Incorrect gravitational vector in the friction models
In the friction models, for instance the manninStrikcler, the vertical component of the gravitational vector is used. This is incorrect, since it is the component normal to the bed which will induce the friction. A thought experiment is to think about a very steep (nearly vertical) slope. For this slope, the friction should be near zero. The normal component of the gravitation vector would be nearly zero and is the right component to use.
In my application I only needed the manningStrickler friction and adapted these files together with the frictionModel to include the normal component of the gravity. I've included the changes I made to these files. Obviously, also the call to the friction constructor should be updated in the solver.
I've also read in the dimensionless manning coefficient and changed the units in the class so the input file becomes less cluttered. I can imagine that incorporating this is somewhat more difficult.