SIR2016-5029_cfwgoshOR_breach: Flood-Inundation Maps for the Coast Fork Willamette River from Creswell, Oregon to Goshen, Oregon (Area of Uncertainty)

This feature class represents inundated area for the Coast Fork of the Willamette River, 
the Row River and Silk Creek (west of Cottage Grove, OR) for eight different flows at the 
gage Willamette River at Goshen, OR (USGS 14157500). The flows are 12,000; 15,000; 
21,450; 27,900; 33,900; 39,900; 46,800 and 62,300 cubic feet per second (cfs). These 
flows correspond to gage heights of 11.8, 13.2, 15.1, 16.3, 17.2, 17.9, 18.6 and 19.8 feet, 
respectively. The domain of the model is as follows: Row River from Dorena dam to the 
confluence with the Coast Fork; Coast Fork from Cottage Grove dam to the confluence 
with the Middle Fork; Silk Creek from River Mile 1.7 to the confluence with the Coast Fork. 
The basis for these features is the Willamette Flood Insurance Study – Phase One (2013). 
The hydraulics and hydrology for the FIS were reused in the production of these polygons; 
the reports and information associated with the FIS are applicable to this product. The 
Digital Elevation Model (DEM) utilized for the Willamette FIS submittal was produced 
by combining multiple overlapping topographic surveys for the Middle Fork and Coast 
Fork of the Willamette River. This DEM was created from four sources: LiDAR of the 
Springfield area that was flown in 2008, LiDAR of Silk Creek that was flown in 2011, 
LiDAR of Fall Creek that was flown in 2012, and photogrammetry of the Middle Fork 
and Coast Fork of the Willamette River that was flown in 2004. In areas where no 
high-resolution elevation data were available, USGS National Elevation Dataset (NED) 
data were used to supplement the DEM. The shapefiles Hi_Res_Extents.shp and 
Low_Res_Extents.shp define the limits of these areas. The horizontal datum of the 
DEM is NAD 1983 State Plane-Oregon South HARN with units of International Feet 
(NAD83). The vertical datum of the elevation model is NAVD 1988 with units of 
international feet (NAVD-88). In addition, some areas show surveyed bathymetry 
within the channel. These can be noted by the sharp increase in apparent depth, 
creating a stripe across the depth grid when compared to the LiDAR data, which 
represents the water surface elevation at the time of the aerial data collection. 
Bridge decks are generally removed from DEMs as standard practice. Therefore, 
these features may be shown as inundated when they are not. An effort to clip 
flood extents on bridge decks was made, but judgement should be used when 
estimating the usefulness of a bridge during flood flow. Comparing the bridge to 
the surrounding ground can be more informative in this respect than simply 
looking at the bridge itself. The features and depth grids stop as the Coast Fork 
approaches the Middle Fork on the northern end of the reach.

Tags: coast-fork-willamette-river, creswell-oregon, elevation, environment, flood, flood-inundation-maps, flooded-area, geoscientificinformation, geospatial-analysis, goshen-oregon, high-water-marks, inlandwaters, oregon, river, stream, usgs-3ca5cbbe-cffc-4e8e-b0c1-7048d7a21d79, willamette-valley
Organization: Department of the Interior