Santa BarbaraUrban Hydrograph

The discussion on this methodology is adapted from the King County Surface Water Design Manual, 1994, King County Washington. The Santa Barbara Urban Hydrograph (SBUH) was developed by the Santa Barbara Flood Control and Water Conservation District in California as a short cut to the SCS Unit hydrograph. The primary difference between the SBUH and the SCSUH method is the use of the unit hydrograph in computing the runoff hydrograph. The SBUH replaces the unit hydrograph with an algorithm. The SBUH method greatly simplifies the computation of runoff hydrographs.

The SBUH uses two (2) steps to synthesize the runoff hydrograph.

1. Computing the instantaneous hydrograph
2. Computing the runoff hydrograph

The instantaneous hydrograph, It, in cfs, at each time step, dt is computed as follows:

Where

• Rt=total runoff depth at time increment dt (inches).
• A=area in acres
• dt=time interval in minutes.

The runoff hydrograph, Qt, is then obtained by routing the instantaneous hydrograph It through an imaginary reservoir with a time delay equal to the time of concentration, Tc of the drainage basin. The following equation estimates the routed flow, Qt. Where dt = time interval in minutes.

Spreadsheet format for SBUH Method

Column	Description
Column 1	Time step number. This is column starting with zero (0) and incrementing by one (1).
Column 2	Time in minutes. For a time step of 10 minutes, the column would increment from zero (0) , 10, 20, 30 etc.
Column 3	Rainfall distribution as a % of the total precipitation.
Column 4	Incremental rainfall (inches). This is just the total precipitation times column 3. Be sure to divide by 100, since column 3 is in %
Column 5	Accumulated Rainfall (inches). Just sum the previous column. We should make a comment about columns 3, 4 and 5. Generally, the rainfall distribution is provided as an accumulated distribution, meaning at time zero, there is zero rainfall and at the last time step, the rainfall is one (1) or 100% accumulated. If that is the case, column 3 can be omitted, column 4 can be the accumulated rainfall ratio and column 5, this column can just be the total precipitation multiplied by the column 4.
Column 6	Accumulated Runoff. This column checks to see if the Column 5 is less than 0.2s. If it is, then zero (0) is returned, otherwise the abstraction is return.
Column 7	Incremental Runoff. Just subtract the previous column 6 from this column 6.
Column 8	Total Runoff. This should be the same as column 7 unless there are several areas that are being combined. In which case, it is the sum of the column 7 of each of those areas.
Column 9	Instant Hydrograph. This is where the SBUH method diverges from the SCSUH method. For the SBUH method, we just apply the equation:Where A is the total project area and dt is the time increment.
Column 10	Design Hydrograph. We apply the remainder of the SBUH method equation: