Author(s): Md. Touhidul Islam; Keisuke Yoshida; Yuki Kajikawa; Satoshi Nishiyama

Linked Author(s): Md. Touhidul Islam, keisuke yoshida, Yuki Kajikawa

Keywords: 3-D vegetation distribution; Airborne topo-bathymetric LiDAR; Diversion weir; River flood modeling; Vertical flow regimes

Abstract: A complicated flow regime has resulted from some fixed weirs, sandbars, and dense trees surrounding a historic diversion weir in the lower reaches of the Asahi River in Okayama Prefecture, Japan. In 2018, a severe flood flow was recorded over the diversion weir for the first time after it was renovated as part of a river improvement project. However, through laboratory experiments and conventional two-dimensional (2-D) depth-averaged flow modeling, hydraulic researchers previously examined the diversion discharge for flood control measures. The existing numerical model could not simulate some phenomena, such as flow resistance attributable to vegetation branches and leaves and vertical flow distribution around the hydraulic structure. Given the limitations of previous studies, a new three-dimensional (3-D) flood flow model with the concept of vegetation resistance porous model was proposed herein by estimating topography, land cover, and vegetation distribution from airborne topo-bathymetric LiDAR data of the targeted river reach. The numerical simulations of the 3-D flow around the diversion area performed during the peak stage of the recent excessive flooding event were compared to referenced 2-D results. Compared to the benchmarked data, the new model-based findings demonstrated that the depth-averaged flow velocity and water level were more reproducible than the 2-D estimates. Furthermore, the proposed model showed a significant advantage over the traditional one by revealing the vertical flow distribution, as well as free surface and near-bed flow fields around the targeted structure. In addition, given the current topographic and vegetation distributions, the diversion discharge designed with changing flow conditions revealed that the newly developed model agreed well with the reference planned values when compared to findings obtained using the conventional model. Overall, the study's findings are expected to inform management tasks for hydraulic structures designed to safely drain heavy rains or flood waters from vegetated rivers. To summarize, the current results can assist policymakers in developing a balanced and rational scenario for flood control measures that take best vegetation management practices into account.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022989

Year: 2022