Actually the exact opposite is true. By picking the right places to build your dams and dikes out to see, you can reduce the total length of [all the dams and d

Actually the exact opposite is true. By picking the right places to build your dams and dikes out to see, you can reduce the total length of [all the dams and dikes], thus total risk is reduced. An example is eg. dutch Delta Works: https://en.wikipedia.org/wiki/Delta_Works https://en.wikipedia.org/wiki/Delta_Works , which (among other things) reduced the total length of sea defenses in Zeeland by 700 km.