Free-Overfall & Weir Flow
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Especially if Supercritical is set to OFF, the percentage of free-overfall and weir flow velocity points should be checked. The review should seek to check that excessive number of points are not free-overfalling, and if so:
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that this is in accordance with the expected flow (eg. weir flow over a levee) – check that the weir option is on if significant weir flow exists; and/or
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the affect on the overall flow patterns is minimal.
The review is best carried out by:
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Monitoring the numbers after “CS” or “FO” on the screen or in the .tlf file – see Section 6.1.1.
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Specifying flow regime output (see Map Output Data Types) to generate the _R.dat file. This file shows the flow regime (see Table 7 .31)
The presence of significant areas of supercritical and/or weirs can be acceptable in large areas of sheet flow. However, care should be taken in interpreting the flow behaviour in these areas, particularly if the flow is supercritical as complex hydraulic processes (eg. hydraulic jumps, surcharging against buildings) can occur.
Typically, most supercritical and weir flow occurs:
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around the edge of a model where it is wetting and drying and has little influence over the general flow behaviour; or
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down steep slopes or over significant drops (eg. over a levee).
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