 Tools for conceptualization of sediment modeling schemes |
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GMS-SED is a package of GMS stratigraphy modeling and geostatistics tools that can be applied for modeling contaminated sediment deposits. Sediment deposits are complex, and accurate delineation of contaminated sediments is crucial for sediment assessment and remediation. GMS-SED is currently being used to delineate contaminated sediments for the Lower Fox River project in Wisconsin (http://dnr.wi.gov/org/water/wm/lowerfox). GMS-SED can:
- retrieve sampling data from a database,
- load large sets bathymetric survey data,
- develop 2D surface layers (TIN and 2D Mesh modules),
- build 3D models of sediment deposits (3D mesh and grid modules), and
- interpolate or krig sample concentrations to the 3D model mesh or grid.
Once a 3D model is constructed, GMS-SED offers great tools for further conceptual modeling (such as dredging scenarios), visualization of sediment layers and cross-sections, and estimation of sediment volumes and material properties.
The GMS interface is separated into several modules; these modules contain tools that allow manipulation and model creation from different data types. The modules of GMS are:
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The Map module is used to import CAD objects, such as DXF files or ArcGIS Shapefiles of the shoreline, and can be used to display map images, such as Geo-referenced TIFF or JPEG orthophotos.
The TIN module is used to define surfaces, such as the top sediment layer or deepest extent of soft sediment. TINs are also used to define the areas for dredge cuts. TINs are also used to create 3D mesh models of the sediment deposits.
The 2D/3D scatter point module allows handling of large 2D and 3D scatter-point datasets, such as bathymetric data that are used to construct sediment surfaces. 2D datasets can include a label, XY coordinates, and sets of dataset values (one of the datasets can be a Z coordinate or elevation). 3D datasets include a label, XYZ coordinates, and sets of dataset values. Datasets can include sample percent solids concentrations, contaminant concentrations, TOC concentrations and other analytical information.
The 2D/3D scatter point module also allows 2D/3D interpolation or kriging to TINs, meshes, or grids. 3D Interpolation methods include several inverse-distance weighting (IDW) methods, Natural Neighbor, and Kriging.
Some of the most useful interpolation options are settings affecting vertical and horizontal anisotropy. Below are some examples IDW interpolation results.
Azimuth = 10 degrees
| anisotropy = 1 | anisotropy = 2 | anisotropy = 3 |
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Azimuth = 30 degrees
| anisotropy = 10 | anisotropy = 3 | anisotropy = 5 |
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Comparison of IDW interpolation results on a horizontal cross-section, with varying levels of horizontal anisotropy. Vertical scale exaggerated 20 times.
Comparison of IDW interpolation results on a profile-view cross-section along the stream centerline, with varying levels of vertical (z-) anisotropy. Vertical scale exaggerated 20 times.
Jack-knife interpolation and other statistical methods can be used to check which interpolation settings are best for the sediment deposits.
GIS-based Model Conceptualization
One of GMS's greatest strengths traditionally has been the conceptual model approach. This approach makes it possible to build a conceptual model in the GMS Map Module using GIS feature objects (points, arcs, and polygons). For contaminated sediment problems in the GMS-SED package, the conceptual model defines the material property zones for a model (typically a 3D mesh or grid). The model data can then be automatically discretized to the model grid or mesh. The conceptual model approach makes it possible to deal with large complex models in a simple and efficient manner.
The GIS Module now available in GMS has made creating conceptual models from GIS data even easier. With direct linkage to ArcGIS and almost any format of GIS data, you can access geometry and attributes faster than ever before.
Whether the GIS data is created in GMS or imported from GIS files, the method of model building remains the same. You edit the model at a GIS object level and let GMS do the hard work of grid or mesh building and parameter assignment to each element of the model.
3D Model Conceptualization
GMS-SED uses new and improved tools within GMS for the creation of complex 3D stratigraphy models and the ability to translate that 3D object directly to a finite-difference grid model or finite-element mesh model. Data gathered from sediment coring results can be used to map gravel zones, sand berms, soft-sediment zones, and firm clay and bedrock base layers. Once developed in a 3D model, each of these zones can be isolated, hidden to better visualize the geometric characteristics of the sediment bed.
