User Guide 013
Hard Bottom Hard Bottom is a morphologic constraint that provides the capability to simulate mixed bottom types within a single simulation. This cell-specific feature limits the erodability of the constrained cells down to a specified depth below the water surface. During sediment transport calculations, exposed hard bottom cells may become covered through deposition. By default, CMS-Flow cells are fully-erodible cells with no specified hard bottom depth (inactive cells; denoted by the CMS-Flow null value of -999.0). Hard bottom only needs to be specified only for computational (ocean) cells.
figure 2-91
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Within the CMS-Flow Model Control window, the hard bottom dataset can be created from the Sediment tab. If the dataset does not exist, it can be created using the Create Dataset button. If a dataset exists (created using the Data Calculator) which represents the intended hard bottom specifications, the Select Dataset button can be used to select such dataset and copy the values to the hard bottom dataset. When specified, cell hard bottom depths will appear in the Project Explorer as a scalar dataset beneath the CMS-Flow grid. This dataset cannot be deleted, though it can be edited like any other dataset. A CMS-Flow simulation must contain the hard bottom dataset (even if it is not specified) so SMS will create a defaulted (inactive cells) dataset if it does not already exist when saving the simulation. The hard bottom dataset can created, edited, viewed and verified using the following SMS interface features.
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example 2-90
Boundary and Initial Conditions In the case of the Equilibrium Total Load sediment transport model, all boundaries are set to the equilibrium transport rate. For the Equilibrium Bed Load plus Advection Diffusion model, the suspended load is specified as the equilibrium concentration at inflow cells and a zero gradient at outflow cells. For the Total load nonequilibrium sediment transport model, the sediment concentration is set to the equilibrium concentration at inflow cells and a zero gradient boundary condition is applied at outflow cells. In the case an initial conditions file is NOT specified both the hydrody-namics and sediment concentrations are initialized as zero. If an initial conditions file is specified, than the initial sediment concentrations are read in. If an initial conditions file is specified but without the sediment concentration, than the initial sediment concentration is set to the equilibrium concentration.
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Step-by-Step Sediment Transport Application for Shark River Inlet, NJ TBC
Numerical Methods Temporal Solution Scheme This refers to the temporal discretization of the hydrodynamic, sediment and salinity transport equations. There are two options in CMS: Implicit and Explicit. The implicit scheme uses a time step on the order of 5-20 minutes and is designed for tidal flow, and mid-term morphology change. The explicit scheme uses a time step on the order of 0.5-1.0 seconds and is appropriate for cases that vary quickly in time such as flooding or barrier island breaching.
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Note: • The second order implicit temporal scheme requires three time step levels. Therefore, for the first time step, the model uses the first order two-level temporal scheme. In addition, if the time step is increased or decreased during the simulation. The first order scheme is used.
Implicit Solver Options The solvers implemented in the implicit temporal solution scheme are the SIP, ICCG, Gauss-Seidel, Gauss-Seidel with Successive-Over-Relaxation, BICGSTAB, and GMRES. Currently, the same solver is applied to flow, sediment and salinity. The default solver is the GMRES. The solver may be changed using the advanced card in the table below. The SIP and ICCG solvers are only available for non-telescoping grids. The maximum number of outer and inner loop iterations may also be changed. The outer loop is the loop over which the governing equations are solved successively, while the inner loop is the loop within the matrix solver for each governing equation.
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Skewness Correction When the lines connecting cell centers do not intercept the cell-face cen-ters, the cells are said to be skewed. When interpolating variables to the cell-face or calculating cell-face gradients a correction is needed for second order accuracy. There are several ways in which the correction can but involves some form of reconstruction of the variable on the cell face or within the neighboring cells. In CMS a linear cell reconstruction is performed within skewed cells for the correction.
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Advection Schemes As in the case of the implicit solution scheme, the same advection scheme is applied for the flow, sediment and salinity transport equations. Future versions of the CMS will allow the user to select different advection schemes for different governing equations. There are several choices for advection schemes with the implicit model which are listed in the table below. The schemes range from first to third order. The hybrid scheme is fast but is the most diffusive. The exponential scheme is based on the 1D analytical solution to a steady-state advection-diffusion equation and produces very stable results. The HLPA is very stable and non-diffusive, but requires slightly more computational time. For most applications, the exponential scheme is recommended and is set as the default. The advection scheme may be change using the advanced card described in the table below.
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Wetting and Drying In CMS, a minimum depth is required for cells to be considered. A cell is classified as wet if the total water depth is larger than this depth. Cell faces are either classified as either open if the two cells neighboring cells are wet or otherwise closed (i.e. cell faces are not classified as wet or dry) , in order to improve stability.
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Parallelization The CMS-Flow is parallelized for PC’s with multi-core processors using OpenMP. The parallelization works by splitting the computational work into “threads” among several cores. Some cores are hyper-threaded, meaning a single core may support two threads. The number of threads is specified in the CMS-Flow Model Control Window. The number of threads must be equal or greater to 1 and cannot be larger than the number of threads available on the machine. If a number is specified which is larger than the maximum number available on the machine, then the code will default to the maximum number available.
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Note: • The OpenMP parallelization requires that compatibility OpenMP run-time library (libiomp5md.dll) be in search paths. • When running multiple simulations at once, the user should keep track of how many threads are being used by each simulation and make sure that the maximum number of threads on the computer is not exceeded. Exceeding the maximum number of threads on the computer will slow down the simulations and also make the computer slow. • It is always recommended to at least leave one thread unused by model simulations so that the computer is responsive while the simulations are running.
Hot Start The term “hot start” refers to starting a simulation with an initial condition other zero (cold start). Hot starts are used for specifying initial conditions other than the default zero value or restarting simulations at intermediate times. The hot start controls are set in the CMS-Flow Model Control window under the Flow tab in SMS 11.0 (see Figure 2 94) and in the General tab in SMS 11.1 and later (see Figure 2 95).
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The hot start options are the input Initial Conditions file, the output time and recurring interval for the Hot Start file. A description of these controls and options are provided in the subsequent sections.