Solar Panels on Slopes and Cliff-Collapse Warning Zones in a 4-Panel View ― Places to Watch in Heavy Rain

Hello, this is the MAPRISE development team. This time we show how to check, in MAPRISE, where solar panels line up on mountain slopes or on ground with large elevation differences, together with cliff-collapse warning zones.

The trigger was the heavy rain of September 2026. Following Typhoon No. 25, a solar power facility next to Lake Inba in Chiba Prefecture was flooded, with most of its panels reported to be under water. That was flood damage, but it made us wonder how panels on slopes fare in heavy rain. Damage to solar facilities on slopes is not new.

Slope-side solar facilities have been hit by heavy rain before

• West Japan heavy rain, July 2018. In Kobe, a 23 kW plant on a slope beside the Sanyo Shinkansen collapsed, and the bullet trains were briefly suspended. In Mihara, Hiroshima, a landslide about 100 m long hit a low-voltage solar plant.
• Aira, Kagoshima, July 2022. A sediment disaster at a solar plant construction site sent soil hundreds of meters onto private land, damaging fields and a warehouse. The national government considered tighter regulation.

Knowing this, it seems worthwhile to check on the map where panels on slopes are.

"Land that used to be forest" and heavy rain

We start from the following premise.

• Forest soil and roots are generally said to store rainwater and release it slowly.
• Where forest was cleared for panels, this capacity may be weaker than in the original forest.
• If so, in rain beyond expectations, water and soil may run off the slope more easily, raising the risk of landslides.

This is a general line of thinking. Actual risk depends heavily on geology, gradient, drainage design and construction, and this article does not speak to the safety of any individual facility.

Four conditions in a 4-panel view

MAPRISE's 4-panel view shows the same place at the same scale in four panels.

The 4-panel view is available on the Starter plan or higher.

Five sites in Kyushu

We matched the AI-detected panel areas against the steep-slope warning zones using the map data, and chose places where panels sit close to a zone.

1. Iwato-cho, Makurazaki, Kagoshima ― a large plant on a slope facing the sea

The aerial photo clearly shows rows of panels on a slope facing the sea. The purple area at top left is what the AI detected. At bottom right, red and yellow zones run in a band along the west edge of the panels. The colored elevation shows the ground dropping toward the sea.

2. Kariyado, Osaki, Kagoshima ― panels on a hilltop, wrapped by a warning zone

A warning zone (red and yellow) wraps around the west and south of the orderly rows of panels on the hilltop. The aerial photo also shows houses scattered around the panels.

3. Yamamuro, Kita-ku, Kumamoto ― a slope next to housing

Panels sit on a hillside within a residential area. At top left you can see the distance to nearby buildings (blue). At bottom right a warning zone runs along the slope, and the aerial photo shows houses around it.

4. Kokubu-shigehisa, Kirishima, Kagoshima ― panels spread over a mountain slope

Panels spread widely over a forested slope. A road runs just south of the panels, and warning zones line the slope south of that road. The colored elevation shows how intricate the slopes are.

5. Kitagawachi-cho, Miyazaki ― cleared land in the hills

A few rows of panels stand on land cleared from the forest. The aerial photo shows bare soil, with forest all around. The warning zone lies on the slope a little way to the south.

Kita-Kanto too

Not only Kyushu. We ran the same matching in Gunma and northern Saitama and chose places where panels overlap or sit close to a warning zone.

6. Sakaguchi, Yoshii, Takasaki, Gunma ― a warning zone overlaps the west side of the panels

In the aerial photo, panels stand in rows along the slope. At bottom right, red and yellow warning zones overlap the west side of the panel area. Another group of panels is visible at the left edge.

7. Gobara, Annaka, Gunma ― a long strip of panels on a riverside slope

Panels line a slope along the river like a band. The warning zone at bottom right runs along this band of panels.

8. Kamikawa, Saitama ― a large plant beside housing

A large array spreads right next to a residential area. The warning zone lies on the slope southeast of the panels, and the aerial photo shows forest continuing behind them.

What to check before heavy rain

• Slope direction and steepness. Use the colored elevation map.
• Position relative to warning zones. See whether zones touch the panel edges, lie below the panels, or cover nearby homes and roads.
• What is downslope. Homes, roads, waterways and fields may be affected by water and soil.
• When heavy rain is forecast. Check evacuation information from your municipality and sediment-disaster alerts from the Japan Meteorological Agency.

About the data

• Panel areas are AI estimates from satellite imagery and may not match actual panel positions.
• Sediment-disaster warning zones come from GSI's hazard map tiles. A place without designation is not necessarily safe.
• This article introduces how to read the map; it does not judge the safety of any facility or land.

Questions or feedback are welcome at info@maprise.jp.

Sources

• Solar panels submerged: flooded solar facility (ABEMA TIMES)
• West Japan heavy rain exposed an unexpected weakness of solar (PVeye)
• Mega solar work resumes after a sediment disaster; the government moves to tighten rules (Nikkei xTECH)
• Maps and data: GSI Tiles (aerial photo, colored elevation) and hazard map (sediment-disaster warning zones) (processed and displayed in MAPRISE; providers and licenses are listed on our Data sources page, and the data you can view on the map on our Layer catalogue)