Water and Life

It has long been thought that the Pont du Gard was built by Augustus’ son-in-law and aide, Marcus Vipsanius Agrippa, around the year 19 BC. Newer excavations, however, suggest the construction may have taken place in the middle of the first century A.D, consequently, opinion is now somewhat divided on the matter. Designed to carry the water across the small Gardon river valley, it was part of a nearly 50 km (31 mi) aqueduct that brought water from the Fontaines d’Eure springs near Uzès to the Castellum in the Roman city of Nemausus (Nîmes). The full aqueduct had a gradient of 34 cm/km (1/3000), descending only 17 m vertically in its entire length and delivering 20,000 cubic meters (5 million gallons) of water daily.

It was constructed entirely without the use of mortar. The aqueduct’s stones – some of which weigh up to 6 tons – were precisely cut to fit perfectly together eliminating the need for mortar. The masonry was lifted into place by block and tackle with a massive human-powered treadmill providing the power for the winch. A complex scaffold was erected to support the aqueduct as it was being built. The face of the aqueduct still bears the mark of its construction, in the form of protruding scaffolding supports and ridges on the piers which supported the semicircular wooden frames on which the arches were constructed. It is believed to have taken about three years to build, employing between 800 and 1,000 workers.

In Ancient Rome, the Aqua Alsietina (sometimes called Aqua Augusta) was the earliest of the two western aqueducts, erected somewhere around 2 BC, during the reign of emperor Augustus. It was the only water supply for the Transtiberine region (the right bank of the river Tiber).

This aqueduct acquired water mainly from a lake just north of Rome called Lacus Alsietinus (a small lake in southern Etruria, currently known as Lago di Martignano) and some from lacus Sabatinus (Lago di Bracciano). The length of this mainly subterranean aqueduct was 22,172 paces (about 32.8 km) and had 358 arches. Its water supply had a diameter of 392 quinariae (about 9 m).

 
An example of a ancient Roman naumachiumThis water was not suitable for drinking, however, and emperor Augustus used it to fill his naumachia in Trastevere. This water supply allowed emperor Augustus and the public to enjoy sham naval battles. The water surplus was used for the irrigation of Caesar’s horti (gardens) and for the irrigation of fields. Such an abundant supply of water gives an idea how much water Rome had at its disposal.

In his chief work (written in 97 CE) De aquis urbis Romae (published in two books), containing a history and description of the water-supply of Rome, [[Sextus Julius Frontinus] as describes only a meager volume to the Aqua Alsietina. This makes sense, if the naumachia was no longer in use in his time (second half of the first century CE).

Some traces of this aqueduct were discovered in 1720. An inscribed stone slab was found inmknn 1887 near the Via Claudia. It is the only written record of the Aqua Alsietina.

The fountain of the Acqua Paola in Rome, built under Pope Paul V announces wrongly on its triumphal arch that “Paul V restored the ancient ducts of the Aqua Alsietina. “. Actually the engineers had rebuilt the old Aqua Traiana, which had run close to the Aqua Alsietina.

Hydroelectricity is electricity generated by hydropower, i.e., the production of power through use of the gravitational force of falling or flowing water. It is the most widely used form of renewable energy. Once a hydroelectric complex is constructed, the project produces no direct waste, and has a considerably lower output level of the greenhouse gas carbon dioxide (CO2) than fossil fuel powered energy plants. Worldwide, hydroelectricity supplied an estimated 816 GWe in 2005. This was approximately 20% of the world’s electricity, and accounted for about 88% of electricity from renewable sources.

Advantages

Economics
The major advantage of hydroelectricity is elimination of the cost of fuel. The cost of operating a hydroelectric plant is nearly immune to increases in the cost of fossil fuels such as oil, natural gas or coal, and no imports are needed.

Hydroelectric plants also tend to have longer economic lives than fuel-fired generation, with some plants now in service which were built 50 to 100 years ago. Operating labor cost is also usually low, as plants are automated and have few personnel on site during normal operation.

Where a dam serves multiple purposes, a hydroelectric plant may be added with relatively low construction cost, providing a useful revenue stream to offset the costs of dam operation. It has been calculated that the sale of electricity from the Three Gorges Dam will cover the construction costs after 5 to 8 years of full generation.

Greenhouse gas emissions
Since hydroelectric dams do not burn fossil fuels, they do not directly produce carbon dioxide (a greenhouse gas). While some carbon dioxide is produced during manufacture and construction of the project, this is a tiny fraction of the operating emissions of equivalent fossil-fuel electricity generation.

Related activities
Reservoirs created by hydroelectric schemes often provide facilities for water sports, and become tourist attractions in themselves. In some countries, aquaculture in reservoirs is common. Multi-use dams installed for irrigation support agriculture with a relatively constant water supply. Large hydro dams can control floods, which would otherwise affect people living downstream of the project.