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QH Structure (Special Weir)

A QH structure uses a user-defined QH relation, that consists of at least two tabulated values for water level above the weir crest, H [m], and corresponding discharge, Q [m3/s]. Intermediate values are linearly interpolated.

QH structures are used to model a structure whenever the standard formulation for a broad crested weir or a culvert is not sufficiently flexible.

The QH relation has to fulfil certain conditions:

  • the first H-value has to be the overflow elevation (weir crest);
  • all Q-values must be positive, or all Q-values must be negative, i.e. water must flow either downstream or upstream for all H-values;
  • the H-values have to be given in a monotonously increasing order, or decreasing is Q-values are negative;
  • the largest H-value given in the table shall not be less than the largest H-value to be computed. The model does not extrapolate beyond the tabulated values.

Note that if all Q-values are negative, then the control water level is taken from the geometric downstream (meaning that the control water level is always where the water is coming from).

Special weirs

Special weirs are QH structures, but with some added functionality:

  1. Two QH relationships can be specified, one for positive flow and one for negative flow. Both relationships use the water level at the upstream (geometric, not flow determined!) h-grid point.

  2. The QH relationships are used for free overflow. For submerged flow, a steady state energy equation is used.

Tabulated structure

Similarly to a QH Structure, the discharge through a Tabulated Structure is calculated based on values in a table. However, for the Tabulated Structure, the table is two dimensional. The values in the table can be one of thee different types:

  1. Discharge as a function of up- and downstream water levels.

  2. Upstream water level as a function of discharge and downstream water level.

  3. Downstream water level as a function of discharge and upstream water level.

The actual discharge through the Tabulated Structure is found by appropriate interpolation in the tabulated values.