1.3 River flood hazards and impacts

What are physical factors cause flooding?

Duration and intensity of precipitation, snowmelt and ice melt, drainage basin characteristics. (See 1.1 for understanding).

What are human factors that cause flooding?

Deforestation, changing agricultural systems and irrigation, urbanisation. (See 1.1 for understanding)

How does changing agricultural systems and irrigation cause flooding?

Agricultural practices such as leaving fields bare in winter reduce interception and vegetation cover, so more rainfall reaches the ground directly, increasing surface runoff. Soil compaction from machinery reduces permeability, limiting infiltration, while removal of hedgerows and field boundaries increases overland flow. Irrigation can also saturate soils, reducing infiltration and increasing runoff into rivers.

What are environmental short term impacts of flooding?

• Environmental destruction (e.g. destruction of crops, temporary drowning of habitats)

• Water pollution (especially if flood water leads to overflowing sewers)

• Damage to infrastructure (e.g. electricity sub-stations flooded, disrupting services)

What are the environmental long term impacts of flooding?

• Environmental recovery or alteration of ecosystems after flooding

• Construction of new flood defences altering natural environments

What are the human short term impacts of flooding?

• Emergency services under pressure to help people

• Injuries and/or death of people

• Short-term economic impacts (e.g. shops closed)

• Daily life disrupted (e.g. children can’t go to school)

• Short-term worries and anxiety

• Spread of disease

• Disruption to food supplies

• Temporary migration

What are the human long term impacts of flooding?

• Long-term illness

• Long-term depression and anxiety

• Homelessness

• Building damage and cost of repairs/rebuilding

• Insurance costs increase

• House prices decline

• Community pressure to stop new building on floodplains

• New flood defences built

• Long-term economic impacts (e.g. rebuilding costs, loss of productivity)

What are human factors of vulnerability to flooding?

• High population density (e.g. towns and cities on floodplains increase exposure)

• Economic development and land value pressures (encourages building on floodplains due to demand for housing)

• Infrastructure and urbanisation (roads, buildings, drains reduce infiltration and increase surface runoff)

• Housing development on floodplains (often due to flat, accessible land and planning pressures)

• Lack of strict national restrictions on floodplain development

• Economic constraints and cost of flood defences (limits protection measures)

• Planning and management decisions (e.g. building in at-risk areas due to housing shortages)

• Insurance and financial systems (spread risk but increase vulnerability when flooding occurs)

What are physical factors of vulnerability to flooding?

• Relief and topography (low-lying floodplains are more prone to flooding)

• Geology and soil type (impermeable rocks/soils reduce infiltration and increase runoff)

• Precipitation intensity and duration (heavy/prolonged rainfall increases river discharge)

• Drainage basin characteristics (size, shape, drainage density affect lag time and discharge)

• Antecedent conditions (saturated or frozen ground reduces infiltration)

• Vegetation cover (low vegetation reduces interception and increases runoff)

• Climate patterns and storm events (e.g. prolonged rainfall or snowmelt increasing discharge)

What are recurrence intervals, and how do they help predict floods?

Recurrence intervals refer to the regularity of a flood of a given size. Small floods may be expected to occur regularly. Larger flood occur less often.

Recurrence can be useful for directing economic resources at those areas that have the highest flood risk, i.e., hard & soft engineering strategies to protect people & property.  An area with a 10 year flood risk has a chance of occurring once every decade. Additionally, they help places / communities to prepare for future floods. Moreover, they remind people that flooding is a natural event – we need to work with water, as much as possible, rather than against it. 

What are hazard maps, and how do they help predict floods.

Hazard maps show areas at risk of flooding by identifying zones based on factors such as flood frequency, river discharge, elevation, and historical flood data. They often display different levels of risk (e.g. high, medium, low) depending on the likelihood and severity of flooding.

Hazard maps are useful for predicting floods because they help planners and governments identify areas most at risk, allowing resources to be directed effectively through flood defences and management strategies. They also help communities prepare by highlighting vulnerable areas, guiding land-use planning, and reducing development in high-risk zones.

