Flood Risk and Management

Why rivers flood

A river floods when its discharge becomes greater than its channel can contain, causing water to spill over the banks. Discharge is the volume of water passing a point each second, measured in cubic metres per second, or cumecs.

Rainfall does not all reach the river immediately. Some is caught by vegetation through interception; some enters the soil through infiltration. Water can then move slowly through soil as throughflow or through rock as groundwater flow. Water travelling across the surface as runoff generally reaches the river more quickly.

Flood risk therefore depends on both the amount of precipitation and how quickly water moves through the drainage basin—the area drained by a river and its tributaries. When large quantities arrive together, discharge rises rapidly and may exceed channel capacity.

Physical factors: precipitation, geology and relief

Precipitation: intense rainfall can arrive faster than water can infiltrate the soil. Prolonged rainfall can saturate the ground, leaving little space for further water. Both increase surface runoff. Rapid snowmelt can also add large quantities of water to rivers.

Geology: permeable rock allows water to pass through it, delaying its journey to the channel. Impermeable rock restricts this movement, encouraging more rapid runoff. Soil conditions matter too: clay-rich, frozen, saturated or compacted soil allows less infiltration.

Relief: on steep slopes, water runs downhill quickly and has less opportunity to infiltrate. More water reaches the channel in a short period, producing a rapid rise in discharge. Gentler slopes usually allow slower movement and more infiltration.

These factors interact. Heavy rainfall over a steep basin with impermeable rock is more likely to produce a rapid river response than the same rainfall over gently sloping, permeable ground.

Human factors: changing land use

Urbanisation replaces vegetation and soil with roofs, concrete and tarmac. These impermeable surfaces reduce infiltration, while drains carry water rapidly towards rivers. Discharge can rise faster and reach a higher peak.

Deforestation removes leaves that intercept rainfall and roots that help water enter the soil. More rainfall reaches the ground directly, and more can travel as surface runoff.

Agriculture can also affect the river's response. Bare ground provides less interception, while compacted soil reduces infiltration. Ploughing downslope can encourage water to flow downhill towards the channel.

Land use also changes what is exposed to flooding. Building homes and businesses on a flood plain places people and valuable property in an area that may flood, even if the river's discharge has not changed.

Reading a flood hydrograph

A flood hydrograph shows the relationship between rainfall and river discharge over time. Time is shown on the horizontal axis. Rainfall is usually shown as bars, measured in millimetres, and discharge as a line, measured in cumecs. These use different vertical scales, so the height of a rainfall bar cannot be directly compared with the height of the discharge line.

Read the graph in sequence:

• Base flow is the background discharge before the storm.
• Peak rainfall is the highest rainfall value shown.
• The rising limb shows discharge increasing as storm water reaches the river.
• Peak discharge is the highest point on the discharge curve.
• Lag time is the time between peak rainfall and peak discharge.
• The falling or recessional limb shows discharge declining as the storm response subsides. Slower throughflow and groundwater flow can continue supplying water after surface runoff decreases.

The constructed illustration uses one rainfall event to compare two possible river responses. Peak rainfall is at hour 2. The rapid-response river peaks at hour 4, so its lag time is 2 hours. The slower-response river peaks at hour 8, giving a lag time of 6 hours.

The rapid response has a steeper rising limb and a higher peak discharge. This is consistent with conditions such as steep slopes, impermeable surfaces or urban drainage. The slower response is consistent with greater infiltration and interception, for example in a vegetated basin with permeable ground.

A hydrograph shows the response, but its shape alone cannot prove which factor caused it. Several physical and human factors can produce similar shapes. A higher peak increases flood risk, but whether flooding actually occurs also depends on the channel's capacity.

Hard engineering: controlling or redirecting water

Hard engineering uses structures or changes to the channel to protect selected areas. It can offer substantial protection, but construction costs and effects elsewhere must be considered.

The downstream effect is particularly important when evaluating channel straightening: moving water away from one place quickly does not necessarily reduce risk across the whole river system.

Soft engineering: slowing water and reducing damage

Soft engineering works with natural processes or helps people avoid and prepare for flooding. It does not always prevent the river from overflowing; some methods reduce the damage instead.

In England, preparation can include signing up for warnings by text, phone or email and making a personal, community or business flood plan. Keeping medication, important documents and contact details together helps people respond when a warning arrives.

Choosing an effective combination

No strategy is best everywhere. A densely built city may need engineered defences because there is little space to allow flooding. An undeveloped flood plain may be better suited to zoning, wetlands and water storage.

A useful evaluation considers effectiveness, cost, who benefits, who loses and environmental effects. A combined scheme can protect buildings while slowing or storing water elsewhere. It must also consider whether protection in one location transfers risk downstream.

UK example: why York needed flood management

York, in northern England, lies at the confluence of the Rivers Ouse and Foss. It has experienced repeated serious floods, including in 1947, 1978, 1991, 1995, 2000 and 2015.

During the 2000 flood, the Ouse rose about 5.5 metres above its normal level. More than 540 properties flooded, and approximately 18,700 hectares of farmland were affected. Overall costs were estimated at more than £12 million, including lost tourism income. These impacts explain the need to protect homes, businesses, transport and the local economy.

York's management strategies

York combines hard engineering with flood plain management:

• The Foss Barrier prevents high water in the Ouse from backing up the Foss. The barrier failed during the 2015 floods and was subsequently upgraded at a cost of £38 million.
• Embankments at Leeman Road raise the banks so the channel can contain more water before overflowing.
• Clifton Ings is retained as an open, undeveloped flood plain with capacity to store about 2.3 million cubic metres of water. This combines zoning with floodwater storage.
• Flood relief channels around Acomb divert water directly into the Ouse downstream of York.
• Floodgates at Marygate close gaps during periods of flood risk and can be removed when they are not needed. Individual properties also use floodgates.

These measures perform different jobs: some block or redirect water, some increase channel capacity, and others provide space for storage.

Evaluating York: social, economic and environmental issues

Socially, the measures increase protection for homes and reduce transport disruption. The Sustrans cycleway across Clifton Ings has also been widened and resurfaced. However, the Foss Barrier's failure in 2015 demonstrates that engineered protection is not infallible.

Economically, the wider flood-management measures have an estimated total cost of £83 million and increase protection for more than 2,000 homes and businesses. Avoiding property damage and disruption benefits residents, businesses and tourism; insurance premiums may also fall. Against this, the Foss Barrier upgrade cost £38 million rather than the estimated £17 million, showing the difficulty of controlling expenditure.

Environmentally, construction of embankments and relief channels has destroyed some habitats. Other measures provide benefits: two ponds connected to the Foss have created wetland habitats, while keeping Clifton Ings undeveloped preserves space for floodwater storage.

Overall, York shows why a combination can be valuable: engineered defences protect a built-up city, while open flood plain provides storage. The benefits must be weighed against high costs, habitat disturbance and the remaining possibility of failure.

Flood risk

Flooding occurs when discharge exceeds channel capacity.

• Precipitation: intense rain exceeds infiltration rates; prolonged rain saturates soil.
• Geology: impermeable rock encourages runoff; permeable rock delays water transfer.
• Relief: steep slopes speed runoff.
• Land use: urban surfaces and drains speed transfer; deforestation reduces interception and infiltration. Flood plain development increases exposure.