The Romeriksporten railway tunnel — Drainage effects on peatlands in the lake Northern Puttjern area

Abstract

This study presents the results of an investigation of drainage effects of the Romeriksporten railway tunnel on peatlands in the Lake Northern Puttjern area. Drainage impacts on peatlands occurred in marked depressions reflecting faults or weakness zones in bedrocks. The impacts were most evident near the tunnel trace, but dry peatlands and streams were observed as far as 600 m from the tunnel trace. Several types of drainage effects were observed on peatland surfaces after tunnelling; surface subsidence and formation of local depressions, peat slides, peat cracking, dry peat holes, and death of vegetation. The effects depended clearly on peatland properties. The greatest and most damaging effects were associated with thick peat layers around open water surface. Here lowering of the water table caused peat slides. Such areas will generally be extremely vulnerable to lowering of the water level. The magnitude of subsidence of peatlands depends on peat thickness, peat decomposition and groundwater lowering. Even after seven years with artificial infiltration in bedrock wells in dry periods to counteract water table lowering, peat subsidence and peat slides were evident, demonstrating that temporary draw down of water can result in irreversible impacts on peatlands. To better be able to avoid damaging effects of tunnel drainage on environment, investigations of properties of peatlands above possible tunnel traces should be integrated in the planning procedure for tunnel constructions. In mires above such traces, the occurrence of floating peat mats, degree of peat decomposition, peat thickness, deliveries of water from surrounding catchments in dry periods and possible pathways of leakage through subsoils and bedrocks should be examined as basis for assessment of vulnerability of peatlands to tunnel drainage and selection of tunnel traces that evade the most vulnerable peatlands. When such areas can not be avoided, requirements should be set to the tightening of the tunnel to prevent damaging water draw downs in dry periods. Observations of drainage effects on peatland surfaces can complement hydrological information from measurements of tunnel leakages and groundwater levels and thus contribute to provide a broader picture of hydrological impacts and bedrock hydrogeology.