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20/10/2015 14:34:59

As part of the Oxford Flood Alleviation Scheme project for England’s Environment Agency, CH2M had to produce output from a Flood Modeller Pro 1D model for use in MODFLOW, the United States Geological Survey’s(USGS)groundwater model.

MODFLOW is a freely available 3D finite-difference groundwater model and is considered an international standard for simulating and predicting groundwater conditions and groundwater interactions.

Due to the new functionality and user-friendly interface provided within our next generation software, it was easy to produce the majority of the required MODFLOW data entirely within Flood Modeller Pro.

The overall objective, in order to provide appropriate inputs for MODFLOW, was to produce a CSV file in the following format:

i, j, tn, WLn

Where i and j are the row and column indices in the MODFLOW grid and the WLn are modelled water levels at time tn for all grid locations where the Flood Modeller Pro and MODFLOW models overlap. The output was required for time outputs every 3 hours.

The results from the 1D model are at discrete point locations and thus don’t directly provide the spatial output for the whole overlapped area. Therefore, to produce the required data, a 1D flood map was required (with a couple of modifications). The key to creating the flood map was having a georeferenced 1D model with, as a minimum, georeferenced point data for each hydraulic unit. Flood Modeller Pro provides a variety of tools for manipulating this data, some of these are described below in the context of generating the MODFLOW input file.

Step 1: Geo-reference the 1D model

If you only have the centreline georeferenced, the next step is to fully georeference the model, using the Toolbox function Georeference all Cross Sections. Once georeferenced, the 1D network (.dat) file can be added to the Layer view simply by dragging it onto the map (or via the Add GIS Layer command)

What exactly does this do? It centres the cross-section on the centreline and extends each point in the cross-section by its distance from the centre point, normal to the centreline.

Step 2: Interpolate the 1D network layer

Unless your 1D model has a very fine spatial resolution, e.g. as fine as your final required output grid and fine enough so that it doesn’t cut across meanders, you will need to add some interpolation points.

The Toolbox function Add Interpolates should be used – simply select your 1D model and centreline from your currently loaded layers, your resolution (minimum interpolation distance) and give your output shapefile a name.

Note: this is different from interpolating the 1D model itself and will not affect the hydrodynamic calculations (but will affect the interpolated “flood map” results).

Step 3: Triangulate

In order to be able to interpolate 1D results onto a 2D surface, the 2D interpolation tool should be used to incorporate the Triangulated Irregular Network (or TIN for short).

Flood Modeller Pro provides two triangulation functions. One is a simple, network-based, interpolate between consecutive cross-sections tool (Toolbox: Flood Mapping > 1D Models > Triangulate Selected File). This is quick, and adequate for many applications, although it doesn’t follow the centre-line nor does it cross structures or junctions.

For a more complex triangulation, right-click on the shapefile in the Layers Panel and select Export > To TIN with TIN Creator. This more complex triangulation approach will make use of the interpolates added in step 2. One drawback is that it tends to over-triangulate, but removal of the extra triangles is a relatively easy process.

Tip: If using the TIN Creator but not the Add Interpolates function, use the Export > To Simplified Shapefile function on the Layers panel in order to reduce the number of points per cross-section and improve the TIN (this process is implicit in the Add Interpolates functionality)

Step 4: Edit the TIN

It’s unlikely that you will produce a perfect TIN straight away. However, the editing of TINs, in particular removal of undesirable triangles, is an easy process. If you select the TIN (file extension .htn) within the Layers panel for editing (Right-click: Start Edit), then selecting and deleting triangles is straightforward. Simply click on the rogue triangle, or drag the mouse across multiple triangles and hit the delete key.

Tip: Keep saving the edited TIN as a different file name at regular intervals. This will save you time if you inadvertently delete the wrong triangles.

Step 5: Add model results to TIN

To overlay the 1D results on the TIN, right click on the TIN file in the Layers Panel and select Add Results. Then navigate to the file name containing the results from the 1D model – this can be the raw binary (.zzn) file. You can also use a CSV file created from the TabularCSV tool to extract the results from the selected time intervals.

Note that the Add Interpolates tool automatically adds dummy nodes not present in the 1D model. Because of this the .dbf file associated with the previously created shapefile was exported and merged with the CSV results file. The 1D model results were then linearly interpolated onto the dummy nodes and then added to the TIN.

Step 6: Export results to grid – Create flood map

Step 6 involved exporting the 1D results in grid format. This allowed us to go from a series of results created at discrete points to interpolating these onto a two dimensional grid covering the entire river network. The TIN created in the previous two steps allowed us to do just that.

We selected the 1D Flood Map from the Results tab on the Ribbon. The menu allows you to select: (i) the TIN; (ii) the underlying DEM; and (iii) the results time step(s) required. The ground grid field was left blank as water levels (not depths) were required.

Step 7: Convert grid results to CSV

Exporting the grid results in CSV format was the one task that could not currently be completed within Flood Modeller Pro. Even so, the format of the ascii file created, namely one row per row of grid data, made it very easy to create a simple script or Excel function to convert this into the required format.

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