Modeling and visualizing uncertainties of flood boundary delineation: algorithm for slope and DEM resolution dependencies of 1D hydraulic models

Abstract

As flood inundation risk maps have become a central piece of information for both urban and risk management planning, also a need to assess the accuracies and uncertainties of these maps has emerged. Most maps show the inundation boundaries as crisp lines on visually appealing maps, whereby many planners and decision makers, among others, automatically believe the boundaries are both accurate and reliable. However, as this study shows, probably all such maps, even those that are based on high-resolution digital elevation models (DEMs), have immanent uncertainties which can be directly related to both DEM resolution and the steepness of terrain slopes perpendicular to the river flow direction. Based on a number of degenerated DEMs, covering areas along the Eskilstuna River, Sweden, these uncertainties have been quantified into an empirically-derived disparity distance equation, yielding values of distance between true and modeled inundation boundary location. Using the inundation polygon, the DEM, a value representing the DEM resolution, and the desired level of confidence as inputs in a new-developed algorithm that utilizes the disparity distance equation, the slope and DEM dependent uncertainties can be directly visualized on a map. The implications of this strategy should benefit planning and help reduce high costs of floods where infrastructure, etc., have been placed in flood-prone areas without enough consideration of map uncertainties.

Visualising DEM-related flood-map uncertainties using a disparity-distance equation algorithm

“…To evaluate the applicability of the disparity-distance equation and algorithm by Brandt (2016) in deriving uncertainty boundaries that account for both the DEM resolution and the slope characteristics of the area, Lim's (2011) earlier modelling results for a part of the Testebo River (i.e. Forsby and Varva), north of Gävle, Sweden, were utilised as test cases.…”

“…Brandt, 2016) concluded that DEMs of between 4 and 10 m resolution (or point spacing) are enough to model flood boundaries with sufficient accuracy (e.g., Werner, 2001;Casas et al, 2006;Raber et al, 2007). However, those recommendations are based on the total characteristics of the model results; i.e.…”

“…A first attempt to see whether the disparities between modelled and real flood boundaries could be linked via the quality of the DEM was reported in Brandt (2009), using two particular river reaches of the Eskilstuna River, Sweden. This was further elaborated in Brandt and Lim (2012), and later quantified by Brandt (2016) through an empirically derived equation together with an algorithm capable of producing uncertainty zones of varying width. As this empirical equation is based on only one single river, there is a need for comparative studies to verify the approach of modelling and visualising DEM and slope-dependent uncertainties.…”

“…Using the Testebo River, Sweden, as test area for the empirical-based disparity-distance equation and algorithm (from Brandt, 2016), that considers both DEM resolution and terrain slope to create prediction-uncertainty zones around resulting inundation extents of a one-dimensional (1-D) hydraulic model, the study's objectives are to:…”

“…Based on the results from the Eskilstuna River, the quantile regression for a number of hydraulic simulations with different DEM resolutions yielded the following empirical equation (Brandt, 2016):…”