In 1979 a river was 27ft below the bridge. Silt build-up in the bottom so the river was 19 1/2ft below in 1989

Question

Please give an equation for d, the distance of the river from the bridge, where t=0 represents 1979? If nothing is done about the silt, what year will the river reach the bridge?

Answer 1

This seems more like a math question. Not to mention that the river might be higher due to more rainfall in a particular year.

Any, if we assume the silt build up is constant over time then the equation would be

d= distance of river surface to bridge

do = depth at time t =0

da = depth at time t = 10 years

t0 = t at time zero

t10 = t at time equals ten years

∆d=da-do/(t10-t0)

∆d= 27-19.5/(10-0)

∆d= 0.75ft/year  

If the river is rising at 0.75 feet per year then it will take

27/0.75 = 36 years to reach the bridge

So it will reach the bridge in 1979+36 = 2015