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Installing a Cuthroat Flume for Submerged Flow

In some applications (i.e. flat grades or the growth of vegitation) it is impossible or impractical to install a Cutthroat Flume so that it can operate under free-flow conditions.    When such a conditon exists, the flume may need to be set so that it operates under submerged flow conditons.

Submerged Flow Advantages

An advantage of operating a flume under submerged flow conditions is the smaller head loss which occurs in the flume versus that under free flow.  The head loss reduction may mean that - for a given maxiumum flow rate - the upstream channel banks may not need to be raised.  

The flat bottomed construction of the Cuthroat flume also means that it can be set directly on the floor of the channel without the need to form a ramp up into the flume.  

Finally, direct floor placement means that upstream seepage losses are reduced as the flume drains quicker.

Submerged Flow Profile

A Cutthroat flume operating under submerged flow condtions will have a very different water surface profile than that operating under free flow conditons.

water surface profiles in a Cutthroat flume under free - transitional - submerged flows 

Profile (a) represents free flow, (b) represents transitional flow, and (c) represents submerged flow.

Measuring Ha and Hb

A Cutthroat flume operating under submerged conditions requires measurement of both the upstream (Ha) and downstream (Hb) water levels.   The ratio of the downstream to upstream levels (Hb/Ha) represents the degree of submergence that the flume is experiencing.  

submergence transition values for various lengths of Cutthroat Flumes

As the length of a Cutthroat flume increases, so does its ability to resist the effect of submergence.  The ratio at which the downstream water levels retard the free discharge out of the flume is the submergence transition (St).


Source:  Skogerboe, G., Cutthroat Flumes for Water Measurement, Agency for International Development, September 1974

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