$6-$ A proposed concrete$-$lined trapezoidal channel will carry flow from a highway culvert to a nearby stream. The engineer uses the Rational Equation to estimate a $25-$year design peak flow rate of $15$ ft$^3/$s$.$ The channel side slopes are $3H$:$1V.$ It will have a $2-ft$ bottom width, a $1\mathrm{.}0\%$ slope, and Manning's $n$ of $0\mathrm{.}035.$ If the total channel depth is $1\mathrm{.}5$ feet, what is most nearly the freeboard above the design flow depth, in feet?