This tool is a reservoirrouting spreadsheet model that uses a standardized NRCS Type 1a 24hour rainfall distribution (typical of Western and parts of Central Oregon) with variables input by the user to approximate the performance of various rain garden configurations. It is to be used for modeling runoff from impervious surfaces of less than 0.25 acres (approx. 11,000 sq. ft.).
Note: This size calculator is for swales with berms that function the same as a linear rain garden, and therefore is sized the same as a vegetated basin. The swale may be at a small slope (<5%), and the berms placed frequently enough to slow the water down and provide infiltration. Vegetated swales designed for conveyance must use a different procedure.
Choose type of rain garden:
Enter the square footage of the impervious surface to be treated. This must be greater than 0 and 11,000 square feet or less:
If you selected maximum raingarden size above, then enter square footage of rain garden. This will calculate the maximum ponding depth necessary. If you selected maximum target ponding depth, then enter the max ponding depth. Note, ponding depth is typically between 612 inches:
(Optional): If you would like to include a gravel layer, check the Gravel Underlayer Present box. You will be presented with additional options concerning the fill in the depth of rock you would like to use:
Gravel Underlayer Present
(Optional):Enter depth of rock trench below rain garden (inches). Rock trenches are sometimes used
to increase the amount of water the garden can hold while it infiltrates the underlying
soil layers. This should be between 0 and 2 feet:
(Optional): Enter the Rock Trench Storage Volume (Cubic Feet):
Raingarden Results 

Raingarden Type:  Stormwater Basin 
Gravel underlayer present:  
Gravel layer ponding level (in):  
Raingarden max ponding depth (in):  
Raingarden Size (Sq Ft):  
Peak Rainfall (in): 
Addtional Optional Steps: Specify an average width and allow the model to calculate the rain garden length. Note: basins and swales should have at least a 5 foot width to compensate for the gradual side slopes (3 feet horizontal to 1 foot vertical, i.e. 3:1 side slopes).
To run the simulation below, press the ENTER key on your keyboard after specifying calculation type and either length or width variables: