0 Members and 1 Guest are viewing this topic.

#### Deskmate

• 174
• I'm loving deskmate, yoh!
• City: Harare
• Current Education: Sabbatical
##### Flood Management
« on: February 10, 2016, 02:51:58 PM »
Rivers, especially big ones, can cause a lot of mayhem. When they flood, they destroy livelihoods, cause economic damage and kill people. It’s no surprise then that humans, being the clever creatures that we are, try and stop rivers from flooding using whatever resources we have. Unfortunately, rivers, like most things in nature, are powerful & unpredictable. Stopping rivers from flooding full stop isn’t possible but we can do a lot to mitigate and manage the risks that arise from flooding.

Flood management strategies generally involve multiple engineering projects that can fall under one of two categories. Hard engineering projects are ones that involve the construction of artificial structures that, through a combination of science, technology and a bit of brute force, prevent a river from flooding. Soft engineering projects are the opposite. These projects use natural resources and local people’s knowledge of the river to reduce the risk posed by a flood.

Each type of project has its advantages and disadvantages. Hard engineering projects are generally very successful and have a large impact on the river. This is one of their downfalls though as the effects of a hard engineering project can disrupt ecological systems in the drainage basin. Hard engineering techniques generally involve the containment of large volumes of water so if they were to fail for some reason, the impacts could be many times worse than if the river had been allowed to flood naturally. There’s also the high cost, technological requirements & maintenance of hard engineering projects that makes them unfeasible in countries without significant economic resources.

Soft engineering projects focus more on reducing the impacts of a flood rather than preventing one. The biggest advantage of soft engineering is cost. Soft engineering projects are significantly cheaper than hard engineering projects making them more suitable for less developed countries. They also have lower education & technology requirements so they can be implemented by local people in remote parts of poor countries.

Soft engineering projects are more sustainable than their hard engineering counterparts. Soft engineering projects are low maintenance and low cost unlike hard engineering projects. In addition, they don’t disturb the natural processes and ecological systems in a river basin instead choosing to integrate with them and in some cases improve them.

Hard Engineering Techniques  Dams Dams are the classic hard engineering solution to flooding problems. A dam is a giant wall built across a river’s channel to impede its flow. Water builds up behind the dam and forms a reservoir which can then be steadily drained at a controlled rate over time. This helps keep discharge downstream of the dam low even during prolonged heavy rainfall.
Besides being highly effective at reducing the risk of flooding, dams can also be used to generate hydroelectric power that can bring economic benefits to an area by attracting manufacturing factories or being exported to other countries. The reservoir that develops behind a dam can be used as a drinking source or for leisure activities.

Despite these benefits, dams have some damn big (so sorry) problems. Dams are among the most expensive of all hard engineering techniques and require access to lots of raw materials like concrete and steel. They also have a huge impact on the local environment. The area behind a dam has to be flooded which destroys habitats and forces people out of their homes. Dams disrupt the processes that take place within a river too by, for example, preventing sediment from being transported downstream. This can result in landforms like deltas being destroyed which can in turn result in the destruction of more habitats. When sediment gets trapped behind a dam, it can change the chemical composition of the water behind the dam which can kill aquatic animals living upstream.

Dams pose a huge risk too. They store thousands of litres of water behind them so if they were to fail, they’d cause widespread death and damage downstream as all the water is released at once.
(Artificial) Levées Artificial levées are, well, artificial versions of their. They act as embankments, essentially extending the channel’s height and increasing its bankfull discharge. Unlike natural levées, artificial levées are significantly larger and are generally constructed out of a material like concrete that is resistant to erosion. The main advantage of an artificial levée is that it allows the floodplain to be built on. This is their downfall though as they encourage the development of the floodplain which can increase the risk of flooding. Furthermore, if they did fail, like the embankments along the Mississippi did in 1927, the damage from the flood would be far worse than if the embankments didn’t exist.
Wing Dykes Wing dykes are slats that are placed in a river’s channel at ~90˚ to the banks. Generally they’ll be placed in pairs on either side of the channel with a gap between them that allows boats to pass through them. Behind dykes, sediment builds up and the channel is narrowed, forcing water to flow faster. This helps reduce the risk of flooding by getting water away from an area at risk of flooding as quickly as possible, preventing a build up of water. They also aid navigation greatly. While wing dykes reduce the risk of flooding in one area, downstream of a river they can increase the risk of flooding making them only useful in sparsely populated areas.

Channel Straightening This technique kind of speaks for itself. By blocking off meanders and constructing alternate, straighter routes across meanders, the river starts to flow faster. Like with wing dykes, this moves water through the river faster preventing it from pooling and so reducing the risk of a flood. A straightened channel is faster to navigate too, a nice benefit of channel straightening.
Like with wing dykes, this technique has several problems. Downstream of a straightened section of a channel, flooding becomes more likely. In addition, erosion is stronger downstream because the river has a lot more kinetic energy.

