Stormflodssikring af Limfjorden: Lukning af Thyborøn Kanal

Abstract

Projektet omhandler Limfjorden, beliggende i den nordlige del af Jylland. Det er undersøgt, om en stormflodssikring opført ved Thyborøn kan mindske ekstrem vandstande inde i Limfjorden under storme, baseret på Kystdirektoratets Løsningsforslag. I projektet har især områderne ved Løgstør og Skive været i fokus, da disse bliver udsat for oversvømmelser ofte. Udvalgte løsningsforslag er simuleret i en simpel boksmodel opbygget af projektgruppen og en kommerciel model opbygget i Mike Zero. Begge modeller returnerer sænkninger af ekstrem vandstanden ved løsningsforslagene. Derudover er ekstremvandstandene også undersøgt ved en klimafremskrivning. Ud over dette er også vandkvaliteten blevet simuleret i to modeller, som er bygget oven på de to hydrodynamiske modeller. På den simple model er der påbygget en simpel empirisk model, med udgangspunkt i tidligere studier, som viste, at denne type model er bedre til at gengive næringsstofkoncentrationen end semideterministiske modeller. På den mere avancerede model er der påsat en ECO-lab model, som gengiver næringsstofferne samt iltkoncentrationer i systemt. Modellerne viste at der ville kommen en lille forskel i næringsstofkoncentrationen ved opførsel af en ny stormflodsbarriere, mens den ved mindsket udledninger af næringsstoffer ville blive bedre, med hensyn til ålegræsdybden.

This project is based on the storm surges in the Limfjord in Denmark. It has been found that the water level within the fjord can cause floods of the lower regions of the coastal areas. The majority of these areas are of agricultural nature, wherefore the focus areas of this report is the cities where floods often occur. The cities that have been chosen are Løgstør og Skive. The first part of the report describes where the location of the project is, and the problems that have been in the past. Furthermore this part describes the solutions, which the Danish Coastal Authority has found to lower the water level during future storm surges. This introduction leads up to the first part of the report, where the water level is described with two different models. These models are afterwards used to simulate different solutions from the introduction, where three solutions have been chosen; expansion of groyne 59, just outside Thyborøn Kanal in the western part of the Limfjord, expansion of groyne 63 within Thyborøn Kanal and a dam with a water gate. The best solution is then used to estimate its effects on water levels in the future with climate changes. It is found that the dam with a water gate is the best solution, and that it will reduce the extreme water levels during storm surges, even in the future with climate changes. The second part of the report focuses on the water quality in the Limfjord, since the ecological water quality is found to be poor in a majority of the fjord. This part simulates the solutions found in the previous part in order to estimate there impact on the water quality, and the solution from the environmental plan for the Limfjord. The solution recommended in the environmental plan for the Limfjord is a reduction of the nutrient input. The simple model returned a slightly worse water quality, but the more advanced model returned a better quality. The solution from the environmental plan could potentially change the water quality to a higher EQR that it is now. Additionally a simple model for the sediment in the fjord is built in order to estimate the changes of the seabed, with the habitat depth of eelgrass as the focus area, however this is found to not change considerably with any of the solutions from the Danish Coastal Authority in question. The conclusion of the report is that the solutions for lowering the extreme water level all will have a positive impact on the water level, and that they could have a positive impact on the water quality in the waterbody. Therefore it is found that a combination of suggested solutions for the Limfjord is the best solution. For the water level a dam with a water gate is the best solution, even when taking the future water rise in to the equation, while the best solution for the water quality is to reduce the discharge of nutrients into the fjord.

A master thesis from Aalborg University

Stormflodssikring af Limfjorden: Lukning af Thyborøn Kanal

[Storm surge protection of the Limfjord: Closure of Thyborøn Channel]