Technical Details

  • Floodplain Forest
  • Channel Complexity
  • Flood Inundation

Floodplain forest: The Pacific Northwest Ecosystem Research Consortium (PNW-ERC) mapped patterns of land use and land cover (LULC) for the Willamette Basin. Using these LULC maps and their LULC classes, we define floodplain forest as the following classes within the pragmatic floodplain of the Willamette River. Note: the unique numeric codes associated with each LULC class are provided for reference as used by the PNW-ERC.

Floodplain Forest Feature Codes
Value

Description

53

Forest closed hardwood

54

Forest closed mixed

55

Upland Forest semi- closed conifer

56

Conifers 0 -  20 yrs.

57

Forest closed conifer 21 -  40 yrs.

58

Forest closed conifer 41 -  60 yrs.

59

Forest closed conifer 61 -  80 yrs.

60

Forest closed conifer 81 -  200 yrs.

61

Forest closed conifer older than 200 yrs.

62

Upland Forest semi- closed hardwood

86

Natural grassland

87

Natural shrub

89

Flooded/marsh

98

Oak Savanna

101

Wet shrub

Channel complexity:  The Pacific Northwest Ecosystem Research Consortium (PNW-ERC) quantified channel complexity in the Willamette River floodplain using two measurements: 1) area of wetted features, and 2) river channel length, using a 1km slice spatial reporting framework for circa 1850 and circa 1995 river conditions. Based on this approach we report channel complexity as area of wetted features and river channel length per slice within the pragmatic floodplain for circa 2010 and for a future condition based on the PNW-ERC Conservation 2050 scenario, using the 100m slice spatial reporting framework. Wetted features classified as contributing to these measurements employ the table below.

Active Channel 2010 Feature Codes
Value Description

1

Main active channel

3

Sloughs, alcoves where one end is connected to main channel

5

Side or secondary channel, both ends connect to main channel

6

Remnant river feature within known floodplain. Disconnected floodplain lakes, sloughs. No apparent connection to main river but within historical floodplain area based on flood extent coverage.

7

Remnant river feature outside known floodplain. Disconnected floodplain lakes, sloughs. No apparent connection to main river and outside historical floodplain area based on flood extent coverage.

8

Human-created water features primarily inside the known floodplain

9

Human-created water features primarily outside the known floodplain

10

Tributary to Willamette River

11

Islands

2 year flood inundation: The regulated 2-year floodplain inundation data represents areas of predicted inundation associated with a regulated 2-year flood event. These data were created by River Design Group (RDG) to be used as a tool in identifying restoration opportunities on the Willamette River and are not intended for floodplain management or regulatory purposes. They are used here with permission of RDG.

Background: RDG is working with the Meyer Memorial Trust, the Oregon Watershed Enhancement Board, Oregon State University, the University of Oregon and The Nature Conservancy to facilitate restoration opportunities on a 62 mile reach of the Willamette River between Eugene and Albany, Oregon. The goal of the project is to use existing information to help guide restoration planning for the Willamette River. RDG used a LiDAR data set gathered between August 2008 and February 2009, provided by DOGAMI and acquired by Watershed Sciences, Inc., as the topographic surface model.

Stream gage data and U.S. Army Corps of Engineers flood frequency analyses were used to calculate water surface trendlines. Trendlines were then used to create a 2-year regulated peak flow water surface layer that was overlaid onto the Willamette River LiDAR surface model and clipped to the pragmatic floodplain extent (i.e. boundary of significant infrastructure such as highways, residential areas, etc.).

The resulting 2-year floodplain inundation layer illustrates areas of predicted inundation associated with a regulated 2-year flood event. Depth represents depth of water, in feet, above the topographic surface captured during LiDAR acquisition and does not necessarily reflect true water depth.