When using a weir to measure the flow of water in a natural channel or man-made structure, one of two flow regimes are present:  free-flow or submerged flow.

Free Weir Flow

Free-flow, also called modular flow, is when the discharge of the weir is not restricted by downstream conditions – where the nappe discharges into the air.

Free-flow exists when the downstream water level is lower than the lowest point of the weir crest elevation. 

In a contracted weir (one who’s notch does not span the full channel), the nappe is automatically aerated and springs free of the weir crest.  In a suppressed weir (where the sides of the channel act as the ends of the weir notch), the nappe may, or may not, be aerated.  Here the nappe should be vented on each side so as to ensure aeration.

Submerged Weir Flow

When downstream conditions are such that the discharge of water over the weir is reduced, the weir is submerged, or non-modular.

When a weir is submerged, the indicated flow rate (based upon measuring at the primary point of measurement) will OVER indicate the actual flow rate. 

To correct for the reduction of flow over the weir, a correction for submergence must be performed.  The calculation of the submerged flow rate requires that the ratio of submergence be calculated by measuring the upstream and downstream heads.  The point at which flow changes from modular to non-modular (free to submerged) is called the submergence transition. 

It is strongly recommended that when weirs are used for flow measurement, that they be designed and installed so that they operate under free-flow conditions at all times.  This may require setting the weir higher then otherwise might be necessary so upstream freeboard should be checked to ensure that this does not cause any problems. 

When considering the relative cost vs benefits of free and submerged flows, remember that the free-flow calibration of weirs is more accurate than the submerged flow calibration.