Denis Dartus

River model calibration, from guidelines to operational support tools

Numerical modelling is now used routinely to make predictions about the behaviour of environmental systems. Model calibration remains a critical step in the modelling process and different approaches have been taken to develop guidelines to support engineers and scientists in this task. This article reviews currently available guidelines for a river hydraulics modeller by dividing them into three types: on the calibration process, on hydraulic parameters, and on the use of hydraulic simulation codes. The article then presents an integration of selected guidelines within a knowledge-based calibration support system. A prototype called CaRMA-1 (Calibration of River Model Assistant) has been developed for supporting the calibration of models based on a specific 1D code. Two case studies illustrate the ability of the prototype to face operational situations in river hydraulics engineering, for which both data quality and quantity are not sufficient for an optimal calibration. Using CaRMA-1 allows the modeller to achieve the calibration task in accordance with good calibration practice implemented in the knowledge base. Relevant reasoning rules can easily be added to the knowledge base to extend the prototype range of applications. This study thus provides a framework for building operational support tools from various types of existing engineering guidelines.

Automatic Differentiation: A Tool for Variational Data Assimilation and Adjoint Sensitivity Analysis for Flood Modeling

Flood modeling involves catchment scale hydrology and river hydraulics. Analysis and reduction of model uncertainties induce sensitivity analysis, reliable initial and boundary conditions, calibration of empirical parameters. A deterministic approach dealing with the aforementioned estimation and sensitivity analysis problems results in the need of computing the derivatives of a function of model output variables with respect to input variables. Modern automatic differentiation (ad) tools such as Tapenade provide an easier and safe way to fulfill this need. Two applications are presented in this paper: variational data assimilation and adjoint sensitivity analysis.

Parameter identification using optimization techniques in open-channel inverse problems

Adverse socio-economic impacts of recent floods both in Europe and other continents emphasize the need for accurate flood forecasting capabilities towards improved flood risk management services. Flood forecasting models are often data-intensive. These models are inherited with (i) conceptual parameters that often cannot be assessed by field measurements, as in conceptual models; and/or (ii) empirical parameters that their direct measurements are either difficult, for example, roughness coefficient or costly, for example, survey data. There is also a category of practical problems, where modelling is required but gauged data are not available. Models, other than purely theoretical ones, for example, Large Eddy Simulation models, need calibration and the problem is even more pronounced in the case of ungauged rivers. Optimal values of these parameters in a mathematical sense can be identified by a number of techniques as discussed and applied in this paper. New generations of satellites are now able to provide observation data that can be useful to implement these techniques. This paper presents the results of synthesized flood data emulating data obtained from remote sensing. A one-dimensional, steady-state flow in a channel of simple geometry is studied. The paper uses optimization methods and the Extended Kalman Filter to ascertain/improve the values of the parameters.

Characterization of catchment behaviour and rainfall selection for flash flood hydrological model calibration: catchments of the eastern Pyrenees

Abstract Accurate flash flood prediction depends heavily on rainfall data quality and knowledge of catchment behaviour. A methodology based on global sensitivity analysis and hydrological similarity is proposed to analyse flash storm-flood events with a mechanistic model. The behaviour of medium-sized catchments is identified in terms of rainfall–runoff conservation. On the basis of this shared behaviour, rainfall products with questionable quantitative precipitation estimation (QPE) are excluded. This facilitates selection of rainfall inputs for calibration, whereas it can be difficult to choose between two rainfall products by direct comparison. A substantial database of 43 flood events on 11 catchment areas was studied. Nash-Sutcliffe efficiencies for this dataset are around 0.9 in calibration and 0.7 in validation for flash flood simulation in 250-km2 catchments with selected QPE. The resulting calibration framework and qualification of possible losses for different bedrock types are also interesting bases for flash flood prediction at ungauged locations. Editor D. Koutsoyiannis

Hydraulic Resistance of Emergent Macroroughness at Large Froude Numbers: Design of Nature-Like Fishpasses

The mean flow in a nature-like fishpass can be highly modified by the Froude number. It is important to understand this evolution to correctly design the structure. The studied configuration is an emergent staggered arrangement of obstacles. The hydraulic resistance of a fishpass is experimentally investigated that depends on several geometric parameters: block shape, ramp slope, block density, and bed roughness. An analytical model based on the balance momentum allows one to quantify the influence of each hydraulic parameter. The bed roughness has a weak influence, whereas the block shape and the Froude number are significant. The variation of the drag coefficient was analyzed to improve the stage-discharge relationship. To this end, a correlation with the block diameter and water level is proposed. The maximal velocity reached in the fishpass can also be estimated. These results have to be compared with the fish swimming ability to assess the fishpass passability.

