A rectangular channel $2$ m wide carries $3$m$3/$s of water at a depth of $1\mathrm{.}2$ m$.$ If an

obstruction $40$ cm wide is placed in the middle of this channel, find the elevation of the

water surface elevation at the construction. What is the minimum width of the

construction that will not cause a rise in the water surface upstream?

Neatness of work $(+10$ points$)$