Sump boxes for rainwater drainage?

Hello Which system is suitable as roof drainage? We have a small soakaway shaft and the rest infiltrates on the surface of the property. The house is on a slope, so much flows onto the lawn. The water also infiltrates. We have sandy soil here. I would like to build an infiltration system. The permeability value is between 10² - 10⁴. The roof area is about 140 sqm. How many boxes would I need approximately? The problem is I have to dig without an excavator. Therefore, I have to estimate how much it will be.

That is hardly answerable, especially since there is a huge difference between having 10^-2 or 10^-4.
A soil with 10^-2 (maybe on a dune) can infiltrate about 36m3/h. With each further negative exponent, you can divide the value by 10.
With a 10^-3 soil, you will get by with 2-3 of the usual 420l seepage boxes, with 10^-4 it will be a little over 20.
So who can tell you the right amount?

1. Calculation of the amount of rain. With 15 liters per sqm for a heavy rain, you already get 2.1 cbm for the roof area. 2. Assume the worst possible infiltration. So 0.1mm of water per second. 3. Since even the worst possible infiltration is still a great value for you, it mainly depends on the amount of water coming from above. The infiltration hardly limits it. 5. So dig holes for five boxes. Or more if you want additional security for heavy rain events.

Can you actually make an overflow with the boxes, or is that not possible with the boxes?