Human Intervention in Coastal Landscapes

Investigating how people change a coastline

Coastal management can protect one place while changing what happens elsewhere. To investigate its impact, we need to locate the intervention, identify changes to the landscape and explain the processes connecting them. Maps show the spatial relationships; GIS allows us to compare information from different dates and measure change.

Mappleton, on the Holderness Coast in East Yorkshire, provides a useful example. It lies approximately 3 km south of Hornsea. Its soft boulder-clay cliffs were retreating rapidly, threatening around 50 properties and the B1242 coastal road. Protecting the existing road was cheaper than constructing a new route.

Mappleton: protection with consequences further south

In 1991, almost £2 million was spent on two rock groynes and a rock revetment at Mappleton. Groynes project across the beach and trap sediment moving along the coast by longshore drift. This retained a substantial beach between the groynes, helping to protect the cliffs from destructive waves. Rock armour at the cliff base absorbs wave energy. The cliffs were also reshaped into gentler slopes and stabilised with vegetation to reduce mass movement.

The scheme reduced erosion locally and protected properties and the B1242. The wide beach also attracted visitors, benefiting local businesses.

However, trapping sediment meant less material continued south. Beaches further downdrift lost protection, allowing waves to attack the cliff base more readily and increasing erosion and slumping. At Cowden, about 1 km downdrift, the owner of Grange Farm reported an increase in average erosion from about 1 m to 3 m per year following construction. This illustrates why an investigation must consider the coastline beyond the defended settlement, not just whether the defences worked locally.

Using OS maps at two scales

An OS map provides a framework for locating the coast, settlements, roads and other features potentially affected by management. Read its key rather than assuming that every line beside the shore represents a defence. Small structures may need confirmation using aerial imagery.

At 1:25 000, one unit on the map represents 25 000 of the same units on the ground: 1 cm represents 250 m. At 1:50 000, 1 cm represents 500 m. The 1:25 000 map is therefore larger-scale and is more useful for examining a small defended frontage in detail. The 1:50 000 map provides wider context, helping you consider neighbouring settlements and stretches of coast.

Locate the intervention and nearby assets, then ask what their positions imply. A road close to an eroding cliff is exposed to risk; a road well inland is less directly threatened. Mappleton’s relationship with the B1242 helps explain why protection mattered. Next, extend the investigation downdrift to look for effects outside the protected frontage. A single current map shows locations, but cannot by itself establish how quickly the coastline has changed.

Comparing dates with GIS

GIS displays geographically located information in layers. These might include a base map, aerial imagery, historical maps and mapped shoreline positions. Historical maps and images provide temporal data: evidence tied to particular dates. Georeferencing places them in the same coordinate framework so that equivalent locations line up.

Begin with a clear question, such as: ‘Has cliff retreat differed between a defended frontage and a nearby undefended frontage?’ Identify when the intervention was built and choose suitable earlier and later evidence. Check the dates and resolution of the imagery, and whether the historical map aligns with stable features such as roads or buildings.

Use a swipe tool to reveal one date beside another, or switch layers on and off. Follow the same feature at each date—for example, the cliff top. A waterline can move with the tide, so a different waterline position does not necessarily indicate erosion. Draw a measurement line perpendicular to the coastline and use the GIS distance tool to measure the separation between the two cliff-top positions. Repeat this at several locations within and beyond the defended frontage.

In the fictional comparison shown, the cliff top retreats 10 m along the defended frontage and 40 m along the undefended frontage over ten years. Dividing distance by elapsed time gives mean retreat rates of 1 m per year and 4 m per year respectively.

Transfer the method to authentic OS maps and GIS imagery by using their actual scales, dates, keys and coordinate information. Historical imagery tools such as Esri Wayback and georeferenced historical maps such as those available through NLS Maps can support comparisons, but their coverage and dates must suit the investigation.

Turning observations into a justified conclusion

Keep observation and explanation distinct. ‘The cliff-top positions are closer together beside the defences’ is an observation. ‘The retained beach reduces wave attack’ is a process-based explanation. Evidence of greater retreat downdrift can support an explanation involving reduced sediment supply, as at Mappleton.

A strong conclusion combines the mapped pattern, the mechanism and the consequences for people. It also recognises limits: storms, cliff material and differences in map accuracy can affect the comparison. Measurements from several locations and dates are more convincing than one measurement. The overall judgement may therefore be that management protects particular assets while redistributing erosion risk along the coast.