Application of Persistent Scatterer Interferometry (PSI) Indicates a Significant Sediment Sink for the Ems Estuary

Abstract

The Ems Estuary enters the North Sea between the Netherlands and Germany. The Estuary has vast tidal flats consisting of fine-grained sediment. It is highly modified e.g. for land reclamation, coastal protection and navigability. Thus, the tidal behavior changed and natural sediment sinks vanished. The latter is identified as the most important reason for the substantial increase in suspended sediment concentration (SSC) from the 1990s onwards. Extensive research is being done to better understand the hydraulic and morphodynamic systems in order to keep the area in a stable state while balancing different concerns and interests, including future sea-level rise. Therefore, sediment budget calculations and morphodynamic modellings are carried out, taking into account all known sediment sinks and sources, such as dredging, land reclamation and fairway deepening. An additional sediment sink, which is hardly considered today, can be caused by the offshore subsidence due to hydrocarbon extraction. So the onshore subsidence above the huge Groningen natural gas field west of the Ems estuary is well known. In this study, we estimate the offshore extension of the subsiding area to the seafloor of the Ems Estuary. To do so, we apply Persistent Scatterer Interferometry (PSI) to generate around the Ems estuary a Wide Area Product (WAP) of interferometric ground motion information. We combined information about the extension of Dutch and German natural gas fields, the WAP-PSI-derived subsidence onshore east and west of the estuary and verified with Global Navigation Satellite Systems (GNSS) Stations including those mounted on tide gauges offshore in the estuary. Interpretation with further thematic data led to the identification and volume estimation of a new significant sediment sink in the Outer Ems. Starting in 1992, the offshore subsidence of the seafloor is estimated. Between 1992 and 2001 a maximum subsidence of 42 mm is detected. An area of approximately 200 km^2 is affected by a subsidence rate > 2 mm/a. The subsidence volume in the 9-year period from 1992 to 2001 adds up to approximately 5.5 Mil. m^3. The resulting average annual volume change of ca. 0.6 Mil. m^3/a is in the same order of magnitude as the estimated annual sediment sink of the total Ems Estuary between 1994 and 2015. With the data of the PSI-based BBD, up-to-date sedimentation volumes can be evaluated. We suggest to take this sediment sink and in general, the subsidence of the seafloor and tidal flats due to natural gas and oil extraction into account in future sediment budget calculations and morphodynamic modelling approaches. The politically decided end of natural gas extraction from the Groningen gas field will also put an end to a huge sediment sink in the Ems estuary probably increasing SSC further.