<Cheonggyecheon Restoration Module 4>
 
Construction and technology, We talked about the decision making process and priority issues, but the technology itself another challenge. The key issues in engineering and design was the scale of stream, how big it should be and types of water front, as we will discuss a little bit later. And allocation of space, time-span, water supply, and how to deal with highly contaminated initial rainfall in an urban setting. Separating and connection, which is the building of bridges to allow people to cross the streams.
 
And lastly, water-friendliness and safety. The first issue is flood control, flood capacity.  Flood capacity depends on the cross-sectional area, the gradient and the surface roughness of the stream. These issues correlate to the types of waterfront, flow velocity and landscaping. Through this process we reached the optimal design. For example, the types of waterfront. The typical types are the natural, lake and urban types. Natural type has trees, shrubs and is ecologically sound but the urban type has more structures for the flow.
 
But there are pros and cons. For example, it is easy to secure enough area for flood protection with urban types.. But while the natural type can provide an ecological environment, it isn't suitable for the Cheonggyecheon area where a big channel for drainage must be built. So it requires heavy investment and a long construction period.
 
This is why Cheonggyecheon took a more urban approach to its restoration.  Allocation of space is also up for some debate. In the Cheonggyecheon area, some space can be used by the people for jogging or bicycling, while some spaces should be left as an ecological space. Some spaces should be intact areas with natural purposes to support the lives of animals, insects and even plants. So how much should be used by the people and how much should be exclusively allocated for animal and plant species is a big concern.
 
In Cheonggyecheon, the up-stream area near downtown is allocated for human activities while the down-stream area is allocated for the animals, insects and plants. Another key issue is that of highly contaminated initial rainfall. As it is in an urban setting, rain swipes all the dust and pollutants into Cheonggyecheon when it rains. The blue line in the graph to the right shows the amount of discharge while the yellow line in the bottom shows the degree of contamination.
 
The initial rainfall has lots of pollutants and that rainfall requires treatment. It is different from a river in a natural setting because in an urban setting there is a large amount of pollutants in the initial rainfall. In the left diagram you can see the double channel system.
 
While waste water is treated with only one channel, the second channel goes to the waste water treatment system and clean rainfall flows into Cheonggyecheon when there is heavy rainfall. By doing this, we can save costs of building a huge waste water treatment systems.
 
This is the more efficient way of treating urban pollution. The water supply issue. This is the dry area, and the challenge is how to supply the water for circulation. The plan was to use underground water from the subway lines into Cheonggyecheon stream, but there wasn't enough volume.
 
So we decided to pump some clean water from the Han river to the Cheonggyecheon's up-stream area. This was also one of the water pollution cleaning processes because when we pour clean water up-stream, it cleans the stream along the line.
 
Usually in waste water treatment, clean water is pumped from up-stream for use and then waste water is discharged in the down-stream area. Even though pumping decreases the volume of water in Han river, water pumped up-stream flows back into the Han river so that Han river's volume can be kept steady.

The construction. The construction was an interesting part of the process. In the beginning, as you see in the top left corner, we built a temporary protection and because this was a densely populated area, construction had to take place within the fence so working space was very small.
 
So we cut each part of the elevated highway and the covered area to restore the bottom part of the stream. This is the final physical cross-section after the restoration. The top blue dotted line is the 200 year flood water level. In urban situations, the regulations allow only 50 year flood level, which is the second dotted blue line.
 
But because of the sensitivity and the importance of the area, the plan was made to raise the standard up to 200 years. That's why Cheonggyecheon is a little deeper. Can you see gray line underneath the blue dotted line? That was Cheonggyecheon's original surface level. We dug a little bit further to secure the drainage volume. Also to the left and right we have some extra room just in case of flood. And we have some lanes and pedestrian paths on top of that area.