Introduction and Coupling

Introduction and Coupling#

Requirements

This tutorial is designed for advanced modelers and before diving into this tutorial make sure to complete the TELEMAC pre-processing and Telemac2d steady hydrodynamic modeling tutorials.

The case featured in this tutorial was established with the following software:

Terminology#

A hydro-morphodynamic simulation implies modeling runoff-driven Sediment transport processes. The previous sections in this eBook focus on hydrodynamics defined as the study of liquids in motion and this section focuses on morphodynamics defined as the study of time-dependent changes in the forms of alluvial beds and their underlying processes.

Sediment Transport Modes#

TELEMAC has a dedicated module called Gaia for modeling morphodynamics. Gaia enables modeling sediment transport and morphological evolution (i.e., Topographic change) in rivers, lakes, and estuaries. It comes with particular routines to consider a spatio-temporal variation of grain sizes, grading curves, and riverbed layering for simulating sediment transport in the form of Bedload (coarse sediment) and/or Suspended load (fine sediment). Bedload is calculated by solving semi-empiric equations, such as the Meyer-Peter and Müller [MPM48] formula (read more later in this tutorial). Suspended load is modeled by solving the Advection-Diffusion equations (typically, the RANS form), which require closures for sediment erosion and deposition fluxes. Figure 189 qualitatively illustrates the two basic modes of sediment transport in the form of suspended load and bedload. Whether a particle is transported in suspension or as bedload can also be determined by calculating of the Rouse number.

sediment transport bedload suspended load

Fig. 189 Qualitative representation of two modes of sediment transport. On the left: suspended load in the form of fine particles moving with the bulk flow; on the right: bedload in the form of particles rolling, jumping, or sliding on the riverbed.#

Sediment is further distinguished between very fine, cohesive sediment and coarser, non-cohesive sediment. In addition, Gaia accounts for bed evolution through an iterative solution of the Exner equation [Exn25] for mass conservation.

The recruitment of sediment for both suspended load and bedload transport requires a detailed look at the riverbed, which will be provided later in the section on the definition of the riverbed composition and the active layer.

Coupling TELEMAC and Gaia#

The morphodynamics module Gaia can be internally coupled with the hydrodynamic models Telemac2d (solving the Shallow water equations) or Telemac3d (solving the Reynolds-averaged Navier-Stokes (RANS) equations). This section explains types of coupling Telemac2d/Telemac3d (hydrodynamics) with Gaia (morphodynamics).

From Sisyphe to Gaia#

Sisyphe is the traditional sediment transport module in TELEMAC, which has largely been replaced by the more unified Gaia module. Gaia is based on the historical module SISYPHE, with a large number of improvements, corrections, and optimizations implemented. Gaia’s unified framework efficiently manages different sediment classes, sand-mud mixtures, and both 2D and 3D spatial dimensions. To get specifications beyond the features presented here in the TELEMAC documentation and the TELEMAC forum, it is useful to know the SISYPHE heritage. SISYPHE routines are still available in recent TELEMAC versions through Gaia, though some keywords require adjustments. Read more in the Gaia manual in Appendix 8.1 and in the gaia.dico (telemac/v9.0.0/sources/gaia/gaia.dico).

Coupling Hydrodynamics (Telemac2d/3d) and Morphodynamics (Gaia)#

A hydro-morphodynamic numerical model can be either fully coupled or decoupled.

Fully coupled model

A fully coupled model solves the hydrodynamic Navier-Stokes equations simultaneously with sediment transport equations (i.e., erosion and deposition fluxes from and to the riverbed through the Exner equation). Bed elevation (i.e., Topographic change) is calculated for every timestep, which leads to long computation times. In addition to the coupling of gravity-driven hydrodynamics (i.e., bulk flow along valley slopes), Sediment transport, and Topographic change, a model can also be coupled with (surface) wave hydrodynamics.

Application range: Rapid morphodynamic processes, such as hyper-concentrated sediment-laden flows or debris flow.

