Development of the tidal parameters within the German North Sea estuaries of Ems, Weser and Elbe under the influence of the tidal dynamics in the North Sea and its surface influxes

The tidal dynamics in the three German North Sea estuaries Ems, Weser and Elbe have changed significantly since the middle of the 20th century. Among the generally accepted causes are anthropogenic changes in the estuary and changes in sea level. To better understand the contribution of the various factors to tidal changes, we use multiple linear regression models to estimate the amount of water level and tidal range changes in the estuaries that can be attributed to external signals. In this way, we are able to separate internal changes from prescribed external influences. The latter comprise the tide signal of the North Sea (represented by tide gauge Leuchtturm Alte Weser) and the respective river discharges upstream the tidally-active area of the estuaries. Trend analyses of tidal characteristics (annual values from 1935 to 2015) show that increasing mean low water levels, as seen at the tide gauge Helgoland, and even stronger increases of mean high water levels have been taking place at the outer tide gauges of the three estuaries. While estuarine tidal high water rose at a similar extent everywhere during the investigation period, tidal low water decreased more rapidly upstream the estuaries. This inverse development leads to a significant increase in tidal range at all estuarine tide gauges, with the largest changes found at the tide gauges St. Pauli (Elbe, 22,3 +/- 1,3 mm/yr), Papenburg (Ems, 31,3 +/- 1,8 mm/yr) and GroBe Weserbrucke (Weser, 17 +/- 1,5 mm/yr). At the same time, river discharges show no significant mean changes. Based on our regression analysis, about 70 % of the observed trends in tidal high water levels in the estuaries is accounted for by the selected North Sea signal and the respective river discharges. On average, we can explain 40 to 85 % of the secular changes in tidal range, with values in the order of 85 % evident only at the outer tide gauges. For tidal low water levels, we find no significant correlation between the external signals and the water levels in the estuaries. Thus, changes in estuarine tidal low water levels cannot be described by external signals using multiple linear regression models. This supports the view that changes in low tidal water levels in the estuaries may be predominantly caused by local developments and interior changes.