What are flood forecasts and what data are they based on?

Flood forecasts predict the likelihood, timing, and magnitude of flooding using a range of hydrological data. This includes river discharge and water levels measured at gauging stations, as well as meteorological inputs such as rainfall intensity, duration, and distribution. Additional inputs from snowmelt and ice melt are also important, as rising temperatures can rapidly increase river discharge. By analysing these factors together, hydrologists can identify rising trends and estimate when a river may exceed bankfull capacity.

What are flood warnings and how do forecasts help reduce flood impacts?

Flood warnings are alerts issued to the public based on forecast data when flooding is likely or imminent. They communicate risk levels and allow individuals, communities, and authorities to take action, such as evacuating, protecting property, or deploying flood defences. Forecasts support warnings by providing advance notice, improving preparedness, reducing damage, and helping to save lives.

What were the causes for the flooding along the River Severn in Feb 2020?

Three named storms (depression systems) during the month: Ciara (7th / 8th), Dennis (15th / 16th), and Jorge (28th / 29th).

Rainfall in Feb 2020 in the UK was 209.01mm – the wettest Feb in a rainfall record that stretches back to 1766.  Not all areas experienced the same amount of rainfall – some got more, some got less.

October – December 2019 was also very wet across most parts of the UK: water stores were exceeded, soils became saturated, and river levels rose – all before the three big storms in Feb 2020.

What are the factors that influenced the flooding along the River Severn in Feb 2020?

The Severn is the UK’s longest river and has the largest drainage basin in the UK.  That means it has a very large catchment.

The River Severn has many tributaries, both small and large, all increasing the overall discharge of the river.

The river’s large drainage basin is mostly agricultural:

• In the Severn uplands grassy pastures for sheep farming cover approximately 60% of the hillsides.

• In the lower Severn section, nearly 50% of the fertile plains are used for arable faming (mainly wheat and barley farming). Other land uses include dairy, poultry and pig farming.

• In the uplands, rainwater reaches the river quickly; in the lower area, depending on the time of year, water can reach the river quickly (in winter with bare ground) or slowly (in summer with crops growing).

Land use: Woodland makes up approximately 10% of the Severn catchment, with the main concentrations around Dyfnant Forest (south of Lake Vyrnwy), the Wyre forest (near Bewdley), Clun Forest and Wenlock Edge. The woodlands are a mixture of young commercial conifer plantations and old deciduous trees. 

Land use: The main communities currently at risk of flooding from the River Severn are the towns that it runs through: Shrewsbury, Ironbridge, Bridgnorth, Bewdley, Stourport on Severn, Worcester, Upton-upon-Severn and Tewkesbury. 

All are historic but have experienced the growth of new housing estates since 2000 as they are popular places to live (& may become even more so in the 2020s as a result of the Covid-19 pandemic & more working from home).  Much of the new housing was built on floodplain land.

For Tewkesbury specifically, the town is situated on the confluence of the River Severn and Avon, both of which have very large drainage basins.

Flood factor: Land use: The risk of flooding in the settlements along the Severn is affected by land use in the catchment, how well it is managed and the time of year.  There are increased throughflow and surface flow speeds wherever vegetation is removed (E.G., when arable crop fields are left bare).  Thus, the river is more likely to flood in winter on the surrounding floodplains – a natural event.  However, building on floodplains has increased the risk of flooding at other times too.

What are the socioeconomic impacts after the flooding along the River Severn in Feb 2020?

Short-term socioeconomic impacts

• Injuries and/or death of people

• Emergency services under pressure

• Short-term economic impacts (e.g. shops closed)

• Daily life disrupted (e.g. schools closed)

• Short-term worry and anxiety

• Temporary migration

• Disruption to food supplies

Medium-term socioeconomic impacts

• Homelessness

• Building damage (homes, businesses)

• Damage to infrastructure (e.g. electricity substations flooded)

• Spread of disease

Long-term socioeconomic impacts

• Long-term illness

• Long-term depression and anxiety

• Long-term economic impacts (cost of rebuilding, loss of productivity)

• Insurance costs increase

• House prices decline

• Community pressure to stop building on floodplains

What are the environmental impacts after the flooding along the River Severn in Feb 2020?

