2.

The permissible tractive force is preferred because it is founded on a more solid theoretical basis.

3.

The typical range of angle of repose of noncohesive materials is 20^o to 40^o, with an average of 30^o.

4.

Because when a canal has clear water, it wants to transport more sediment ("hungry water"). Therefore, for the same slope and flow depth, a clear-water canal will have more eroding capacity than a normal-sediment-load (200 ppm) canal.

5.

S_o (geomoprphology/gravity), S_c (friction), and Froude number (hydraulics).

6.

S_o, S_c, 0, +∞ and -∞

In the upstream end, S_y is asymptotic to S_c and 0.

In the downstream end, S_y is asymptotic to S_o and 0.