Performed by Eben Dennis - GIS Coordinator, ICON Engineering, Inc. for the City of Boulder and Urban Drainage Flood Control District
Background & Initial Setup
The focus of this study is a comparison between the City of Boulder’s pre-flood 2013 LiDAR dataset and the FEMA-collected post-flood 2013 LiDAR dataset. In compliance with best practices for raster processing, the FEMA data was resampled to match the Boulder raster’s resolution and location. A raster calculation of FEMA - Boulder was performed in order to provide a difference raster. Two separate rasters were then created, one for values +0.5 ft (fill), one for -0.5 ft (cut), and clipped to the study area. The elimination of lower values is done to mitigate some of the margin of error noise between the two datasets. The cut and fill difference rasters were each multiplied by 4 (resolution of all processed rasters is 2’x2’) to generate volume rasters.
The area of interest for this study was defined as a 50-ft buffer around the City’s 2013 Flood Extents layer, combined with a 100-ft buffer of all Hydrology Line features (with the exception of "DXF_LAYER" == 'LATERAL' ) located outside of the 2013 Flood Extents.
Processing
At the outset of this analysis, ICON determined that the best way to communicate areas of local erosion and deposition would be to aggregate the raster data and perform a cluster analysis. A grid of 20 ft hexagons that intersects the study area was created, and then zonal statistics for both volume rasters were calculated, with SUM being the statistical derivation used for the analysis. The zonal statistics table was joined to the hexagonal grid, and a new grid was created for both cut and fill. Polygons selected by the query "RANGE" = 0 AND "COUNT" > 1 were deleted from this layer, as this would flag any uniform change in rasters from breaklining around ponds or buildings.
Next, a cluster and outlier analysis was performed on each dataset in order to identify statistically significant clusters of high value polygons. Polygons from the cluster analysis results where "COType" == 'HH' comprised a new shapefile, and these polygons were dissolved to generate the highlight areas for this analysis.
Zonal statistics were then calculated again from the volume rasters, this time with the dissolved high volume areas, and again SUM was used to generate our final volume for the statistically significant areas of erosion and deposition.
Map Layers
Areas of Erosion/Deposition - Detailed above. Volume available on click.
Points of Erosion/Deposition - Centroids of the hexagonal grid/zonal statistics join. Size based on volume.
Streams - From the UDFCD stream dataset.
City Bounds - City of Boulder municipal boundaries.
Area of Interest - Buffered 2013 Flood Extents and selected Hydrology Lines.
Drainageways - Buffered 2013 Flood Extents with detailed information available on click.
