Parshall Flume Settling
One requirement for a Parshall Flume to accurately measure flow is that the converging floor be set level from front-to-back and from side-to-side. However, because of the material weight, the unreinforced nature of natural channels, long-term consolidation of the foundation soils, and general installation errors, a flume may be installed off-level or may experience settlement over time. When this happens, the standard flow equation is no longer accurate.
Research at Colorado State University has resulted in the development of a generalized correction for settling which is applicable to both free and submerged flow conditions.
The true or measured discharge (Qm) may be determined by:
Where (with level measurements taken on the LEFT side of the flume looking downstream):
Parshall Settling Defined
A negative lateral (crest) slope is when the RIGHT side of the flume - looking downstream, in the direction of flow - is lower than the flume-floor centerline.
A positive lateral slope is where the LEFT side is lower than the flume-floor centerline.
A negative longitudinal slope is where the flume entrance is LOWER than the exit of the flume. A positive longitudinal slope is where the flume entrance is HIGHER than the exit of the flume.
Correction for Parshall Flume Free-Flow Conditions
Correction for Parshall Flume Submerged Flow Conditions
Where:
For submergence of less than 90%, the flow in an off-level Parshall Flume can be corrected to within +/- 3% of the true discharge. For submergence of 90% or greater, the flow can be corrected no better than +/- 5% of the true discharge.
These corrections were determined for:
- Parshall Flumes 1”, 2”, 3”, 12”, and 24” in size (although researches state their belief that the procedure should be applicable to larger flume sizes)
- Lateral slopes of -11.6% to 11.6%
- Longitudinal slopes of -10.5% to 10.5%
Sources: Correction for Settlement of Parshall Flume, Settlement and Submergence Adjustments for Parshall Flume
