High throughput water pumps (not 240volt) availability

We've recently found out that in the very near future with any marine surge flood about 0.1m higher than the one in 2024, SSEN would pre-emptively remotely cut the local electricity supply to protect their lowest local substation transformer. So the 30 or so subfloor residential sump pumps will stop when most needed. So perhaps buy a few petrol driven pumps, but the problem there its unlikely they would be maintained and would lie idle in sheds for 5 years say, and not run when required. So how high a capacity/throughput/amperage of pumps using 12volt (from car batteries) and where can they be obtained eg for large garden water features perhaps, ie bigger than windscreen washer pumps say. Or even perhaps human powered pumps capable of lifting one metre , efficiently by perhaps 2 people, an action like those "wild west" railway human powered personel carriers, or old sailing ship human powered bilge pumps, and where to obtain such. An easy job to extend vertically the sumps, as all the same make and size, just remove the covers and bolt on a small vertical extender , for access for such supplemental pump piping.