BEGIN:VCALENDAR VERSION:2.0 PRODID:-//pretalx//talks.osgeo.org//foss4g-2026//talk//3CZCNP BEGIN:VTIMEZONE TZID:JST BEGIN:STANDARD DTSTART:20000101T000000 RRULE:FREQ=YEARLY;BYMONTH=1 TZNAME:JST TZOFFSETFROM:+0900 TZOFFSETTO:+0900 END:STANDARD END:VTIMEZONE BEGIN:VEVENT UID:pretalx-foss4g-2026-3CZCNP@talks.osgeo.org DTSTART;TZID=JST:20260902T143000 DTEND;TZID=JST:20260902T150000 DESCRIPTION:# Background\nFlood inundation models are an essential componen t of flood hazard assessment\, emergency management\, infrastructure plann ing\, and climate adaptation. Most contemporary two-dimensional flood mode ls operate on structured raster grids or unstructured meshes and typically require repeated conversion between raster\, vector\, and computational r epresentations throughout the modelling workflow. While these approaches a re mature and widely adopted\, they can introduce complexity in data manag ement and interoperability. Discrete Global Grid Systems (DGGS) provide an alternative spatial framework based on hierarchical tessellations that su pport globally consistent indexing\, multi-resolution analysis\, and stand ardised spatial referencing. Although DGGS have been widely applied in Ear th observation\, geospatial analytics\, and environmental data management\ , comparatively little work has explored their use as the primary computat ional mesh for hydrodynamic simulation.\nThis paper presents FloodA5\, an open-source flood modelling framework built on the A5 equal-area pentagona l DGGS. The A5 DGGS provides an equal-area hierarchical tessellation of th e Earth composed of pentagonal cells. At any given resolution\, all cells possess identical area\, while refinement follows a strict parent–child hierarchy in which each cell subdivides into five children. Interior cells possess a uniform five-neighbour topology\, and compact hierarchical iden tifiers provide efficient indexing and storage. These properties make the grid attractive for hydrodynamic modelling because water storage calculati ons can be performed directly from cell area and depth\, while the hierarc hical structure provides a potential pathway towards future adaptive multi -resolution simulations.\n# Methods\nFloodA5 was developed to investigate the feasibility of performing flood inundation modelling within a fully DG GS-native workflow\, maintaining a single spatial representation from mesh generation through terrain processing\, simulation\, storage\, and visual isation. The framework is implemented primarily in Julia\, with DGGS opera tions provided through the pya5 ecosystem via a lightweight Python interop erability layer. FloodA5 integrates mesh generation\, digital elevation mo del (DEM) processing\, hydrodynamic simulation\, sub-grid terrain represen tation\, visualisation\, and data storage within a unified software archit ecture.\nFloodA5 currently supports two hydrodynamic formulations. The sta ndard solver applies the inertial shallow-water approximation of Bates et al. (2010) on the A5 mesh. Because A5 cells form a non-orthogonal polygona l grid\, a first-order correction based on the angle between the edge norm al and the cell-centre connection vector is applied when calculating water -surface gradients. The framework also includes an optional Sub-Grid Sampl ing (SGS) formulation intended to represent terrain variability below the computational mesh resolution. Rather than storing a single representative elevation for each cell\, the SGS approach derives hypsometric relationsh ips from high-resolution DEM samples and pre-computes volume–elevation\, wetted-area\, hydraulic-radius\, conveyance\, and edge-sill relationships . During simulation\, water storage is tracked as volume and converted to water-surface elevation through inversion of the hypsometric curves\, whil e flow routing uses hydraulic properties derived from the pre-computed SGS tables.\nThe framework was evaluated using two synthetic benchmark proble ms and a real-world flood case study. The first benchmark consisted of a p oint-source injection on a flat domain. Under isotropic conditions\, the r esulting inundation pattern should be circular and therefore provides a si mple test of directional bias. The second benchmark consisted of a planar slope intersected by a perpendicular embankment. This benchmark was design ed to evaluate routing behaviour under a known flow direction and assess t he ability of the SGS formulation to represent sub-cell topographic barrie rs. A larger-scale evaluation was performed using the January 2005 Carlisl e flood event\, using the same domain configuration and inflow hydrographs employed in previous LISFLOOD-FP studies. Three FloodA5 configurations we re tested: a resolution 18 standard solver (approx. 