The “Horizons” approach allows you to create complex solids from sediment coring (borehole) and cross section data quickly and easily. These tools allow you to create solids with complex stratigraphy such as pinch out zones, truncations, and outcroppings.
You can transfer the results (material properties) of a solid model directly to a numerical sediment transport or hydrodynamic model, or quickly image or map sediment layers in GMS and 2D descriptions of layers to CAD and other applications.
Site Visualization
GMS-SED uses GMS’s powerful graphical tools for model creation and visualization of results. 3D rendering of the sediment elevations, physical features, and contaminant concentrations are critical to a good understanding of the contaminated sediment problem. GMS provides a variety of colorization and section schemes, all of which can be viewed from any angle.
A sediment deposit model can be displayed in plan view or 3D oblique view, and rotated interactively. Cross-sections and fence diagrams may be cut arbitrarily anywhere in the model. Hidden surface removal, and color and light source shading can be used to generate highly photorealistic rendered images. Contours and color fringes can be used to display the variation of input data or computed results. Cross-sections and iso-surfaces (surfaces with a consistent concentration or material property) can be interactively generated from 3D meshes, grids, and solids, allowing the user to quickly visualize the 3D model.
Graphical visualization is a critical tool for the sediment modeler, not only in developing the methods for interpreting sampling results and delineating contaminated regions, but also for communicating the results and discussing alternative modeling outcomes with the engineering team. When a remedial action scenario is developed, graphical visualization of the job is also a powerful tool for training the remedial action team.
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| Top-of-sediment elevations in segment of Lower Fox River, Wisconsin |
Visualization of near-surface contaminant concentrations |
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Card-house sections of contaminated sediments for visualization of contaminant distribution.
Engineering cross-sections directly from GMS, showing soft-sediment and upper layer for removal or capping.
Tools for conceptualization of sediment modeling scheme
Since the GMS-SED package is quite versatile, it is challenging for the sediment modeler to select from all the modeling options available to develop an efficient sediment modeling scheme. Visualization of the sediment deposit geometry and bathymetry is essential – as is investigating the spatial distribution of contamination.
One useful strategy for model conceptualization is map out sediment elevation, contaminant concentration, and sediment depth information in sets of side-by-side plots (see example below). The steps for model-building for large systems may be as follows:
- import shoreline geometry from GIS or CAD, or develop within GMS from an orthophoto. Then. Use GMS to create a surface TIN. The TIN density should be at least several times greater than the sampling density.
- import sediment sampling data as 2D scatter-point sets (XY position of core, top-of-sediment elevation, maximum contaminant concentration in core, soft-sediment depth, and maximum depth to a threshold concentration).
- compose new 2D scatter-point set for sediment elevation, combining core elevations with estimates for shoreline elevations.
- interpolate top-of-sediment elevations and other sediment sampling station data to the TIN, and develop contour images (see examples below). Experiment with a variety of interpolation settings – starting with the Classic IDW interpolation method.
- From side-by-side images, delineate approximate boundaries of remedial action areas (sub-divide the model into sub-areas for large systems). In addition to contaminant concentration and sediment elevation information, plot spatial patterns of percent solids, TOC, and other bulk sediment properties to help delineate sub-areas. Because changes in interpolation settings and follow-up sampling can expand or contract the extent of the sub-area, allow the boundary of the sub-area to extend beyond the areas that are clearly contaminated.
- Sediment volumes can be estimated from the average depth to a threshold concentration (calculated automatically by GMS), then multiplying this average by the total area of the TIN.
- While the distribution of contamination in plan-view is often correlated with sediment elevation, contaminant concentrations are often more strongly correlated with sediment depth. So, use sediment elevation trends to help delineate the sub-area, and then consider developing the 3D sediment model for the sub-area on the basis of sediment depth.
- use the 3D mesh tools to develop 3D models of the sub-areas. Each sub-area may have different interpolation settings. Each sub-area model will have its own GMS project file.
This general process was used to model contaminated sediments in the Lower Fox River Project (http://dnr.wi.gov/org/water/wm/lowerfox).
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Little Lake Butte des Morts, Lower Fox River, Wisconsin. Side-by-side imaging of top-of-sediment elevations (left), maximum contaminant concentrations at any depth (middle), and depths to a set threshold contaminant level (right).
Approx. scale: Top-to-bottom length of river segment is approx. 3.1 miles.