Vulnerability analysis method of vegetation due to groundwater table drawdown induced by tunnel drainage

Abstract

Tunnel drainage demonstrates a behavior that constantly drains the high-ground subsurface water that feeds the overlying vegetative cover. The environmental impact of construction process on groundwater table drawdown can be evaluated using groundwater models. However, considering only the effects of drawdown is insufficient to evaluate the vulnerability between the terrain surface and groundwater table. We proposed a vulnerability analysis method based on soil water pressure by including the Soil-Plant-Atmosphere Continuum (SPAC) approach. We defined the vulnerability based on the wilting point of the terrain vegetation. A theoretical case study representing the groundwater level alterations that induce permanent wilting was investigated. The vulnerability analysis method takes account of two important factors; vegetation type and soil textures. The results show that wilting is more infiltration dependent, therefore, it does not occur immediately once the water table is lower than rooting depth. Soil water pressure losses caused by groundwater table drawdowns are compensated when sufficient infiltration occurs. Without sufficient infiltration, soil texture is an important factor for the resilience of vegetation cover under the influence of groundwater table drawdown. In our case study, layered soils lead to a flow behavior which makes the vegetation reach permanent wilting earlier than the homogeneous counterpart. Vulnerability analysis of vegetation cover can be regarded as an important step for an environment-friendly tunnel drainage design.