Human Impacts and Secondary Data in River Investigations

Connecting the river to people in its catchment

A catchment is the area drained by a river and its tributaries. A river investigation therefore concerns more than the channel at your measurement site: river conditions can have consequences for homes, businesses and services elsewhere in the catchment.

Discharge is the volume of water passing a point each second. Flooding occurs when river water spreads beyond the channel onto surrounding land. This connects the physical river landscape with a human question: what lies in the area that could flood, and how would people be affected?

Homes may need repairs, businesses may have their premises and operations disrupted, and roads or essential services may be affected. A flooded road can matter to people whose own homes remain dry because they depend on it for access. The Environment Agency assesses exposure not only for houses but also for schools, hospitals, industry, roads, railways and water and energy infrastructure.

The consequences depend on the characteristics of the flood as well as what is exposed. Greater flood depths can create a more serious hazard. The Environment Agency notes that just 30 cm of floodwater can be enough to move a car during a heavy rainfall event. This is why an investigation should consider potential impacts, rather than only whether an area is coloured on a map.

Sediment, channel capacity and people

Water can wash soil and fine gravel from surrounding land into a river. These materials are sediment. Their removal is erosion, their movement is transportation, and their accumulation on the river bed is deposition. These processes connect land elsewhere in the catchment with changes at your fieldwork site.

Accumulated sediment occupies space in the channel, reducing the space available to carry water. This matters to people concerned about flooding and to people using rivers for navigation. Removing sediment can increase channel capacity temporarily, but managing a river is not simply a matter of making one stretch deeper.

Dredging removes accumulated material and may also remove bed and bank material to enlarge a channel. In the UK it has been used to help drain land, maintain flows to mills, improve navigation and reduce flooding. However, natural processes can return sediment to the same places quickly—sometimes within weeks—so the increase in capacity may be short-lived.

Dredging can also increase flood risk for downstream communities. It may make banks unstable, damage fish spawning grounds and disturb contaminated sediment, affecting water quality downstream. A response intended to help one location can therefore have costs elsewhere in the catchment.

The Environment Agency assesses dredging location by location rather than treating it as a universal solution. Other approaches include temporarily storing floodwater upstream and slowing its movement towards communities. For example, flood storage areas can hold water on land such as parkland that will recover more readily from flooding. This illustrates why an investigation should consider the whole catchment, not only the channel beside the homes at risk.

What secondary data adds to fieldwork

Secondary data is information collected or produced by someone else. Your measurements describe river conditions at particular sites and times. Secondary evidence can place those observations within a wider picture of the catchment and its flood risk.

For this river investigation, use a flood risk map and one other secondary source. An Environment Agency flood risk map is a suitable example in England. A flood risk management plan can provide the additional source: it explains the flood risks being addressed and the measures proposed to manage them.

These sources answer different questions. A map helps identify where flooding could affect people and infrastructure. A management plan helps explain what authorities and communities intend to do about that risk.

Reading a flood risk map

Begin by checking the map’s title, provider, date and key. Identify the source of flooding represented: river flooding and surface-water flooding are not interchangeable. A location with little mapped river risk may still face another source of flooding.

Read the categories using the map’s own definitions. For example, the Environment Agency defines very low risk as an annual chance below 0.1%, or less than 1 in 1,000. This describes a chance in each year, not a fixed interval between floods.

Next, locate your fieldwork sites and follow the mapped pattern along the river. Identify any homes, businesses, roads or services within the areas shown as potentially affected. Compare locations and explain the consequences of their exposure.

Flood hazard and flood risk are closely related but distinct. A hazard map describes the likelihood of flooding in a location; a risk map identifies what could be affected, such as people, infrastructure or the environment. Check what your particular map actually shows before drawing a conclusion.

In this fictional teaching sketch, the home and road lie within the shaded modelled flood area. This suggests possible damage to the home and disruption to access. The school lies outside that shaded area, so the sketch does not identify it as exposed to this particular modelled flood extent. However, the road serving it could still be disrupted.

To transfer this reasoning to a real map, replace the sketch’s simple shading with the actual map key and locate real buildings, roads and fieldwork sites accurately. The sketch does not represent an actual catchment, an Environment Agency map or measured flood observations.

Using a second source to explain responses

A flood risk management plan sets out objectives and proposed actions for managing flood risk. Select information relevant to the catchment you are investigating, rather than summarising a whole national document.

Record the flood source being addressed, the proposed measure, who is responsible and any stated timescale. Then connect this information to the map. For example, an area containing homes exposed to flooding creates a reason to investigate how warning arrangements or property flood resilience could reduce the consequences.

Flood resilience means preparing to withstand flooding and recover from it. Measures can include flood warnings and changes that make properties more resilient. Developers also have a role through sustainable drainage and avoiding inappropriate development on floodplains.

A proposal is not proof of an achieved outcome. If a plan describes an intended measure, report it as planned unless you have evidence that it has been completed.

Combining evidence without overstating it

A useful conclusion combines where flooding may occur, what could be affected and how risk is being managed. Your river measurements and landform observations add local evidence, but one visit cannot establish the long-term probability of flooding. Likewise, observing deposited sediment does not by itself prove that it causes flooding: consider channel conditions and wider catchment evidence before making that claim.

Maps also have limitations. Check their date and level of detail: the flood risk maps used for the second-cycle management plans used information available in December 2019 and did not provide property-level assessments. More recent assessments use improved data and modelling, so a change between maps does not automatically demonstrate a physical increase in flooding.

Where mapped flood depths are available, they may refer to outside ground levels. A building’s floor can be higher than the surrounding land, so a mapped depth does not necessarily mean water will enter that building. Use the evidence to support a careful conclusion about potential exposure, not a guarantee about what will happen.