Evaluating the impact of tunnelling on surface-water resources: update of the Drawdown Hazard Index (DHI) method

The Drawdown Hazard Index (DHI) method implements the rock engineering system introduced by Hudson in 1992 to quantify, through interaction matrices between the system variables, the potential for water inflow into a tunnel and the possible effect that this drainage may have on a spring, a groundwater-dependent ecosystem or a stream on the surface. The method is designed to identify and quantify these potential impacts and address the compensation measures from the early stages of the project. The method does not implement analytical or numerical calculations that require knowledge of precipitation, infiltration, and flows, but exploits the cause–effect relationships between the system variables to obtain the DHI index. This paper aims to complement previous papers that have presented DHI methods by providing all of the theoretical details of the method not covered previously, and by including recent updates, including the analysis of streams and groundwater-dependent ecosystem, in addition to the springs that are already included in the previous versions of the DHI method.