This study adopts the 1D–2D model SOBEK to investigate the impact of drainage density on river flow. Coupled 1D–2D hydrodynamic models are widely utilized in food hazard mapping. Previous studies adopted conceptual hydrological models or 1D hydrodynamic models to evaluate the impact of drainage density on river flow. However, the drainage density affects not only river flow, but also the flooded area and location. The uncertainty of drainage density in food hazard mapping is assessed by a designed case and a real case, Yanshuixi Drainage in Tainan, Taiwan.
The study's results indicate that under the same return period rainfall, reduction in tributary drainages in a model (indicating a lower drainage density) results in an underestimate of the flooded area in tributary drainages. This underestimate causes higher peak discharges and total volume of discharges in the drainages, leading to flooding in certain downstream reaches, thereby overestimating the flooded area. The uncertainty of drainage density decreases with increased rainfall. The authors suggest that modeling food hazard mapping with low return period rainfalls requires tributary drainages. For extreme rainfall events, a lower drainage density could be selected, but the drainage density of local key areas should be raised.