Short-term environmental impacts

• Water pollution (e.g. sewage overflow)

• Temporary destruction of habitats

• Crop damage

Medium-term environmental impacts

• Continued ecosystem disruption

• Soil degradation and contamination

Long-term environmental impacts

• Permanent changes to ecosystems

• Altered river channels and landscapes

• Construction of flood defences altering natural processes

What are the predictions used to manage the flooding along the River Severn in Feb 2020?

The Environment Agency uses data from the Met Office about rainfall and collects data from its own gauging stations on river velocity and discharge. The EA issued a record 632 flood warnings and alerts on February 16th 2020, with 19 severe flood warnings issued in total during the month. According to its own evaluative reports, these warnings and alerts helped individuals and communities prepare for potential flooding and take necessary safety precautions.

What were the Environmental Agency’s active management towards the River Severn Feb 2020 flooding?

Flood Defences and Operations:

1. The EA mobilized 1,000 staff per day to operate flood defences, clear debris, and make repairs, including deploying over 6km of flood barriers and 90 mobile pumps.

2. Flood defences protected an estimated 128,000 homes across England.

3. Temporary barriers were deployed and adjusted to prevent overtopping, and pumps were used to dewater areas behind defences.

Communication and Public Advice:

• The EA provided safety advice to the public, urging them to stay informed, sign up for flood warnings, and prepare flood plans.

• The EA also offered advice on how to protect homes from flooding and what to do in case of evacuation.

• They urged people not to drive or walk through flood water and to follow the advice of emergency services.

Coordination and Support:

• The EA worked closely with local authorities, emergency services, and other agencies to manage the flood response.

• They provided technical support, including river flow data and feedback.

• The EA also played a role in assessing the impacts of the flooding and providing support for recovery efforts.

What were the response of Tewkesbury Council & Gloucestershire County Council towards the River Severn Feb 2020 flooding?

They provided:

• Rest centres with food, drink and support

• Offices were open 24hrs a day for six days and then 6am - 10 pm for two weeks

• Distributed sandbags

• Provided housing advice and support

• Environmental health advice to individuals and businesses

• Bottled water distribution via distribution points and direct distribution to vulnerable people

• Collected flood damaged goods

• Provided 100+ Porta-loos

• Temporary satellite doctors’ surgeries for one week

• Information to public via press releases and website

• Clear up the aftermath of flooding

• Repaired damaged assets

They received help from:

• Councillors, communities, and volunteers

• Community facilities, I.E., using village halls for water distribution

• Food and water donations from supermarkets

What was the Environmental Agency’s long-term planning for the River Severn.

What they want to do (future aims):

• Develop long-term flood risk strategies

• Adapt to climate change (warmer, wetter conditions)

• Increase use of nature-based solutions

• Invest further in flood defences

• Prepare for more frequent extreme weather events

• Support farmers in flood prevention

What they have done (actions taken):

• Recognised increasing flood risk due to climate change

• Emphasised need for flood defence investment

• Promoted peatland restoration

• Highlighted role of nature-based solutions in reports

What has taken place to help manage the River Severn after this flooding event?

• £5.5 million spent protecting Mythe water treatment works → successful (did not flood in 2020 vs flooded in 2007)

• Hard engineering works well for specific high-value sites, but is expensive and localised

• No major flood defences for Tewkesbury itself → limited overall protection due to cost, visual impact, and risk of increasing flooding downstream

• Reliance on Environment Agency warnings and alerts → improves preparation but does not stop flooding

• Increased use of property-level protection (e.g. flood barriers, insurance requirements) → reduces damage but shifts responsibility to individuals

• Use of Sustainable Drainage Systems (SuDS) and allowing areas (e.g. fields, car parks) to flood → helps protect higher-value areas → effective soft engineering

• Attempts to limit building on floodplains, but development still occurs → management not fully effective

• New developments can increase surface runoff, worsening flooding in surrounding areas

• Local support (grants, mental health services) → aids recovery but does not reduce flood risk

What are dams and how effective are they in flood management?

