$**$SOLUTION: x$=2\mathrm{.}75$ ft$**$ Continuous flow measurement is needed in a long, concrete irrigation canal with a bottom slope of $0\mathrm{.}001.$ The maximum flow rate expected in the $15-$ft$-$wide canal will not exceed the design $($normal$)$ depth of $6$ ft$.$ Determine the maximum height of a flow measurement weir that keeps the weir backwater from exceeding the $3$ ft of freeboard $($i$.$e$.,$ a channel depth of $9$ ft$).$ Assume the weir friction and velocity head upstream of the weir are negligible.