Dam inspection: one record for a tall wall, deep water and dark galleries

Dams are broadly concrete gravity or arch dams and embankment (fill) dams built of earth or rock. Concrete dams need watching for cracks, leakage at joints, efflorescence and surface deterioration (freeze–thaw in cold regions); fill dams for settlement, changes in seepage, and displaced riprap. In both, deterioration of gates and spillways — the equipment that releases water — bears directly on safety when it matters most.

In Japan, dam operators carry out daily patrols and instrument monitoring under the River Act, and dams also receive periodic government inspections. Since 2013, dams that are river management facilities also undergo a “comprehensive dam inspection” within 30 years of entering service and roughly every 30 years after, assessing long-term ageing and the condition of the dam body. Hydropower dams are additionally covered by electricity-sector inspection rules.

The pain is height, water and tight spaces. Tall downstream faces are inspected by rope access or from gondolas. Below the waterline on the upstream face, divers or ROVs are needed, and deep cold water is dangerous for people. Galleries are dark and damp, and the inside of gates and outlet conduits is cramped. Windows when the reservoir can be drawn down for inspection are limited.

At dams the rule is to combine aerial and underwater drones. Aircraft take the downstream face and the exposed upstream face; caged aircraft take galleries, outlet conduits and penstocks; underwater ROVs and uncrewed surface vessels take what lies below the waterline.

The downstream face is flown as a grid at constant stand-off. Walls over a hundred metres are split into vertical bands, divided at the joints, which makes mapping onto drawings easier. When the reservoir is drawn down, parts of the upstream face normally under water can be photographed from the air — don’t miss that chance.

A dam gathers almost every condition drones find hard. Deep valleys narrow the sky and degrade GNSS; valley winds are strong and shifting. The foot of the downstream face has spray from releases, galleries are pitch dark and humid, and conduits and penstocks are confined.

• GNSS-denied: GNSS fails at the toe and in galleries. Use aircraft that localise from LiDAR or cameras, and check satellite reception at the valley floor in advance. GNSS-denied
• Height: the dam’s height strains sight lines and links. Consider crew both on the crest and downstream. Height
• Wind & turbulence: winds funnel through the valley and roll off the dam face. Keep an escape route away from the wall. Wind & turbulence
• Water & humidity: spray and humidity fog lenses and wet aircraft; watch for condensation in galleries. Avoid flying during releases. Water, spray & humidity
• Darkness: galleries and penstocks rely on onboard lighting. Lock exposure and repeat last time’s conditions. Darkness
• Confined spaces: conduits, penstocks and gate chambers have close walls. Use caged aircraft and battery plans with turn-back margin. Confined spaces

Dam inspection data comes alive when images, point clouds and instrument records are overlaid on one model of the dam. Divide crack maps by joint number and check whether zones of rising leakage coincide with growing cracks. AI is good at proposing cracks, leak stains and deposits and at showing change. Evaluating dam safety is the responsibility of dam engineers and specialists.

For a template that turns images and AI findings into a report the owner can act on, see writing up drone and AI findings as an inspection report (Airffic Flight Institute).

In Japan, dams are managed under the River Act and its rules on structure and operation, with electricity-sector rules also applying to hydropower dams. There are few nationwide rules specifically governing drone use in dam inspection, so which records are used and how is up to the operator and the inspecting authority. Dams are mostly in mountains outside densely inhabited districts, but flights close to the dam and its equipment, or beyond visual line of sight, must be checked against the specified-flight rules.

Abroad, the US Federal Energy Regulatory Commission (FERC) requires independent consultants to inspect certain licensed hydropower dams (for example, those over 10 m high) every five years, with a comprehensive assessment every ten. In England and Wales the Reservoirs Act 1975 requires inspection and supervision by qualified engineers for high-risk large reservoirs (now those holding 10,000 m³ or more). The International Commission on Large Dams (ICOLD) publishes technical guidance on dam safety. Compare countries in world regulations.

• Inside the body and the foundation: internal cracks, foundation rock and seepage paths do not appear in photos. They belong to instruments, boreholes and geophysics.
• Sounding & touch: tapping concrete for delamination is still done by people up close.
• Turbid water: underwater drones see nothing in heavy turbidity; combine with sonar and divers.
• Gate operation tests: actually opening and closing gates and checking hoists is mechanical inspection work.
• Safety evaluation and operating decisions: water level and releases are decided by operators and specialists.

In the Airffic editors’ view, it goes like this. A drone port sits on the roof of the dam control office and photographs the downstream face along the same route each season. A caged aircraft waits in the gallery and, on any day the leakage readings change, flies that section and records it. Once a year an underwater drone dives, adding the upstream face and intakes to the same dam model.

Instead of hanging from ropes, engineers trace a century of change on the model. Dives and descents down the wall become rare. Dam inspection turns from a job of going to dangerous places into a job of carrying a hundred-year record forward.