If the width of the sump was the same as the width of the roof then you were just raising the water level by an equal amount, probably making the sump harder as well as the rubble from the roof wouldn't pack 100% efficiently, so the floor of the sump would rise more than the ceiling of the passage, and hence the airspace would shrink. Of course, you can remove the rubble from the sump which was probably the point of the operation.