Interpolate

The Interpolated Section calculates a weighted average of the river or conduit section properties upstream and downstream to produce a hybrid section according to its location. For example, in the case of sparse cross sectional data, interpolates can be used to ensure smooth gradations between channel properties.

Data

Theory and Guidance

The Interpolated Section calculates a weighted average of the river or conduit section properties upstream and downstream to produce a hybrid section according to the location of the interpolated section.

Interpolated Sections are usually required for two reasons. First, in the case of sparse cross sectional data, they ensure smooth gradations between channel properties thus avoiding sudden variations which can cause sensitivity in a model. An example of this use is shown in the diagram below. Thus they should be used when properties vary radically between sections.

The other reason to use Interpolated Sections is to ensure adequate spatial resolution. In regions where there is likely to be significant water surface curvature, as a Froude number approaches unity for example, or in a backwater profile, Interpolated Sections should be inserted.

If the Direct Method steady state solver is used, this gives an indication of how many extra sections are required and at which locations. If more than three extra sections are recommended, this suggests either a data anomaly or very strong water surface curvature.

The simulation engine also performs a check on the suitability of the upstream and downstream sections for interpolation. For an example of two incompatible sections, see the diagram below.

Since interpolation is based on depths, the projection of Section One onto Section Two in the diagram above does not include any information on flood plain in Section Two . This indicates a possible data anomaly if the sections are close together. If they are not, an extra surveyed section is required. Details of poor interpolation are sent to the zzd file.

It is possible to choose the locations of Interpolated Sections in a reach if it is known where the large surface curvature occurs, for example upstream of a weir or a bridge. In general however, it is recommended that sudden changes in chainage are avoided since they may induce additional errors.

General

The units upstream and downstream of a single or consecutive sequence of Interpolated Sections must be:

• either a River section or CES section (but not VPMC Cross Section ).
• Symmetrical Conduits (but not any other type of conduit)

The units upstream and downstream of an Interpolated Section must be of the same type.

Datafile Format

Line 1 - keyword `INTERPOLATE' [comment]

Line 2 - Label 1, [Label 2, Label 3]

Line 3 - dx, easting, northing