3.14.4.4Weirs
Weir channels do not require data for length, Manning’s n, divergence or bed slope (they are effectively zero-length channels, although the length is used for automatically determining nodal storages – see Section 3.13.2.2).
A calibration factor is available for weirs. For a given flow the backwater (head increment) of the weir channel is proportional to the factor. It is normally set to 1.0 by default, and modified if required for calibration purposes. This factor is not the weir coefficient, rather a calibration factor to adjust the standard broad-crested weir equation.
For weirs a standard weir flow formula (from “Hydraulics of Bridge Water ways”) is used, and no additional input is required. The weir is assumed to be broad-crested, such as a causeway or an embankment. A weir with significantly different characteristics can be modelled using a non-inertial channel with carefully selected section properties.
Weirs have three flow regimes of zero flow (dry), upstream controlled flow (unsubmerged) and downstream controlled flow (submerged). The weirs invert and a calibration factor are entered using the 1d_nwk attributes in Table 4 .10.
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