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  1. An alternative approach delineating floodplains is to create a water-surface digital elevation model (WSDEM) for the flood of interest and a binary raster indicating whether the land surface elevation is above or below the water surface elevation. The best method for creating a WSDEM would be observations (e.g. Dartmouth Flood Observatory, http://floodobservatory.colorado.edu/), but a model would likely be the only practical option especially for consistency across a region. Spatial extrapolation of the water-surface elevation of rivers + a nominal increase in stage for the flood of interest is a viable approach for applications where hydraulic model output are not available. This method has been demonstrated at regional scales for mapping floodplains along the 17 the major rivers in the Puget Sound Basin (HUC4, ~37,000 sq km) with validation using maps of Special Flood Hazard Areas (aka FEMA 100-yr floodplains) (Konrad, 2015, https://pubs.usgs.gov/sir/2015/5033/) and for mapping side channel along the Cedar River, WA (Konrad et al., 2017, https://doi.org/10.1002/rra.3230) using either 10 m NED or LiDAR data. In both of these applications, the interest was in frequently flooded areas (<0.1 annual probability of inundation) as they disproportionately account for many ecological functions of floodplains and flood hazard and represent a more tractable management focus than 100-year floodplains that extend valley wall to valley wall in some places.


    Two key technical issues for this approach are 1) how to extrapolate water-surface elevations from a river across a valley to create a WSDEM and 2) what to use as the nominal increase in stage for the flood of interest (the flood height issue). While the extrapolation of the water surface elevations of a river across a valley is computationally intensive, once the WSDEM raster is created, floodplains can be delineated rapidly and repeatedly for various flood heights. The increase in stage for a specified flood (e.g., median annual) can be determined at gaged sites and applied to ungaged sites using spatial interpolation as Greg Schwarz suggests or a relation based on drainage area for a homogeneous region (see figure 2 from SIR 2015-5033 below). Regardless, any of these estimation methods that do not account for at-site flood hydraulics will have positive bias (over-estimating flood heights and floodplain area) in unconfined reaches because gages are generally located in more confined locations and negative bias (under-estimating flood heights and floodplain area) upstream of constrictions that create backwater conditions during floods. Floodplains for reaches upstream of severe constrictions had the largest errors for the Puget Sound mapping relative to FEMA maps.