How much of the Netherlands is affected by the problem regulated by the Hoogheemraadschap van Rijnland? 
Hoogheemraadschap van Rijnland:  It conducts water control activities in the general area known as Rijnland. 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