Parshall Flumes Solutions

Flumes are specially shaped, engineered, and static piece of equipment that restrict flow causing an acceleration. This restriction of flow creates a relationship between the fluid level in the flume and the rate of flow allowing for easy sight measurement.

Flumes accelerate fluid from a subcritical flow to a critical or supercritical flow through a few different methodologies:

1. Constricting the side walls of the flume.
2. Creating slope by manipulating the floor.
3. A combination of the above.

Once at a critical or supercritical flow a single head measurement can be used to determine the rate of flow at a single, defined point, notated as Ha. The relationship of fluid level at Ha and rate of flow can be either calculated through theory and formula (long-throated) or determined through lab testing (short-throated).

Accuracy of Flumes

Under perfect conditions flumes can be accurate within + (2-5%). Once installed, field conditions can cause accuracy to be closer to + 10%. These are similar accuracies to other open channel flow measurement devices such as wiers.

Advantages of Flumes

While both weirs and flumes measure open channel flow, flumes have some clear advantages.

1. Measure higher rates of flow.
2. Less head loss.
3. Easily pass debris.
4. Smaller footprint
5. Available in many styles and sizes
6. Easier to maintain
7. More resistant to downstream effects such as submergence.

Classes of Flume

Depending on the style of flume, each of these sections will have a different shape and proportion. The key difference lies in the type of throat, thus breaking flumes into two classes:

• Long-throated Flumes – Control the rate of flow with a throat long enough to create a laminar flow while still contained within the throat section. By creating a laminar flow, the discharge rate of long throated flumes can be calculated using fluid mechanics concepts and Ha can be measured anywhere above the throat section of the flume.
• Short-throated Flumes – A bit of a misnomer, short-throated flumes may actually be quite large in specified length. They control the flow rate in a curvilinear manner rather than laminar-ly. Due to their curvilinear nature, each style and size of flume has its own highly specific relationship between rate of flow and Ha. Unlike their long-throated counterparts, each style and size of short throated flume has their own set point of measurement Ha. Calibration for this class of flume has been done empirically using other more accurate flow measurement devices to create each flumes discharge table.

Most flumes have the same 3 basic sections.

Converging section
This section funnels subcritical flow (FLOW MUST BE SUBCRITICAL) into the throat section.
The point of measurement is in this section for all types of flumes. This point is fixed for short throated flumes while variable for long throated flumes

Throat section
Further constricting flow and accelerating it from subcritical to critical or supercritical, this section is essential for the proper functioning of the flume. It is here where the most drastic construction of the sidewalls and elevation changes occur.

Diverging section
Now that the fluid is at a supercritical flow, it can potentially cause downstream scour and erosion. It is the job of the diverging section to slow the flow and help protect the downstream channel.
Note that not all flumes have this section, most notably the H-style and Montana Flumes. These two styles are recommended to discharge vertically into a collection bowl or a pool.