The battle against the changing

Photo b y Ch ri s G al l a gh e r o n U n s plas h

The battle against the changing environment - flooding

Raghav (Lower Sixth)

Firstly, I am going to address the misconception that many of us have about environmental change: the environment has always changed throughout Earth’s history, it has not started because of humans. The greenhouse effect is not caused by humans. The reason life exists on Earth is because of it. The human effects on environmental change are called the enhanced greenhouse effect. The environment is, however, is reaching more extremes and is becoming more unpredictable.

Air temperatures are rising because of increased greenhouse gas emissions from vehicles, industries and supplying energy. The emissions are in form of Carbon Dioxide, Nitrogen, Chlorofluorocarbons, and Methane which are forming a layer in the atmosphere insulating the earth by trapping greenhouse gases. The earth naturally warms and cools on a cyclical basis however, emissions caused by humans is resulting in only warming.

The reason why the environment is changing around us is due to ‘climate change’ driven by humans. This article will address what we can do to protect people from the consequences of our actions.

Flooding is one of the worst types of natural disaster, it is amongst the deadliest along with earthquakes. Flooding is caused by increased rainfall and rapid snowmelt, sea floods associated with storm surges and flash floods following landslides, dam, and glacier bursts.

The climate crisis means that we are making flooding more disastrous and less natural.

One way in which we see that the environment is changing is through the increased frequency of flooding. Flooding occurs when heavy rainfall occurs, and the river channel overflows or ‘bursts its banks.’ The faster the rainwater reaches the river channel, the more likely the channel is to flood. The topography and nature of the landscape influence how quickly the rainwater reaches the channel. The time taken for rainwater to reach the river is called the lag-time. The lag-time is the time between the peak rainfall and peak discharge in the river. Discharge is the volume of water that passes a point in the river’s course every second.

The lag-time is a way to show the time taken for the river to reach its peak discharge from the peak rainfall. A quick lag-time means that river reaches a high volume of water very quickly and so flash floods can occur. The natural factors that affect the lag-time are associated with the changing environmental conditions:

1. Frequent and heavy rainfall events can cause surrounding land and floodplains to become saturated- the water is absorbed by the ground and the ground no longer has the capacity for any more water. This means that there is very little infiltration of water once it hits the ground during a rain event, this results in surfacerun-off or overland flow. Overland flow is the fastest type of flow in the hydrological cycle which means that the rainwater reaches the river very quickly. This adds to the volume of the river, and it will soon overflow, and flooding occurs.

2. Warmer air can hold more water vapour. During convectional rainfall, the sun heats the ground, and the water evaporates into the air. As the air rises higher up, it begins to cool and condense into clouds. These clouds can hold large amounts of water and once they reach their dew point, water is released via precipitation in the form of snow, rain, and hail. Now due to the air being warmer due to trapped heat in the atmosphere from the sun, there is more water vapour and so more clouds are being formed, which can trap heat, in the form of longwave radiation, keeping the earth warmer, especially at night. Since there are more clouds with more water vapour, there are more frequent, intense rains that are also long-lasting. This means that the ground becomes saturated more quickly and so the short lag time and high volume of rainfall results in large floods.

3. Coastal flooding is caused by rising sea levels. The rise in sea levels is caused by the melting of polar ice caps. The glaciers and icebergs melt because of an increase in air temperature. The sea increases in volume and the sea expands which results in rising sea levels. When cyclones, hurricanes and other extreme weather events occur at the coast, the higher level of the sea gives it a higher starting point. This means the floods reach further inland and so there is greater damage because of flooding.

4. Cyclones can cause storm surges. Cyclones are affected in two ways by environmental change. The formation of tropical cyclones occurs when there are very warm conditions at the ocean surface and when the vertical temperature gradients through the atmosphere are strong. As the climate continues to warm, the difference between the temperature near the surface and the temperature higher up in the atmosphere is likely to decrease. The temperature gradient weakens and so reducing the frequency causing the cyclones to be more damaging. This will be because the increasing temperature of the ocean surface will increase the maximum wind speeds and intensity of rain. Storm surges and heavy rainfall will result in flooding. A storm surge is a rise above normal sea level resulting from strong onshore winds and/or reduced atmospheric pressure. Cyclones are most likely to occur during late summer and early autumn in tropical regions. For example, floods in Bangladesh occur when cyclones are funnelled northwards up the Bay of Bengal. The Bay get narrower and shallower (due to silt deposition) and so storm surges can sweep over the islands of the delta causing catastrophic floods as these deltas are low lying.

A six metre storm surge resulting from the 1991 cyclone killed 138,000 people.

Impacts of flooding

Flooding can cause large amounts of damage. It tends to cause a lot more financial damage in urban areas. Rural areas tend to have less flooding since there is a lot more land for infiltration to occur and there are fewer houses and people there.

