Abstract
Coupled 1D–2D hydrodynamic models are widely utilized in flood 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. Therefore, this work adopts the 1D–2D model SOBEK to investigate the impact of drainage density on river flow. The uncertainty of drainage density in flood hazard mapping is assessed by a designed case and a real case, Yanshuixi Drainage in Tainan, Taiwan. Analytical 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. We suggest that modeling flood 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.
Highlights
In coupled one-dimensional and two-dimensional hydrodynamics models (1D–2D model), the channel flows are simulated in a 1D model, and the floodplain flows are simultaneously simulated in 2D
Drainage density has a significant impact on the accuracy of the 1D–2D model simulation, but has not been discussed in previous studies
In the case study of Yanshuixi Drainage, Effect 1 caused an underestimate of the flooded area, and Effect 2 caused an overestimate of the flooded area
Summary
In coupled one-dimensional and two-dimensional hydrodynamics models (1D–2D model), the channel flows are simulated in a 1D model, and the floodplain flows are simultaneously simulated in 2D. Their research result revealed that the channel and floodplain friction, and inflow discharge were the main uncertainties in flood propagation Another major source of model uncertainty is the model input, which include both hydrological and geological data. Wong et al (2015) used a 1D–2D model (LISFLOOD-FP) to study the effect of river channel cross-section and longitudinal-section geometry on flood dynamics Their results indicated that uncertainty on channel longitudinal-section variability only affected the local flood dynamics, but did not significantly affect the friction sensitivity or inundation mapping. Ogden et al (2011) and used the GSSHA model to evaluate the impact of impervious area, drainage density, width function and subsurface storm drainage on peak flood flows using the Dead Run watershed in Lanzhou State, USA as a case Those studies, which were based on the concept hydrological model or 1D model, found that a larger drainage density leads to an increase in peak discharge. This investigation designed two experiments using the SOBEK model to measure the effect of drainage density on two-dimensional flooding simulation
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