Decoupled model

A decoupled model alternates between solving hydrodynamics and morphodynamics (i.e., the Exner equation). The riverbed is considered fixed when hydrodynamic variables are computed, and then bed elevation changes are calculated separately based on the computed flow field. This asynchronous approach is computationally more efficient than full coupling.

Application range: Most river models, and in particular, lake or oceanic models where morphodynamic timescales are much longer than hydrodynamic timescales.

Gaia follows the decoupled approach. The time step used for morphodynamic computation is the same as for hydrodynamics (specified in the Telemac2d or Telemac3d steering file). At each time step, hydrodynamics are solved first with the bed frozen, then the sediment transport equations and bed evolution (Exner equation) are solved based on the computed flow field.

Coupling period for wave-current-sediment interactions

When coupling Gaia with the wave module TOMAWAC, a coupling period can be specified to control how frequently wave fields are updated. This is relevant because wave computations can be expensive and wave conditions may not change as rapidly as currents. For basic Telemac2d/3d-Gaia coupling without waves, the morphodynamics are computed at every hydrodynamic time step. Read more about wave coupling in section 5.1 of the Gaia manual.

File Requirements for Coupling Gaia#

In addition to the standard Telemac2d steering, boundaries, and geometry mesh files, coupling hydrodynamics with Gaia requires a new steering (*.cas) file that needs to be referenced in the main steering file of the simulation. To this end, create a new folder for the Gaia tutorial (e.g., called /gaia2d-tutorial/), copy the dry-initialized steady2d simulation and results files (or clone the gaia2d-tutorial repository), and create a new Gaia steering file (e.g., called gaia-morphodynamics.cas). Thus, the following files should live in the modeling folder for this tutorial:

Gaia simulation file repository

The simulation files used in this tutorial are available at hydro-informatics/telemac.

Couple Gaia in the Hydrodynamics Steering File#

To programmatically implement the coupling of Gaia with a Telemac2d/Telemac3d simulation, a couple of new keywords need to be defined in addition to the keywords explained in the steady2d chapter. The first additional keyword is the baseline for any coupling with Telemac2d or Telemac3d steering file:

/ steady2d-gaia.cas
COUPLING WITH : 'GAIA'

steady2d-gaia.cas is the hydrodynamics (Telemac2d or Telemac3d) steering file

In this tutorial the hydrodynamics (Telemac2d or Telemac3d) steering file is referred to as steady2d-gaia.cas and the morphodynamics (Gaia) steering file is referred to as gaia-morphodynamics.cas.

In addition, the GAIA STEERING FILE keyword links the above-created gaia-morphodynamics.cas in the Telemac2d (or Telemac3d) hydrodynamics steering file:

/ steady2d-gaia.cas
/ ...
GAIA STEERING FILE : gaia-morphodynamics.cas

Hotstart#

This tutorial builds on the results of the dry-initialized steady2d model because Gaia simulations typically require a well-developed flow field as initial condition (see the above definitions). Using a former simulation result for model initialization is called hotstart, which requires a results file from a previous simulation. For this purpose, make sure that the dry-initialized steady2d results file is in the simulation folder (download r2dsteady.slf). Then define the hotstart in the Telemac2d steering file with the following keywords:

/ steady2d-gaia.cas
/ ...
PREVIOUS COMPUTATION FILE : r2dsteady.slf / results of 35 CMS steady simulation
INITIAL TIME SET TO ZERO : YES / avoid restarting at 15000

COMPUTATION CONTINUED is obsolete in TELEMAC v9.0

Since TELEMAC v9.0, the keyword COMPUTATION CONTINUED has been deleted. The continuation step is now automatically activated when PREVIOUS COMPUTATION FILE is specified in the steering file. Simply providing the previous computation file triggers the hotstart behavior.

