Anaviduct is a special class ofnavigable aqueduct, in which thewaterway also includes alock. One example of a naviduct has been built atEnkhuizen on theHoutribdijk in theNetherlands on the instructions of theRijkswaterstaat. This cost over €55 million and was completed in 2003. It is big enough to simultaneously transfer two largeRhine river-barges from theMarkermeer to theIJsselmeer or vice versa. The structure was chosen because a busy waterway crosses adam carrying a busy road, but the space available for separate aqueduct and lock facilities was restricted.
The term is derived from the words for navigation and aqueduct, which are in turn partly derived from the Latin wordsnavis meaning ship andducere meaning to lead or conduct.[1] It can also be thought of as a lock system with an underpass under the lock for road traffic. The term is introduced and used extensively in the Netherlands for referring to the works near Enkhuizen in Dutch and English language texts.[2]
The world's only operational naviduct is near Enkhuizen, and is part of works commissioned in 1995 and administered in the Netherlands by the Rijkswaterstaat.[1][3] This location is at the western end of the Houtribdijk in the province ofFlevoland, which separates the Markermeer from the IJsselmeer. This dam was built as a possible northerndike to build theMarkerwaardpolder in 1975. Although construction of this polder has been permanently shelved and was officially abandoned in 2003, the dam is still called a dike. Until the building of the naviduct, marine transport between the two lakes had to cross the dike at the Krabbersgat lock (near Enkhuizen) or the Houtrib lock (nearLelystad) due to the differing lake levels. The two lakes have the same target level, but wind surge and feed flows affect the two water bodies differently. Without a lock on the naviduct, the flow of water through the aqueduct could become too high for navigation. Highway N302 runs along the dike between the two towns, withbascule bridges at the two lock locations. Prior to the official opening of the naviduct at 11am on 26 April 2003 byRoelf de Boer, delays often arose for both road and water traffic, which could not pass the lock location simultaneously, and this was a particularly acute problem at the Enkhuizen end of the dike.[4][5]
The naviduct is constructed in a specially designedpolder, having an artificially managed water table depth of -12.00 mNAP. The polder is diamond-shaped with a length of 500m and a width of 160m, while its embankment is elevated to +6m NAP. The road under the naviduct reaches a depth of -10.80m NAP. The polder guide dams were built using thegeotextile tube method, where porous sausage-shaped bags are pumped full of sand slurry. The slurry isdewatered as the pumps fill the bags, forming a stable structure that is subsequently covered with quarried stone.[6]
Both locks have a length of 125 meters, a width of 12.5 meters and meet theVa requirements of theCEMT Class (i.e. each is suitable for a large Rhineriver-barge).[3] Special waterproof concrete was used for the locks, as they are designed to operate without the need for maintenance resulting from surface wear.[7] The completed project cost €55 million before tax.[8]
TheSeneca Aqueduct on theChesapeake and Ohio Canal is a disused naviduct, consisting of a lock (Riley's Lock) and an aqueduct. The lock is located next to the aqueduct, but both are part of a single structure made out of sandstone. It was used from 1833 to 1924. One of the arches of the aqueduct collapsed in 1971, so it can no longer contain water.
The Dutch government is currently considering whether to replace some of its existing lock and opening bridge combinations with naviducts. Naviducts have a high construction cost compared to standard locks and bridges, but benefit the economy by reducing delays. The formerState Secretary forInfrastructure and the Environment,Joop Atsma, issued a statement in 2012 detailing the factors influencing the decision to build a new naviduct on theAfsluitdijk atKornwerderzand. Strengthening of the existing locks was costed at €33 million, building new locks at €91 million and a naviduct and new locks at €200-450 million, depending on the size of locks and size of highway. A naviduct would give waiting time annualised benefits of €3 million to maritime traffic and €13 million to road traffic, based on the bridge opening 7,000 times a year.Cost–benefit analysis of all potential solutions were considered, and for the present time, the locks will be strengthened. However, it is quite possible that a naviduct will be built to replace the locks when they reach the end of their useful life in 2050.[9]
52°41′26.40″N5°17′43.78″E / 52.6906667°N 5.2954944°E /52.6906667; 5.2954944