Compute the discharge for a rectangular channel that is $20$ m wide, the flow depth is $1\mathrm{.}5$ m $,$ the channel is concrete lined $($float finished$),$ and the slope of the channel bed is $0\mathrm{.}015\frac{m}{m}.$ You should

a$)$ Calculate the discharge for that same channel, assuming it is an excavated earthen channel with short grass and limited weeds.