Since the “Great Deluge of ‘06″ (last summer’s HUGE rain storm) I have been trying to find out just how prone our little patch of Alexandria is to flooding. There are three main components to determining this – our elevation above sea level, the drainage systems and how hilly we are.
How hilly we are is important because the flatter the ground, the slower water will flow away. A steep hill is less prone to flooding than a flat bottomland. The drainage systems we know about from last summer – they can get backed up. This is partly due to their age but also due to how low and flat our area is – the city needs pumps to keep the water moving. If we lived in a more hilly part of town, gravity would do the work. Lastly is elevation above sea level. This is more important for rising sea levels and when hurricanes come through than for events like last summer.
Each factor feeds into and effects the other factors. Last summer’s storm/flooding could be blamed partly on all three. Those closer to Dewitt & Nelson are a little lower than those further away from this intersection as evidenced by whose basement flooded.
In my search to learn if we’re in a flood zone (according to FEMA, we’re not) I wanted to know what our elevation above sea level is. This is information which isn’t very important most of the time but is critical to know when a storm surge from a hurricane is coming through. If NOAA tells us that there is a 12 foot storm surge coming up the Potomac should we worry? How about a 20 foot storm surge?
I *finally* found a trustable resource which tells us how high above sea level we are. The USGS has an online “viewer” (http://seamless.usgs.gov/Website/Seamless/viewer.htm) which provides all kinds of great information about geography for the whole US. It takes a little while to zoom into Nelson Ave. but once there you can use the elevation tool to find out that we are between ~40 and ~50 feet above sea level.
This is good to know for all kinds of reasons. If that hypothetical hurricane with a 20′ storm surge comes through, we’re not safe. We’re safer than Old Town but a 20′ storm surge will greatly decrease our drainage system’s ability to work. We’ll be above a lot of the flooding but we’ll still have to worry about drainage. And, as we know from last summer… 12″ of rain in 3 hours is enough to overwhelm our drainage systems.
