Badge: I'm not challenging your logic; I just don't understand it. So . . .
1. Why is cross-sectional area reduced when the river is carrying more silt?
2. If we let the river take its course, would the delta stop subsiding?
It seems to me that if we undammed all the upstream tributaries and removed all the dikes and levees, the Mississippi would be muddier than it is now and would deposit more of its load along the way while also accumulating more from erosion. It would build a wider flood plain and it would meander more. And what was left of that increased load would be deposited at the mouth, building up the delta.
For me, liberal-arts major, it is a very counter-intuitive idea that reducing the load in the river by damming its upstream tributaries can have no effect on what's happening at the mouth of the river. I'm not an engineer, but the guys I worked with in D/G&EnE, USMA, were, and what you are saying seems (24 years later) to contradict what they said about flood-control, etc.
1. Because, at lower velocities, the silt will settle at the bottom and reduce the depth. The hydraulic grade line would not change, and as a result, the velocities would increase.
2. For every action, there is a counter-reaction. The key to keeping the rivers from getting too "muddy" is to minimize the amount of soils/fertilizers/pollutants entering. This involves conservation practices by farmers (which they are subsidized to do already, but don't do enough of it), minimizing fertilization (which has been done but not enough) and minimizing pollutants from storm water (parking lots, roads, etc.).
Of course, if we took out all the dams and levees, floodplains would get wider. Think of floodplains as natural storage - not conveyance bodies. When you take away all the natural storage with levees, all the water is forced into the main channel with zero storage. This all comes to a head when it finally gets to the Mississippi, which then start raging. If all of the floodplain were opened back up, we would see no flooding on the Mississippi - hence, no need for flood "control" (as if we can "control" flooding).
If we let the Mississippi take its course, the delta would slowly come back. Right now it's getting beat up by high velocities in extreme events. These are not natural disasters - they are man made disasters.
Finally, this flood "control" thing is big business for the Military guys (and USACE, of course). Not to mention the big contractors who make tons of money off of these flood "control" efforts.
Of course they would contradict me!!
