re: NYT: Editorial on Climate Change and Louisiana

Miami is sinking because it is built on one of two types of subsoil environments common to Florida: karst or delta

Karst geography is when there is porous limestone with a high water table. As uplift forces or human activity lower the water table relative to the surface, the water errodes the limestone leaving void spaces. As more and more is built on top and the water level continues to decline, the sediment above collapses into the void and sinkholes form, often creating lakes. Florida is filled with these sinkhole lakes.

Where that karst linestone has been covered with shifting sand or silt deposits, those sinkholes are then filled in with the sand and silt above causing sinking.

If Miami is built on delta, then the following process similar to Louisiana is occuring: when delta sediment is deposited, it is very wet, but as more sediment is deposited on top, the water is squeezed out, resulting in compaction. Deltas are built when the volume of sediment added on top is greater than the volume lost from compaction. In areas where water is being pumped out for human purposes, compaction increases more rapidly (the inverse is also true. In New Orleans, it is believed that the water system leaks are so widespread that they are causing soil in some parts of the city to swell and rehydrate, slowing the rate of subsidence).

What can also increase subsidence and compaction is adding greater weight of buildings on top of the soil. More concrete means less surface area for sediment to build on. More buildings means more weight increasing compaction. More water wells means a faster declining water table. All of these factors speed up compaction resulting in widespread subsidence relative to a fairly static mean sea level.