Drape and Merge

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Drape and Merge

In the context of merging surfaces, the drape and merge tool will allow you to combine multiple surfaces (eg the 100yr flood surfaces) in a number of ways:

1) Create a single, seamless surface (eg create a maximum 100yr envelope providing the 100yr flood level across the entire LGA), or

2) Create a virtual merge within a single catchment by clipping source files to a polygon and using a composite layer (often used for creating a time varying surface using AR&R ensemble hydraulics)

 

Single, Seamless Surface

To achieve a single, seamless surface, a common framework is required onto which the separate surfaces are draped. This is usually a DEM of the entire Council area. For example, the screen below is a merge of 8 separate flood studies.

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The mapping/overlaying operation will only work if the base file and all water surface files are in the same coordinate projection, or if each layer has had its projection specified. Also note that grids (.wrr) and TINs (.wrb) can be interchangeably mapped to each other.

To operate:

Base File

select the base "DEM" file to drape onto (a waterRIDE™ file containing terrain only or terrain with water - a copy of the terrain component will be extracted to a new start file).

 

Drape:

select the water surface file(s) to map. The order can be altered in the list.

 

Drape Multiple Surfaces:

Next, select how to process the mapping of using multiple surfaces:

Maximums Envelope: Will map the first file, then overlay subsequent files if the value is greater than what is already there.

Sequentially: Sequentially overlays valid values (non delete value) from the water surfaces, replacing any values from previous iterations as it goes

Generally, this will be the maximums envelope approach.

 

Options:

Drape using TIN Interpolation: When draping gridded surfaces, you can specify whether the surface should be mapped as a grid (ie flat, discontinuous, terraced water surface), or as a localised TIN surface (continuous, dynamically triangulated surface between cell centroids). This may be important when mapping to a much finer DEM framework, especially in areas of shallow flow to avoid a mapped product with alternating. TIN interpolation is almost always recommended.

Drape Terrain: Update the terrain in the base file using the terrain from the source files being draped. If selected, this can be controlled using "always" and "only if base is blank".

Save PEAKS Only: select whether a peak surface only should be generated (no time series), and if so, should the peaks of the source file(s) be used (quick) or should the full time series from the source files be used (thorough)

Use existing water surface data in BASE as initial conditions: when draping, the initial conditions of the BASE file (water levels, velocities etc) will be used as the starting point for the process.

 

Notes:

This action will automatically set a default output surface name, adding a '-mapped' to the base name if mapping and using the same base name if overlaying. If you are overlaying the first water surface and want to preserve the base name, simply select a different output filename to save to and a copy of the base file will be made before the water surfaces are processed.

When overlaying a source timeseries file to a base timeseries file, the timesteps must be the same (the process will check and let you know if they are not) and only the overlapping elapsed time steps will be output.

Mapping a coarse TIN water surface with many timesteps to a large fine grid DEM can generate very large files. In such cases it may be more efficient in disk space (at the cost of some speed loss) to map the water surface to the DEM on the fly.  The Setup Map-on-the-fly option should be used in this case, and this process creates a cross-reference index file between the TIN and the grid, and a peak properties file for rapid display of peaks. All the timeseries surfaces are mapped from the TIN to the grid as needed.

 

Virtual (AR&R Ensemble Hydraulics)

The aim of the approach is to create a representative time varying surface from which dyanmic hydraulic information and full flow hydrographs can be readily extracted. Normally, the outputs of ensemble hydraulics runs include only the peak envelope, which is unsuitable (incorrect) to use for calculating flows.

In this approach, a GIS file of polygons is used to define which hydraulic runs should be used in various parts of the catchment. The tool then clips the specified files to the polygons and merges them virtually using a composite layer.

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To use the tool, you will need to create a GIS layer of polygons that includes a field containing the relative (or full) path to the waterRIDE surface (wrr/wrb) to use within that polygon.

A Cleaned/Cutoff grid can be further specified to mask/clean the results (useful for direct rainfall models). This can save time if this grid was delivered as part of the modelling results. For TUFLOW, such grids often have the suffix "cutoff".

An example GIS layer is provided below, along with a masked/unmasked version of the resulting composite surface.

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Example GIS layer with relative (or full) path to the waterRIDE surface to use in each polygon

 

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Unmasked composite results
 

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Masked (cleaned) composite results