Diversion Spillways Diversion spillways are artificial channels that a river can flow into when its discharge rises. These channels move water around an area at risk of flooding and send it either back into the river (but further downstream) or into another river. Spillways generally have floodgates on them that can be used to control the volume of water in the spillway.
Spillways pose a threat to areas near the confluence between the spillway and whichever river it flows into as the discharge here will be increased and so too will the risk of flooding. In addition, the path that spillways take can take water around areas not usually used to flooding. If the spillway was to fail for some reason, this could cause widespread damage.

Soft Engineering Techniques  Floodplain Zoning Floodplain zoning involves placing restrictions on land usage in the areas surrounding a river. Essentially, construction is unlimited outside of the floodplain but the floodplain itself is limited to the construction of public outdoor facilities like playing fields and parks. This has several effects in reducing the risk and impact of flooding. Obviously if no one is allowed to build on a floodplain then the damage caused by the river flooding will be greatly reduced because there isn’t much to damage. In addition, floodplain zoning ensures that land on the floodplain isn’t urbanised so infiltration can occur and surface run off is reduced. This reduces the likelihood of a river flooding.
The problem with floodplain zoning is that it limits development to certain areas. In addition, if a floodplain has already been developed on, there isn’t anything that can be done to un-develop it without forcing people off of the floodplain.

Afforestation Afforestation involves the planting of trees in a drainage basin to increase interception and storage while reducing surface run off. This reduces a river’s discharge and so makes it less likely to flood. Afforestation also prevents mass wasting which reduces the amount of soil entering the river and keeps the river’s capacity high. When combined with floodplain zoning, afforestation can be very effective at reducing the risk of flooding.
Afforestation has the benefit of creating new habitats for animals and improving water quality by filtering pollutants out of rainwater. The issue with afforestation is that it requires quite a lot of space to be effective and could prove unpopular among famers on the floodplain as they won’t be too happy with trees sapping nutrients from the soil. This issue can be combated somewhat by creating riparian buffers, thin vegetated strips of land that run adjacent to a river’s channel.

Wetland Restoration Wetland restoration involves creating conditions that are favourable for the development of wetlands (marshes or swamps). Wetlands are able to store large volumes of water which helps to reduce the discharge in a river. Wetlands don’t reduce flooding where they are located but rather downstream of the river. They have the added benefit of creating new habitats for animals and greatly increasing biodiversity. They reduce the area of land available for farming though which makes them unpopular among farmers.

River Restoration River restoration involves restoring a river that has undergone hard engineering back to its original course. This can involve un-straightening a channel, removing artificial levees, diversion spillways & wing dykes. This may seem silly as it is going to reintroduce flooding back in the area where these structures were located but, if this land is no longer valuable, river restoration can help reduce the risk of flooding downstream.
River restoration is a good thing if it’s done properly as allowing the river to take its natural course prevents and reverts any environmental and ecological damage introduced by hard engineering projects. River restoration also has as good as 0 maintenance costs making it very cheap.

The issues arise when river restoration takes place in areas that are still being used by people. I said previously that river restoration can be used where land is no longer valuable. Well, what defines valuable? Farmland? A small village? A town? This decision comes down to the local environmental agency. If they make the wrong call, the restoration project can cause a lot of damage.
Hard Engineering - The Three Gorges Dam The Three Gorges Dam is a mega-dam located on the Yangtze River in central China. It’s 2.3km wide, 185m high and took over 15 years to build. The dam was built in response to the seasonal flooding that takes place along the Yangtze that has killed hundreds of thousands of people. A flood in 1998 killed over 3,500 people, left 13M homeless and caused billions of dollars of damage. With several of China’s major economic cities (e.g. Nanjing) located along the Yangtze, it has become increasingly important for the Chinese government to do something to reduce the risk of flooding.

Impacts  Economic The overall cost of the dam is estimated at roughly ¥180 billion. It will take nearly a decade for the dam to pay for itself by generating electricity. The project was funded by the China Development Bank and profits from another dam located on the Yangtze.
Overall the dam will have a positive effect on China’s economy as the dam aids the navigability of the Yangtze by making it slower and deepening the channel. This will help increase the trade capacity of the river which acts as one of the largest transportation networks for trade within China. The dam will also reduce China’s dependency on coal meaning that it doesn’t have to spend as much on mining or importing coal.

The reduction in the risk of flooding will save money in the long run because there will be less economic damage in the future. The inundation of land behind the dam however is costly because many factories were located behind the dam and had to be relocated, often at a high cost. In addition, the lack of annual flooding means that much of the farmland that is located on the Yangtze’s floodplain will gradually become less fertile, reducing agricultural yield.
Social The dam has succeeded in reducing the risk of flooding, taking the recurrence interval for large floods from 1 in 10 years to 1 in 100 years. Nonetheless, the Three Gorges Dam has had some huge negative social impacts. Over 1.2 million people were required to leave their homes as they were going to be inundated by the dam’s reservoir. The people who refused to leave were forcibly removed from their homes. The reservoir ended up flooding 13 cities and hundreds of villages. Those that were displaced were relocated to cities that had been specially constructed for the dam.