C. Villaret

Morphodynamic modeling using the Telemac finite-element system

The open-source finite-element system Telemac has been applied to simulate various complex hydrodynamic and morphodynamic situations including waterways, curved channels, recirculating flows and wave-induced littoral transport. In the applications presented here, the sediment transport model is mainly restricted to the transport of non-cohesive sediments, which relies on classical semi-empirical concepts including sand grading effects, parameterization of secondary currents and wave effects. In comparison with other comprehensive modeling systems (Delft-3D, Mike-21, etc.), the main originality lies in the efficiency and flexibility of the finite elements. Thanks to the optimization of numerical schemes, parallelism, as well as tremendous progress in the performance of computers, bed evolution can be calculated on basin scale (10-100km) and for the medium term (years to decades), without the use of hydrodynamic filtering methods. As a novelty in release 6.0, we present a method of feedback for the bed roughness, which reduces uncertainty in the prediction of both transport rates and flow velocities.

Sand Transport by Waves and Currents: Predictions of Research and Engineering Models

This paper concerns work being undertaken as part of the EU MAST3 SEDMOC project (1998–2001). One of the main aims of this project has been to improve our ability to predict sediment transport rates throughout the physical parameter ranges of importance in coastal engineering practice. Here the transport predictions of a local, one-dimensional (vertical), numerical research model, and also Bijkers practical sand transport model, are compared with data from two field sites. The importance of the wave-related component of sediment transport is also assessed.

First-order uncertainty analysis using Algorithmic Differentiation of morphodynamic models

We present here an efficient first-order second moment method using Algorithmic Differentiation (FOSM/AD) which can be applied to quantify uncertainty/sensitivities in morphodynamic models. Changes with respect to variable flow and sediment input parameters are estimated with machine accuracy using the technique of Algorithmic Differentiation (AD). This method is particularly attractive for process-based morphodynamic models like the Telemac-2D/Sisyphe model considering the large number of input parameters and CPU time associated to each simulation.The FOSM/AD method is applied to identify the relevant processes in a trench migration experiment (van Rijn, 1987). A Tangent Linear Model (TLM) of the Telemac-2D/Sisyphe morphodynamic model (release 6.2) was generated using the AD-enabled NAG Fortran compiler. One single run of the TLM is required per variable input parameter and results are then combined to calculate the total uncertainty.The limits of the FOSM/AD method have been assessed by comparison with Monte Carlo (MC) simulations. Similar results were obtained assuming small standard deviation of the variable input parameters. Both settling velocity and grain size have been identified as the most sensitive input parameters and the uncertainty as measured by the standard deviation of the calculated bed evolution increases with time. A first-order second moment method (FOSM) is applied to quantify uncertainty.This method uses Algorithmic Differentiation (AD) and a Tangent Linear Model (TLM).The method is compared with Monte Carlo analysis in a trench migration test case.A TLM of the Telemac-2d/Sisyphe morphodynamic model has been applied.The FOSM/AD method is an efficient alternative to Monte Carlo simulations.

A large scale morphodynamic process-based model of the Gironde estuary

We present here our effort to develop a morphodynamic model of the Gironde estuary, using a process-based approach. In this complex and strongly dynamical environment, internal coupling between the flow (Telemac-2d) and sediment transport models (Sisyphe) allows the representation of detailed sediment processes and interactions with the bed, using a bed roughness feedback method. In this complex and highly heterogeneous environment, the sediment bed composition had to be schematized assuming 3 classes of bed material. Model results could be further improved by accounting for the cohesive sediment (consolidation processes). Thanks to parallelization of the codes, the simulation takes only 18 hrs on 8 processors (linux station) to calculate the 5-year bed evolution, at the basin scale (150 km).