22 m cell spacing)\, a resolution 20 standard solver (approx. 5.6 m cell spacing)\, and a resolu tion 18 SGS solver. Results were compared against a 5 m LISFLOOD-FP refere nce simulation using inundation extent intersection-over-union (IoU) and d epth RMSE metrics.\n# Results\nThe synthetic benchmarks demonstrated that physically plausible flood propagation can be simulated on the A5 DGGS. In the point-source benchmark\, the resulting inundation pattern exhibited a high Polsby–Popper circularity score of 0.96\, indicating near-circular expansion. However\, visual inspection revealed a preferred northwest–s outheast propagation axis\, suggesting the presence of directional routing bias. The planar slope benchmark provided stronger evidence of this behav iour\, with the flood wave deviating approximately 30–60 degrees from th e expected downslope direction. These results indicate limitations in the current treatment of non-orthogonal gradients and suggest that more sophis ticated gradient reconstruction approaches may be required for accurate ro uting on pentagonal meshes.\nThe SGS benchmark revealed a second important limitation. Although the SGS formulation successfully represented sub-cel l elevation variability\, it failed to reproduce the hydraulic effect of a n embankment located entirely within individual cells. Water was able to p ass through the barrier because the current SGS representation preserves e levation distributions but not the spatial arrangement of topographic feat ures. This finding highlights a potential limitation of storage-based sub- grid approaches when internal barriers are important controls on flow rout ing.\nComparison with the Carlisle reference simulation demonstrated that useful flood simulations can nevertheless be produced using the current im plementation. The Resolution 18 standard solver produced the closest corre spondence with the LISFLOOD-FP reference\, achieving an IoU of 0.67 and a depth RMSE of 1.17 m. While these results indicate only reasonable rather than strong agreement\, they demonstrate that a DGGS-native flood model ca n reproduce broad inundation patterns within a real-world catchment. Inter estingly\, neither increased resolution nor the SGS formulation improved p erformance\, suggesting that numerical formulation errors currently domina te resolution-related effects.\nComputational performance was also evaluat ed. A Resolution 18 mesh containing approximately 30\,000 cells was genera ted in approximately two minutes and completed a 120-hour flood simulation in 39 minutes on a workstation-class AMD Ryzen Threadripper system. These results indicate that DGGS-native flood modelling can be performed effici ently without specialised high-performance computing infrastructure.\n# Co nclusions\nThe results demonstrate the feasibility of hydrodynamic flood m odelling on an equal-area pentagonal DGGS and establish FloodA5 as one of the first open-source frameworks to provide a complete DGGS-native flood-m odelling workflow. At the same time\, the synthetic benchmarks identify im portant methodological challenges\, particularly in relation to non-orthog onal gradient treatment and sub-grid representation of internal topographi c barriers. These findings should not be interpreted as limitations of DGG S-based modelling in general. Rather\, they highlight the importance of nu merical formulations specifically designed for non-orthogonal polygonal me shes. Beyond flood modelling\, FloodA5 illustrates the broader potential o f DGGS-native environmental simulation frameworks and provides an open-sou rce platform for future research into multi-scale environmental modelling\ , integrated geospatial analysis\, and environmental digital twins. DTSTAMP:20260718T004250Z LOCATION:Cosmos1 SUMMARY:FloodA5: An Open-Source Framework for Flood Modelling on an Equal-A rea Pentagonal DGGS - Matthew Wilson URL:https://talks.osgeo.org/foss4g-2026/talk/3CZCNP/ END:VEVENT END:VCALENDAR