What is flooding?
River flooding is an overflow of water that submerges land that is typically dry. They can vary between a few inches of water to several feet / metres. While some floods can build gradually, others can happen very quickly and are referred to as ‘flash flooding’, which is usually a result of heavy precipitation over a short duration of time. If flooding occurs inland, such as afar from the coast or river, it is referred to as ‘inland flooding’. Both excess precipitation, burst dams or levees, and ice or snow melt can cause flooding, but precipitation is a more common cause.
A flood occurs when a river bursts its banks and the water spills onto the floodplain. There are many causes of flooding, and it primarily concerns the speed of water, like rainfall, which is more than the river and ground can handle and absorb. Flash-flooding is one example where heavy rainfall or excessive rainfall can causes areas to flood due to the speed of water. Flooding can be caused by both human and physical factors.
What impact does flooding have?
Flooding can have a serious impact on both human populations and the environment. Flooding can cause damage to homes and possessions, reduce the value of property and can disrupt access to utilities and communication. However, it can also increase fertility of some land by the deposition of fine silt (alluvium) and provide forms of irrigation.
Flooding can cause serious injury or death as some can get trapped by the rising waters, not to mention the stress felt by victims thereafter. Damage and loss of property is another consequence of flooding, and often living in flood prone areas can reduce or limit the level of insurance to cover this. The cost of insurance can be very expensive, while the cost of rebuilding lives after flooding can be equally costly. Some people can therefore be priced out of living in areas and can make some places undesirable, which can have further consequences on the economy and development.
When the river floods the environment, it can deposit material which sometimes can be fertile and beneficial, but other times it can empty sewage and chemicals. The deposition can mean lands can become polluted and this can be a further issue for the environment, nature and public health. Nevertheless flooding brings water to dry areas that may not get water regularly and could provide a form of irrigation. Flooding can be very disruptive to species, like breeding grounds, and can impact species numbers. River temperatures and water patterns can disrupt natural ecosystems, which can have an impact on other species and humans.
What causes river flooding?
Physical factors are those caused by nature and the environment.
When there is more rainfall, especially over a short period of time, there is increased risk of flooding. More rain and the same drainage means it is likely to be overwhelmed.
There are different types of rock and soil, some that absorb water (porous), some that allow water to pas through them (permeable) and some that block water completely (impermeable). The geology of the river determines whether water goes into the soil, or remains on the surface. If water remains on the surface, the water will become surface run-off and will run into river channels quickly.
Land Use and Vegetation
Vegetation can intercept water and prevent flooding. Therefore, areas with less vegetation may be more likely to experience flooding. Land use therefore determines the likelihood of flooding risk
Topography, Altitude and Relief
Areas with steep hills and are narrow or small are likely to return water to the river quickly, increasing the risk of flooding. Gentler gradients and slopes are more likely to increase the chance of absorption and decrease flooding. In contrast, steeper hills increase the water flow and likelihood of surface run-off.
Human factors are those that are caused by human intervention or interference, which disrupt the natural system.
Trees and vegetation provide a means to prevent flooding through interception of rain, while they also absorb water overtime. Removing trees will reduce interception and increase the amount of water going into the river.
Managing rivers can take many forms, and river management can increase the amount of water within a river, and can prevent absorption and increase a rivers speed. Channelisation is the process of straightening rivers and often reinforcing with concrete, which increases the speed of rivers and increases the risk of flooding.
Climate Change (and global warming)
Climate Change has a significant impact on water sources, distribution, the hydrological cycles and the amount of water available. There will be an estimated increase in water availability as sea levels rise, and this could greatly impact how much water falls and where it falls.
Urbanisation and Population Growth
Urbanisation sees the increase in the use of concrete and tarmac to build roads and buildings, coupled with land use changes and deforestation or decreasing green spaces. The increase of impermeable rock and drainage systems means water travels increasingly fast back to the river. The lack of absorption and increased water being returned to the river increases flooding.
How do you measure flooding?
A hydrograph is one method to show the way a river is affected by a storm or flooding events. The graph shows two metrics, the rainfall and amount of discharge before, during and after a storm. Understanding the discharge patterns of drainage basins help predict flooding and how it would impact somewhere – improving prevention measures and plans.
Peak rainfall refers to the time of highest rainfall
Peak discharge is the time when the river reaches its highest flow
Lag Time refers to the delay for the water to return to the river.
