In fact, this is a simple Pythagorean theorem topic. The water surface width of 70.2cm under the bridge should be 70.2 meters.

First, find the radius of the circular arch bridge. Connect AO. There is AO = AD+DO in the triangle AOD, where AD=70.2/2=35. 1 m, DO=AO-CD=AO-2.4, and the solution is AO=257.86875 m and DO = 255.46875 m.

Connect MO, suppose the intersection of MN and CD is G, and there is Mo = Mg+Go in the triangle MOG, where MO=AO=257.86875 m, MG=3/2= 1.5 m, and GO=257.86439 m to solve.

In which GD = go-do = 257.86439-255.4875 = 2.39564m.

Because GD is higher than AB above the water surface.

2 meters cargo ship, so the cargo ship can pass through the arch bridge smoothly.