WMS:OC Unit Hydro Small Area Hydrographs

Orange County Small Area Hydrograph Wizard – Step 1 of 2

Small area hydrographs can be used in lieu of developing and applying a unit hydrograph when the time of concentration is less than 25 minutes. A flow rate for each computational time interval is computed using the Orange County rational method equation. Effective precipitation is calculated by subtracting unit losses (the smaller of the two loss values, Fm or Ybar) from the unit rainfall (Orange County IDF curves). The Rational unit hydrograph is applied to each flow rate and the runoff hydrograph results from arranging each of the unit hydrographs according to the design storm pattern used in Orange County, with the peak flow unit hydrograph beginning at hour 16.

1. Click the Update Frequency button to select the return period and IDF curves
2. Computational time interval must be equal to or greater than the time of concentration
3. Use the Computes Losses button to compute losses using GIS data
4. Select Next to view a tabular output of the calculations
5. Exit all dialogs and select the hydrograph icon to view the runoff hydrograph

Related Topics

• Orange County Rational Method Equation
• Rational Hydrograph