Hurricane Isaac left a tremendous amount of rain in southeast Louisiana, and the Tangipahoa River flooded very quickly from rainfall alone. For the past 24 hours I’ve been hearing a lot of very vague warnings and evacuation orders. At first we were told “everyone along the Tangipahoa River has 90 minutes to evacuate.” The river was expected to rise 4-7 feet above its current level (which was already 3 feet over flood level, because of the incredible amount of rainfall. It is currently well-above flood stage at 22.5 ft.) Then it turned into “everyone within 2 miles of the river,” then 1 mile, then a half mile, and now Governor Jindal reassuring us not to worry. He says that if the water on Lake Tangipahoa is released slowly into a nearby “wooded area,” then it will pose a much reduced risk of rapid water level rise than if the dam breaks. At times like this, having a clear understanding of watershed dynamics is incredibly helpful. As it is, this flood event is going to rank as the 3rd highest water level on record. It could be more than that in the (increasingly unlikely) event of a dam breach.
First of all, those 2-mile, 1-mile and .5 mile distances given were arbitrary, and probably more related to “floodway” expectations (very quickly moving water) than potential flooding. Elevations along the river vary greatly. There may be residents on hills 10 feet or higher above catastrophic flood levels, who may be only a quarter of a mile or less from the overflowing riverbanks. Yet, people 5 miles or more out from the riverbanks may be at elevations so low that they may have to contend with backflowing streams and no drainage of additional rainfall for days. The further south you go, the wider and shallower the floodplain becomes– meaning greater risk for people further away from the river– until it eventually hits the 5-8 foot storm surges from Lake Ponchartrain.
The bridges across the river are nearly submerged already. The point of writing this is not to counteract the emergency orders, but to point out how incredibly dangerous it can be to live inside of a poorly understood watershed during major storms. Sure, we have FIRM maps and USGS river gauge data, but for residents and officials inside of Tangipahoa Parish, there is very little access to helpful information to determine who is really at high and low risk within our watershed when a major dam is at risk of being breached. We’ve had no comprehensive watershed studies to guide disaster planning, and as a result we have only vague and alarming, but not really helpful, emergency warnings.
The truth is, we just don’t have enough understanding of our watershed. For the past two years I have been studying and working in the Tangipahoa River watershed, primarily to target conservation areas that would protect buffer zones along the river to keep people out of reach of disastrous flooding. It’s hard to explain my work when people ask, but in a emergency like the potentially catastrophic flooding of the Tangipahoa River after Hurricane Isaac, I think it is clear to everyone just how important it is to keep undeveloped green space along major rivers.
Tangipahoa Parish has developed but has never fully adopted or implemented its Comprehensive Master Plan, which designates a mere 100-feet of green buffer zone along the river. That means that developers and landowners can continue to build extremely close to the banks of this relatively large local river. In 10-20 years, the number of people affected and the potential loss of lives to residents and their rescuers can increase dramatically in this rapidly-populating area. Information is not distributed explaining how fast moving water can quickly knock out a bridge or a roadway, and my husband was still able to get across the highway bridge this morning (though I certainly wouldn’t have recommended it!) in spite of water levels ABOVE THE LEVEL OF THE ROAD. The water is being held back by the concrete guards. Why hasn’t the parish closed off the bridges and roads within the advisory area? Certainly, people should be using common sense, but when information is lacking, why aren’t our public officials giving more guidance?
We have the expertise and we have the capacity to begin this kind of study in Tangipahoa Parish, but it just hasn’t been done. The resistance seems mostly political. There is more incentive to continue encouraging growth than to take time to evaluate the safety of our current activities within the watershed. One big, final question I have is: Why is there an enormous dam at the head of a river that has had so little comprehensive analysis?
Okay, enough of that. Looking at elevations and historical information from the Tangipahoa River flood of 1983, I’ve put together a map of my property and the likelihood of flooding nearby if the dam breaks and the water does rise 4-7 feet or so. The boundaries are approximate, but behind our house there will be a 75+ acre lake where the old swamp used to be in the 1970’s, north of Bell’s Branch, before it was cleared and drained for pasture. The riverbanks tend to have high bluffs that can hold the water back for those living along the river, but the force of the water accelerates erosion and can eventually eat away at those bluffs pretty quickly. It’s possible with a big flood like this that the course of the river may actually change in some areas.
Right now I am “hunkering down” on the opposite side of the river, waiting it out. The river bridges are finally being shut down as I type because of high water, so it’s likely that I won’t make it back across for another 3 days to a week anyway– and even more if the bridges are knocked out entirely.
We did make it back to the property yesterday and so far everything has held up incredibly well in the storm. One of my fig trees is split three ways and lying on the ground, but I’m expecting it to recover. I’ll post some photos soon, and I’ll keep updating as there are new developments.