Sand filters are usually rated at 15 gpm/sqft maximum for commercial and 20 gpm/sqft for residential.
Backwashing is generally considered to be optimal at about 15 gpm/sqft and typically should not exceed 20 gpm/sqft.
The SD40 has 1.9 sqft. 1.9 x 20 = 38. So, your filter is rated at 38 gpm. The manual says 40 gpm.
The pump will do about 80 gpm when there is low resistance to water flow.
During backwash, the resistance is low and you will get about 80 gpm of flow.
That's twice the rated flow rated for the filter.
You say that it's a lot of sand with not much water. That's odd and might point to other issues more than just an oversized pump.
It's hard to tell if the entire issue is due to an oversized pump or if there is also something else.
It's very common for pumps to be oversized. Your pump probably uses 1,500 watts when a pump using 150 watts would work.
Some states like California actually outlawed single speed pool pumps for filtration unless the pump was less than 1 hp total.
In my opinion, all states should adopt the same rules as California.
