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Flood control strategies: Hamburg as a case study

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I have mentioned before that the city of Hamburg is vulnerable to flooding, and that it has developed some interesting approaches to manage floods.  There is public education. There are initiatives to increase infiltration, turn the city into more of a sponge during downpours, create storage; I have described some of the flood control strategies for the Alster watershed.  But what about the big monster at the door: the Elbe river?

The Elbe is why Hamburg exists (it is, above all, a merchant port) but also why the city is so vulnerable to flooding, as the above map shows.  The river conveys downstream the impacts of storms that may originate in Saxony or even further upstream, past the mountains, in the Czech Republic. It also funnels storm surges from the North Sea downstream.  What is unique about Hamburg’s approach is its acceptance of the idea that floods are unavoidable – that allowing the water in and minimizing the damage is far more sustainable than higher dikes and storm barriers.  Some of this comes from the early work of Erik Pasche (1955-2010).

Pasche developed the concept of cascading flood compartments and adaptive response (yes, a bit of a mouthful).  What this translates into is the development of a hierarchy of where flood waters should go, and in what sequence; identify and protect what is vulnerable within that.  This can be seen in this diagram below:

The idea is that the flood is routed to pre-selected areas, and only to those if the flood waters can be managed this way; a worse flood may cause other areas to be inundated, in sequence, always with the idea that the more vulnerable areas are the last to be flooded.  This way the expected damage resulting from any given flood is minimized, as showed on the right side of the diagram.

But what does that look like in practice?  The concept applies best to the low-lying island of Wilhelmsburg.  This was the part of Hamburg worst hit by the 1962 flood, which killed 315 and left over 60,000 homeless.  The island is now home to a large part of the city’s harbour, heavy industry facilities, but also a sizeable residential sector (including the new development of the 2013 IBA), as well as farm fields.  The diagram of the island below shows the areas that would be flooded with and without compartments; the vulnerable residential areas are indicated with two dotted perimeters.  Without the compartment approach, which prioritizes flooding farm fields, it is clear that much of the flooding (in darker blue, right diagram) would occur in the residential sectors.

There are other innovations – in particular, for asset protection.  If you are going to decree that a particular place is where flood waters will be let in, it will be easier to accept if the various buildings found there can be somehow protected.  At the same time, the strategy needs to expand to areas that are not designed to be flooded; prudent design always assumes system failure.  This is expressed in the diagram below.

But how can one design buildings to withstand a flood?  In rural areas, houses and barns may be built on an artificial mound (these are called terpen, and the Dutch have been building them as well).  But in built up areas, buildings with stilt foundations, floodable ground floors (such as garages), or reinforced doors and windows may work well.  This is actually the strategy followed in HafenCity and the Fishmarket area, and it has been shown to be successful.  For instance, on November 9 2007, urban developer Thorsten Gödtel had taken a seat inside a café in HafenCity just as a North Sea gale had sent a 5- meter high storm surge of water up the river.  He recounts that

as he sipped a cappuccino, the turbid river rose outside, creeping up the establishment’s extra-thick windows, and temporarily turning the cafe into an aquarium. Later, he’d exited the building onto the street through a door one flight up, without even wetting the soles of his feet.

I like to imagine what such as strategy would mean for us.  How would Richmond fare with a flood managed using a Pasche cascade? Where would the inner dikes be installed?  And could the approach of HafenCity be used for False Creek or Coal Harbour?

Written by enviropaul

May 2, 2020 at 8:39 pm

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