Generic Unit Hydrographs
In calculating a runoff hydrograph from rainfall data, users can use a default triangular or curvilinear hydrograph such as provided by the SCS or RTK methods. However, if a hydrograph does not closely follow those shapes, the generic unit hydrograph method can be used to develop a more accurate unit hydrograph if the user has sufficient rainfall and runoff data for the catchment.
This method is selected for a catchment by setting the Runoff Method to Unit Hydrograph and the Unit Hydrograph Method to Generic Unit Hydrograph. The values of the hydrograph entered by the user are a collection corresponding to the runoff per unit of excess precipitation (rainfall minus loss) over time. These values should be based on field data collection. These values are unique to each catchment and are not scaled by the area.
The time step size on which the resulting hydrograph is based is referred to as the "convolution time step". It must be greater than zero and should be less than or equal to the rainfall increment. The hydrograph will be smoother if the time step is smaller but there is no benefit in making it smaller than the output increment set in the Calculation options.
For each plug of runoff generated over a single time step, an individual runoff hydrograph is generated. All the successive unit hydrographs are superimposed to form the ultimate runoff hydrograph for the catchment. The underlying theory is described in Stormwater Conveyance Modeling and Design, by Bentley Institute Press (pp. 158-162) or Wastewater Collection System Modeling and Design, by Bentley Institute Press (pp. 252-254).
The fundamental equation for this method is show below.
| Qk | = | flow at time step k, cfs | |
| Pi | = | precipitation during time step i, in./hr | |
| U k-i + 1 | = | flow at time step k from rain during time step I, cfs/in | |
| k | = | duration of rain in time steps + duration of hydrograph |
The theory behind unit hydrographs is that the volume of water, calculated as the area under the hydrograph curve, should correspond to 1 inch of excess precipitation over the area. The user needs to check if this is true.
For example, over a 2 acre area, the volume of water calculated under the unit hydrograph should be 2 acre-in (7,260 cubic feet). If there is 1.5 in of excess precipitation (precipitation – losses) over this catchment, the volume of water calculated using the unit hydrograph method should be 3 acre-in (10,890 cubic feet). This runoff volume is displayed under the Catchment tab of the Detailed Calculation Summary.