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 drainage line, 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
(LP). 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.