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<br />366 ComputJlional Fluid Dynamics <br /> <br />14.4 Model grids <br /> <br />Channel bathymetry was measured in the study reaches by the Grand Canyon <br />Monitoring and Research Center (GCM RC"). A fathometer mounted on a man. <br />oeuvrable boat recorded local water depth. The location of the fathometer, including <br />elevation, was tracked from a shore station with a theodolite manually trained on a <br />target mounted directly above the fathometer. The boat followed streamwise and <br />cross-stream lines spaced about 10m apart Isee Andrews etal.. 1999, for a more <br />detailed de,cription of surveying methods). In addition. the shoreline was surveyed <br />to outline channel shape; additional measurements were made around and over sand <br />deposits (Figure 14.2). <br />The water discharge was about 425 m"js during the measurements of channel <br />shape. In order to form topographic maps that extend to elevations in exceSs of <br />the river stage at 2830 mlls , the field-surveyed bathymetric and shoreline data were <br />combined with GCMRC photogrammetrically generated contour data (Figure \4.2). <br />Where the data sets overlapped, which typically occurred near the channel margin. <br />the field-surveyed data were used to generate the topographic surface because these <br />data were considered to be more accurate. A Triangulated Irregular Network (TIN) <br />surface model was created using the Delauney method of triangulation in which <br />topographic features are developed into a series of connected triangles where the <br />nodes of the triangles correspond to measured locations and the facets of the <br />triangles correspond to changes in slope. Contours were generated from the TIN <br />surface and corresponded well with the photogrammetric contours in the areas of <br />overlap. The TIN surface was then interpolated using a bivariate quintic interpol- <br />ation scheme, implemented by the ARC/INFO Geographical Information System <br />software (Environmental Systems Research Institute. Inc., 1991), in order to gen- <br />erate 10-m resolution grids used as the basis for model calculations (Figure 14.3). <br />The choice of grid spacing requires a balance between computational efficiency, <br />espedally given the demands of a time-stepping model, and sufficient detail. The <br />10-m grid is sufficient for computing sand volumes and locations, but docs not capture <br />metre-scale detail sw:h as backwater channels. Comparisons with model results using a <br />5-m grid showed smoother representation of channel shape, but provided no significant <br />improvement on calculated volumes. Tighter grids are more compatible with the <br />calculation of flow fields only, without time-stepping erosion and deposition, and can <br />be helpful for some applications such as habitat studies (e.g. Korman ot al.. 2004). <br /> <br />14.5 Model application and results <br /> <br />In addition to channel shape, application of the model requires specification of <br />discharge, downstream water surface elevation at that discharge, sand flux and grain <br />size into the reach, and the initial thickness of sand deposits. Two water discharges <br />were modelled in this study: 1270mJ/s. corresponding to the 1996 test flow. and <br />2830 mJ Is, which is close to the pre-dam average annual nood and the highest <br />discharge (in 1983) since the closure of Glen Canyon Dam in \963. Three sand <br />conditions (Figure 14.4) were modelled in each reach. These sand conditions are <br /> <br />- <br /> <br />. <br /> <br />. <br /> <br />. <br />