Type
Valve
Alpha zero
Distribution
The required formula to be applied for the specific weir-structure is defined in the attributes. A range of formulas are available:
· Broad Crested Weir formula
· Weir formula 1
· Weir formula 2 (Honma formula)
The standard formulations for flow over a broad crested weir are established on the basis of the weir geometry and the specified head loss and calibration coefficients. These formulations assume a hydrostatic pressure distribution on the weir crests. Different algorithms are used for drowned flow and free overflow, with an automatic switching between the two.
Weir formula 1 is based on a standard weir expression, reduced according to the Villemonte formula.
For the Weir formula 2 the discharge is calculated using the Honma formula.
For the Weir formula 1 and Weir formula 2 a number of weir formula parameters must be specified for the geometry and model parameters.
For more information see the scientific documentation.
Valve regulation of the structure flow can be defined as part of the structure definition.
Four different valve regulation types are available:
· None: No valve regulation applies (flow is not regulated).
· Only Negative Flow
Only flow in negative flow direction is allowed. Valve regulation does not allow flow in positive flow direction and the flow through the structure will be zero in this case.
· Only Positive Flow
Only flow in positive flow direction is allowed. Valve regulation does not allow flow in negative flow direction and the flow through the structure will be zero in this case.
· No Flow
No flow is allowed in the structure. Valve regulation closes completely the structure.
For definition of flow direction please see Flow direction.
When the water level gradient across a structure is small the corresponding gradient of the discharge with respect to the water levels is large. This in turn may result in a very rapid flow response to minor changes in the water level upstream and downstream.
Alpha zero is the water level difference at which the discharge calculation is described by a linear variation. If the water level difference is below this value the discharge gradients are suppressed.
The default setting is 0.01 meter. If a structure shows oscillatory behavior it is recommended to increase this value slightly.
The distribution of the calculated discharge along the section of structure faces can be specified in two ways
· Uniform
· Non-uniform
When non-uniform distribution is selected the discharge will be distributed as it would have been in a uniform flow field with the Manning resistance law applied, i.e. is relative to h5/3, where h is the depth. This distribution is, in most cases, a good approximation. This does not apply if there are very large variations over the bathymetry or the geometry. The distribution of the discharge only includes the faces for with the element to the left and the right of the face is a real wet element. In no elements on the downstream side of the structure are real wet elements the distribution is determined based on the upstream information.
A uniform distribution is applied for the vertical distribution.
For Composite structures the distribution for the first structure is applied.