A framework for assessing tunnel drainage-induced impact on terrestrial vegetation

Abstract

The tunnel projects elevate the transport of large cities, but they also bring negative impacts on the terrestrial vegetation, particularly in mountainous rural areas. Aside from the immediate effects during construction, the tunnel drainage can have long-term effects on the terrestrial vegetation by depleting shallow groundwater resources adjacent to the tunnel. Previous research proposed a number of analytical and numerical methods for determining allowable tunnel drainage rates in order to keep terrestrial vegetation from wilting. These methods, however, only provide local estimates of drainage-induced drawdown and an approximate steady-state groundwater budget based on tunnel design parameters, ignoring soil water dynamics that have a direct influence on vegetation roots. As a result, we propose a comprehensive integral vulnerability assessment framework (IVAF) for assessing tunnel drainage's long-term regional impact on terrestrial vegetation. This framework couples a stochastic groundwater model, including tunnel parameters, with a topsoil model that includes soil–plant-atmosphere continuum (SPAC) components and creates regional vegetation vulnerability maps based on the vulnerability concept. It aims to provide a tool for tunneling experts by revealing the long-term tunnel drainage-induced changes between regional groundwater and soil water. The vulnerability assessment was applied to an example tunnel project in a moderately mountainous region. Vulnerability maps reveal the most likely influenced zones by tunnel drainage. The vulnerability index has been summarized via rooting depth segmentation, demonstrating the drawdown effect on topsoil. The assessment results show that IVAF is capable of establishing detailed groundwater and land cover connectivity on topsoil levels in poorly-sampled areas, as well as identifying potential vulnerability caused by tunnel drainage-induced drawdown.