The INITIAL TIME SET TO ZERO keyword resets the simulation time to 0. Next, make sure that all INITIAL CONDITIONS keywords are commented out with a / (alternatively delete these lines from steady2d-gaia.cas):

/ steady2d-gaia.cas
/ ...
/ INITIAL CONDITIONS - not required (hotstart)
/ ------------------------------------------------------------------
/ INITIAL CONDITIONS : 'ZERO DEPTH' / use ZERO DEPTH to start with dry model conditions
/ INITIAL DEPTH : 0.005 / use INTEGER for speeding up calculations

Bottom elevation must be available in the hotstart geometry (SLF)

The bottom elevation must be printed out in the results file of the simulation used for the hotstart. To this end, make sure that the list of values for the VARIABLES FOR GRAPHIC PRINTOUTS keyword contains B as indicated in the explanations for the setup of the dry-initialized model.

Continuing a Gaia computation (sedimentological hotstart)

To continue a Gaia simulation from a previous sedimentological computation (i.e., to restart with existing bed composition and layer data), use the PREVIOUS SEDIMENTOLOGICAL COMPUTATION FILE keyword in the Gaia steering file. Since v9.0, specifying this file automatically activates continuation without needing any additional keyword. The previous file should contain the bottom elevation (B), layer thicknesses (*ES), and ideally the sediment masses (*S* or *M*) or ratios (*A*, *R*) for proper continuation.

The dry-initialized steering file prescribes flowrates and elevations, which requires modifications in steady2d-gaia.cas to prescribed Q only. The reason for the Q-only prescription is that with Gaia, we want to model-predict changes in water depths and riverbed elevation, which means that the water surface elevation must not be constrained (i.e., not prescribed) as a boundary condition. Thus, the setup of boundary conditions for Gaia also requires slight modifications of the boundary (*.cli) file(s), which will be explained in the next section on the Basic Setup of Gaia. To this end, make sure that in the hydrodynamics steering file only the flowrate prescription keyword is activated and the elevation prescription is deactivated (comment out with /):

/ steady2d-gaia.cas
/ ...
/ Liquid boundaries
PRESCRIBED FLOWRATES  : 35.;35.
/ PRESCRIBED ELEVATIONS : 374.805626;371.33

Control Sections#

Control sections are sequences of node numbers (or node coordinates) at which TELEMAC sums up fluxes, for instance, to verify inflow and outflow mass balances. The unsteady simulation section provides detailed instructions for defining control sections and this tutorial re-uses the control sections file from the unsteady simulation (download control-sections.txt).

Expand to view the file control-sections.txt
# control sections steady2d
2 -1
Inflow_boundary
144 32
Outflow_boundary
34 5

To use the control sections for the Gaia simulation add the following to the hydrodynamics steering file:

/ steady2d-gaia.cas
/ ...
SECTIONS INPUT FILE :  control-sections.txt
SECTIONS OUTPUT FILE : r-control-flows.txt

Thus, re-running the simulation will write the fluxes across the two defined control sections to a file called r-control-flows.txt.

Hydrodynamic Steering Summary#

With the above adaptions and using a simulation length of 30000 timesteps (to observe morphodynamic evolution) with a graphical printout period of every 5000 timesteps (to reduce the output file size), the final hydrodynamic steering file should look like this:

