The thing about I-5 is way over blown IMO. The extreme case, which has never occurred, is estimated that I-5 could be out of service in that limited area for UP to 10 days about once every 10 years. The total number of days that I-5 has been out of service in the Chehalis area would be 65 days since I-5 was built if that extreme case were true. I-5 has only been closed a hand full of days over the entire history of the freeway, and most importantly, there are viable alternate routes for those few days that the interstate is closed. It adds a few hours to delivery schedules for trucks coming from LA or the Central Valley. No one goes hungry. Vegetables still get to the grocery store. Lettuce might cost a dime extra for that week, worst case scenario.

Raising I-5 would be expensive, but feasible from an engineering perspective. Not so feasible is how to manage Salzar Creek (the small stream drainage). It drains all the floodwater from the hills of Chehalis onto the floodplain, and that water needs to get to the river. If it's impeded by dikes, gates, or other obstructions, it floods all the area that hasn't been filled along the freeway over the last 20 years. That fill never should have been allowed, but the movers and shakers around Chehalis want to be like California, where water flows uphill toward money.