After a 50-year career in agricultural engineering, I believe flooding with the regular rainfall in Puget Sound could be mitigated quite simply, but level building is not the answer. For cost effectiveness I would suggest a combination of dredging and controlling runoff in upper watersheds by making every culvert a firepond. (This would entail using an open-topped box on the upstream side of a culvert with a self-closing tidegate).
This water storage would cut down on erosive runoff, help build up the aquifer, prevent water cumulative layering with grade change and conserve soil and water. Runoff would be regulated by having control after storms subside.
How would such a program be paid for? The money saved by averting flood damage would go a long way in providing an incentive for such a flood-control program. Flood damage is costly and a hardship for property owners.
All this experience makes me feel that all possible options should be looked into for flood control.