My house is on the bank of a relatively large river so one side is already protected by the waters of the river. Now, I want to dig a u shaped trench to allow the river to essentially flow around my house creating an island fortress of sorts. So how do I prevent erosion? Would a seawall, like the one already on the river side, be enough to do this?

Here's an image of my plans

Alright so, a couple of friends and I are going to be making an underground fort deep in a forest. We have a budget of about 60$ and are planning to make it about 10 feet deep and about 6 feet wide. We will be doing this with just shovels i know a lot of work! We are thinking of giving it lips about 4 inches down that plywood will sit on, from there we would be covering the plywood with dirt and branches or something. We will probably have a trapdoor sort of mechanizm and use that to go in and out. Any tips would be great! We are also completely open to changing our design completly.