Why might flood defenses increase damage during floods? How and where do flooding impacts occur

This isn't my expertise, but from what I researched, some flood defences limit or impede access to water bodies and completely clog channels and drainage ditches. As a result, this may increase the risk of flooding elsewhere.

Take, for example, sea walls. Sea walls can change the flow of sea waves as well cause an increase in beach sand erosion. Because they are  permanent structures, they tend to interfere with some natural processes like habitat migration. The change influences the movements in sea level and therefore, sea walls act as a hindrance.

Sea walls also cause variations to the area besieged by the tides in estuaries and hence reducing the size of the area to be occupied by water during high tide.

Furthermore, levees comes with many disadvantages as well. While providing protection from flooding in one place, they can divert the water elsewhere causing flooding in another place. Crops can drown and be killed in the flood causing problems for farmers. Levees can also fail. They can fail by overtopping, underflow, slumping, or erosion. Underflow is when water permeates the sand deposits underneath the levee and water rises up on the other side. When the weight of the water is too heavy for the levee, it can push over the sides. This is called slumping. Erosion is when the sides and top of the levee slowly start to wear down due to the waves of the water.

From my understanding:

\(T=\frac{n+1}{m}\)

We're told that \(n=70\), and \(m=1\)... Sub those in:

\(T=\frac{70+1}{1}\)

Therefore \(T=71\).

\(T=\frac{70+1}{2}\)

\(T_{1990}=35.5\)

\(T=\frac{70+1}{3}\)

\(T_{1980}=23.66\)

Hope this works, and was helpful. I'm not entirely sure because I've never used this formula before.