On the flow conditions requiring detailed geometric modeling for multiscale evaluation of coastal forests

The multiscale evaluation method is applied to assess the influence of detailed geometric modeling of trees on their macroscopic attenuation effect against tsunami-like flow. Specifically, we conduct a series of numerical flow tests (NFTs), i.e., 3D flow simulations in a local test domain (LTD), under various inflow conditions to evaluate the macroscopic flow characteristics in the LTD accommodating an array of either simple cylinder or detailed tree models that mimic a coastal forest. After the procedure of NFTs in the multiscale evaluation method is briefly summarized and the corresponding governing equations and analysis conditions are presented, we introduce two indices for evaluating the macroscopic flow characteristics within the framework of multiscale modeling. Based on the NFT results, we discuss how the modeling scheme for trees influences the macroscopic flow characteristics in terms of these indices and clarify the microscopic mechanisms that influence the macroscopic attenuation property. Additionally, the NFT results are compared with the experimental results to justify these discussions, and key factors are explored in terms of reproducing real phenomena. In addition, to apply the multiscale evaluation method to assess the disaster mitigation performance of various types of ecosystems in a realistic situation, we discuss under what circumstances and with what level of detail vegetation should be modeled.