Local historian and architect William Serfaty describes their function:7
The level of the bottom of the tanks is high enough to empty out to ships or lighters berthed at Rosia Harbour by a sophisticated gravity feed running under what is now the road to Camp Bay. The entire structure was built without access to Portland cement. The construction is excellently executed in brick and sand-lime mortar with a complicated finish to waterproof the tanks. The vaulted roofs of the tanks are a wonderful sight, and also serve to provide a sloping catchment surface (which catches the light beautifully), directing water to the appropriate settlement tank, from which it is then directed to storage tanks.
It was important to keep the water pure, so the system was kept secure, and access to the catchment roof restricted to the employed personnel by the provision of a high wall, which has kept the site out of the public eye all these years.
So well did the contractor, Juan Maria Boschetti, build the reservoir, that 150 years later in the 1950s the navy built Rosia Distillery in the space at sea level below and in front of the Rosia Cottages between the two eighteenth century defensive walls. Its site is now a slope, constructed about 2000 with rubble from demolished buildings at Cumberland Road and Tower Buildings, for vehicles to reach Rosia Harbour. It continued to supply the resultant stored water from the tanks to lighters, which would pull alongside at Rosia Harbour to load up with fresh water for naval vessels from the reservoir.
There is ample evidence these tanks were operational by the time of the Battle of Trafalgar in 1805. In 1804, for example, Captain William Otway, Commissioner of Gibraltar Yard was able to inform the acting Governor Sir Thomas Trigge that:8
…the late heavy rains having above half-filled the Great Tank at Rosia: I think that His Majesty’s Ships may take water from thence whenever Your Excellency has reason to suppose that there is a probability of the Wells at Ragged Staff becoming dry.
Otway also remarked on:
…the necessity of having some Careful Person constantly to reside on the Spot for the care of the Works, and for whom some sort of habitation must be built…