Modelling Accuracy Uncertainties Impact Mapping: Difference between revisions

*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>

 

=I am running existing and developed case and see differences away from the model changes. Why?=

Any geometry changes between models, no matter how small, will affect results, sometimes to a greater degree than that occurring in the area of change. For example, a few millimetres increase in water level can determine whether or not overtopping of an embankment occurs, and this can consequently cause even larger impacts on the downstream side of the embankment. Furthermore, these changes can be compounded by subsequent changes in timestepping when using the adaptive timestepping option (the default in TUFLOW HPC), especially at fringes of the flood extent, where cells are constantly wetting and drying. Modellers and reviewers should be judicious and pragmatic when assessing which impacts are real and which are numerical noise.<br>