Stretch

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Stretch

waterRIDE™ identifies flood extents by the intersection of the water surface with the terrain and this process require the water surface to extend into the landscape beyond this intersection. Depending on the model type and setup, the network in some models may stop short of this intersection at some locations, with 2D grid models often stopping one cell 'short'.

To overcome this problem, the stretching tool can be used to stretch the water surface beyond its intersection with the terrain. This process is usually only feasible when the model has been transferred to a broader DTM that encompasses the extent of flooding. A water surface cannot be extended beyond the limits of its spatial framework.

When stretching larger grids (greater than 175,000,000 cells), only stretch by a buffer is available. As these large grids cannot be stretched in memory, a fast SSD local hard drive (or cloud based dataset) is recommended as their is considerable disk I/O involved. Only the peaks will be stretched in such grids.

A number of options are available:

 1. to extend the surface to its the wet extents using a DTM

 2. to extend the surface by a set number of grid cells or TIN triangles as a buffer around the current wet cells.

 3. extending the surface to the flood extents of another hydraulic surface (commonly used in flood forecasting or surface interpolation); or

 4. dynamically stretch the active water surface to the wet extents within a polygon drawn directly on the screen

With options 1 and 2, depending on the terrain in the stretched area and adjacent water levels in the wet area, some 'leakage' can occur across the mesh. To counter this, a combination of GIS polylines/polygons and a minimum depth threshold for stretching can be used.

Limit Lines: GIS "limit lines" can be readily created using the GIS tools. A "limit line" will not allow stretching across the line. For grids, a cell underneath a limit line can be stretched onto, but not passed. For TIN's, the limit lines define edges or connections between nodes that become impassable for stretching. Note that limit lines cannot be used when stretching large grids.

Minimum Depth Threshold: The minimum depth threshold is used to determine what calls surrounding a cells being considered for stretching are considered "available" for stretching. For example, if a threshold of 0.1m was used, and all surrounding cells are less than 0.1m deep, then the water surface will not be stretched onto that cell.

Often, it is worthwhile using a combination of method 1 and 2 to identify unwanted leakage. An iterative use of Method 2 is generally yields the best results.

The third option is typically used in flood forecasting where predicted surfaces are interpolated between design surfaces. The two design surfaces must have overlapping extents otherwise an interpolated value cannot be determined. For example, all design flood surfaces could be stretched to the PMF, or each surface could be stretched to the next highest surface.

Option 4, introduced in waterRIDE 10, is a fast way to "tidy up" a water surface.