How much of the Netherlands is affected by the problem regulated by the Hoogheemraadschap van Rijnland? 
Hoogheemraadschap van Rijnland: The Hoogheemraadschap van Rijnland is the oldest water board of the Netherlands, having received its first commission to protect the land from flooding back in 1248 from William II of Holland. Flood control in the Netherlands: Flood control is an important issue for the Netherlands, as about two thirds of its area is vulnerable to flooding, while the country is among the most densely populated on Earth.
i don't know