The bread and butter of terrain data and DEMs have always been hydrologic modeling. The reason is simple, water follows the rules, it goes downhill and terrain will determine where the water will go, or so I thought…..
As I write this, I am in the middle of an actual flood. Boulder, Colorado is experiencing a 100 year flood event and it has been an eye opening experience to see the difference between a model and the real world.
Our house sits on top of a ridge about 80 feet above flood level and 1200 feet away from the nearest drainage channels. If you evaluated a typical fluvial flood model, our house would be shown as well above flood levels of even a 500 year event. The problem is that the typical fluvial model only works when you focus on the drainage channel as the source of flood. In a pluvial event, where rain comes down faster than the drainage infrastructures can handle, the use of terrain data alone as an input for predicting the water movement is not enough. Not only must you know drainage infrastructure capacity but you need understand geology and the permeability of the soil, as well as the actual soil saturation.