Channels, weirs and barrages (continued)

The Goolwa Barrage, (Fig. 5) some 10 km from the Murray mouth, in South Australia, is another unique structure. Construction of this barrage restricted the ingress of seawater to the lower reaches of the river, converting Lakes Albert and Alexandrina into freshwater storages extending 150 km upstream to Blanchetown. These major structures have been in use for many years and are still giving good service. Conventional alternatives would have been much more costly because of the extremes of flow in the River Murray and the shape of its channel cross-section.

5 Goolwa Barrage, River Murray Mouth, South Australia

Distribution channels in irrigation areas reflected the technology of the late 19th century and early 20th century. Masonry and brick were the principal materials for large channel structures, with cast iron being used for gates and superstructure. For smaller structures, e.g., bridges over channels and channel regulators, the very strong and durable local red gum timber was used extensively. Many of these remained in service for over 50 years and some still exist.

Extensive use has been made of precast and prestressed channel structures since the early 1960s. Standard designs were developed for a wide range of bridge components, regulators and other structures. These units were usually fabricated at a central workshop for a large region, thus facilitating a high standard of quality control and economies of scale. Earthworks were carried out with steam shovels for deep cuttings and horse teams pulling ploughs and scrapers for the remainder. The horses' feet provided excellent compaction of channel banks, and the highly picturesque horse teams were used in Victoria and New South Wales up to 1939. They were inevitably replaced by mechanized equipment in the late 1940s as tractors, bulldozers, scrapers and draglines became generally available.

The profile of the early channels, with relatively flat side slopes, reflected the need to provide for ease of movement by horses rather than the most efficient hydraulic performance. This profile, however, also suited the type of channel regulator required for effective flow regulation in flat country, so that an acceptable compromise may well have been achieved by accident.

Australian Academy of Technological Sciences and Engineering