Catchment

The first catchment area of this type (an entirely original idea conceived by the then City Engineer of Gibraltar) was constructed in 1903 on the sand slopes of the east side of the Rock, which has an average inclination of 1½ to 1. These slopes had big boulders embedded in them, which were blasted away, the surface trimmed as even as possible and a channel and footpath constructed at the lower perimeter of the collecting area. Into these trimmed sand slopes timber piles 1500mm x 150mm x 40mm were driven their full length, to these a timber framing of 75mm x 75mm x 4500mm purlins and 75mm x 75mm x 2400mm rafters were nailed and on these, corrugated galvanised iron sheets 2400mm x 900mm were secured by means of drive-screws all round their edges. All timbers had been previously treated with creosote forced in at a pressure of 170 lbs per square inch.

Roughly each hectare covered took: 5928 rafters, 1112 purlins, 1161 piles; 5928 sheets, 2920 kgs of screws and 850 kgs of washers.

CHANNELS

The collecting channels were designed to convey a maximum of 102 mm of rain per hour on an area of about 14.97 hectares (this being the area available for eventual conversion into catchments). The access footpaths along the channels were incorporated in the design to act as a relief channel fed by an overflow system and itself overflowing through pipes down to sea. The main channel in the east west tunnel has a valve via which water can be diverted into a natural fissure and eventually down to the sea. Water from the first rains, which washes the dirt from the catchments, is flushed away in this manner. The catchments and channels ceased operations as a water source in October 1993.