I think these are flood control barriers. Maybe for tides or surges. The have curved tracks on the side that looks leads me to believe they rotate down into position.
I found two more, each on different rivers nearby:
34.675472,135.455775
34.658305,135.463498
Flood control barrier on the Kizu River
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By kjfitz @ 2006-01-04 15:28:32
(updated 2006-01-06)