Weirs and control structures

Hydrographers inspect the condition of control structures on a routine basis to ensure that they're functioning as expected and are clean of debris. Some control structures are incredibly sensitive to the effects of debris, where even the slightest build-up of algae can change the stage-discharge relationship.

There may be significant changes in water level and flow data immediately after a site visit if the hydrographers have removed a large amount of debris from a control, such as a tree or rocks.

These large changes in water level and flow caused by debris on controls may be present in near-real time unverified data but will be edited out by the hydrographers as part of the quality coding process.

Artificial controls have their drawbacks. While increasing low flow accuracy, they can increase sedimentation and disrupt fish migrations.

Water managers are increasingly required to consider fish passage around control structures. If we identify a control as disrupting native fish passage, the site may need a fish friendly alternative.

Fish friendly controls, fish ladders and fish ways, allow fish to migrate upstream and downstream of the site. They are designed to create ideal conditions for native fish movement, including rapids and resting pools.

The following principles are best practice for considering new fish friendly controls:

• should be suitable for all native fish that move through the site.
• should be designed to provide year-round passage for fish over the full range of flows or, as a minimum, ensure passage when fish are migrating.
• should provide both upstream and downstream fish passage.
• should be designed to minimise fish injury.
• Resting habitat should be provided within the fishway

The RWMP works closely with the Arthur Rylah Institute for Environmental Research (ARI) in identifying the sites that pose the largest risk to native species, and to design and install fish friendly controls.

ARI have a range of resources and guidelines on fish way design, construction and performance.

While most controls work by restricting the water over a known barrier (in height and width) flumes constrict the waterway horizontally, that is they push more water through a narrow channel without much vertical change in bank height.

As they don’t have the knife edge or crest seen on other types of weirs, flumes can provide greater flow accuracy in a channel, but are susceptible to being less accurate if there are back up effects present.

Some of the RWMP sites monitor regulated channels, culverts or pipes. The controls on these sites can be gates, valves, drop-boards or culverts and are often operated by third parties.

The stage-discharge relationships at these sites are often complex and dependent on local operations.

Constructing an artificial control isn't always possible, or required, at all sites.

At these sites, it's the natural shape of the waterway that determines the stage-discharge relationship.

We simply refer to these sites as having ‘natural’ controls.

We try to pick the best possible location to install our sites, and often weigh data accuracy against environmental impacts.

There are a range of weir types across our monitoring network, including varieties such as v-notch weirs, rectangular weir plates, and concrete hump weirs. These all function by having an engineered structure in the waterway with a known height and width, and therefore a known area.

Weirs are designed to provide the optimal hydraulic conditions to develop and maintain a stage-discharge relationship, and are often installed at sites where highly accurate flow data is required.