The Great East-side Sand Slopes and former Water Catchments

A brief history

The Great Eastside Sand Slopes form an extensive area (approximately 45 hectares) of largely consolidated windblown sands that extend from above Sandy Bay in the south to Catalan Bay to the North. These sands, which contain a high percentage of uniform quartz grains, originated outside Gibraltar, since there are almost no quartz-bearing strata on the Rock. The sand slope was formed during the Quaternary period, when the area to the east was a dry sandy plain and wind action deposited the sand upon existing scree breccias and boulder conglomerate (Rose & Rosenbaum, 1991). At one time, the Talus slopes to the north and south, together with the Sand Slopes, formed one contiguous mass. However, the Catalan and Sandy Bay quarries, opened by the Admiralty in 1895 to provide material for the Dockyard extensions, isolated the Eastside Sand Slopes from the Talus slopes. The talus extremities, located below the major cliff faces - namely Spyglass and Rock Gun - seem to have accumulated the largest quantity of rock boulder material. This has formed the conglomerate scree breccias but is still covered by a sandy layer. The central portion, where the Eastside Sand Slopes are located, has undergone less rock deposition from above, but has a greater accumulation of windblown sands, substantially differentiating this geological structure from the adjacent Talus slopes. Drawings from the 1800s depicting Catalan Bay show that the Sand Slope was almost devoid of vegetation and this confirms the presence of goats and grazing activity. In 1903, the City's Chief Engineer came up with a plan to cover the 10 acre slope with corrugated iron sheets to collect potable water. This resulted in most of the Sand Slope habitat being lost and with it, a number of plant and animal species including the Black Wheatear (Oenanthe leucura) which probably relied on this habitat.

The water catchments were deemed obsolete in 1991 with the advent of desalination plants in Gibraltar, thus promulgating their removal and restoration of the habitat in the 1990s. The restoration process was extremely laborious and consisted of the removal of the corrugated iron sheets, followed by the installation of a biodegradable mesh to stabilise the slope. A reseeding programme followed thereafter, using native grasses and shrubs in close consultation with the GONHS. This was carried out by experts from the Gibraltar Botanic Gardens. In addition to the removal of the water catchment and the reseeding of the slopes, a complex network of strong rockfall protection fencing was installed.

World Heritage Status

The Gorham’s Cave Complex is the name given to the area covering some 28 hectares on the eastside of Gibraltar from sea level to the top of the Rock. In July 2016, it was inscribed as a UNESCO World Heritage Site for its exceptional testimony to the occupation, cultural traditions and material culture of Neanderthal and early modern human populations through a period spanning approximately 120,000 years. The striking cluster of sea level caves contain archaeological deposits that provide evidence of Neanderthal and early modern human occupation of Gibraltar, and the landscape setting and natural species which assist in presenting the natural resources and environmental context, including climatic conditions, of Neanderthal life. The Gibraltar Nature Reserve, including the Great Eastside Sand Slopes, form part of the buffer zone to the World Heritage site and together they represent over 40% of the territory of Gibraltar.

Water Catchments – A colossal feat of engineering

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 91500mm x 150mm x 40mm) were driven their full length, to these a timber framing of purlins (75mm x 75mm x 4500mm) and rafters (75mm x 75mm x 2400mm) 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 had a valve via which water could be diverted into a natural fissure and eventually down to the sea. Water from the first rains, which washed the dirt from the catchments, was flushed away in this manner.

Reservoirs: Between 1911 and 1914 reservoir No 5, of 9,091 cubic metres capacity, was excavated from the rock by means of enlarging a tunnel driven parallel to the channel tunnel and some 7.6 metres below it. The tunnel was enlarged on either side to form a chamber some 12.2 metres high 13.1 metres wide and 121.9 metres in length. After the excavation works were completed and all loose rocks removed, the floor was concreted in two layers, a 150 mm levelling layer of mass concrete followed by a 150 mm slab of 1:2:4 concrete using Portland cement and 35mm stone as coarse aggregate. The walls were constructed using two skins each of 114 mm space between the back wall and the rock face was filled with mass concrete and the 150 mm cavity filled with a Portland cement, 1:1 mortar mixed with 5% water proofing agent. The floor received a 50 mm Portland cement, 1:1 mortar screed and finally rendered with a 19 mm layer containing 5% of water proofing agent. The walls were cement plastered in 3 coats using a 3:1, 2:1 and 1:1 with 5% water proofing agent mix. The last layer of wall plaster and floor render was steel trowelled. Nothing was done to the roof as the rock over the span used is self-supporting.

The catchment area was increased by a further 5.66 hectares and, in 1928, a further reservoir (No 6) was constructed similar to reservoir No 5, but of half its length and with a capacity of 4,545 cubic metres. Each reservoir is connected to the main channel by means of large diameter pipes with valves to control the entry of water into them. They are now also connected to the pumping mains conveying water from other sources. Between 1928 and 1945 four more reservoirs (Nos 7 to 10) were excavated from the rock adding a further 18,181 cubic metres to the storage capacity. It is interesting to note that reservoir No 10, which had been excavated but not completed at the time of World War II, contained a barracks built to house a detachment of the "Black Watch" regiment under bombproof conditions. The construction of these reservoirs follow the original pattern except that they are offset from the pilot tunnel thus providing access to their supply mains. Outlet valves for supply and cleansing are operated from within the reservoirs by means of long spindled hand wheels.

Each reservoir has an overflow onto the pilot tunnel and via the tunnel into the natural fissure. The last reservoirs (Nos 11 and 12) were constructed 1958-1961 at a lower level, reached from the east side, opposite Catalan Bay. These were constructed to receive rain from a further extension of 4.05 hectares of catchment at a lower level on the east side slopes. These brought up the total number of potable water reservoirs to 12 plus the Moorish Castle reservoir providing a total storage capacity of some 72,727 cubic metres.

Following the ceasing of operations, of the catchments as a source of potable water the reservoirs are currently used as service reservoirs and as storage reservoirs providing substantial water reserves.