Q$3\mathrm{.}2.$ Based on the maximum flow rate found in Q$1$:

$($a$)$ Find the Area of the river cross section without piers.

$5$

$($b$)$ Find the Area of the river cross section with piers.

$($c$)$ Find the average velocity of river during the $100-$year flood without bridge piers.

$($d$)$ Find the average velocity of river during the $100-$year flood with bridge piers.

$($e$)$ Accordingly, find the afflux height and the length of backwater.

Q$3\mathrm{.}3.$ Consider the pier in the middle of the bridge is submerged to a certain depth during

the flood with a return period of $100$ years. Calculate the drag force on the middle pier.

Assume the drag coefficient $($CD$)$ is $0\mathrm{.}36.$ In drag force equation, use the average velocity

of river during the $100-$year flood with bridge piers, calculated in Q$3\mathrm{.}2$