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Musings about the environment and all it touches, from education to city planning

The weir on the Elbe

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Just before reaching Hamburg, the Elbe River encounters a major obstacle on its way to the sea: the dam at Geesthacht. It is actually the only dam that the river encounters through its 630 km course across Germany. And thought it is a very low dam, as dams go, it is quite a sight.

I wanted to see it for myself. I find the Elbe fascinating. It may not have the mystique of the Rhine or the Danube. But here, in the middle of the tamed and regulated nature of northern Germany, the Elbe remains wild, unpredictable, and quite capable of regularly wreaking havoc.

Technically, the Geesthacht dam is a weir, since it rises only four meters above sea level . It was built in 1960 in an attempt to regulate the flow of the river. The Elbe is notorious for its wild swings between high and low flows (over twenty times higher). The very low levels can be an obstacle to navigation (during the summer of 2015, we saw the shores exposed, the flow reduced to record lows; in contrast, the record high flows of June 2013 can be seen here). The weir, with its articulated vanes, ensures that the flow stays at the minimum level that ships and barges need – or so it is hoped. A double set of locks enable river traffic to travel all the way between Hamburg and Prague. Hamburg, just downstream, is a deep-water port. To accommodate ocean-going ships, the river is continuously dredged up to Hamburg. In the 50s, Hamburg was planning to dredge its harbour deeper for the new generation of container ships. There was concern that this could lower the river level upstream, stranding river traffic; hence the decision to build the dam.

Repairs on one of the weir's vanes

Repairs on one of the weir’s vanes

The original design was meant to be higher, at 5.65 meters above sea level. With such a drop it would have made sense to produce hydroelectricity, another source of green energy for the country. Why was this not done?

As it is, the weir backs up the waters about 30 kilometers upstream. But the original higher design would have raised the level of the Elbe all the way into what used to be East Germany. In the middle of cold war tensions, the East Germans protests were heeded. Nevertheless, the more modest level agreed upon was enough for year-round ship traffic in the Elbe-Lübeck canal, as well as for the intake of the Geesthacht pumped energy storage plant.

But the protests against the proposed hydroelectricity were of local origin: fishermen and biologists worried that the turbines would turn the river fish into mince meat. I find it remarkable that their concerns were heard back then. At the time, the Elbe was grossly polluted; there were barely any fish of significance living in the river. But the turbines could have been the last straw for the few fish clinging to life. The proponents backed down, a decision remarkable for its foresight.

Fish passages on both sides of the river

Fish passages on both sides of the river

There is a causeway above the weir, and this is what I wanted to see – or rather, feel the raw power of the rushing waters. Tall dams are impressive, but they are static monuments of concrete. Not so the Geesthacht weir. Here, everything is about the dynamic waters. In december when I visited, there was well over one thousand cubic meters of water flowing over the bridge, every second. That’s a thousand tonnes of rushing liquid pushing over the weir, jumping over it. The causeway, barely two meters above, thrummed with energy.

But what about the fish? They have been spared the turbines, but no fish can swim upstream against such rushing waters. A fish passage was installed on the south side of the river – a nice, meandering slow stream, very natural looking. Unfortunately it was never up to the task and whatever fish populations existed were badly impacted by the construction of the weir. Grass carp, catfish, flounder and a few other fish were found to use the passage, but trout and pike, in particular, were missing. As for the hoped-for return of sturgeon, the passage was just too small.

The new fish passage, on the north side of the river, opened in 2010. It was built by the power company Vattenfall, in compensation for habitat lost to the Moorburg power plant (habitat compensation, in German, goes by the mouthful ökologische Schadensvermeidungs- und -begrenzungsmaßnahme). This new passage is 550 meters in length, 45 separate pools, and is the largest fishway in Europe. The design challenges were fairly daunting: the fish way has to be large enough for the biggest fish (the three meter long atlantic sturgeon), while having a current that can accommodate the weakest swimmers such as migrating stickleback, smelt, and eel. The strong tides mean that the water level in the downstream end has a high range, something that had to be mitigated in order to allow small eels through (eels need a surface against which to push through). It is hoped that the new fish passage will allow not only sturgeon, but also salmon and the endangered maraena whitefish, to recolonize the river.

The new fish passage

The new fish passage

One can only hope. With the new fish passage, Vatenfall is angling to resurrect the hydroelectric project. They suggest installing low speed, large clearance turbines so as to spare fish, with a fish deflection system. Call me dubious.

mittens crabs on the old fish passage

mittens crabs on the old fish passage

But there’s another character that has found the old fish passage totally fine: the invasive chinese mitten crab. During migration time (yes, crabs too migrate), their numbers are so large that people gather to watch the spectacle. A very impressive sight, if a bit creepy: the introduced crab is very destructive 9among other things, its digging habits cause shoreline erosion). The waters behind the weir also hold the biggest concentration of another invasive species, the zebra mussel, this one coming via the Czech republic. These two mean that the ecological balance of the Elbe has shifted. But I expect the Elbe, only partially tamed by the weir, still has many surprises in store.

Technical details can be found here, here, and here.


Written by enviropaul

March 13, 2016 at 5:08 pm

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