Free Weir Flow vs Submerged Weir Flow

When you install a weir box to measure flow rate, you’ll be faced with one of two circumstances: Either the flow will be free, or it will be submerged. These flow types offer very different experiences for your weir and any subsequent measurements, so you need to make sure you know the benefits and downsides of each. Learn everything you need to know with our free weir flow vs. submerged weir flow comparison.

Free Weir Flow

In a free weir flow system, the discharge of the flow over the weir isn’t dependent on downstream conditions. The nappe of the weir would discharge into the air. Put simply, a weir is free flowing when the water level downstream is lower than the lowest point of the weir’s crest. This will be the case for most weirs because they’re typically used on a slope. 

One of the primary differences among free weir flow options is between contracted and suppressed weir systems. A contracted weir is one in which the notch isn’t the full width of the channel. A suppressed weir, on the other hand, uses the sides of the channel as the notch’s ends, making it span the entire channel. The former allows the nappe to be automatically aerated, but the latter doesn’t guarantee that. If your flow isn’t naturally aerated in a suppressed weir, you’ll have to vent the sides.

Submerged Weir Flow

As the name suggests, weir flow is submerged when the weir itself is underwater, though it’s important to remember that submerged weir flow technically encompasses all conditions in which the discharge of water from the weir is impacted by the downstream conditions. The problem with submergence is that it alone will throw off your measurements.

If flow-rate measurement is thrown off by submergence, it can be accounted for without too much effort. Due to the submergence, the measurement based on the primary point will give you a measurement that’s too high. To correct this, you just have to calculate the submergence transition or the ratio between the upstream and downstream heads. 

Which Is Best?

In general, a free weir flow is preferable to one that is submerged. While a submerged weir can still provide accurate measurements, you have to account for a new point of measurement, and calibrations can be trickier than they would be in an offset free-flow system. The downside is that adjusting a free weir flow is more costly than changing the calculations for a submerged weir flow.

If you’re looking to ensure that your weir doesn’t become submerged, you have to make sure that the downstream conditions don’t affect the flow at the point of measurement. This may mean setting the weir up higher than you initially expected. This can be an easy solution when installing a weir, but you have to make sure that the upstream conditions aren’t compromised by this or you’ll be getting inaccurate measurements for an entirely different reason.

Find a Weir With Tracom

After reading our free weir flow vs. submerged weir flow comparison, you probably know which setup is best for your needs. To get the most out of your weir, look no further than Tracom, no matter what kind of channel you’re looking to measure. Our team can offer customized solutions that work for your unique situation. Contact us today to get started!

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