The calculation is done automatically by specifying a longitudinal slope for the ditch or swale in the Biofilter box. This takes into account that the volume available for detaining peak flows decreases more the greater the longitudinal slope.

It is also possible to use the functions with series connection of ditches or export of the ditch to an area downstream, the latter if stormwater outlets are added along the length of the ditch. This makes it possible to simulate the effect of dividing the ditch with weirs to increase the flow detention effect by obtaining a larger total available volume for flow detention.

In general, the flow detention effect of ditches and swales decreases with increasing size of the catchment area. The calculation of both the required and available detention volume of ditches and swales also takes into account, as before, the thickness and pore content of the underlying plant bed and the hydraulic conductivity of the surrounding soil, the area of ​​the ditch for direct precipitation on the surface, side slope and ditch length. It is also possible to see the effect of any specified raised well in the ditch/swale on the flow detention, and now also taking into account the longitudinal slope.

This is described in more detail and visualized in the following article:

Larm T., Wahlsten A. Nordgren M. and Kjellin J. (2024). Optimized design of biofilters and swales/ditches for flow detention and treatment. Tidskriften Vatten (In Swedish).