Weirs and sloping cell faces

[Guest Chris Nielsen]

The autoweir and weir_dz structures allow for weir of varying elevations determined from the model geometry - either from the cell centres, the vertices contained in the 2dm file, or via specification of an external polyline (using the external nodestring command). See for more information.

 

There is a complication in the weir flow calculation when the vertices of a cell face are of different elevations.  If they are then the following checks are made:

 

• If the water level is higher than both vertices (H1 and H2), the crest level is (H1 + H2) / 2 (ie the average) 
• If the water level is between the two vertices (H1 < WL < H2 for example), the crest level is adjusted so that the flow area of the weir (assumed to be flat across the face) is consistent with the actual flow area (which forms a triangle). H = (WL + H1)/2

The following diagram illustrates this adjustment.