Floods in Colorado have caused more than $US2 billion ($2.13 billion) of economic damage, according to catastrophe risk modeller Eqecat.

Heavy rain and flash flooding that began on September 9 and intensified over the next week caused eight deaths and the destroyed 1500 homes. A further 17,500 homes were damaged.

Hundreds of kilometres of roads were flooded, with many bridges and at least four dams damaged or destroyed.

Eqecat says the damage, when calculated as a percentage of the area’s GDP, is comparable to Superstorm Sandy.

The Rocky Mountains are known for flash floods because of their heavy rainfall and steep canyons, which concentrate rainwater run-off. Recent forest fires have reduced the terrain’s ability to retain water.

Eqecat says the Colorado flood losses will be largely uninsured, unlike recent catastrophes along the US east coast.

The US National Flood Insurance Program lets homeowners establish cover within defined flood zones as a part of their mortgages.

But most of the areas hit in Colorado are not in defined zones, and flood insurance take-up outside the zones has historically been small.