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Breach Modelling

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I am after some general advice about modelling breaches. We have successfully been using variable geometry to define breaches in fluvial and coastal defences. I have heard several times recently that TUFLOW shouldn't be used for breach modelling and I'd like to ascertain if this is a generalisation or whether there is some validity behind it. I can appreciate that if I want to model the minutiae of the breach propogation and the flow processess immediately downstream of the breach then a dedicated breach modelling package or 3D modelling is possibly more appropriate. But is saying that TUFLOW shouldnt be used for breach modelling a generalisation like saying 1D models shouldnt be used for representation of floodplains? or is it more likely to be from the mouths of those with a vested interest in selling different software to us?? Or should we be more careful when assuming confidence in the breach model results?

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I have used Tuflow for breach modelling in the past for a number of different projects. The work was broad scale modelling and the client only required approximate flood envelopes and hazard mapping. I can understand the reluctance to use Tuflow for breach modelling as there is better suited software currently on the market; however Tuflow did provide adequate results which were fit for purpose.

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Another good question (two in one night!)

Yes, the hydraulic processes of a breach are in reality a complicated 3D problem, one that for any 2D scheme is going to be a challenge (and at best an approximation). TUFLOW model's the flow across a breach in the same way as it models flow over a levee. It is testing every timestep as to whether the flow across the cell sides is upstream controlled (by comparing energy levels and Froude No.), and if so uses either the broad-crested weir equation or upstream controlled supercritical flow depending on the slope of the topography at the cell sides (an adverse slope to the direction of flow uses the weir equation). In the case of a breach, the flow control is invariably at the crest of the breach, and if not downstream controlled (drowned out) will use the weir equation.

So provided that, as you say, you're not interested in the "minutiae of the breach propagation and the flow processes immediately downstream of the breach", and the flow over the breach is adequately represented by the weir equation, TUFLOW should give an accurate representation of the flooding due to the breach. If you wish to adjust the weir equation, you can use the tgc commands "Set WrF ==" and "Read MI WrF ==" to vary the weir equation coefficient spatially. Some of the these features are briefly discussed and benchmarked in a paper I wrote in 2001 (see http://www.tuflow.com/Downloads/Modelling%...me,%202001.pdf).

If you are interested in the minute details, then a 3D CFD (computational fluid dynamics) solution is probably what you need, but make sure you have a Cray and lots of patience for this modelling.

On a precautionary note, yes, we should be "more careful". Breaches are often hypothetical, and if they occur the processes are very complicated. There is therefore a lot of uncertainty just in the process of deriving a breach scenario, or in trying to reproduce a breach that actually occurred. It is therefore very important to understand and appreciate these uncertainties prior to carrying out any modelling so that the modelling is fit-for-purpose. I would imagine that in nearly all cases, TUFLOW's modelling of a breach is more than adequate for the purposes of helping to establish the flooding patterns and flood risks. However, the substantial uncertainties associated with the timing and formation of a breach need to be covered through sensitivity testing of these uncertainties, and careful interpretation of the results.

FYI, the 2008 release of TUFLOW has several new and very powerful features for manipulating topography, including the ability to easily simulate changes in topography due to breaches (eg. erosion of large sections of a levee using a single GIS object) based on time or on triggers such as exceeding a water level.

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