CMS/Hard Bottom

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Background

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 erodibility 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.

Within the CMS-Flow Model Control window, the hard bottom dataset can be created from the Sediment Transport tab. If the dataset does not exist, it can be created using the Create Dataset button. If a dataset already exists 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 can be edited like any other dataset. The hard bottom dataset can created, edited, viewed and verified using various techniques depending on the SMS version number.

All versions of SMS

A capability to manually edit the hard bottom dataset exists. The user can make the Hard Bottom dataset active and then select a cell (or group of cells), then changing the scalar (S) value in the Edit Window.

Sometimes, the user may wish to make decisions for hard bottom values for larger groups of cells. Hand editing (as mentioned above) is crude and may allow for a poorly defined hard bottom layer.

SMS 10.x to 11.x

In these versions, there was a user-friendly tool which provided for a couple of specification options. With the Select Grid Cell tool active, make a selection, right click to bring up the tool menu and choose the Specify Hard Bottom... option.

This opened a CMS-Flow Hard Bottom Specification window. The following options were provided in the Hard Bottom Specification window:

  • Use bathymetric cell depth - Sets the cell hard bottom depth to be the cell geometry value thereby creating an exposed non-erodible condition. If multiple cells were selected, then each cell will use its respective bathymetric depth.
  • Specified distance below bathymetric cell depth - Sets the cell hard bottom depth to be the cell geometry value plus the specified distance thereby creating a sediment-covered non-erodible condition. The distance is limited to positive values to ensure the hard bottom depth is greater than the geometry value. The cell can provide sediment for transportation, however, the amount of erosion is limited. If multiple cells were selected, then each cell will use its respective bathymetric depth.
  • Specified depth - Sets the cell hard bottom depth to the specified depth thereby creating a sediment-covered non-erodible condition similar to specified distance. The depth is limited to greater than the geometry value. If multiple cells were selected, then the depth is limited to greater than the largest geometry value and all cells will have the same value.
  • Unspecified - Resets to an inactive hard bottom condition. The cell hard bottom depth is set to the CMS-Flow null value. If multiple cells were selected, then all cells will be reset.

If no cells are selected when opening the Hard Bottom Specification window, then all computational (ocean) cells will be used. If a selection of only non-computational cells, then specification cannot occur. If a selection contains computational and non-computational cells, then the specification will only apply to the computational cells.

If multiple computational cells with differing specifications are selected, the window will not display a selected specification type and the OK button will be disabled. This is to protect the previous specifications from being overwritten by mistake. The OK button will be enabled when an option is selected. The minimum hard bottom depth of the multiple computational cells selected will be displayed in the Depth edit field and the minimum hard bottom depth minus the maximum geometry depth of the multiple computational cells selected will be displayed in the Distance edit field.

SMS 12.0 - SMS 13.3

This added capability and interface enhancement was lost with the release of SMS 12.0 and up through version SMS 13.3. This left the only option for the user to manually manipulate individual (or small groups) of cells at a time.

SMS 13.4+

With the release of SMS 13.4. A new hard bottom coverage and a hard bottom helper tool have been introduced. Guidance for usage is given in the section below.

Hard Bottom Coverage

To begin, the user should create a new map coverage within SMS 13.4.

  • Right click, "Map Data" in the data tree, and choose "New Coverage"
  • From the list under the "Models" Group, open the CMS-Flow item and choose "Hard Bottom".

Once created, the user should create polygons (feature arcs which close on themselves) to delineate sections of the domain that will be assigned as hard bottom. The user should select each polygon, right click and choose "Assign Polygon attributes". The three options that come up are:

List of potential Hard bottom coverage types.
Type Description
Non-erodible cell each cell inside the polygon will not be allowed to erode.
Specified distance erodible cell each cell inside the polygon will have hard bottom located at the specified distance below the present Z (bed elevation) value. This value should be positive to indicate a position below the present bed.
Specified elevation erodible cell each cell inside the polygon will have hard bottom located at a fixed elevation. This value should be negative if it is below the mean water level as it is an elevation and not a depth.

Note: Any areas that are not covered by one of the three hard bottom types, will be assigned the fully-erodible flag, -999.0.

Video Clip

The process for creating and defining Hard Bottom for a real-world case is shown in the short clip below.

https://www.youtube.com/watch?v=kh4hfzHNZNU

Hard Bottom Dataset Tool

Once there is a hard bottom coverage and all hard bottom polygons have been defined, the user can then run the CMS-Flow Hard Bottom Dataset Tool from the Toolbox.

  • Click the toolbox icon, SMS13 Toolbox.png.
  • Under the CMS-Flow folder, click "CMS-Flow Hard Bottom Dataset Tool" then click "Run Tool..." at the bottom.
  • From the resulting dialog, choose the CMS-Flow grid, choose the hard bottom dataset, enter a new name for the resulting dataset to be added to the CMS-Flow grid.
  • Click OK to run the tool and to create the dataset. If a dataset already exists with the same name, a new dataset will be created with some incremented number at the end.

For Hard Bottom to be utilized by the CMS, the user must remember to enter the CMS-Flow simulation model control and enable, then assign this new CMS-Flow grid dataset.



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