Urban areas can face flooding more frequently because they are built near or along rivers and so when a river burst its banks the water travels a shorter distance before reaching a settlement. Impacts can be split into economic, social, or environmental:

Economic Damage to property as water can enter low-lying properties. The river could be carrying large amounts of pollution and silt that will destroy floors, walls, and electrical connections. E.g. the Somerset floods in 2014 caused significant economic damage: 600 homes and 6880 hectares of agricultural land were flooded. People who were not insured suffered the most. Flood response also carried significant costs; the fuel used to power emergency pumps cost £200,000 per weak. The floods cost the country £147.5 million to respond to and rebuild. Social Some people who live close to rivers and coasts, struggle to get insurance whilst others cannot afford the insurance premiums. The cost of repairing and rebuilding damaged buildings can push up insurance premiums. E.g. Shrewsbury is in a high to medium flood risk part of the river and so businesses and people will be affected during flooding events and so will face damages. Therefore, some insurance companies choose not to ensure company or residential areas since they will face losses during flood damage. Environmental Floods will wash chemicals and sewage into the water. The contaminated water will spread quickly over an area- causing a public health issue and killing marine life. Land and waterways can experience long term damage where they cannot recover.

Clean up tasks such as removal of sediment and litter can cost the government a lot of money. E.g. £15 million was made available by the UK government to help clean up after the 2014 Somerset floods. Small businesses run by locals that are damaged by the floods can mean that people lose their only source of income and must either use their savings to live off or to rebuild their business. E.g. as a result of the Sommerset flood of 2014, an estimated £1 million was lost by local businesses due to lack of access from the closed main roads and train services like the main Bristol line between Taunton and Bridgwater. Floods will destroy places where river wildlife and fish might have breeding grounds. Changes to river temperature and patterns can upset natural ecosystems. Extreme flood events can affect freshwater ecosystems, E.g. populations of the freshwater pearl mussel, an endangered species in the North of the UK, can significantly be affected by extreme floods, which remove mussels from riverbeds.

Fig1. An image showing areas of high, medium, low and very low risk of flooding from the River Severn flowing through Shrewsbury.

What humans can do to nitigate the damage of floods

Humans can carry out two types of flood management. This can be through hard engineering or soft engineering. Within these two types, we can either prevent future floods from happening or we can respond more quickly and effectively to future floods.

Channelisation is a form of hard engineering. It directs the river flow by building concrete blocks as artificial levees. This way we raise the banks and speed up the flow of the river, this way water will flow away quickly from the area and not overflow, causing damage. Spillways are often built when rivers are channelled. The diversion spillways can reduce floods in the location they are built to reduce the pressure on channels, sluice gates allow more control of the channel the discharge of the river. The concrete channels also protect the banks from erosion which is very important in urban areas as it will endanger surrounding infrastructure, this will also reduce the load in the river, this there will be less deposition and so the river will have a greater cross-sectional area. Instead of channelisation, Wing dykes/river groynes can be alternatives that change the natural flow by disturbing the thalweg of the river. Water is deflected away from eroding the banks; velocity is also reduced. This can be a cheaper alternative and will serve the same purpose of protecting banks and reducing erosion. The river will not have sufficient energy to erode the banks by erosion. Concrete channels can cause flash floods further downstream and have other unexpected effects like decreased water quality, increased sedimentation, bed and bank erosion and loss of habitats. Wing dykes are often built of wood which can rot and wear easily. Which is why, sometimes, instead of controlling the direction, the output may be controlled with a more Dams collect water in the reservoir behind when there is heavy rainfall. This protects the settlements further downstream, which are not affected as all the rainwater is managed by the dam. The dam can be used for hydroelectric power once water is let out which can help in improving reliance on renewable energy. However, unlike channels dams can be very costly to build and maintain. Like channels, dams can have a negative impact on the settlement downstream: if the dam gets too full it could burst or the dam can hold up a lot of silt and sediment which is useful for farming on the floodplains.

Sometimes, the cheaper option can be to change policies and create temporary or permanent flood damage mitigation methods. Flood profiling can be temporary or permanent. New buildings can be constructed with flood-proof ground floor walls or have temporary flood gates that can be mounted and taken down to maintain accessibility to areas but also to protect important areas during floods. For example, in Shrewsbury, mountable flood walls have been introduced to protect the Severn Theatre and Shrewsbury University and the surrounding car park is designed to flood to decrease flooding in other areas. This is an example of land-use zoning which is effective in protecting new housing and infrastructure. Many of the new structures that may be at risk of damage from flooding have built-in measures such as some houses may be elevated, or some have floodwalls surrounding the house. This is both a cheaper alternative and with fewer downsides as it reduces future damage and disruption to current systems.

