This would require changing the laws of physics, gravity, specifically. It's not like we can dig a gravity fed hole in the ground to store floodwaters. The reason being that a big enough hole that is also deep enough would exposed the groundwater table and fill with water long before the flood even happens. I applaud you for thinking about the issue, and a lot of people have studied flood issues for a very long time.

What the agency leaders seem reluctant to say is the clear, but not so obvious truth about flood water. That truth is: "The key to flood management is to divert the flood water away from my property onto your property." In other words, flood water has to go somewhere, and generally there is no place for it to go that won't negatively impact someone. If there were easy answers, the engineers would have identified them long ago.

People won't stop building in flood ways and flood plains. So they want flood control dams (i.e. move the water off their property onto someone else's). They are dumbstruck that the cost of those dams is too high to meet federal C.O.E. cost:benefit requirements where every dollar of cost must provide at least one dollar of benefit. So the perceived Chehalis dam solutions seems to be to build it with OPM, also known as "Other People's Money." That would require special Legislative or Congressional action since most taxpayers don't want to shell out their money for an uneconomic public works project.