Free vs. Submerged Weir Flow Boxes

When you choose a weir box to measure your flow rate, you’ll have to determine whether you want to install a free flow box or a submerged flow box. Installing the wrong type of box will result in erroneous, inaccurate readings and can create problems like undesired runoff, so it’s essential that you understand the right applications for each and their benefits and drawbacks. Learn about the differences between submerged and free weir flow and how each one works.

Submerged Weir Flow Boxes

Submerged weir flow boxes see the entire box placed underwater. If you have any situation where the discharge of water from your weir box can be impacted by downstream conditions — that is, there is a point where downstream conditions are deeper than the conditions where your weir is positioned, even if the weir itself is not physically submerged — you’re dealing with submerged weir flow. It’s important to remember that submergence under any conditions can affect your measurements.

If your measurements are off due to submergence, you will need to account for this. When you measure at the primary point, it will be too high. You will need to calculate the ratio between upstream and downstream heads to correct for this erroneous measurement and get the proper numbers.

Free Weir Flow Boxes

If you are using a free weir flow system, your flow discharge will not be impacted by the conditions downstream. If the water level downstream is always lower than the lowest point of the crest at the weir, you have free flow. Because most weirs are positioned on a slope upstream, the majority of systems experience free flow.

Free weir flow systems are split between suppressed and contracted systems. A suppressed system uses the sides of the notch ends to span the entire channel, while a contracted system’s notch does not span the full width of the channel. In a contracted system, the nappe can be automatically aerated. A suppressed system that doesn’t have some form of natural aeration will need to be side vented.

Why Free Flow Is Better Than Submerged

Generally, free weir flow boxes are better than submerged weir flow boxes. The reason is that they allow you to get the most accurate measurements without the need for additional calculations. While a submerged box can provide you with good measurements, you have to perform additional calibrations and calculations, accounting for different measurement points. A free system lets you just take your measurement and be done.

The downside of a free flow system is that if it does provide inaccurate readings, it’s more costly to adjust than it is to just change calculations in a submerged system.

When installing your weir, make sure that your downstream conditions don’t affect your measurement point. That may require rethinking where you install the weir box. The trick is making sure that you don’t compromise upstream conditions with the box.

Fiberglass Weir Boxes at Tracom

If you’re looking for a fiberglass weir box to suit your submerged or free flow needs, Tracom is ready to set you up. Check out our complete selection of weir boxes, options and accessories, and get in touch with us to place your order today!