Flat-bottomed Trapezoidal Flumes were first developed in the early 1960’s to be placed in canals. Their shape was chosen to more closely match that of the typical small canal shapes. 

The initial Trapezoidal flumes were – like short-throated flumes – calibrated in the laboratory.  More recently, computer analysis has shown that for theses flumes, under some circumstances, the procedures for rating long-throated flumes also provides satisfactory results.

The caveat here is that the long-throated analysis is valid for heads less than about 50% of maximum.  For heads greater than this, the throats of the flumes become too short for precise long-throated flume analysis.  Above this point and the laboratory ratings must be used.

As a result, Openchannelflow classifies the traditional sizes of Trapezoidal flumes as being short-throated as most of our customers use the flumes over most of their flow ranges.

However, newly developed Trapezoidal flumes with longer throats have been developed for specific sites. These flumes lend themselves to compute (long-throated) analysis and can rightly be called long-throated flumes.  Keep in mind, though, that these flumes are usually one-off designs – with the associated drawbacks of operator training and increased fabrication cost.

In short:  Trapezoidal flumes can be BOTH long-throated and short-throated….it depends upon the flume and the application!