Panels even in reservoir

The huge reservoir in the south site was constructed around the projecting central slope in a U-shape and, if you approach the reservoir, you can find panels set up at about 1m from the paved ground. Combiner boxes were also set up slightly high on the assumption that the area under the panels would submerge (Fig. 5).

Fig. 5: Panels and combiner boxes set up in reservoir (source: Nikkei BP)

At the north end of this "panel area/reservoir," there are no panels but you can find another deeper reservoir and a rainwater exit hole in the middle. Rain on the south site initially flows into the deep reservoir at the bottom, starts submerging the reservoir under the panels only after the deep reservoir becomes full and is discharged downstream through the exit hole before the water reaches the panels (Fig. 6).

Fig. 6: Deeper reservoir at north end with rainwater exit hole (source: Nikkei BP)

Flood control-related technology generally required in the Forestland Development Permission System is stipulated to "be able to keep the peak amount of water flow in the heaviest rain possible in 30 years within the downstream flowing capacity." In other words, no floods are supposed to occur in the downstream areas of the developed site even in the heaviest rain that would only strike once in 30 years.

The company reportedly designed the drainage and flood control facility in the south site of the Fujiwara Yurino Power Plant with due allowance, assuming a higher amount of rain compared with the technical requirements. In fact, water has never submerged the area under the panels since the plant began operation, even though it experienced a major typhoon last summer.

Slopes hardened by spraying concrete

According to Hijidenki, it struggled with the rocky ground during such large-scale reclamation. As for foundations to support mounting systems, the company used both pile foundations and concrete stationary foundations in accordance with the terrain. Examining the ground in advance, Hijidenki used stationary foundations in areas that contained a lot of rocks (Fig. 7).

Fig. 7: Piles and stationary foundations used for different terrain (source: Nikkei BP)

Also, Hijidenki hardened the slopes by spraying concrete all over the surface in areas with many rocks near the surface with the aim of preventing rockfalls due to heavy rain and other factors.

"In civil engineering and reclamation, we paid almost excessive attention to design in preparation for natural disasters," Watanabe said (Fig. 8).

Fig. 8: Rocky slopes hardened by spraying concrete (source: Nikkei BP)