Iron Trunk AqueductNearby Tourist Attraction,  in or near to Milton Keynes

Cosgrove Aqueduct or the Iron Trunk Aqueduct

Total length   101 feet
Width              15 feet
Height            60 feet
Water depth  6 feet 6 inches

Cosgrove aqueduct is a navigable cast iron trough navigable aqueduct that carries the Grand Union Canal over the River Great Ouse, on the borders between Buckinghamshire and Northamptonshire at the northwest margin of Milton Keynes.

The present structure was built in 1811, to replace a previous brick structure that had failed.

When the present structure was erected, it was known as the "Iron Trunk". The structure has two cast iron trough spans, with a single central masonry pier. The abutments were constructed in masonry but have been refaced in brick during the twentieth century. The trough is 15 feet (4.6 m) wide, 6 ft 6 in deep, with a total length of 101 feet (31 m). The canal surface is about 40 feet above the river.

There are large approach earthworks about 36 feet high above the valley floor and 150 feet wide, with a total length of half a mile.

The Grand Junction Canal which later was absorbed into the Grand Union Canal, required to cross the course of the River Great Ouse, the lowest point between the summits at Tring and Braunston.

Initially flights of locks, four at the southeast and five at the northwest, were used to allow the canal to descend to cross the river on the level, and this arrangement came into action in 1800.

However William Jessop, the canal company’s engineer, designed a three-arch brick viaduct so that the canal could cross at a higher level, reducing the water loss and delay in locking down to river level. His structure was opened on 26 August 1805.

A section of the canal embankment collapsed in January 1806. This was repaired, but the aqueduct structure actually collapsed in February 1808, cutting the canal and all traffic.

Although Jessop is sometimes blamed for the failure of the first structure, the collapse actually led to a legal dispute with the original contractor. The issue went to trial, with damages eventually being awarded to the Grand Junction Company for loss of trade while the canal was out of action, and the cost of the replacement aqueduct. As a temporary measure the lock system, was brought back into use, and Benjamin Bevan, an engineer employed by the canal company, designed a replacement structure.

By this time, cast iron trough aqueducts designed by Thomas Telford for the Pontcysyllte Aqueduct and others, had proven their worthiness, and Bevan adopted the system for his structure.

However, the canal was then operating as a wide canal in contrast to many northern narrow canals. This meant the troughs had to be considerably larger and stronger.

The cast iron units were cast at the Ketley foundry at Coalbrookdale; who had already been involved with Telford on the Longdon-on-Tern Aqueduct.

They were transported to Cosgrove by the canal itself, and assembled on site.

The new structure was completed in January 1811.