Galle Fort 18th-century VOC underground storm sewer system: twice-daily ocean tidal flushing?

Dutch military engineers in the 1700s constructed a sophisticated subterranean drainage and sanitation network beneath Galle Fort that operated entirely on ocean tidal energy. Large vaulted culverts engineered from coral stone, laterite (Kabook), and Dutch clinker bricks run beneath the street grid. The main sewer channels feature intake and discharge portals opening through outer rampart sea walls. As high tide rises, seawater surges through one-way wooden flap valves, scouring municipal effluent out of street channels and flushing sewage straight into deep ocean currents at low tide, preventing cholera epidemics.