Case
When cloudbursts, groundwater and the river come knocking
It was only at the 11th hour that the historic Svostrup Kro inn by Gudenåen river was saved from a devastating flood in the winter of 2019-2020.
In the aftermath, both regulars and dedicated representatives of the contractor, the engineer and manufacturers have fought alongside an innkeeper who refuses to succumb to climate change, bureaucracy and quick fixes. The result protects the inn from 100-year events, integrates well with the the landscape and looks to the future.
The Svostrup Kro inn is located about ten kilometres from the town of Silkeborg. Inside, it is an idyllic inn from the 1800s with a kitchen that serves delicious food for all ages. Outside, there is a stunning view of Gudenåen river from the terrace. The river runs peacefully at the end of the garden.
However, in the autumn and winter of 2019-2020, the tranquility was broken. For months, Gudenåen had flooded the entire garden with water levels high above ground level. Only large water-filled barriers and pumps kept the water from reaching the inn's foundation. In February, the water level at Svostrup bridge was recorded at 19.20 metres – which represented a 100-year event.
River not the only threat
Meanwhile, in the low-lying rooms – furthest from the river – the water reached to 20 cm above floor level, and in the old wine cellar, the water protruded through the floor – despite the three pumps working around the clock to pump out the water. This water came from the hills behind the inn, which caused the groundwater under the inn to rise to a dangerous level.
The Svostrup Kro inn – Kathrine Leth's childhood home – was in grave danger, a news story that broke throughout the country. And for her husband, Dragan Sljivic, it marked the beginning of a three-year struggle for their livelihood and the preservation of a gem.
Taking matters into own hands
Dragan was 17 when he came to Denmark in 1997. He fled the civil war in the Balkans, where he, at age 12, had become a volunteer in an aid organization that built dams out of sandbags when the rivers overflowed their banks. The innkeeper and trained chef therefore knew the power of water well, so when rules on nature protection, lake and river protection lines and seven disagreeing municipalities along Gudenåen delayed a quick decision, Dragan took matters into his own hands – together with the 'Association for the Preservation of Svostrup Kro', which was founded for this purpose by passionate regulars.
To get a sense of the scale of the project, they contacted contractor Christian Brendstrup from Christians Anlæg in Silkeborg. Being a contractor, sewer master and landscaper, Christian and his 25 employees have helped many homeowners at the Silkeborg lakes protect their properties against flooding and rising groundwater.
Christian outlined the solution, and with this in hand, Dragan got a sense of the costs involved. Around DKK 2 million!
Christian outlined the solution, and with this in hand, Dragan got a sense of the costs involved. Around DKK 2 million!
Can the ideas be realised?
The outlined solution also marked the start of a complex process of obtaining exemptions from national and European regulations for lakes, streams and nature protection, but the ideas also turned into comprehensive preparatory works before the final solution could be designed and scaled. This work was assigned to engineers from Sweco in Silkeborg and Kolding, headed by project manager Bo Michael Frankø. He says:
"Christian had outlined a great solution, so we initiated six geotechnical drillings to investigate the soil composition on the site. We also performed subsequent simple test drilling and gradation tests to assess the drainage options. We installed data loggers that gave us a picture of variations in the groundwater table over an extended period of time – and compared it to the water level in Gudenåen river so we could generate a potential map. Ground and river water samples were also taken to assess the effect of drainage, and 244 measurements of terrain contours gave us a picture of how much the site should be raised."
With all the data in place, Sweco was able to make modelling that showed what the situation would have been like in February 2020 if the intended climate protection had already been established.
Christian's thoughts and ideas passed the test!
Looming groundwater
"With the inn being located on low grounds, I knew that this was not just about keeping Gudenåen from entering the inn, it was also about directing the water from the hills away from the inn," says Christian, and this was confirmed by Sweco's surveys. Christian had outlined a drainage solution for the inn, which after Sweco's data collection was expanded to include a 270-metre drain pipe around the entire inn.
The drain will collect the water from the hills before it reaches the inn, and on the other side, towards the garden, the drain must keep the groundwater level at an acceptable level. If too much water is removed, empty holes may form, which could cause the building to collapse. So the drains cannot go deeper than one metre, but when calculating the final drain levels, Sweco allowed for draining at higher water levels.
The problem of not going below one metre is that the 400-year-old wine cellar goes deeper, so during the flood, three pumps were working around the clock, sending electricity costs through the roof. Measurements show that without any pumping, the groundwater level in the cellar will reach 40-50 cm. Today, a single pump handles the water, while the other two are ready to step in during cloudbursts.
Although the towpath that runs along the river was already raised 70 cm, Christian suggested building a 100-metre clay dike between the inn and the river. Originally, the dike was supposed to withstand a 20-year event, but while calculating the height, in 2020, water levels corresponding to a 100-year event were measured. So the height was changed to 19.70 metres.
To prevent the dike from standing out too much in the surroundings, Sweco designed it with low and grassy slopes that blend in with the natural terrain. Between the dike and the inn, the terrain was raised 0.3-0.8 metres. A total of 2,000 tonnes of soil and sand of a carefully chosen composition was deposited around the drainage solution.
Well away with the water
A pumping pit is central to the collection of water from the drain pipes – up to 360 m3/hour according to the hydrogeological calculations. The water flows to a newly established concrete well holding two Grundfos SL.1 pumps on the ready.
"The dialogue between all parties has been very constructive throughout the preparatory work, so all elements are dimensioned for the tasks," says Ole Brauner Karlsen, Front Line Specialist at Grundfos. "From the well, the water flows naturally into a small lake in the garden – situated right next to the well – and from there on to Gudenåen river.
Here, a return valve has been established, which closes if the water level in the river rises above the outlet level. At the same time, two drain lines between the inn and the dike will open to lower the water level behind the dike. When the pressure transmitter detects that the water is rising in the well, the pumps start, pushing the water through a separate pump line directly into Gudenåen river – 7,200 litres per minute."
When the pumps are working, you can see the enormous pressure through the glass pane that forms part of the cover of the Ø2000 well.
There has been a very constructive dialogue between all parties throughout the preparatory work, so that all elements are dimensioned for the tasks.
From groundwater to heating solution
Dragan Sljivic can now look back on the three years and a debt he would rather have been without. Despite the support association's strenuous efforts, the association had to give up when potential and necessary applications for funds were rejected. Therefore, the collection only brought in DKK 50,000, forcing Dragan to take out a bank loan of DKK 2 million for climate protection.
But as he puts it, he has gained a new hobby. The sympathetic innkeeper has become a climate-protection geek. He has also learned a lot about politics and bureaucracy and about what can be achieved when professionals cooperate successfully.
"For example, Sweco came up with the idea that the drainage water from the wine cellar could be used for heating. A heat pump can extract about 3°C from the 5-7°C groundwater. And the oil boiler had to be replaced anyway..."
It may be just a drop in the river, but it’s better than nothing. And better than cloudbursts.