CMS-Flow Bottom Friction

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Bottom Friction Dataset

Overview

In SMS versions 10.1 and earlier, the bottom roughness is specified using the Manning's n coefficient in the Cells tab of the CMS-Flow Model Control window (see Figure 1). In SMS 11.0, the option is provided to use a roughness height, or bottom friction coefficient in addition to the Manning's coefficient. In SMS 11.0 the bottom roughness is specified in the Bottom Friction Dataset section of within the Input tab of the CMS-Flow Model Control window (see Figure 2). The Bottom Friction Dataset is specified at every computational (ocean) cell and is required for each model simulation.

Figure 1. Specification of Manning's n coefficient in the Cells tab of the CMS-Flow Model Control window in SMS 10.1
Figure 2 Specification of the Bottom Friction Dataset the Input tab of the CMS-Flow Model Control window in SMS 11.0


Creating

Figure 2.4. Initialization of the Bottom Friction Dataset in SMS 11.0

In SMS 11.0 each time a new CMS-Flow simulation is created a bottom friction (roughness) dataset needs to be created using the Create Dataset button. The bottom friction parameter (related to Manning n) is spatially varying (cell-specific) over the grid domain. The default value upon grid creation is 0.025. At times a user may desire to represent locations where added friction is needed due to structures or increased turbulence due to sharp changes in current speed. More information on using this feature of CMS-Flow can be found here.


Figure 3. Three SMS Project Explorer showing the Manning's n, Bottom Friction Coefficient and Roughness Height datasets.

Once the Bottom Friction Dataset is created and the CMS-Flow Model Control window closed, a new auxiliary dataset will appear in the SMS Project Explorer (see Figure 3). Table 1. shows a list of the CMS cards related to the bottom friction datasets.


Table 1. SMS Cards related to the bottom friction datasets

Card Arguments Default Range Description Versions
USE_WALL_FRICTION CHARACTER ON ON | OFF Turns on or off wall friction >=3.5
MANNING_N_DATASET CHARACTER CHARACTER [<grid file>] [<grid name>//"Datasets/ManningsN"] none Grid file name and dataset path for the input Manning's n dataset >=v3.5
BOTTOM_FRICTION_COEF_DATASET CHARACTER CHARACTER [<grid file>] [<grid name>//"Datasets/BottomFricCoef"] none Grid file name and dataset path for the input bottom friction coefficient dataset >=v4.0
ROUGHNESS_HEIGHT_DATASET CHARACTER CHARACTER [<grid file>] [<grid name>//"Datasets/RoughnessHeight"] none Grid file name and dataset path for the input roughness height dataset >=v4.0


Editing

Figure 4. Manning n contours after modifying the areas under all three bridges.

Bottom roughness datasets may be edited manually, using the Data Calculator or by interpolating from a scatter set. Figure 4 shows an example where the Manning's n coefficient is increased for Shark River Inlet to account for the presence of vertical piles.

To interpolate bottom roughness data from a scatter set


Exporting

Exporting the bottom roughness (friction) datasets is useful for creating different project alternatives or when switching from different bottom roughness datasets types such as from Manning's n to Bottom Friction Coefficient and back. It is also useful for scripting multiple runs with different project alternatives.

Figure 5. Manning n contours after modifying the areas under all three bridges.
Figure 6. Exporting the Bottom Friction Coefficient to an XMDF file.


Advanced Cards

Table 2. Advanced Cards related to the bottom friction (roughness) dataset

Card Arguments Default Range Description CMS Versions
MANNING_N_CONSTANT real number none >=0.0 Specifies a constant input Manning's n coefficient. Over-rides any previous bottom friction cards. >=v4.0
BOTTOM_FRICTION_COEF_CONSTANT real number none >=0.0 Specifies a constant input Bottom Friction Coefficient. Over-rides any previous bottom friction cards. >=v4.0
ROUGHNESS_HEIGHT_CONSTANT real number none >=0.0 Specifies a constant Roughness Height in m. Over-rides any previous bottom friction cards. >=v4.0


Wall Friction

Figure 6. Wall friction specification in the SMS 11.0 CMS-Flow Model Control window.

The wall friction enhances the flow drag perpendicular to any dry boundary. The wall friction may be turned On or Off in the Flow tab of the CMS-Flow Model Control window (see Figure 7). The default in SMS is for the wall friction to be ON. Table 3 shows the CMS card used activating the wall friction.

Table 3. SMS Cards related to the bottom friction datasets

Card Arguments Default Range Description Versions
USE_WALL_FRICTION CHARACTER ON ON | OFF Turns on or off wall friction >=3.5


Wave-enhanced Bottom Friction (Advanced)

In the presence of waves, the mean (short-wave averaged) bottom shear stress which is used in the momentum equations is enhanced. There are 5 different options in CMS for estimating the wave-enhanced bottom friction. The default is a simple quadratic formula but can be changes using the Advanced Card described in Table 3.

Table 3. Advanced Card used for specifying the wave-current mean stress formulation

Card Arguments Default Range Description CMS Versions
WAVE-CURRENT_MEAN_STRESS CHARACTER W09 W09 | DATA2 | DATA13 | F84 | HT91 Defines the model used for calculating the mean bottom shear stress used in hydro >=4.0
WAVE_BOTTOM_FRICTION_COEFFICIENT REAL 0.5 0.3-0.7 Wave bottom friction coefficient used for quadratic combined wave-current mean bed shear stress calculation. >=4.0


Bed Slope Friction Factor (Advanced)

The bed slope friction factor accounts for the increased surface area over sloping beds as compared to the horizontal area. This feature may be turned On or Off through the Advanced Card described in Table 4.

Table 4. Advanced Card used for activating the bed slope friction factor

Card Arguments Default Range Description CMS Versions
BED_SLOPE_FRICTION_FACTOR CHARACTER OFF ON| OFF Specifies whether to include the bed slope friction factor or not in the calculation of the bed friction >=4.0


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