/ steady2d-gaia.cas
/
TITLE : 'gaia2d steady'
/
/ HOTSTART - continuation is automatic when PREVIOUS COMPUTATION FILE is specified (v9.0+)
PREVIOUS COMPUTATION FILE : r2dsteady.slf / here - 35 CMS initialization after t 15000
INITIAL TIME SET TO ZERO : YES / avoid restarting at 15000
/
COUPLING WITH : 'GAIA'
GAIA STEERING FILE : gaia-morphodynamics.cas
/
/ DEFAULTS FROM STEADY2D
/
/------------------------------------------------------------------/
/     COMPUTATION ENVIRONMENT
/------------------------------------------------------------------/
/
BOUNDARY CONDITIONS FILE : boundaries.cli
GEOMETRY FILE            : qgismesh.slf
RESULTS FILE           : r2dsteady-gaia.slf
/
MASS-BALANCE : YES / activates mass balance printouts - does not enforce mass balance
VARIABLES FOR GRAPHIC PRINTOUTS : U,V,H,S,Q,F / Q enables boundary flux equilibrium controls
/
/ CONTROL SECTIONS
SECTIONS INPUT FILE :  control-sections.txt
SECTIONS OUTPUT FILE : r-control-flows.txt
/
/------------------------------------------------------------------/
/     GENERAL PARAMETERS
/------------------------------------------------------------------/
TIME STEP : 1.
NUMBER OF TIME STEPS : 30000
GRAPHIC PRINTOUT PERIOD : 5000
LISTING PRINTOUT PERIOD : 5000
/
/------------------------------------------------------------------/
/     NUMERICAL PARAMETERS
/------------------------------------------------------------------/
/ General solver parameters from section 7.1
DISCRETIZATIONS IN SPACE : 11;11
FREE SURFACE GRADIENT COMPATIBILITY : 0.1  / default 1.
ADVECTION : YES
/
/ FINITE ELEMENT SCHEME PARAMETERS - section 7.2.1 in the manual
/------------------------------------------------------------------
TREATMENT OF THE LINEAR SYSTEM : 2 / default is 2 - use 1 to avoid smoothened results
SCHEME FOR ADVECTION OF VELOCITIES : 14 / alternatively keep 1
SCHEME FOR ADVECTION OF TRACERS : 5
SCHEME FOR ADVECTION OF K-EPSILON : 14
IMPLICITATION FOR DEPTH : 0.55 / should be between 0.55 and 0.6
IMPLICITATION FOR VELOCITY : 0.55 / should be between 0.55 and 0.6
IMPLICITATION FOR DIFFUSION OF VELOCITY : 1. / v9p0 default
IMPLICITATION COEFFICIENT OF TRACERS : 0.6 / v9p0 default
MASS-LUMPING ON H : 1.
MASS-LUMPING ON VELOCITY : 1.
MASS-LUMPING ON TRACERS : 1.
/ MASS-LUMPING FOR WEAK CHARACTERISTICS : 1. / enabling leads to weak characteristics
SUPG OPTION : 0;0;2;2  / classic supg for U and V
/
/ SOLVER
/------------------------------------------------------------------
INFORMATION ABOUT SOLVER : YES
SOLVER : 1
/
/ TIDAL FLATS  - see section 7.5
TIDAL FLATS : YES
CONTINUITY CORRECTION : YES / default is NO
OPTION FOR THE TREATMENT OF TIDAL FLATS : 1
TREATMENT OF NEGATIVE DEPTHS : 2 / value 2 or 3 is required with tidal flats - default is 1
/
/ MATRIX HANDLING - see section 7.6
MATRIX STORAGE : 3 / default is 3
/
/ BOUNDARY CONDITIONS
/------------------------------------------------------------------
/
LAW OF BOTTOM FRICTION : 4 / 4-Manning
FRICTION COEFFICIENT : 0.03 / Roughness coefficient
/
/ Liquid boundaries
PRESCRIBED FLOWRATES  : 35.;35.
/ PRESCRIBED ELEVATIONS : 374.805626;0.
/
/ Type of velocity profile can be 1-constant normal profile (default) and (cli) 4-vector is proportional to root (water depth, only for Q)
VELOCITY PROFILES : 4;1
/
/ INITIAL CONDITIONS - not required (hotstart)
/ ------------------------------------------------------------------
/ INITIAL CONDITIONS : 'ZERO DEPTH' / use ZERO DEPTH to start with dry model conditions
/ INITIAL DEPTH : 0.005 / use INTEGER for speeding up calculations
/
/ STABILITY CONTROLS
/ ------------------------------------------------------------------
PRINTING CUMULATED FLOWRATES : YES
/
/------------------------------------------------------------------/
/     TURBULENCE
/------------------------------------------------------------------/
/
DIFFUSION OF VELOCITY : YES / default is YES
TURBULENCE MODEL : 3
/
&ETA
Contents