You may not be able to tell so far this year, but we’ve had ample evidence over the last couple of years that May is the high season for flooding in Tennessee. Today is the two year anniversary of the huge flood that swept Nashville clean two years ago, and various parts of west Tennessee (and the rest of the Ohio River and lower Mississippi) spent most of May a year ago under water. In light of that recent history it seems like a good time to introduce the idea of hydrology.
Hydrology is the study of the most plentiful resource on the planet. Water. Nearly everyone is aware of the hydrologic cycle to some extent. It’s a never ending cycle of water falling from the sky, being used, and evaporating back into the sky. It’s a subject as deep as the ocean but I’m only going to cover it here as it is usually implemented by a civil engineer. The US Geological Survey has a pretty good discussion of what a dedicated hydrologist does. (The difference between the USGS description and mine goes back to the difference between the scientist and the engineer.)
For a civil engineer, hydrology is important because it answers one staggeringly important quest, “How much water am I going to have to drain off my project?”. There are countless different mathematical models available to figure this out. They vary from extremely simple to eye numbingly complex.
The general purpose of hydrologic modeling is to get from rainfall to runoff. You start with the amount of rainfall and determine all the losses due to various physical and chemical processes and you end up with runoff. The aptly named runoff is the amount of water which is actually draining from the surface of your land. Some of the losses hydrologic modeling attempts to account for are:
- Infiltration into the ground
- Interception by vegetation
- Depression storage in low spots (ponds, sinkholes)
- Soaking of the plan
Detailed, accurate runoff modeling is staggeringly complex due to the enormous number of variables involved. Doing it well requires a lot of experience on the part of the modeler because factors as simple as how wet the ground is before the storm or how the storm crosses the drainage basin can markedly change the results.
We’re coming to the border of ‘too long: didn’t read’ so I’m going to continue the discussion in a few upcoming posts. I’ve got some future posts in the works to discuss different methods as well as some background on the elusive elusive ‘100 Year Flood’.