Two figures in, a clear answer out. The broad crested weir calculator uses acceleration due to gravity (g), height of stream above the weir crest (h) and weir length (l) to produce coefficient (c) and discharge (q), applying the standard relationship for this problem in the order the calculation actually needs it.
What Is the Broad Crested Weir Calculator?
This broad crested weir calculator is for the kind of problem where you know the physical quantities but not the number you need. It turns acceleration due to gravity (g), height of stream above the weir crest (h) and weir length (l) into coefficient (c) and discharge (q).
Physics calculations go wrong in predictable ways, usually through mixing units or dropping a digit. Work through the result one step at a time and check that the figures are the size you would expect at each stage.
How the Broad Crested Weir Physics Works
The relationship is applied step by step, and each step is a standard piece of physics rather than a single opaque formula. That means you can check the intermediate values against what the theory predicts, which is usually faster than checking only the final answer.
Keep every value in the same unit system throughout the calculation. Mixing metres and feet part way through is the single most common reason a correct formula gives a wrong answer.
Units, Assumptions and the Limits of the Model
The calculation assumes ideal conditions: no air resistance, no heat loss, and dimensions that do not change while you work. Those assumptions are reasonable for most classroom and engineering estimates, and unreasonable for anything more precise.
Where a value is a rate, it has to be quoted for the same period as everything else. A per-hour figure cannot be combined with a per-day figure without converting one of them first.
A Worked Example
Working through an example, with acceleration due to gravity (g) set to 10, height of stream above the weir crest (h) set to 5, weir length (l) set to 2, the calculator returns coefficient (c) of 1.7213, and discharge (q) of 38.49. Substitute your own figures and the same steps produce the same kind of answer.
Frequently Asked Questions
What do I need to work out coefficient (c)?
Enter acceleration due to gravity (g), height of stream above the weir crest (h) and weir length (l). The calculator then works through the relationship and reports coefficient (c) and discharge (q).
What does the broad crested weir actually calculate?
It takes acceleration due to gravity (g), height of stream above the weir crest (h) and weir length (l) and converts them into coefficient (c) and discharge (q) using the standard relationship for this problem. Each step is applied in turn, so every output is traceable back to the numbers you typed in.
How should I enter acceleration due to gravity (g)?
Use the value you actually have, in the unit shown next to the field. Enter every value in the same unit system. The calculator assumes the figures you supply are consistent, so it cannot correct a mistake that happens before the calculation starts.
Why does a small change in one value move the answer so much?
That happens when an output depends on a ratio, because dividing by a small number magnifies any change in it. Check the units first, then check whether the value you entered is in the range the relationship was designed for.
Is the answer exact?
It is as exact as the formula and the inputs allow. The method itself is the standard one for this problem, so the only sources of error are the figures you enter and the precision you ask for.