A stream goes from a cement channel to an earthen channel, all other factors remain the same. What happens?

Question

a. The velocity increases
b. The velocity decreases
c. The gradient increases
d. The cross-sectional area decreases.
e. None of the above

Answer 1

At the juncture where the water leaves the cement channel the earthen channel will quickly erode, creating a drop and therefore an increase of velocity and accelerating erosion which both deepens and widens the earthen channel for some unspecified distance down stream. Since the volume of water per unit of time leaving the concrete channel remains constant then the water moving though the deeper/wider earthen channel (after its initial drop) will have a decreased velocity while moving through the pool. This decreased velocity results in lessened erosion as it proceeds down stream and thus a narrower eroded channel and consequently increased velocity and erosion creating a gully that will continue the process until the entire land mass upon which the concrete sits has been undermined and the concrete channel will fall. At that point the parameters of your question have changed beyond the defined scope of your question.
By the way, where is all this water coming from?

Answer 2

The velocity decreases because the resistance to flow is likely to be higher in an earthen channel than concrete.

The statements about gradient and area are spurious as no information is given about these physical parameters.

Answer 3

The earthen channel will have a higher Manning's "n" value, so the velocity will decrease.

Answer 4

all of the above

Answer 5

I think A.