I am sandwiched by rivers. The Stilly on the north, the Snohomish on the south. Then Puget Sound via Possession Sound immediately to the west. These are large flow rivers draining their Cascade watersheds immediately to the east.Winters are flood times here. Big floods happen when there has been a lot of snow (check), then warming with rain (check). This is true for most of western Washington with the difference being the relative increase in miles for the rivers to reach the ocean and the relative small difference in feet above sea level for the river's course. When a lot of snow melts and combines with a lot of rain, these rivers fill fast and just as quickly top their banks and fill their flood plains.
I took this picture just below the junction of the north and south forks of the Stillaguamish, what we call the Stilly. I'm looking north from Haller Park at the old and now unused railroad bridge that still spans the river here. The water is flowing from right to left, east to west. The water is moving between 19 and 20 feet. I just Flip-filmed a 100 foot tree, roots still attached, shooting past this point at a pretty good clip. It zipped through the railroad bridge abutments but hit the central abutment on the Highway 9 bridge sideways and backed up a huge wave for several minutes. Then another cut log with what looked like a two or three foot diameter swept down the current and rammed it on one side snapping it in two. The halves squirted on downstream.
Calculating the exact force that bridge abutment had to withstand would be an interesting exercise for an engineer and that would be exactly what the building specification would require. But think about it. Force is calculated by multiplying the surface area on the log (which changes as the water flows around its circular trunk and a drag factor also would have to be added) by the density of the water (water weighs about 2.2 pounds per liter) by the square of the water's velocity. I think that's pretty close to the right calculation, maybe another reader can add corrections, if needed. But the resulting force is then also transferred to the point of contact on that central pier.
In Path of the Paddle, Bill Mason says the force of an eight mph current on a canoe caught in a similar predicament can be two tons. I know from experience that you won't be moving that canoe against that current. The water flowing in the Stilly is traveling at what looks to be at least 20 mph, maybe faster.
