Boulder Sediment Assessment
Base Map
  • Off/On
  • Deposition bubble_chart
  • Erosion bubble_chart
  • Streams remove
  • City Bounds crop_16_9
  • Area of Interest crop_16_9
  • Drainageways
Highlight Areas of Deposition Larger Than: 500 yd3
Highlight Areas of Erosion Larger Than: 500 yd3

Boulder Sediment Assessment

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.


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.

Sediment Volume by Drainageway
Drainageway Cut (yd3) Fill (yd3)
Bear Canyon Creek 15,498 14,318
Bluebell Canyon Creek 5,218 11,793
Boulder Creek 217,779 175,240
Boulder Slough 7,159 1,797
Elmers Twomile Creek 1,816 2,342
Fourmile Canyon Creek 46,055 122,353
Goose Creek 8,754 10,034
Gregory Canyon Creek 9,653 13,568
Kings Gulch 1,860 1,352
Skunk Creek 13,197 18,994
South Boulder Creek 130,458 83,324
Sunshine Canyon Creek 3,570 13,209
Twomile Canyon Creek 12,825 42,843
Viele Channel 1,198 1,387
Wonderland Creek 10,421 14,042
Total in Drainageways 485,461 526,596
Rest of AOI 257,105 403,888
Total 742,566 930,483