What are the most important flood prevention points?

What are the most important flood prevention points?　｜　Flood prevention construction method

Important flood prevention points are the places of utmost danger during floods and require careful inspection. According to levee condition they can be divided into [Levee Height], [Levee Cross-Section] and [Water Leakage] subcategories. On top of that, depending on the situation, they are also divided into two levels of importance.

Importance A (most important for flood prevention)
Importance B (important for flood prevention)

Incidentally, for [New Levee], points with washout history [Destroyed Levee Traces] and places which were river channels before being turned into levee [Old River Channel Traces] are filed under [Caution] category.

■Important flood prevention points

Type	Importance
Type	Ａ	Ｂ
Levee height (discharge capacity)	Points where the water level might exceed the height of levee during a flood of current river channel (※１) according to estimated flood discharge level（※２）.	Points which although don’t overflow but still need some leeway in their height (difference between current levee height and water level) during a flood of current river channel according to estimated flood discharge level.
Levee cross-section	Points where the current levee’s cross-sectional area or crown fall below half of the estimated needed width.	Points where the current levee’s cross-sectional area or crown fall below the estimated needed width, but are still above half of it.
Water leakage (※water leaks through the levee during flood)	Points where the water leakage has occurred in the past, that haven’t been exposed to proper measures yet.	Points where the water leakage is being taken care of, but the process has yet to finish. Points which have no history of water leakage, but being situated on old river channels can potentially leak and haven’t been exposed to proper measures yet.
Construction	Points of aging contraptions such as weirs, bridges and gutter tubes which are in severe need of improvement that haven’t been exposed to proper measures yet. Bridges in danger of having their girders sunk due to being below the estimated water discharge level.	Points of bridges, which although are in no danger of having their girders sunk according to the estimated water discharge level, need some leeway in their height (distance between girders and water surface).

※1　River channel	: a channel with levees at both sides through which the water flows during floods.
※2　Estimated flood discharge level	：Maximum amount of water which can flow through a river. In case of Abukumugawa River it is the amount it could reach only once in 150 years due to unprecedented heavy rain.
※3　Levee crown	：Levee crown is its flat portion on the top

■Caution

Construction work	Points where open-cut construction can negatively affect the whole levee.
New levee	Points which are still within three years of their construction. It takes time for levee to fit in with the soil and other bordering levees. Also in case it has no flood history there is no affirmation to its functionality. A three year time cautionary period is recommended.
Destroyed levee traces	Points which have a history of levee getting destroyed in the past. Soil is usually weak at such places thus a cautionary period is recommended even after proper measures are taken.
Old river channel	Points which were originally a river. Soil is usually weak at such places thus a cautionary period is recommended even after proper measures are taken.
Floodwall gate	Floodwall gate is something akin to a door leading inside the levee. In case of flood possibility they should be closed for levee to be functional.

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