A nested (or dual-range) Parshall flume is one where a smaller, lower flow range flume has been mounted into a larger, higher flow range flume. By nesting the two different flumes together, over time two substantially different ranges of flow can be measured in one installation.
The flat wall and drop throat of the Parshall flume make them particularly well adapted to nesting. The smaller, inner flume is usually attached to the larger, outer flume through the use of sheet metal screws through wing walls and by anchoring angle at the top of the flume; while the void space between the inner and outer flumes are usually filled with sand capped by grout to allow for later removal.
In order to allow the inner flume to sit flush on the outer flume’s floor, the end flanges and encapsulated ribs on the inner flume’s floor are omitted. This omission, although necessary, does make the flume more susceptible to distortion. During the backfill process, therefore, it is important to make sure the inner flume is sufficiently braced / cribbed to ensure that the sidewalls do not distort. Distortion of the inner flume’s floor is not as great a concern as the sidewalls are as the inner flume is intended to sit flush and inner flume’s floor is screwed to the outer flume’s floor.
Wing walls are used to direct flow smoothly from the outer flume (or channel) into the inner flume. The wing walls are integral to the inner flume and usually are screwed / bolted to the outer flume or channel.
For retrofit nesting installations, factory pre-drilling and counter-sinking the attachment points eases installation. Without wing wall flanges and factory drilling, retrofit nesting can be considerably more difficult.
There are three general uses for nested Parshall flumes:
- Low initial flow to higher long term flows. Examples include subdivisions and plant expansions.
- Higher initial flows to lower current flows. Collection system changes, improved plant efficiencies, changes in plant output, I&I reduction, water conservation, population decline are all examples of these flows.
- Seasonal flows. These flows are less common, but include resorts, vegetable / seafood processors, and snowmelt applications. For these flows, the larger, outer flume is sized to handle considerably more flow than the inner flume – much more so than other applications.
For additonal information on nested (dual-range) flumes, vist Nested Flumes.