Trench DocsUsing monthly rainfall figures
Trench uses three types of monthly rainfall figures:
- adopted rainfall
- the monthly rainfall you choose for a site.
- retained rainfall
- the adopted rainfall that has been corrected for slope angle.
- mean rainfall
- the recorded rainfall for the site.
- mean rainfall is an optional input for Trench.
The lower the rainfall you adopt, the smaller the disposal area needed to accept wastewater. Using mean monthly rainfall (which is approx. 50 percentile rain) may not be conservative enough, particularly in lower permeability soils where one high rainfall event might cause the system to (temporarily) overflow. For in-ground systems, mean monthly rainfall is probably acceptable where the soil is permeable enough to accept almost any rainfall or wastewater loading - for example, sands and silty sands, and structured, red-brown, porous and friable clayey silts and silty clays developed on basalt.
TIP
Consider using higher-than average rainfall for most uses of this application. You could. for example, add 10% or 20% to all monthly figures. You might decide to use 90 percentile rainfall figures for one or more months. (The 90 percentile figure is that monthly rainfall record which exceeds 90% of the rainfall records for a station ranked in increasing order. There is thus about a 1 in 10 chance of 90 percentile rainfall occurring in any specified month, and approx. a 1 in 100 chance of 90 percentile rainfall in any two months, in any year.) Using three or more months of 90 percentile rainfall may be excessively conservative.