A new, three-parameter friction model at the boundaries of free surface flow is proposed. The model is valid for all flow and roughness characteristics and it is proposed to replace the widely used Manning equation in the flood simulation models. In the gauged flow domains, the parameters of the model can be calibrated directly using the appropriate field data. For the ungauged flow domains, the parameters are estimated following a four-step procedure: (1) selection of roughness zones; (2) generation of synthetic data from water depth-flow velocity-roughness height combinations; (3) calculation of shear stresses using a physically based equation; and (4) estimation of the model parameters through regression toward the results of this equation. The proposed friction model is used in the case study of the Tous dam break in Spain in 1982, as it was simulated by the two-dimensional hydrodynamic model FLOW-R2D. The results of this application show that the proposed friction model performs slightly better than the commonly used Manning equation.
Dam break studies consist of two submodels: a) the dam breach submodel which derives the flood hydrograph and b) the hydrodynamic submodel which, using the flood hydrograph, derives the flood peaks and maximum water depths in the downstream reaches of the river. In this paper, a thorough investigation of the uncertainty observed in the output of the hydrodynamic model, due to the seven dam breach parameters, is performed in a real-world case study (Papadiana Dam, located at Tavronitis River in Crete, Greece). Three levels of uncertainty are examined (flow peak of the flood hydrograph at the dam location, flow peaks and maximum water depths downstream along the river) with two methods: a) a Morris-based sensitivity analysis for investigating the influence of each parameter on the final results; b) a Monte Carlo-based forward uncertainty analysis for defining the distribution of uncertainty band and its statistical characteristics. Among others, it is found that uncertainty of the flow peaks is greater than the uncertainty of the maximum water depths, whereas there is a decreasing trend of uncertainty as we move downstream along the river.
Abstract:A methodology is presented which can be used in the evaluation of parametric uncertainty in urban flooding simulation. Due to the fact that such simulations are time consuming, the following methodology is proposed: (a) simplification of the description of the physical process; (b) derivation of a training data set; (c) development of a data-driven surrogate model; (d) use of a forward uncertainty propagation scheme. The simplification comprises the following steps: (a) unit hydrograph derivation using a 2D hydrodynamic model; (b) calculation of the losses in order to determine the effective rainfall depth; (c) flood event simulation using the principle of the proportionality and superposition. The above methodology was implemented in an urban catchment located in the city of Athens, Greece. The model used for the first step of the simplification was FLOW-R2D, whereas the well-known SWMM software (US Environmental Protection Agency, Washington, DC, USA) was used for the second step of the simplification. For the training data set derivation, an ensemble of 100 Unit Hydrographs was derived with the FLOW-R2D model. The parameters which were modified in order to produce this ensemble were the Manning coefficients in the two friction zones (residential and urban open space areas). The surrogate model used to replicate the unit hydrograph derivation, using the Manning coefficients as an input, was based on the Polynomial Chaos Expansion technique. It was found that, although the uncertainties in the derived results have to be taken into account, the proposed methodology can be a fast and efficient way to cope with dynamic flood simulation in an urban catchment.
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