Whether you call them nested, dual-range, or flume reducer inserts, nested flumes are an innovative solution to the problem of measure two substantially different flow ranges over time. By nesting a small, lower flow rate flume inside of a larger, higher flow rate flume; both sets of flows can be accurately measured. The most common use for nested flumes is where initial flow rates are expected to be significantly less than the final operating flow rate, but not always!
Nested flumes can be used for:
- Low-to-high flows (sub-division build-outs, plant expansions)
- High-to-low flows (collection system changes, I&I reduction, water conservation, population decline)
- Seasonal flows (resorts, vegetable / seafood processors, snow-melt runoff)
Nesting is most common in Parshall flumes, but is also available with Montana and Cutthroat flumes. The nested, or inner, flume need not be of the same type as the outer flume. Trapezoidal and HS / H flumes, while not nesting outer flume candidates in themselves, nest easily given their flat-bottomed designs.
The nested flume may be:
- Factory installed - temporary (low-to-high flows)
- Field installed - permanent (high-to-low flows)
- Removable / reinstallable (seasonal flows)
When factory installed, the inner flume is usually bolted into place, with the connection caulked for watertightness. As the flow rates rise over time, the smaller, inner flume can be unbolted and removed, leaving the larger, outer flume permanently in place.
Smaller flumes can be retrofitted into oversized flumes through the use of retrofit kits, which include transition wing walls, nesting support reinforcement, anchors, and sealant. Here the smaller retrofit flume is permanently secured into the larger flume to hold the flume in place.
For seasonal, removable / reinstallable installations, the smaller, inner flume is designed to nest as snugly as possible into the larger, outer flume so that friction holds the flume in place. The connections between the flumes are then caulked as necessary so that all flow is directed into the smaller, inner flume.