Modelling Accuracy Uncertainties Impact Mapping: Difference between revisions
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=What are the reasons
* The level of uncertainty/inaccuracy in the input data, especially uncertainties in hydrological inflows (which can be considerable) and topography.
* Whether the model is calibrated, and if calibrated, the range of calibration events and quantity/quality/type of calibration data. A model well calibrated to a range of flood events will be much more accurate than an uncalibrated model. More information on calibration can be found in <u>[https://www.tuflow.com/Download/Publications/October2016_FMA_Newsletter_HuxleyRyan.pdf Flood Modelling: How Accurate is Your Model?]</u> or an Australian Water School webinar <u>[https://www.tuflow.com/library/webinars/#maximise_accuracy Maximising Hydraulic Model Accuracy]</u>.
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*Accuracy of the input data (e.g. terrain, landuse, hydrologic inflows).
*Approach used for solving the underlying mathematical equations describing free surface fluid flow.
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Understanding the degree of uncertainty is important for setting absolute metrics such as design levels and freeboard. While less important for impact mapping because the uncertainties are present in both sides of the comparison. The hydraulic modelling carried out should be based on recommended industry and software guidelines, and follow sound modelling practices.<br> An overview on uncertainties in flood modelling can be viewed in Australian Water School webinar <u>[https://www.tuflow.com/library/webinars/#jul2019_how_wrong How Wrong is Your Flood Model?]</u> <br> <br>
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=What is numerical noise?=
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=What approaches are
Establishing guidelines for mapping tolerances for flood impact assessments typically follow one of the approaches below, with the approach taken dependent on the objectives and the type of hydraulic modelling output field being mapped:
*A percentage, for example, a maximum increase in velocity of 10%. A threshold or cutoff is sometimes used below which the impact is assumed to be inconsequential or to discard slight changes to near zero values.
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*For a change in output to be significant, it needs to represent the risk to the sensitive receptor. For example, erosion potential is best measured by an output field such as bed shear stress. However, due to the complexity of the equation there are complications in interpreting bed shear stress, especially where depths are shallow and where Manning’s n is representative of the vegetation rather than the soil surface. More complicated output fields are also difficult to present and convey to stakeholders. Therefore, tolerances and thresholds tend to be set using output fields more easily understood by all stakeholders.
*The potential cumulative impact of multiple changes in the floodplain. For example, flood behaviour changes associated with a single development in isolation may be negligible, dozens of neighbouring developments over decades may however cause a significant change in flood behaviour relative to the pre-developed catchment state.<br>
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* Use depth varying manning's n (lower manning's n for shallow water depths), specifically for direct rainfall models.
* Set appropriate <font color="blue"><tt>Map Cutoff Depth </tt></font> for the modelling task. e.g. direct rainfall models might have higher values to avoid undesirable noise on the wet/dry interface.<br>
* Use smaller 1D timestep for models with 1D features.
* Try double precision, specifically for models with higher elevation, 1D features and or very small flow/rainfall depth increments.
* When running HPC and or Quadtree, test control number factor smaller than 1.
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=Why seemingly identical models can produce non-identical results?=
Generally speaking single path numerical solvers such as those used for hydraulic modelling should be able to produce the same numerical results twice to the last bit of every binary number calculated and output. However, this can become difficult with parallel computations as the order in which a list of single or double precision numbers are summated can produce slightly different rounding errors and thereby produce very slightly different results. For the vast majority of models TUFLOW Classic
Prior to 2020-10-AB release, the new boundary method introduced in TUFLOW HPC 2020-01-AA release for inflowing HT and QT boundaries (refer see Section 6.1 of the 2020 Release Notes) can in rare situations be affected by bitwise reproducibility when parallelised. When this issue occurs, very slight numerical differences can occur throughout the model, noting that they will be of a much smaller magnitude than those that occur when carrying out impact assessments, but will cause undesirable numerical noise in the impact mapping.<br>
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