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（Asahi River）上的历史分流堰，近期在翻新工程完工后首次记录到超量过堰洪水流量。河川研究人员通过实验室研究与常规二维（2-D）深度平均数值模拟，分析了用于防洪减灾的分流流量特性。现有模型无法充分复现植被枝叶引发的水流阻力、河道周边垂向流速分布等特定流态现象。为此，研究团队基于机载激光雷达地形测深（airborne light detection and ranging (LiDAR) topo-bathymetry, ALB）数据估算研究区域的地形、土地覆盖与植被分布，开发了三维（3-D）植被阻力多孔模型。将该模型结果与时空图像测速（space-time image velocimetry, STIV）数据及现场实测结果对比可知，其对水位与流态的重现精度优于参考二维模拟结果。基于所提模型设计的分流流量方案，在当前河床与植被条件下具备工程可行性。