As promised last blog, I will talk about sea level rise. With that comes the endlessness of dykes. Since day 2 I’ve followed the seadyke a lot. But once in a while I turn landinwards a bit and come across beautiful old villages, former seadykes or riverdykes. Only by the change in building style, I can now feel I’ve left the Netherlands. But then again, the dykes never end. What’s on either side of them does change. From sea to salt marshes on one side to agriculture, villages and nature reserves on the other side. While nature reserves inland make for beautiful days of bird watching, on the other side of the dyke lies one of my… hmm… I might even say passions. I can’t explain why, but whenever I am on a salt marsh, it makes me happy. And hopefully next year I’ll be working on one again! But first, I tell you all about the last week and how dykes and salt marshes relate to sea level rise.
Sehestedt to Rudbøl
Day 6 – 91 km to Otterndorf – via Bremerhaven and Cuxhaven. Still feels like Friesland most of the time
Day 7 – 74 km to Steinkirchen – turned landinwards somewhat. Hit 500 km and my first hills. Preparation for Norway has started.
Day 8 – 99 km to Kollmar – skipped visiting Hamburg. First proper rain! Well, proper… let’s say it was refreshing whilst cycling.
Day 9 – 87 km to Meldorf – definitely not my best day. Rain, a lot of wind, grey, and overall a pretty boring landscape. Although the “stuwwallen” came as a nice variation in the route.
Day 10 – 61 km to Sankt Peter Ording – on my “day off”. Something with campsites that were not open. But had a lovely day of bird watching!
Day 11 – 65 km to Husum – Visited the beautiful lighthouse of Westerhever.
Day 12 – 77 to Rudbøl – So fed up with the wind I cut off a bit of the route. The plan was to go to Denmark tomorrow, but repair works on a dyke forced me to deviate from the route. It made then more sense to ride directly to Denmark.
For the last 12 days I’ve been mainly cycling along dikes. This includes a lot sheep shit covering my bike and gear, a lot of wind and sometimes some boredom especially when it starts raining. I deviated from the North Sea Cycle Route, mainly to follow the dyke more. The route does not always really follow the sea. After Cuxhaven, for instance, we turned land inward and from Hamburg to the north followed the Elbe. After Cuxhaven, there were so many orchards! At the campsite I even pitched my tent in a cherry orchard. The Elbe was a nice deviation from the dykes. But from now I’ll slowly be expecting some more dunes too.
The Dutch are known for creating land out of sea, and keeping the sea from the land. And one of the most important reasons for this are dykes. But the Dutch are not the only ones with dykes. Welcome to Germany!
By now I’ve seen hunderds of kilometers of seadyke. These are large structures separating the sea from the land. But not always does this work. Extremely high tides and storms may cause water spray over the dyke, or actually cause dykes to breach. Once that happens, you’re screwed. Along my route, there are many reminders of high tides, or from floods. Some in the form of height measurements, but – to me – more impressing are old photos of floods on the site were they were taken. It basically shows you what is at stake. And cycling through some crowded villages does that too.
With the current global change, sea levels will rise. Extreme storms will also be more frequent. If nothing changes with our sea protection, the chances of land flooding will only increase. A seemingly easy option is to increase the height of the dykes. This has been done in the past. Cycling through Germany, you could clearly see that the old dykes were much smaller. The old houses so close to the dyke seem very vulnerable.
When sea and river meet
Once you build a sea dyke, you also need to allow water from the land to flow back into the sea. This is always tricky, and quite technical. So, someone else may explain how that works precisely. I did come across some of these outflows. Some are simple pumping stations. Sluices are already somewhat bigger and allow ships to go through.
One of the more impressive I came across, is the Eidersperrwerk. I did look at the structure, but was way more interested by the birds. At the south end, there was a colony of black-headed gulls. At the north end, there was a colony of common terns. It is not by chance that they breed there. Because of the sperrwerk, not only water has to pass through concentrated openings. Aquatic life has to do so too. Fish get trapped, which is basically a fast food service to the birds. I spent quite some time here photographing the terns. I’ll show those pictures in about 3 months.
If you’re Dutch-speaking and want to know more about how aquatic life deals with these Deltawerken, there is a nice documentary “Holland – Natuur in de Delta”, which also includes the Deltawerken as barriers for migrating sticklebacks.
To quickly come back to the theme ‘sea level rise’, when a river meets the sea, but there is some water regulation in between, problems inland can still arise when extremely high tides are simultaneous with high water levels in the river. The river water can’t flow out to the sea any more, which could potentially lead to riverfloods too. A very visual and fun game to play and understand the effect of sea level rise and water protection (including rivers) is Pandemic: Rising Tide.
While making a dyke larger is often a go-to solution, it might not be the best one. And that is where salt marshes come in.
What is a salt marsh? If I would let my heart speak, it would be something like “a beautiful grassland with purple flowers, smelling like fresh mothballs, with cool insects, some grazers and a ton of birds”. But that may not help much… Besides, I apparently only note the cool things, and leave out the horse-flies, the inability to shelter against rain, sun or wind, and getting stuck in the mud… And this last part is key in explaining what salt marshes are.
Salt marshes start as a little build up of mud particles as a result of slow-flowing sea water in an intertidal area. At first diatoms and later plants will settle on these slightly higher formed ridges of mud. Once plants settle, they cause an increase in sedimentation and the land will grow. More plants arrive, more mud is trapped, until the land is by hardly ever flooded. The soil is still salty, so only a particular set of plants grows here. Once the sea water can’t reach the salt marsh any more, for instance due to building a dyke, the soil and vegetation turn brackish and will slowly loose all salt.
Most of the areas through which I’ve cycled here and in the north of the Netherlands would have been salt marsh in absence of dykes. These salt marshes used to be very nice for cattle grazing. The grass is of high quality. So, men actually started building salt marshes to create more land. Building structures on the mudflats initiates the trapping of sediment and thereby the growth of salt marshes.
So, what has this all to do with sea level rise and dykes? Well, the plants on the salt marsh help reduce the water speed of very high tides. It reduces the wave action. This means that a dyke with a salt marsh in front of it, needs to be a little less high. It also reduces the chance of waves demolishing parts of the dyke, potentially causing a breach. Along the German coast, restorations of these man-made salt marsh structures is actually happening.
There is much more going on with salt marshes and sea level rise. How well do they keep up with sea level rise (studied in the Dutch waddensea as a result of the subsidence of the salt marsh at Ameland due to gas extraction). How will they respond to climate change? How does grazing come in to play with this all? But it is too much to go over everything we know so far about salt marshes. For now, I wl say goodbye to them. There will be some more in Denmark. But mostly I’ll say: I’ll be back! Well, at least, it’s the plan that I go back to study the salt marsh again. More specifically, the insects in the very cool (or hot) warming experiment on Hamburger Hallig.
The road goes ever on and on…
So now, I’m sitting just across the border in Denmark. With the German part of the North Sea Cycle Route now behind me, the road will bring me some new adventures in Denmark. And of course you’ll hear about it next blog!