Slopes, levees and rivers: seeing long earth structures before the rain comes

Unlike concrete or steel structures, slopes and levees deteriorate through the struggle between soil and water. Rain soaks in and weakens the soil, surfaces erode, and water finds paths through the body. Protective shotcrete, frame works and revetments, and supporting retaining walls and ground anchors, crack and loosen over the years.

For river levees, the 2013 revision of Japan’s River Act clarified the duty to maintain, repair and inspect river management facilities; inspection at least once a year is the norm, with emphasis before the rainy and typhoon seasons. Road slopes and retaining walls are inspected under the national inspection guideline for road earthworks. In both cases the basis is walking, looking and feeling the ground underfoot.

The pain is length and grass. Levees run for tens or hundreds of kilometres and must be mown before inspection. Tall cut slopes are inspected hanging from ropes. After heavy rain, the most dangerous places are the hardest to approach. And the municipalities and prefectures managing local rivers and roads are short of civil engineers.

Long, wide earth structures are a natural fit for drones. Build 3D models from photographs and compare shape with last time. Drone LiDAR can recover the ground under vegetation to a useful degree, and aircraft carrying green (bathymetric) lasers can now measure the riverbed under shallow water.

The basic method is to repeat the same route, the same height, the same season. Vegetation changes how much ground you can see, so compare surveys taken after mowing or in winter dieback. Combine RTK with ground control and check points to keep positional accuracy consistent.

Rivers and hillsides have open sky, so GNSS is usually good. The hard parts are height, water and wind. Tall cuttings mean big elevation differences, and valleys bring strong, shifting winds. Over rivers, water reflections fool sensors, and fog and rain linger after floods.

• Height: on tall cuttings, terrain-following flight keeps image detail consistent. Check the pilot’s line of sight too. Height
• Water & humidity: reflections disturb downward range sensors and image matching. Take care with altitude hold over water and choose weather-resistant aircraft. Water, spray & humidity
• Wind & turbulence: valleys and levee crests are windy, and post-flood inspections often meet bad weather. Set a wind limit in advance. Wind & turbulence
• Long distances: kilometres of flight mean beyond visual line of sight. Plan observers, or BVLOS approval and reliable links.

For earth structures, the difference over time matters more than any single image. Overlay point clouds to reveal settlement and bulging; compare orthomosaics to follow crack growth. AI is good at proposing cracks, slips and vegetation change. Judging whether a change is *dangerous* still needs people who know the geology, past disasters and rainfall records.

River administrators (the national government, prefectures and others) are responsible for levees under the River Act; road administrators are responsible for road slopes. In disasters, teams such as MLIT’s TEC-FORCE now routinely survey damage by drone. If drone-derived terrain is used as survey output, it must also follow surveying rules such as the Geospatial Information Authority of Japan’s UAV survey manuals.

Rivers and hillsides are mostly outside densely inhabited districts, but long sections tend to become beyond-visual-line-of-sight flights. Japan allows BVLOS over unpopulated areas (Level 3) and, since December 2023, “Level 3.5”, where an onboard camera checking for people below can replace ground spotters and signage. Abroad, the US Army Corps of Engineers runs a levee safety program and a national levee database (about 6,000 systems, some 23,000 miles), and the Netherlands periodically assesses its primary flood defences against statutory safety standards. Compare countries in world regulations.

• What your feet feel: soft ground, small depressions and animal burrows are often found only by walking.
• Inside the levee: soil conditions and seepage paths inside do not show in photos or laser scans; boreholes and geophysics are needed.
• Deep or turbid riverbeds: green lasers struggle with turbidity and depth; boat-based sonar is still needed.
• Judgement during a flood: identifying seepage and sand boils and flood-fighting remain jobs for people on site.
• Risk evaluation and evacuation decisions: interpreting terrain change is a human job.

The Airffic editors imagine it like this. Drone ports sit on the roofs of floodgates and pumping stations along the river. Before the flood season, and right after heavy rain ends, aircraft fly fixed routes over levees and slopes. The difference from the previous terrain is coloured station by station. “Five centimetres of settlement here”, “more seepage there” reach the river office and city hall by morning.

People walk only the coloured sections to confirm, and decide on repairs and evacuation preparations. Hillsides and levees shift from “repair after failure” to “act when movement starts”. Nights of heavy rain become a little less frightening.