(lively music) Helping the water flow more quickly downstream may help reduce flood risk here, but it may increase flood risk downstream where the river channel will struggle to cope with the increased flow of water. If you enable more water to be stored within the river channel during flood events, then the stream power or erosive energy of the river will be increased, and it may start to erode its bed and its banks. That eroded material may then be transported downstream by the river and deposited in the river channel, decreasing channel depth and channel capacity and leading to an increase in flood risk. Removing pools, riffles, gravel bars, and vegetation and introducing unnatural, hard engineering material to rivers can destroy wildlife and natural habitats along a river. And these impacts can extend for many kilometres upstream and downstream. So, in recent years, these traditional methods have been replaced by softer engineering. (lively music) This kind of softer engineering can include keeping hard engineering structures to just one side of the river channel, allowing floodplains like this on the other side to flood naturally. Also, maintaining the natural pool and riffle morphology of the river channel, which helps to keep river ecosystems healthy. Rather than using hard engineering materials like concrete and block stone to protect the river bank, more natural materials like willow can be used to reinforce the bank instead. If hard engineering and river channelization has resulted in problems downstream, this can often be solved by restoring the natural river channel, by reintroducing meanders, removing dams and weirs and encouraging wetland formation along the banks of the river to slow the flow of water. However, it can take decades, if not longer, for the river to return to some kind of pre-engineering state. (lively music) Flood overfill basins like this one beside the River Rheidol in Aberystwyth is used to store excess water during flood events. This helps to prevent the flooding of the surrounding buildings and also helps to slow the flow of water down the drainage basin. This flood overflow basin is set back about 100 metres from the main river channel. This provides a buffer zone of land, which can hold water during flood events even before the flood waters reach the overflow basin. (lively music) Natural measures that slow the flow of storm rainfall can also help prevent flooding downstream. Protected peat bogs, like this one at Cors Caron, act like natural sponges. All the ground beneath this walkway and spread out in front of us is completely saturated with water. It's like a giant sponge that stops the water from rushing down the drainage network. The peat bog also provides a more natural solution to slowing the flow of water than a hard engineered reservoir dam. Another form of natural flood management involves planting trees, or afforestation. Planting trees on flood plains or in upper catchments like this increases the interception of rain water by vegetation and makes the ground surface rougher. And all this slows the flow of water on its way down towards the river channel. (lively music) This woody debris is creating a natural log jam. It's acting to slow the flow of water in the river. Now, this is actually a very useful effect that we can recreate artificially by adding engineered log jams into river channels. When engineers do this, they're looking at the useful effects of this natural process and using it as inspiration to find a softer engineering approach. There have even been discussions about reintroducing beavers to the UK. Beavers build dams across rivers that slow the flow of water, a bit like this log jam, and help create wetlands upstream. In fact, many appropriate catchments for this have been identified and some beavers have already been introduced during trials in Devon. Soft engineering solutions generally cost less than hard engineering. Measures that use wood to protect the bed and banks of the river, and to slow the flow of water, can cost tens to hundreds of pounds per meter. Whereas the hard engineering alternatives can cost thousands of pounds per metre or even millions in the case of major reservoir dams. Although this is an attractive approach, especially the benefit to wildlife through creating different habitats, there is some debate about whether this approach could really have an effect on large-scale flooding. (lively music) It's not always possible to create effective soft engineering solutions, especially in densely populated urban areas where using hard engineering that can withstand severe flooding is necessary to protect homes and infrastructure. So, rivers can often be most effectively managed by using a combination of hard and soft engineering techniques that work together to reduce the risk of flooding.