Impact of river construction projects on the groundwater balance: A case study of a river widening project in the Alpine Rhine valley

The St.Gallen Office for Water and Energy and the Liechtenstein Office for Civil Protection plan to widen the riverbed of the Alpine Rhine over a section of 1.7 kilometers. Recently, Emch+Berger AG completed the planning phase for a preliminary study. Given the strong interaction between the Alpine Rhine and groundwater, the study also evaluated the project's impact on the groundwater balance.

TK CONSULT AG was commissioned to support the study by conducting a scenario analysis based on the existing numerical groundwater model of the Alpine Rhine valley. The project scenario was based on the optimized future width and the derived vertical position of the riverbed, as calculated by Hunziker, Zarn & Partner AG. The reference scenarios included: (i) the current groundwater model, (ii) a model reflecting the expected natural development of the riverbed without widening, and (iii) a model exploring an extension of the planned widening section.

The results of the groundwater modeling were summarized as follows:

• A spatial representation of the simulated differences in groundwater levels between the project and reference scenarios.
• A spatial depiction of areas requiring further investigation regarding groundwater level rise, depth to groundwater table, and land use.
• A qualitative assessment of the predicted impact on the public water supply.

Overall, the numerical groundwater modeling highlighted the key aspects that must be considered in further planning of the project and identified the need for additional investigations regarding the groundwater balance.