Recently, the New Morris Method has been presented as an effective sensitivity analysis tool for mathematical models. The New Morris Method estimates the sensitivity of an output parameter to a given set of input parameters (first-order effects) and the extent these parameters interact with each other (secondorder effects). This method requires the specification of two parameters (runs and resolution) that control the sampling of the output parameter to determine its sensitivity to various inputs. The criteria for these parameters have been set on the analysis of a wellbehaved analytical function (see Cropp and Braddock, Reliab. Eng. Syst. Saf. 78:77-83, 2002), which may not be applicable to other physical models that describe complex processes. This paper will investigate the appropriateness of the criteria from (Cropp and Braddock, 2002) and hence the effectiveness of the New Morris Method to determine the sensitivity behaviour of two hydrologic models: the Soil Erosion and Deposition System and Griffith University Representation of Urban Hydrology. In the first case, this paper will separately analyse the sensitivity of an output parameter on a set of input parameters (first-and second-order effects) for each model and discuss the physical meaning of these sensitivities. This will be followed by an investigation into the sampling criteria by exploring the convergence of the sensitivity behaviour for each model as the sampling of the parameter space is increased. By comparing these trends to the convergence behaviour from Cropp and Braddock (2002), we will determine how well the New Morris Method estimates the sensitivity for each model and whether the sampling criteria are appropriate for these models. It will be shown that the New Morris Method can provide additional insight into the functioning of these models, and that, under a different metric, the sensitivity behaviour of these models does converge confirming the sampling criteria set by Cropp and Braddock.
Observed reductions in pollutant concentrations through stormwater treatment devices commonly display the characteristic form of exponential decay, in which the rate of decrease of pollutant concentration with distance is proportional to the concentration. The observation of an apparently irreducible or background pollutant concentration, C*, in many devices has led to development of the two-parameter "k-C*" model. It is known that this model is too simplistic because the parameters k and C* are not constant but can vary greatly with pollutant concentration and hydraulic conditions. This paper presents an alternative exponential decay model for filtration of particulate pollutants, which is based on simple mathematical descriptions of key removal processes. The model delivers a process-based method for estimating the exponential decay constant. Moreover, the need to specify a background concentration is eliminated. To test the theory, the model is applied to the removal of clay and silica particles from horizontal flow through an experimental gravel trench. Particle concentrations were measured at nine locations along a 7.2 m long flume. The model agrees very well with the observed change in suspended solids concentration for the two pollutant materials and the range of flow rates tested. A single model parameter, notionally representing the "stickiness" of pollutant particles, is required for different pollutant materials.
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