Rainwater Soakaway Sizing Spreadsheet xls

Download free excel sheet calculator for the design and sizing of soakaway for rainwater drainage. Soakaways is a method for disposing rainwater, it is a hole or a pit in the ground into which you run the rainwater drainage.

Download also: Rainwater Drainage Design Calculation Excel Sheets XLS

Rainwater Soakaway Sizing

There are three determining factors for the design of a soakaway drainage system:

1. The size of the area, which relates to the volume of water to be drained (Area =  m²). Note that soakaways should be provided for all roof areas and hard standing areas within the building plot.

2. Surface run off coefficient (C), it is the percolation rate of the soil/sub-soil

3. Storm intensity I=mm/Hr. Note that the storm intensity depends on Storm Return Period (The occurring probability of floods, windstorms or tornadoes)

And the total volume for soakaways is calculated from the following formula:

V = (C*I*A)/1000

Download Soakaway Sizing Excel Sheet 

Soakaway Sizing Example:

Area = 850 m²

Surface run off coefficient C=0.9

Storm return period = 2 years

Storm intensity I=25mm/Hr

V = (C*I*A)/1000

V= (0.9x25x3000)/1000=19.125 m3

No. of Soakaway=total volume/volume of soakawy

Assume the standard volume of the single soakaway = 10 m3

Then No. of Soakaways = 19.125/10= 1.91 

Then 2 Soakaways with volume of 10 m3 will be suitable in this situation.