An integrated evaluation of the National Water Model (NWM)–Height Above Nearest Drainage (HAND) flood mapping methodology

Abstract

Abstract. Flood maps are needed for emergency response, research, and planning. The Height Above Nearest Drainage (HAND) technique is a low-complexity, terrain-based approach for inundation mapping using elevation data, discharge–height relationships, and streamflow inputs. The recent operational capacities of the NOAA National Water Model (NWM) and preprocessed HAND products from the University of Texas offer an operational framework for real-time and forecast flood guidance across the US. In this study, we evaluate the integrated National Water Model –Height Above Nearest Drainage (NWM–HAND) flood mapping approach using 28 remotely sensed inundation maps and 54 reach-level catchments. The results show the NWM–HAND method tends to underpredict inundated cells in 4th-order and lower-order reaches but does better with a slight tendency to overpredict in high-order reaches. An evaluation of the roughness coefficient used in the production of synthetic rating curves suggests it is the most important parameter for correcting these errors. Persistent inaccuracies do occur when NWM streamflow predictions are substantially biased (>60 % mean absolute error between NWM and observed streamflow) and in regions of low relief. Overall, the NWM–HAND method does not accurately capture inundated cells but is quite capable of highlighting regions likely to be at risk in 4th-order streams and higher. While NWM–HAND should be used with caution when identifying flood boundaries or making decisions of whether a cell is dry or wet, its applicability as a high-level guidance tool along larger rivers is noteworthy.

Improving flood hazard datasets using a low-complexity, probabilistic floodplain mapping approach

“…Our mapping approach, while simple in its representation of process, produces floodplain maps with a high rate of agreement to those created with a hydraulic model, which has a more robust representation of process, but is data intensive and has greater computational demands. As such, our work supports the assessment that a new generation of floodplain mapping approaches, notably those classified as low-complexity, has great promise to improve upon our understanding and communication of flood risk and conservation practices [18,20,24,48].…”

“…In its current form, the probHAND product is unlikely suitable to support the NFIP and the determination of flood insurance rates. Although the probHAND had better agreement with the calibrated hydraulic model, than did the DFIRM, documented local uncertainties, both in our model and elsewhere, are likely too high to identify the flood risk for a single property [ 24 , 50 ]. It may, however, provide a good resource for other flood risk management applications, including public education and emergency planning, notably over regional scales.…”

“…Many of the alternative approaches simplify the representation of process [19][20][21] or of underlying terrain [17], making their application over large spatial scales possible [22,23]. Such frameworks hold promise for improving flood risk planning and emergency response as well as for conservation applications [21,24].…”

Performance of automated flood inundation mapping methods in a context of flash floods: a comparison of three methods based either on the Height Above Nearest Drainage (HAND) concept, or on 1D/2D shallow water equations

“…Another possible source of uncertainty is the absence of connection between floodplains associated with each river reach: water cannot flow into pixels which are not connected to the considered river section, which may be particularly problematic at confluences of flat and wide floodplains. Johnson et al (2019) conclude that significant errors may be observed for both low-order upstream river reaches, and downstream and flat floodplains.…”

“…These methods are applied either directly from the DTM for the AutoRoute method (Follum et al, 2017(Follum et al, , 2020, or from a Height Above Nearest Drainage raster (Nobre et al, 2011) derived from the DTM: f2HAND (Speckhann et al, 2017); Geoflood ; MHYST (Rebolho et al, 2018); Hydrogeomorphic FHM (Tavares da Costa et al, 2019). All these methods determine a local discharge/height relationship from i) the cross-section and longitudinal profile geometries extracted from the DTM (locally for Autoroute, averaged at river reach scale for HAND-based approaches), and ii) a local hydraulic formula: Manning-Strickler (Zheng Xing et al, 2018;Johnson et al, 2019;Garousi-Nejad et al, 2019) or Debord (Rebolho et al, 2018). These approaches are very efficient in terms of computation times, and can therefore be suitable for real time inundation forecasting at continental scales (Liu Yan Y. et al, 2018), or for probabilistic or multi-scenario modelling (Teng et al, 2017).…”