Just released on Labs for your feedback is Project Boulder for flood analysis and simulation
Read the blog from Scott Sheppard on the topic and click the image above for a direct link to join the project
I have done a quick test with the capability and had results in minutes.
I live in Shrewsbury, UK where we have the River Severn run through the town.
The water flows from the Welsh mountains and creates massive flood problems almost every year in Shrewsbury and other locations down its path.
This video I run a flood simulation on data provided from the free Ordnance Survey Open data and then compare to a surface where I have combined a topo survey and sonar scan of the river bed.
This video is the shortened version of just the simulation on the accurate surface and against a point cloud of the area (its an old scan, so no colour)