I’m not sure why these workers were flooding the area near the hotel shuttle pickup at Schiphol, but I wonder if it has something to do with what the tour guide mentioned in Rotterdam. The Netherlands has a very high water table and is prone to flooding in heavy rains, so many modern buildings (like the Rotterdam Centraal Train Station) have a system of cisterns and holding tanks to collect the water from the roof during heavy rains, and then release it gradually so it doesn’t overwhelm the storm drains and flood the surrounding area. Perhaps these workers were doing a controlled release of storm run-off, or some other related utility work.