ᐅ Soakaway crates for rainwater drainage?

Hello
Which system is suitable for roof drainage? We have a small soakaway tank, and the rest drains away on the surface of the property. The house is located on a slope, so much of the water flows onto the lawn. The water also infiltrates there. We have sandy soil here.

I would like to build an infiltration system.
The infiltration rate is between 10² and 10⁴.
The roof area is approximately 140 m² (1507 sq ft).

How many infiltration boxes would I need roughly?
The problem is that I have to dig without an excavator. Therefore, I need to estimate how much excavation will be required.

This is hardly answerable, especially since there is a huge difference between having 10^-2 or 10^-4.
A soil with 10^-2 (perhaps on a dune) can allow about 36 m³/h (42 yd³/h) to infiltrate. With each further negative exponent, you can divide that value by 10.
For a 10^-3 soil, 2-3 of the usual 420 l (110 gal) infiltration boxes will suffice, while for 10^-4, it will be a bit over 20.
So who can tell you the correct amount?

1. Calculation of the amount of rain. With 15 liters per square meter (1.6 gallons per square yard) for a heavy rain, the roof area results in already 2.1 cubic meters (74 cubic feet).
2. Assume the worst-case infiltration rate: 0.1 millimeters (0.004 inches) of water per second.
3. Since even the worst-case infiltration rate is still an excellent value in your case, the main factor is the volume of water coming from above. Infiltration hardly limits this.
5. So dig holes for five boxes. Or more if you want extra safety for heavy rainfall events.

Is it possible to install an overflow with these boxes, or is that not an option for this type of box?