Infiltration: Calculation of Swale Size

Expert question:

Our soil report determined a design coefficient of 5x10^-6. There are cohesive sands, waterlogging may occur, therefore only a swale infiltration is possible. No soakaway etc. So far so good.

According to the calculation, the swale for the driveway area (93 sqm), since insurable paving is used, should be 30 sqm in size.

For the house (123 sqm roof area, garage 40 sqm) another swale is planned, which is supposed to be 90!!! sqm in size.

Who can calculate this and verify the result?

What ponding depth, rainfall amount, and duration did the calculator assume?

Formula:
As = Au x 10^-7 x r_D(n) / [z_M / (D x 60 x 1.2) – 10^-7 x r_D(n) + k_f / 2]
As = pond area in sqm
Au = connected area in sqm
R_D(n) = rainfall amount in l / (s x ha)
Z_M = ponding depth in m
D = rainfall duration in min
K_f = infiltration coefficient in m/s

And how do I get the values that I am allowed to use?

For example, ask the person who calculated your data what assumptions they made. Then you can question the values and check the calculation.