Three-dimensional numerical modelling of floods in river corridor with complex vegetation quantified using airborne LiDAR imagery

Excessive flood flow over the historic diversion weir in the vegetated Asahi River in Okayama Prefecture, Japan, was recently recorded for the first time after its renovation work. Fluvial researchers analysed the diversion discharge for flood mitigation measures through laboratory studies and conventional two-dimensional (2-D) depth-averaged simulations. The existing model was insufficient for simulation of certain phenomena such as flow resistance caused by vegetation branches and leaves and vertical flow distribution around the river corridor. Therefore, we developed a three-dimensional (3-D) vegetation resistance porous model by estimating topography, land cover, and vegetation distribution from airborne light detection and ranging (LiDAR) topo-bathymetry (ALB) data. Results show that the water level and flow regime were more reproducible than by referenced 2-D calculations when compared to space-time image velocimetry (STIV) data and field measurements. The diversion discharge designed using the proposed model is feasible with the current riverbed and vegetation conditions.

日本冈山县植被覆盖的旭河上的历史性分水堰，在完成修缮工程后，近日首次监测到超量洪水过堰流量。河流学研究者通过室内试验与传统二维（2-D）水深平均数值模拟，分析了用于防洪减灾措施的分水流量。现有模型无法准确模拟部分复杂水动力现象，例如植被枝叶产生的水流阻力，以及河道周边的垂向水流分布。为此，研究团队通过机载激光雷达（airborne light detection and ranging, LiDAR）地形测深（airborne light detection and ranging topo-bathymetry, ALB）数据反演地形、土地覆盖与植被分布，构建了三维（3-D）植被阻力多孔模型。研究结果表明，相较于传统二维数值模拟结果，结合时空图像测速法（space-time image velocimetry, STIV）实测数据与现场监测数据验证后，该模型对水位与水流流态的复现精度更高。基于所提模型设计的分水流量，在当前河床与植被覆盖条件下具备工程可行性。