The run-off / discharge of the river is measured in cumecs – cubic metres per second
There are other factors that affect flooding and measurements in a hydrograph:
- Type of drainage basin – larger basins get more water than smaller, meaning more runoff and also longer lag-time due to the distance. Longer basins will produce lower peak flows and longer lag than say a round basin. Higher drainage density (numbers of sources and tributaries) would collect more water and in shorter periods of time, lessening lag-time.
- Type of slope – steeper slopes will affect speed and lag-time – namely will have shorter lag-time.
- Geology – rock types can affect absorption and speed, affecting lag-time. Permeable rocks increase infiltration and less surface-runoff, meaning a shallow ‘rising limb’. Impermeable rocks mean greater surface run-off and a shallow ‘rising limb’
- Land Use and Urbanisation – concreate and tarmac, much like land usage and geology, can impact leg-time greatly, shortening it and increasing surface run-off. Moreover, in urban areas there would be drainage systems to direct water. to rivers. In forest type areas, interception is higher and this creates a ‘shallow limb’ and lengthened time lag.
The eroding and weathering riverbanks can pose many challenges to society, such as loss of homes, loss of habitat and loss of income. There are a few management strategies that can be employed to ‘save’ the river and prevent flooding – there is hard engineering which is when you attempt to control natural processes, and soft engineering which tends to accommodate more natural and sustainable methods, working with natural processes. Both methods have pros and cons, and you have likely seen many examples of both types without even realising it.
River management can take many forms, and some can be more impactful than others.
Dams are large constructs built across a river to control the amount of discharge downstream, and is released in a controlled way. Building across the river causing a build of water, stored in a reservoir. The water stored in the dam can be used for other processes such as agricultural or industrial. Dams are useful at controlling water flow and can produce hydroelectricity when the water is released. Moreover, some reservoir can be used for recreation or leisure, forming a local resource or tourist attraction. Dams are however expensive to build, they’re are unsightly, and can result in lost land to the reservoir – they can be unpopular as people may lose homes, while they disrupt the river flow and those relying on it downstream.
Flood walls is an artificial barrier or wall designed to temporarily contain water from a burst riverbank during times of extreme or seasonal weather events. They can be made permanent and / or incorporated into the design of an area to virtually become invisible. Flood walls can also be made temporary where flood gates or stoplogs can be built to protect a stretch of river. Stoplogs are hydraulic mechanisms to adjust the water level or discharge in a river.
Levees and Embankments
Levees are naturally occurring banks that run alongside a river when it flood, comprised of deposited sediment. With every flooding event, the levee grows bigger and bigger. They also mean that not every time river waters rise, it floods, as the levee is the highest flood point. Artificial levees can be constructed as well, but they can be expensive and look unattractive. Flood embankments are a form of levee, usually covered in grass and popular in rural areas. They can take up some space, but are cheaper than flood walls. The issue with embankments is they can speed of water, increasing the likelihood of flooding downstream.
Straightening and Deepening
The river channel may be widened or deepened allowing it to carry more water, and straightening a river means it can travel faster downstream, instead of meandering. By increasing a rivers capacity, it is less likely to flood as it can contain more than previous. Moreover, the channel of the river can be altered to divert it away from settlements. The issue with changing the river metrics or channelisation is that it can lead to greater flood risk downstream as the water is carried faster.
Washlands are areas permitted to flood, where some water is directed too away from settlements. They are typically found downstream where the water is directed too. Sluice gates would be opened to allow for more water. These areas are useful to act as a floodplain, however they are not easy to find and do not use in built up areas where most land is occupied or protected.
Land Use Zoning
Land-Use zoning is an effective way of determining flood risk zones and those of high and low severity. This can be accomplished in new settlements when you can dedicate certain land uses and floodplains. Red areas are those most at risk and may be used for farming or leisure fields, yellow zones may get flooded but unlikely, and green areas are extremely unlikely to flood so suited for homes and businesses. The zones are a useful way of town planning and land-use, but does not work well for built-up areas like London.
Afforestation is the action of growing further vegetation and tress. Vegetation and trees act as a very useful medium for interception, slowing the rate of rainfall and decreasing the likelihood of surface run-off. Afforestation is a low cost and effective option, enhancing the quality of the drainage basin and biodiversity in many respects. Afforestation cannot prevent flooding, but it can mitigate flooding and possibly reduce it’s impact.