
      2. Create a SCS Type II rainfall with a total rainfall of 8.6 inches, duration of 24 hours, and a time step of 10 minutes.
      3. Using the Hydrograph Generation Routine develop a hydrograph using this rainfall and this watershed. Use the SCS 484 Hydrograph generation technique.
      4. Time step and time of concentration are important parameters in the use of this method. By trying different time steps (modify these in the rainfall window), develop a 'Rule of Thumb' for the ratio of time step to time of concentration. This rule of thumb should ensure stable and consistent results in the hydrograph generation. Document the data which supports this rule of thumb.
        Hint: Plot hydrograph and look for instabilities in the plot, also try plotting Peak Flow vs. Time Step.
      5. Determine a Composite Curve Number (CCN) for your watershed. Using the composite curve number method, the entire watershed is assumed to be pervious. What affect does this have on your hydrograph and why?
      6. Will the use of the composite curve number affect the rule of thumb that you developed in the previous steps? Support your results with data.

      OUTPUT

      It is recommended that you plot peak flow (y axis) vs. time step/Tc (x axis). This plot should give you some ideas of where the calculations are most stable. Report your rule of thumb as a ratio (i.e. 5:1,3:1, etc...). You do not need to include the output or plot of each hydrograph, just a report of the peak flow for each combination of time step and Tc.

      You should include the SMADA watershed printout with your report for both the watershed modeled with combined (CCN) and separate routing.