Land-use zoning involves mapping areas that are likely to flood:

Zone A: Prohibitive Zones- these areas are nearer to the channel with a relatively high risk of flooding. Essential waterfront developments may be permitted but future developments will not happen.

Zone B: Restrictive Zones- little development is allowed and what is permitted should be flood-proofed. They are best suited to low-intensity or low-value land such as pasture, playing fields and car parking.

Zone C: Warning Zones-areas further away and situated on higher land may be used for a wider range of developments, from residential to public buildings. Inhabitants are made aware of imminent flood danger and are instructed on how to react when floods occur.

Majority of these methods can damage river wildlife and therefore, a more sustainable method of flood management can be through utilising sustainable urban drainage and collection methods such as creating a ‘sponge city’- this method is a crossover between hard engineering and soft engineering. The concept has become very popular in China as the country has had an increase in flooding rates in recent years. “A sponge city is one that can hold, clean and naturally drain water – using an ecological approach”, according to Kongjian Yu, the Dean of Peking University’s College of Architecture. The method will involve retaining rainwater to be used domestically. The uses include irrigating rooftop and vertical gardens, urban farms, recharging depleted aquifers, replacing, or replenishing water used in toilets, and processing it so that it can be clean enough to use as drinking water.

Most river management strategies that use constructional hard engineering approaches can have considerable financial and environmental costs. However, hard engineering methods tend to be focused on areas of greatest need, particularly with urban areas with high land values and population densities.

Some countries may wish to invest in more sustainable options that

cause minimal damage to the environment. Which is why soft engineering can be a preferred method of flood management. Soft engineering is working with natural processes and features rather than attempting to control them. These can often be cheaper methods with fewer negative impacts.

Planting vegetation and plants near rivers and close/ in urban areas to increase interception and infiltration. The roots uptake water and so the ground has more capacity for water and so there may be less surface run-off. This is the same principle behind the use of green roofs/ rooftop gardens to absorb rainwater, and the excess water can be harvested for use in the building. This method reduces stormwater runoff, and it also benefits the environment as it prevents combined sewer overflow, it also neutralises the acid rain effect and removes nitrogen pollution from rainwater.

Flood diversion is where certain areas of land are allowed to flood such as wetlands and floodplains. Water pumps can pump water here from other places. These areas are either not built on or can benefit from flooding. Such examples of land include paddy plains. Paddy plains are used to grow rice crops which grow in flooded fields. This method is part of land use zoning, where certain fields or land are seen to be unsuitable for development and so unaffected by flooding. Water pumps and pumping stations are used to transport water from flooded areas or directly from a near capacity river to the wetlands or floodplains.

Now with the changing environment, flooding is becoming more frequent and intense. Development and the growing population mean flooding can have severe consequences. Hard engineering and soft engineering methods are the two main ways humans can reduce damage from flooding. Hard engineering is the most difficult method of flood prevention/ mitigation. This is because it generally is very expensive, requires a lot of maintenance, can affect the environment and the economy of the area. However, unlike with proper management and funding, hard engineering is the most effective way to combat flooding. This is because they last longer than soft engineering methods and are stronger so are of better use in urban areas. With the world rapidly urbanising with development and population growth, more people are migrating to urban areas: 4.35 billion people in the world live in urban areas compared to 3.40 billion. Since there are more people living in urban areas, there are more people at risk and so, hard engineering is worth the cost as it is saving future costs from damages and recovery.

Number of people living in urban and rural areas, World

4 billion

3 billion

Urban population

Rural population References:

Cambridge International AS and A level Geography Hodder Education Textbook

Oxford University Press Geography for Cambridge International AS and A level Textbook

3cf983377b8043ff1ecf15709eebf298.pdf (climatecouncil. org.au)

What is convectional rainfall? - Internet Geography

What causes a flood? - BBC Bitesize

Somerset floods crisis: How the story unfolded - BBC News

The Somerset Levels Flood Case Study - Internet Geography

https://www.bing.com/videos/ search?q=impacts+of+flooding&do cid=608011444793846861&mid=3D7CAE8BBC2DE14EEF653D7CAE8BBC2DE14EEF65&view=detail&FORM=VIRE

How does flooding affect humans and the environment? - Internet Geography

10 measures to prevent (urban) flooding (wavin.com)

2 billion

1 billion

0

1960 1970 1980 1990 2000 2010 2020

Source: World Bank based on data from the UN Population Division Note: Urban populations are defined based on the definition of urban areas by national statistical offices. OurWorldInData.org/urbanization • CC BY Fig 2: Graph showing where the majority of the global population lives across the world, until 2010 majority of the world lived in Rural areas, since then there has been linear growth in the urban population. https://ourworldindata.org/ urbanization