Floods effects on rivers morphological changes application to the Medjerda River in Tunisia

Abstract In Tunisia especially in the Medjerda watershed the recurring of floods becoming more remarkable. In order to limit this risk, several studies were performed to examine the Medjerda hydrodynamic. The analysis of results showed that the recurrences of floods at the Medjerda watershed is strongly related to the sediment transport phenomena. Initially, a one dimensional modelling was conducted in order to determine the sediment transport rate, and to visualize the river morphological changes during major floods. In continuity of this work, we will consider a two-dimensional model for predicting the amounts of materials transported by the Medjerda River. The goal is to visualize the Medjerda behaviour during extreme events and morphological changes occurred following the passage of the spectacular flood of January 2003. As a conclusion for this study, a comparative analysis was performed between 1D and 2D models results. The objective of these comparisons is to visualize the benefits and limitations of tested models. The analysis of the results demonstrate that 2D model is able to calculate the flow variation, sediment transport rates, and river morphological changes during extreme events for complicated natural domains with high accuracy comparing with 1D Model.

Modelling errors calculation adapted to rainfall Runoff model user expectations and discharge data uncertainties

A novel objective function for rainfall-runoff model calibration, named Discharge Envelop Catching (DEC), is proposed. DEC meets the objectives of: i) taking into account uncertainty of discharge observations, ii) enabling the end-user to define an acceptable uncertainty, that best fits his needs, for each part of the simulated hydrograph. A calibration methodology based on DEC is demonstrated on MARINE, an existing hydrological model dedicated to flash floods. Calibration results of state-of-the-art objective functions are benchmarked against the proposed objective function. The demonstration highlights the usefulness of the DEC objective function in identifying the strengths and weaknesses of a model in reproducing hydrological processes. These results emphasize the added value of considering uncertainty of discharge observations during calibration and of refining the measure of model error according to the objectives of the hydrological model. A novel objective function taking into account discharge observations uncertainty, model specifics and user-defined tolerance.A resulting calibration methodology is demonstrated on an existing hydrological model dedicated to flash floods.The results of state-of-the-art objective functions are benchmarked against the proposed objective function.

Langmuir circulations and enhanced turbulence beneath wind-waves

Abstract This paper presents a set of experiments which describes the mutual influences between wind-wave enhanced turbulence and secondary motions like Langmuir circulations (LCs). Different external driven forces were produced in laboratory wind-wave flumes, and mean and turbulent fields were measured for each experimental situation. A (k–e) model coupled to momentum equations are numerically integrated to investigate the influences of a diffusion–dissipation (Diff=e) equilibrium of turbulent kinetic energy (TKE) to represent surface boundary conditions under wind-waves. A new formulation for estimating surface energy is obtained from wave and wind stress parameters, expressed in non-dimensional grounds as k S =0.12(σa/u ∗ ) 2 +C μ −1/2 . The close fit between measurements and numerical results suggests that Craik–Leibovichs equations associated with (k–e) turbulence model provides a good description of flow characteristics. Model issues confirm that secondary motions have remarkable influences over the distribution of momentum and turbulence, indicating that wind-wavy boundary conditions resulting from a balance between diffusion and dissipation of TKE is capable of reproducing the realistic scales of phenomena.

On the influence of longitudinal mean flow over Langmuir circulations

The Craik-Leibovich equations including vortex forcing terms and a pressure gradient force in x-momentum are integrated numerically. Some typical environmental problems with strong longitudinal currents are analysed. Results are compared with Leibovich & Paoluccis stability analysis which allows to identify situations where Langmuir cells exist. An establishing characteristic time for the setting of cells has the same order of magnitude as in field observations. Its variation with Langmuir number (La) and nondimensional depth is pointed out. A scaling analysis of the momentum equations is made for vanishing La. Influence of the longitudinal current intensities over Langmuir circulations structure is showed and confirmed by numerical results : more energetic convective cells are verified for situations where stronger longitudinal velocities are present. Results indicate how the redistribution of the shear stress induced by secondary flows is important for mixing in the water column and for coastal sediment transport.