River response to mining-induced subsidence

Abstract

Land subsidence is a phenomenon commonly associated with water withdrawal and mining. In river valleys it increases the risk of flooding, which is usually reduced by channel alignment and raising of flood dykes. In this work, we describe the natural, advanced processes of sedimentation and channel adjustment to ground level changes in the Chechło River reach within an about 30-year-old subsidence basin in southern Poland. Physicochemical properties of sediments, channel geometry and its changes over a 5-year period have been analyzed. These sediments of the ponded depression differ markedly from the laminated overbank deposits of the unaffected river reach in respect of their stratigraphy and accretion rate. Formation of a levee is the characteristic feature of the ponded reach. The levee deposits are silt-dominated and highly organic and overlay thick sandy strata. The ponded parts of the basin outside the levee zone are characterized by slow deposition of fine grained organic sediments. A local increase in a channel slope induces relatively intensive erosion at the knickpoint and the formation of sediment wave on the channel at the downstream end of the pond. These changes are associated with a rapid increase of the Width/Depth ratio. The sediment wave has a tendency to disperse in place due to channel downcutting and the formation of the over 100 m-long downstream reach with the geometry similar to that observed in unaffected river reaches. These investigations indicate that even small subsidence ponds can exist in a highly dynamic fluvial environment for decades before they disappear. They can enrich the local habitat and landscape for long enough to be important for biodiversity in regions heavily impacted by human activity. Decisions regarding the restoration of subsided lands should be site-specific and preceded by considerations for achieving the best possible benefits instead of mandatory liquidation of subsidence depressions.