Flumen: an agentic, GPU-accelerated program for flood and coastal hydrodynamics, its architecture and its calibration against laboratory and inter-model benchmarks

Flumen is a hydrodynamic program written in Rust that solves the two-dimensionalshallow-water equations with rain, infiltration and a full set of boundaries, couples themto one-dimensional river networks, extends them to three-dimensional hydrostatic flow onσ layers with a k–ε closure and Coriolis force, and adds a depth-integrated non-hydrostaticpressure for dispersive waves. A double-precision CPU engine serves as the reference, anda CUDA engine compiled at run time carries the same physics on the GPU, where a 1.08-million-cell flood runs for one hour of simulated time in 3.1 s and a 48-hour estuary in threedimensions takes 13 s against 546 s on the CPU. A desktop interface, a Model ContextProtocol server with 43 tools and a remote compute server let an engineer or a language-model agent build, run and calibrate models on a rented GPU. Twenty-one verificationtests pass on both engines. On the Environment Agency benchmark the depths of Tests 4and 5 agree with LISFLOOD-FP to 2 mm and 24 mm, which is within the spread betweenLISFLOOD-FP’s own solvers. On two NTHMP laboratory benchmarks the calibrated modelreproduces the gauges of a solitary wave on a composite beach to 1.0–10.3 mm and the Monaivalley runup to within 1 % at the highest point, and a wall-impact treatment of breaking,calibrated on the GPU, raises the runup of the largest breaking wave on a vertical wall from40 % low to within 1 % on a 10 mm grid and 13 % high on a 5 mm grid, although a splashtwice the water depth remains 60–67 % low. The benchmarks also exposed a defect that theanalytical tests had missed, a depth limit of the non-hydrostatic solver that held water 15–20 mm too high on a wetting shelf, and they left one discrepancy unexplained, a drawdownat one Monai gauge that no physical or numerical setting reproduces.