Increased Water Levels Due to Morphodynamic Changes; The Limfjord, Denmark

Abstract

The Limfjord is an estuary in Denmark between The North Sea and Kattegat. The Thyboroen Channel connects the fjord with The North Sea. The water levels in the western part of the Limfjord are strongly correlated with the water level in the sea at Thyboroen. Analyses revealed a close to threefold increase in the channel cross section over the last 100 years. This has led to a detailed investigation into the effects of the channel cross section on the extreme high water levels in the Limfjord now and in the future. For Lemvig in the western part of the Limfjord the water level with a return period of 100 years would have been 1.73 m with the 1958 channel bathymetry throughout the whole period and 1.99 m with the 2005 channel bathymetry. With the extrapolated 2060-bathymetry the 100 years water level will be 2.38 m. A number of measures to counteract the consequences of this development have been evaluated.

Ocean Circulation Model Applications for the Estuary-Coastal-Open Sea Continuum

“…Highest recorded sea level in Denmark occurred during storm surges in the North Sea, with the record being the October storm 1634, with up to 6.34 m above Danish Normal Null in the inner part of the German Bight. Wind driven sea level variations in the Limfjord can be strong in the storm cases (Knudsen et al, 2012). In the years following 1960, modeled sea level maxima exceeded well beyond 2.5 m in northern Jutland, at the western entrance to the Limfjord, near Thyborøn.…”

“…The modeling studies investigated the functioning and dynamics of benthic filter feeders (e.g., Maar et al, 2010), estimated the effects of mussel fishing activities on the sediment dynamics (Holmer et al, 2003;Pastor et al, 2020), studied the species connectivity (Pastor et al, 2021) and investigated physical drivers of stratification and de-stratification (Wiles et al, 2006;Hofmeister et al, 2009). Other modeling applications were dedicated to estimate changes in the frequency and magnitude of storm surges and water levels as a consequence of climate change (Knudsen et al, 2012). Hofmeister et al (2009) investigated the hydrodynamics in the Limfjord using the General Estuary Transport Model (GETM).…”

Assessing Future Flood Hazards for Adaptation Planning in a Northern European Coastal Community

“…An expanding channel cross section (Bruun and Gerritsen, 1960;Knudsen et al, 2011) leads to increased water transport into and higher surge levels in the fjord during storms. Between 1958 and 2005 a statistical 100-year extreme water level in the town of Lemvig has increased from 1.73 to 1.99 m DVR90, and it is expected to increase further to 2.38 m DVR90 by 2060 excluding effects of sea level rise, if the channel is allowed to develop naturally (Christensen, 2011a,b;Knudsen et al, 2012;Sorensen et al, 2013). All heights refer to the Danish Datum DVR90 which corresponds to Danish mean sea level around 1990 (Schmidt, 2000) unless otherwise stated.…”

Aspects of inlet geometry and dynamics

“…After that, from 24-26 January the increase of hydraulic efficiency takes place. This was supported by measured entrance cross section as presented in Knudsen et al (2012).…”

“…Thyborøn inlet in Denmark, which has been steadily opening and becoming more hydraulically efficient (Knudsen et al, 2012) over a number of years experiences a different kind of major flushing events, which might be the cause of the long term opening. That is, while the Limfjord system, Figure 6.18, has a small catchment compared to its surface area and therefore does not flood due to rain fall, it experiences large high water events (1.5 m to 2 m above MSL, see Figure 6.19) driven by storm surges in the North Sea.…”