Well, I guess I mean "WHEN". That's what rivers do... wash out pipelines. I hope you're not trying to say that pipeline washouts don't happen. 3 major product pipelines were washed out on the Trinity River just north of Houston, Texas. (Colonial Pipeline, for one.) Rivers are not static systems and constantly seek lower levels. You just hope they do not do reach the level of your pipeline during the design lifetime of your particular project. What usually happens is that most pipelines are generally well maintained and easily outlast their design lifetime. This is when washouts usually occur, unless the river cross section is monitored and the line is lowered to levels below any new scour depths that fall within the river's new potentials.
Did you hit the zero too many times? Unless the the crossing is installed using some horizontal directional drilling technique, you will NEVER EVER have 100 ft below the river bottom (I assume you mean scour depth). At least in the 30 years I've been doing this, that has been the case. I mean do you have the time and money to dig a 120 foot deep pit to place the thrust boring machine on one side of a river and another 100 foot deep pit on the other side to receive the cutting head??? That doesn't sound reasonable to me. I was construction engineering manager for 750 miles of the BTC Pipeline Project and we did not have any such design (at least one that was not horizontally drilled (ie. not thrust bored) for any of our river crossings.
In fact, I give you a challange. I do not believe you can show me any AFC drawing of a crossing, river, canal, railroad, highway or anything else, where the pipeline is/was/to be/ installed using, lay-in, drop in, cut and cover, or horizontal thrust boring, (or any method other than the relatively very expensive and horizontal drilling method) 100 ft below any level marker. Most pipeline river crossings are placed across rivers by overtrenching to 10 ft below scour depth and pulling them in place, then sinking them to the required level. If you can show me such a drawing, I will show you a company that will go broke within the next 1 to 2 years.
OK back to actual river crossing design. Rivers are not always static systems. Rivers may temporarily reach a static or steady state where the bottom profile is in equilibrium with the current energy level of its particle transport dynamic. River "bottoms", as I think you use the term, are never the critical design case, as a river bottom profile at any given instant in time is dependent on the actual flow state at that instant in time only. Maximum Scour depth for the design lifetime of your pipeline is the design condition. When rivers are flowing at a low flow state, sand particles, pebbles and rocks are deposited, positioned and transported to and from and around the river bottom according the velocity prevalent at that flow state and an equilibrium condition is/can be achieved if an identical flow condition is/will be prevalent for a sufficient length of time. That will be the "river bottom" at the low flow state. Of course, when the river reaches the high flow state, the particle transportation dynamics is usually much different and a new equilibrium state becomes the target river profile, which will be reached, if the new flow condition prevails for the necessary length of time. Hence there is a given scour depth, usually defining a different depth profile, for each given state of flow of any given river. The only time they are (usually) the same is when the river has a sufficiently "hard" bottom in relation to any given flow state the river is likely to achieve. That condition usually means the current state of the river is that it is flowing on bedrock, or some other material that is not likely to change during the design lifetime of the pipeline. The Stokes particle settling velocities will determine the scour depth at your design storm condition, so unless your river is not at bedrock, or some other particle flow insensitive material, you will be able to determine your scour depth for any given flow state prevalent.
So, don't think of rivers as static systems.
