Extending Height Above Nearest Drainage to Model Multiple Fluvial Sources in Flood Inundation Mapping Applications for the U.S. National Water Model

Abstract

Height Above Nearest Drainage (HAND), a drainage normalizing terrain index, is a means able of producing flood inundation maps (FIMs) from the National Water Model (NWM) at large scales and high resolutions using reach‐averaged synthetic rating curves. We highlight here that HAND is limited to producing inundation only when sourced from its nearest flowpath, thus lacks the ability to source inundation from multiple fluvial sources. A version of HAND, known as Generalized Mainstems (GMS), is proposed that discretizes a target stream network into segments of unit Horton‐Strahler stream order known as level paths (LPs). The FIMs associated with each independent LP are then mosaiced together, effectively turning the stream network into discrete groups of homogeneous unit stream order by removing the influence of neighboring tributaries. Improvement in mapping skill is observed by significantly reducing false negatives at river junctions when the inundation extents are compared to FIMs from that of benchmarks. A more marginal reduction in the false alarm rate is also observed due to a shift introduced in the stage‐discharge relationship by increasing the size of the catchments. We observe that the improvement of this method applied at 4%–5% of the entire stream network to 100% of the network is about the same magnitude improvement as going from no drainage order reduction to 4%–5% of the network. This novel contribution is framed in a new open‐source implementation that utilizes the latest combination of hydro‐conditioning techniques to enforce drainage and counter limitations in the input data.

Using Inundation Extents to Predict Microbial Contamination in Private Wells after Flooding Events

“…The HAND boundary was derived from a combination of elevation data, storm-specific discharges, and reach-averaged relationships between water depth and discharge, rendering the boundary storm-specific. However, HAND only captured overbank, riverine flooding, leading to an underprediction of flooded areas. Further, the boundary was dependent on estimated parameter values (e.g., channel roughness, assumed uniformly to be 0.05) .…”

“…However, as the 100 year boundary was not storm-specific, it overestimated flooded areas, as evidenced by the lower specificities. The HAND boundary captured only overbank flooding along rivers, 22 which led to higher performance in the inland cluster and poor sensitivity across the tracts. Further, HAND had poor performance in the coastal cluster because HAND has an elevation threshold to account for uncertainty propagation, 27 which results in underestimating flooding in low-lying areas where flooding depths are shallow.…”

“…In this study, we used an uncalibrated HAND boundary, which assumes constant hydraulic parameters, such as channel roughness. The optimization of HAND-modeled inundation maps , to adjust roughness coefficients, and other hydraulic parameter estimations could potentially produce a storm-specific boundary with improved predictive power. This boundary can be developed in near real time, with predicted boundary values days to weeks prior to the storm.…”

“…After Hurricane Harvey, microbial contamination was 8 times higher in flooded versus nonflooded wells based on user-reported data compared to only 2–3 times higher using inundation extents . Inundation extents may not fully capture flooding risks for the well community due to techniques and limitations used to develop the boundaries (e.g., rainfall-derived, coastal storm surge, overbank flooding). − Further, there is no knowledge of how differences in inundation extents impact the assessment of well water impacts (e.g., contamination and damage) after a flooding event.…”

Optimizing Height Above Nearest Drainage parameters to enable rapid flood mapping in North Carolina

“…Both GFPLAIN and GFI inform the varying flood depth used for floodplain delineation using a contributing drainage area scaling relationship based on the work of Leopold and Maddock (1953), but GFPLAIN flags floodplain cells as those with elevations less than the maximum flood elevation, while GFI classifies floodplain cells by comparing the natural logarithm of the ratio between the maximum flood depth and the height above the nearest hydrologically connected river cell to a specific threshold value for binary classification. One geomorphic method used for mapping flood extents from large scale models such as the NWM is Height Above Nearest Drainage (HAND) (Afshari et al, 2018;Garousi-Nejad et al, 2019;Johnson et al, 2019;Aristizabal et al, 2023). HAND is a geomorphic method that takes a digital elevation model (DEM) as an input and converts the elevation data into a relative elevation wherein every cell represents the local height above the nearest hydrologically connected drainage channel (Rennó et al, 2008;Nobre et al, 2011).…”