The river crested at 24.02 feet about a mile from my house, a full 9 feet above flood level, and that’s without the dam ever breaking. Now the water level is dropping dramatically and most of the creeks are near their base levels. For some, life goes back to normal. For others, this hurricane has changed everything.
I’ve posted two graphs below, one with the forecast levels from Aug 30 to Sept 7, and the other with the actual levels through the 4th. One thing I noticed was that the crest immediately after the storm was expected to be 23.5 ft but then rose to the crest you see in both graphs below about .5 ft higher. In receding, the water has left rather quickly, and today is almost 3 feet below what was forecast for the 4th of September. Principles of hydrology typically follow a pattern where natural channels with forested buffers will rise slowly and empty slowly, whereas more developed watersheds with lots of hard surfaces (more houses with roof runoff, more roads and parking lots, more ditches and drainage pipes, etc) will have much more rapid rising and falling, and with higher crests. See: Effects of Development on Floods.
The fact that our river rose very quickly and well above the first forecast, and then fell more rapidly than expected, has me wondering what data (other than rainfall) NOAA/USGS use to forecast river levels, and whether this data no longer reflects development in the watershed. This is especially important after the growth we’ve seen in this area post-Katrina.
According to the Census, growth in Tangipahoa Parish has been about 20% from ~100,000 to ~120,000.
20,000 new people (I am one of them!) makes for a lot of new houses and commercial areas in the watershed, and a lot of new, hard surfaces to shed the almost 12 inches of rain that fell in Hammond by Sept 1– or about 7.48 gallons per square foot. To illustrate, my 990 sq ft roof shed 7400+ gallons of water over just a few days! That’s a lot of water, and I’m glad to be surrounded by several acres of pasture to soak it up.
Now imagine the load on a drainage system with 100 small buildings like mine, and 100 driveways of about 200 square feet each, plus another 50,000 sq feet of roads and improvements and you’re talking something like 1.2 million gallons of water shedding from the surface and hitting the creeks/ditches (or just sitting and flooding) all at once. That’s just one imaginary subdivision. Now imagine 20 of these developments in one small town, or 50 new developments like this across the parish.
Looking at it in this way makes all that flooding a little less mind-boggling, and perhaps offers some hope that rainwater catchment practices, green infrastructure and development restrictions in certain areas can have life-saving impacts in flood mitigation.