Error Assessment for Height Above the Nearest Drainage Inundation Mapping

Abstract

Real‐time flood inundation mapping is vital for emergency response to help protect life and property. Inundation mapping transforms rainfall forecasts into meaningful spatial information that can be utilized before, during, and after disasters. While inundation mapping has traditionally been conducted on a local scale, automated algorithms using topography data can be utilized to efficiently produce flood maps across the continental scale. The Height Above the Nearest Drainage method can be used in conjunction with synthetic rating curves (SRCs) to produce inundation maps, but the performance of these inundation maps needs to be assessed. Here we assess the accuracy of the SRCs and calculate statistics for comparing the SRCs to rating curves obtained from hydrodynamic models calibrated against observed stage heights. We find SRCs are accurate enough for large‐scale approximate inundation mapping while not as accurate when assessing individual reaches or cross sections. We investigate the effect of terrain and channel characteristics and observe reach length and slope predict divergence between the two types of rating curves, and SRCs perform poorly for short reaches with extreme slope values. We propose an approach to recalculate the slope in Manning’s equation as the weighted average over a minimum distance and assess accuracy for a range of moving window lengths.

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

“…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.…”

“…More thoughtful reach delineations may also reduce uncertainty [ 47 , 50 ]. While short reaches are often not long enough to adequately represent slope [ 50 ], longer reaches could mask important valley and channel morphology.…”

“…More thoughtful reach delineations may also reduce uncertainty [ 47 , 50 ]. While short reaches are often not long enough to adequately represent slope [ 50 ], longer reaches could mask important valley and channel morphology. Numerous segmentation approaches have been proposed (e.g., [ 64 ] and may be adopted to a watershed, or region to create geomorphically-consistent reaches.…”

Featured Collection Introduction: National Water Model II

“…The height above the nearest drainage (Liu et al ) in conjunction with synthetic rating curves (SRC) currently are used to determine the extent of inundation in NWM. Godbout et al () evaluate the accuracy of this inundation forecasting method using data from the 2015 Memorial Day flood in Austin, Texas. They find SRCs are accurate enough for large‐scale approximation of inundation but not so accurate to predict street‐level inundation, particularly in short reaches with extreme slopes.…”

Interactive comment on "A comprehensive evaluation for the National Water Model (NWM) Height above Nearest Drainage (HAND) Flood Mapping Methodology" by Johnson et al.

“…Printer-friendly version Discussion paper Some may say that there have been earlier evaluations (e.g. Zheng et al, 2018a;Zheng et al, 2018b;Godbout, 2018) P2, L33-35. Please state how the relief between each cell and the nearest stream is calculated.…”