Case study: Vulnerability Assessment, Zealand

Description of the application

The combined effects of climate change are difficult to access and require integrated impact models. The objective of the present study is to quantify the effects of sea level rise and climate changes on the water quantity and quality of a Danish lake located close to the sea. The lake is presently used for water supply and is threatened by saltwater inflow through the stream connecting the lake to the sea. In the present study the delta change approach has been applied on results from the regional climate model HIRHAM to obtain reliable projections of temperature, precipitation, and reference evapotranspiration, and sea level rise, both for the period 2071‐2100. With respect to temperature and sea level rise the absolute changes projected by the climate model from the present to the future period are added to daily observations of the two variables. Prior to bias‐correction the projected sea level rise is adjusted to account for isostatic changes to obtain result for the relative sea level change which is the relevant variable when vulnerability is examined. The distributed and integrated modeling system MIKE SHE is used for hydrological modeling. In the present application the model description includes: 2D diffusive wave approximation of the Saint Vernant equations for overland flow; Muskingum‐Cunge routing for flow in the upstream part of the river system and the full Saint Vernant equations for the downstream part of the river system, and 3D Boussinesq equation for flow in the saturated zone.

Lessons learned

It was expected that abstraction of water from Lake Tissø would be problematic in a future climate because of decreasing precipitation during the summer period. Results from other studies in Denmark have shown that the summer stream discharge is vulnerable to this change in rainfall pattern. Hence, it was expected that both the water level in the lake and the discharge from the lake into Halleby stream would be reduced in the future resulting in restrictions on the abstraction of water from the lake. However, Lake Tissø acts as a reservoir that is able to store relatively large quantities of winter precipitation that falls in the A2 climate. The water stored during winter is slowly released during the summer period resulting in both increasing minimum water levels and outflow from the lake. Hence, the results indicate that with respect to water resource considerations the production of water from the lake is sustainable also in a future climate. A critical quality issues was however found to threaten the production of water from the lake. Saltwater from the Great Belt was found to enter to lake during summer and fall where the water level in the lake is relatively low. At a mean sea level rise of 0.73 m, events with saltwater intrusion happen approximately once a year on average. It is likely that the intrusion of saltwater will be critical to both the ecological status of the lake and the possibility to abstract water from the lake. The inflow of saltwater could be prevented if a sluice gate was installed at the down‐stream end of the stream between Lake Tissø and the sea. The gate will close in cases where the sea water level is higher than the fresh water level and ensure that the saltwater only affects the outlet of the stream. This solution is presently used at several locations in Denmark and would be a relatively low cost adaptation measure.

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Last modified: Sept. 13, 2016, 4:44 a.m.



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