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

Trabrennbahn: green infrastructure with impossible soils

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Thr Tranbrennbahn complex on Google view.  Note the solar panels.

The Tranbrennbahn complex on Google view. Note the two ponds in the courtyard and the solar panels on the north side.

Imagine you’re building a residential complex. How do you make sure it doesn’t flood in a downpour?

In the old days, you would just dig storm drains, and let the city sewers handle it. But what if the sewers can’t? Either you get flooded as the sewers back up, or there is a sewage spill into a creek somewhere downstream. Not good either way, and increasingly riskier, as climate change is bringing on bigger storms.

The other approach, one now mandated by many city governments, is to imitate nature. Let rain water infiltrate into the ground, recharge the ground water. This is done with green roofs, infiltration basins, rain gardens and the like (there are many good examples; here’s a good source).

A pond in the Trabrennbahn complex

A pond in the Trabrennbahn complex

But what if the ground refuses to let water through? That was the problem for the developers of the Trabrennbahn complex, which I visited recently. The complex, built in 1998, is made up of 1,160 in separate low-rise buildings, and include a daycare, a school, and an old-age home. District heating with a co-generation system is used for the whole development, and of course the buildings adhered to the latest insulation and energy conservation standards (see below).

Like Lansdowne in Richmond, the complex is built over a former horse racetrack. The developers chose to retain the shape of the track in their design, as can be seen from the air view. This preserved a large internal area, a large green space from which cars are excluded but kids can play safely. The complex is built outside of Hamburg’s dense core; the air photo shows that the area is mostly single-family housing, but well served by transit.

The challenge for the developers: the soils are made of clay or glacial till, so the usual infiltration design could not be used. Instead, excess rainwater is stored on the soil surface. The grounds already had two large ponds inside the tracks; the developers added numerous ditches with weirs, which can hold a substantial amount of water. All that is needed to prevent flooding is to hold back the water a bit, and then slowly release it to the storm drains (or in this case,a creek). So instead of having a sudden rush of water, followed by no water after the storm passes, you get a sponge-like effect, a gentle increase in flow over a longer period.  During the storm, the water level in your ponds and ditches is a few centimeter higher, that’s all.

An example of a storage and drainage ditche

An example of a storage and drainage ditche

It’s simple, and it also makes for a very pretty, inviting landscape, as I could see when I visited. The central open courtyard is mostly lawn, but its surface is bordered by a mixture of un-mowed tall grass, bushes, and copses of trees.  This provides habitat for wildlife, but also breaks the monotony that a plain lawn would create. As it is, there is ample surface for kids to play games. The area has also been landscaped into gentle rises and dips; the landscape is designed so that water collects and flows in the dips, obviously, but the visual effect is that of a pleasing, natural landscape, even if you know it has been engineering from an original pancake-flat topography.

The designers added an extra touch, which may appear minor but makes all the difference: the ponds and ditches are shallow enough to be safe, so no fences or railings are needed (nothing worse than a fence to make a landscape feel hostile). Copses of boxwood trees add visual appeal, but also ensure that geese find the landscape uninviting (geese prefer large open areas). Conversely, few trees were planted next to the ponds where vegetation is thinned so that the open water of the ponds remains visible.

Design is one thing, follow-up is another. This is why I like visiting older environmental sites: have they aged well? Have they lived up to expectations? In Trabrennbahn’s case, the answer is a resounding yes. It was raining when I visited, but scattered toys and scooters showed that the play areas are well used by kids. The ponds and ditches were in fine shape, and no wonder: maintenance crews were on site during my visit, cleaning ditches and drain outlets, something they do regularly. In fact, inspections and maintenance on a regular basis, as well as planned removal of excess algae (which clog drains) was required in order to get a building permit. And so was the requirement of having a limnologist (a lake and river expert) approve the plan; this was all mandated in a legally binding land-use plan. So no sneaky changes after everyone has moved in – smart.

Another air photo of the site

Another air photo of the site

By the numbers:
District heating: natural gas fueled co-generation system, 1.25 MW electrical, 1.6 MW thermal
Drainage: 1,660 metres of ditches, 2,540 metres of waterside, over 200 underground drain pipes
Open ponds: 12,800 m2 of open water from two ponds where brick clay was originally extracted

Details can be found here (in English), here and here (in German).


Written by enviropaul

November 21, 2015 at 2:58 am

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