The Pul-i-Khaju at Isfahan.
The use of a bridge to serve also as a dam is not uncommon in Persia, where the scarcity of water necessitates impounding it in reservoirs for irrigation of the land (H. Shirley 26).
travellers made their parched and dusty way through thte guarded gateways of the bridge, seeking rest and refreshment in teh cool shaded pavilions and caravanserai to be found upon it. For to the Persians, bridges were much more than simply a means of crossing a river. By the seventeenth century some of them, such as the Allahverdi-Khan and Pul-i-Khaju, were designed as delightful retreats form teh heat and dust of the desert (H. Shirley 27).
The superstructure is built on a dam which impounds the river to a height fo six feet. From this reservoir irrigation channels lead the water to the fertile lands on either side. The dam is of stone, pierced by narrow openings, the flow through which is regulated by sluices. The bridge is 85 feet wide and consists of some twenty-four brick arches with an overall length of 462 feet... Together with Old London bridge in its prime and the Ponte Vecchio in Florence, it constitutes one fo the great bridges that were closely linked by the buildings on them to the multitudinous life of their times (H. Shirley 28).
"Ponty" as it's known to the locals, is famous for its old bridge, which was, when built, the longest single spanning bridge in the world. The bridge, built in 1750 by William Edwards (a self taught mason) was so long (45m / 140 feet span) that it took three attempts to get it right. The first, a wooden bridge was washed away by floods, the second, of stone, collapsed during construction because of its weight. The third design was also stone, but much lighter because it had 6 large holes in it... 3 on each side, of diameter 9, 6 and 3 feet. Edwards was paid 50 pounds to maintain it for seven years. In 1857, a three-arch bridge was built alongside to make it easier for traffic to cross the river (the old bridge was a bit too steep).
Alcantara Bridge over the Tagus was built by the Roman, Caius Julius Lacer, for the Emperor Trajan.
“Some of the mightiest and most impressive Roman Bridges, which fall broadly into two groups, are to be found in Spain. On the one hand there is the long low bridge of many nearly equal arches acros a broad river valley. Of this kind are the bridges of Salamanca, Cardova, and merida. As a contrast there is themajestic majestic lofty bridge of few and unequal arches thrown across a wide rocky gorge (H. Shirley 21).”