Dams are large barriers built across rivers to store water in a reservoir and control discharge. They are effective because they regulate river flow, reduce peak discharge, and can release water gradually, lowering flood risk downstream. However, they are expensive to build and maintain, can displace communities, and may fail or overflow during extreme events. They also disrupt natural sediment flow and ecosystems.

What is river straightening and how effective is it?

River straightening involves modifying a river channel to make it straighter, increasing velocity so water moves through the system more quickly. This reduces flood risk in the modified section. However, it can increase flood risk downstream as water arrives faster, and it disrupts natural river processes and habitats.

What are artificial levées and how effective are they?

Artificial levées are raised embankments built along river channels to increase channel capacity and contain floodwater. They are effective at preventing local flooding and protecting settlements. However, if they fail or are overtopped, flooding can be severe. They also increase discharge downstream and can give a false sense of security, encouraging development in high-risk areas.

What are diversion spillways and how effective are they?

Diversion spillways are channels that redirect excess water away from the main river into other areas or basins during high discharge. They are effective at reducing pressure on the main channel and lowering flood risk in vulnerable areas. However, they can transfer flood risk to other locations and may damage environments or land where the water is diverted.

What are emergency responses to flooding and how effective are they?

Emergency responses are immediate actions taken during or just before a flood, such as evacuations, temporary shelters, sandbagging, and deployment of emergency services. They are effective at reducing loss of life and protecting property in the short term. However, they do not prevent flooding and depend heavily on accurate forecasts and public response.

What is flood recovery and how effective is it?

Recovery involves restoring affected areas after a flood, including repairing infrastructure, rebuilding homes, and providing financial aid. It is effective in helping communities return to normal and reducing long-term impacts. However, recovery can be costly, time-consuming, and may not fully restore economic or social conditions.

What are the limitations of emergency response and recovery strategies?

These strategies focus on reacting to floods rather than preventing them. Their effectiveness depends on preparedness, funding, and coordination between agencies. In some cases, repeated flooding can make recovery unsustainable, especially in high-risk areas.

What is floodplain zoning and how effective is it?

Floodplain zoning involves controlling land use by restricting high-value development in high-risk flood areas. It is effective because it reduces potential damage and exposure to flooding by keeping vulnerable infrastructure away from flood-prone zones. However, it is not always fully enforced, and development pressures may still lead to building in flood-risk areas, reducing its overall effectiveness.

What is drainage basin management and how effective is it?

Drainage basin management involves managing land use and water flow within a basin to reduce runoff and lag time. This includes measures such as controlling urbanisation and improving land management to increase infiltration. It is effective because it addresses flood risk at a basin-wide scale, but it requires coordination between stakeholders and can be difficult to implement consistently across large areas.

What is afforestation and how effective is it in flood management?

Afforestation involves planting trees to increase interception, evapotranspiration, and infiltration. This reduces surface runoff and slows the movement of water through the drainage basin, lowering peak discharge. However, it takes time for forests to mature, and its effectiveness can be limited during extreme rainfall events when soils become saturated.

What is wetland conservation and how effective is it?

Wetland conservation involves protecting or restoring wetlands to act as natural storage areas for excess water. Wetlands increase lag time by storing water and releasing it slowly, reducing flood peaks downstream. However, wetlands require space and may conflict with development, and their capacity can be exceeded during prolonged or extreme flooding.

What is riverbank conservation and how effective is it?

Riverbank conservation includes managing vegetation and stabilising banks using natural methods to reduce erosion and slow runoff into the river. Vegetation increases interception and helps bind soils, reducing sediment input and stabilising channels. However, it is less effective during high-magnitude flood events when erosion and bank overtopping can still occur.