Laserfiche WebLink
1.2.2 Development of Contours in Areas not Covered by Historical Sources <br />Contoured data of the bottom of alluvium and the groundwater level were not available from <br />historic reports for the alluvium of Crow Creek, Upper Box Elder Creek, Upper Beaver Creek, <br />and the South Platte River just below Chatfield Reservoir. In these areas, picks from DWR well <br />permit images and other sources were used in this task to manually draw contours of the <br />bottom of the alluvium and groundwater level surfaces. Additionally, the contours from <br />adjacent historic reports were used to guide the geometry of the contouring. These areas are <br />identified as SPDSS Task 42 in Figure 2. <br />1.2.3 Merging of Contours <br />The contours obtained from historical reports and the contours created in this TM were merged <br />together into one dataset. Contours from one report that overlapped or were adjacent to the <br />contours of another report were evaluated to determine how well the contours agreed between <br />the reports. Incases where there was not good agreement, the electronic contours were <br />modified to seamlessly merge the historic report contours. If picks were present in the area of <br />interest they were used to guide the merging, and where no picks existed then engineering <br />judgment based on the trend of the contours was used. Contours from more recent reports were <br />given preference over contours from more historic reports. The areas where contours were <br />modified to merge the datasets are shown in Figure 3. <br />1.2.4 Gridding of Bottom of Alluvium and Groundwater Level Contours <br />Gridding is the process by which randomly scattered data are projected onto a uniformly <br />spaced grid using an algorithm of choice. For the South Platte Alluvium Region sufficient data <br />picks were not available to create an accurate grid for the entire study area. However, sufficient <br />contour data from the historical reports and those contours created for this TM in the areas of <br />Crow Creek, Upper Box Elder Creek, Upper Beaver Creek, and the South Platte River just below <br />Chatfield Reservoir were available to accurately define the region. <br />Grids were created by first converting the contours into regularly spaced points along the <br />contour line using GIS. This allowed the development of a horizontal coordinate (X,Y) and <br />elevation value (Z) data set. The XYZ data set was used to grid a comprehensive bottom of the <br />alluvium and groundwater level surface in Surfer® (version 8; Golden Software, Inc.) a software <br />Bidding package. A statistical analysis using kriging techniques was used to develop <br />variograms for each surface. Variograms define the variation in data related to distance between <br />pairs of data points. A function was then fit to each variogram, which was used to interpolate <br />estimated elevation values onto a uniform grid. <br />For the bottom of alluvium, an exponential variogram model was applied with a sill of 17,800 <br />and a range of 1,900. For the groundwater level data, the gridding was conducted with the <br />kriging method using a linear variogram model with a slope of 1.40 and no nugget effect. The <br />sill, range, and nugget are terms that describe the kriging nonlinear curve-fitting parameters <br />and are defined in geostatistical textbooks and in the Surfer software manual. Both sets of data <br />were gridded on a 100-meter spacing, which is sufficient to capture the detail of the narrowly <br />incised paleochannels previous studies have shown to exist in the region. <br />After the grids were developed, contours lines were created by using Surfer® to fit lines of equal <br />value to the grid cells values. These contour lines were imported into ArcGIS and compared to <br />SPDSS Phase 3 Task 42.3 TM -Final <br />11/30/2006 <br />