Box Vale - The Weirs Wherigo Cache

Earthigo Cache

Weirs & Dams

A weir is an impervious barrier across a river designed to alter its flow characteristics. In most cases, weirs take the form of obstructions smaller than conventional dams, pooling water behind them while also allowing it to flow steadily over their tops. A weir is anchored in the bedrock so that no water can flow under it.

Weirs are commonly used to alter the flow of rivers to prevent flooding, measure discharge, and help render rivers navigable. The Southern Highlands is rife with weirs (and dams) and most were built for water catchment for industry and domestic use, and they can be found here along the Box Vale Walking Track as well as between Bargo and Tahmor, between Moss Vale and Bowral (at Bong Bong), and near Medway, Berrima and Bundanoon.

In casual conversation the words weir and dam can be used synonymously but there are technical differences between the two. A weir is typically a solid structure and the water level is constant as it flows freely over the top. On the other hand, a dam typically has sluice gates which control the rate of water flow as well as the water level on the upstream side of the dam. Some weirs have adjustable gates at the top which can be adjusted to control water flow. These weirs are called barrages.

Types of weirs

There are different types of weir. It may be a simple metal plate with a V-notch cut into it, or it may be a concrete and steel structure across the bed of a river. The shape, structure, and appearance of weirs vary with their intended function or use.

Broad-crested weirs are robust structures that are generally constructed from reinforced concrete and which usually span the full width of the channel.

A sharp-crested weir allows the water to fall cleanly away from the weir. Sharp crested weirs are typically 5mm or thinner metal plates. These weirs come in many different shapes such as the rectangular weir, the V- or triangular-notch, and special notches (trapezoidal, circular or parabolic weirs).

A notch weir is any weir where the physical barrier is significantly higher than the water level except for a specific notch (often V-shaped) cut into the panel. At times of normal flow all the water must pass through the notch.

A crump weir is a fixed weir. The water flows over the weir without the down-stream level being below the weir crest. If this is the case, such a weir is termed an underwater weir.

A needle dam is a weir designed to maintain the level or flow of a river through the use of thin "needles" of wood. The needles are leaned against a solid frame and are not intended to be water-tight. Individual needles can be added or removed by hand to constrict the flow of the river, forming a sluice.

A compound (or combination) weir is any weir that comprises several different designs into one structure. They are commonly seen in locations where a river has multiple users who may need to bypass the structure. A common design would be one where a weir is broad-crested for much of its length, but has a section where the weir stops or is 'open' so that small boats and fish can traverse the structure.

Drop Structures and Check Dams are other types of weirs but their primary purpose is erosion prevention rather than water impoundment.

Local History

Both Box Vale weirs were constructed to supply water to Mittagong.

Work began in 1908 on the first of the weirs and it was completed in 1909. It is a cylindrical-arch style wall 30-feet high (at its highest point) designed to hold seven million gallons of water in the Baker's Creek gully (now known as Nattai Creek).

Complications involving a two million gallon concrete pressure reservoir located on Welby Hill (then called Fitzroy Village) delayed the project but by 1910 the reticulated supply became available to all residents of Mittagong.

By 1928 increased consumption threatened the water supply so Council proposed raising the wall of the existing dam by four and a half feet. State engineers estimated this plan risked collapse so they insisted that a second dam be built. In 1931 this dam was constructed in a natural recession of land about 1km upstream from the first dam.

In 1965 the Medway Dam near Berrima was completed and supplied parts of both Bowral and Mittagong. This supplemented the Box Vale/Welby system.

In the 1970s negotiations were entered into for the local water supply to be purchased from the new Wingecarribee Reservoir (located east of Bowral and supplemented from Tallowa Dam in Kangaroo Valley) and consequently the larger towns and smaller Northern Villages were connected to this supply.

In 1991 the Box Vale/Welby water system was de-commissioned in favor of the water supply from the Wingecarribee Reservoir which can be mixed with Medway water to augment the Mittagong supply when necessary. The two weirs located in Box Vale have been abandon ever since.

Wherigo

This whereigo is a simple walking tour that will take you to both Box Vale weirs.

Due to tree cover and changing weather conditions GPS interference can be quite high. Accordingly, zones are large and they should not pose problems as long as you stay on the paths and trails (until it's time to go bush for the cache).

Clicking the "Download Cartridge" button will take you to wherigo.com where you can, ahem, download the cartridge you need to complete this cache.

Have fun.

** Congrats to Muppet95 for FTF! and to minniek, fitzy_1965, Nibletricecakes, mizmaz, Sensible Sis, Bonfire7, Brewmaker, Seemyshell, and The Bushies for a 9-way 2TF!**