Last week, Louisiana sustained a major disaster in the form of extreme flooding in central and southern regions of our state. President Obama declared a major disaster for severe storms and flooding for 20 parishes in Louisiana. Now that the flood waters have largely receded, people in our communities affected by these devastating floods are trying to get back to their homes, clean up and assess damages.
Here at the LSU College of Science, we’ve asked researchers from the LSU community, to the extent of their expertise, to provide answers to questions our community members might have about the 2016 flood and issues arising in the aftermath. For more information or if you need help related to flooding issues, visit www.lsu.edu/floodrelief.
Q: What is a flash flood?
Carol Wicks, department chair and professor in the Department of Geology & Geophysics: A flash flood is a very rapid flooding of rivers (or streams, bayous, etc.) and the surrounding low-lying areas. These are usually caused by severe thunderstorms, tropical storms or excessive precipitation within a short time.
Carol Wicks conducts research to understand the linkages between hydrology and ecology of carbonate systems.
Q: Why are flash floods difficult to predict?
Carol Wicks, department chair and professor in the Department of Geology & Geophysics: Some of the rainwater can seep into the ground, if the ground (soil) has capacity to absorb the rain and if the rate at which the rain is falling is less than the rate at which the water can seep into the ground.
In terms of the soil’s capacity to absorb the rainwater, we know from our everyday experiences that dry soil can absorb water (or has the capacity to absorb water) and that water-logged soil cannot. Our everyday experience also gives us insight into the rates of rainwater absorption. Rain that falls as a gentle rain event can be absorbed into dry or even relatively moist soil with very little water running off. During torrential downpours, even on dry soils, the rate at which the rain falls exceeds the capacity of the seepage rate of the soil. Thus, water that cannot seep downward into the soil flows across the land surface and into nearby streams, bayous and rivers. So, rain can seep downward or flow as runoff, if we neglect for the moment evaporation and the role of vegetation.
During the Louisiana Flood of 2016, some locations in southern Louisiana received more than 20 inches of rain over a two to three-day period. To put that into perspective, Hurricane Katrina delivered over 15 inches of rain to some areas over a two to three day period. The rainfall totals were similar and the rainfall durations were similar. All that rain could not be absorbed by the soil, therefore there was massive runoff into local streams and bayous.
Again, our everyday experience tells us that streams and bayous can accommodate a certain amount of water. We have all watched as stream levels rise after rainfall events and then slowly drop. When a rainfall event is at the intensity of 20 inches in two to three days, not only does the water not seep into the ground, but the streams overflow their natural banks. Then we have localized flooding.
But during this particular rainfall, the area that received so much rain was so large that all of the local streams, bayous and rivers were at capacity. Localized flooding became massive flooding and all of this flooding happened fast (flash).
Accurate prediction depends on predicting how much rain will fall during particular periods, understanding soil moisture conditions and seepage rates in the areas of rainfall, and knowing how much of the land surface can actually allow water to seep downward (soil can; parking lots cannot). It also depends on knowing water levels in streams, being able to assess how much more water can be accommodated in a stream, and then predicting how quickly that mass of water will flow downstream (flood routing). None of those steps are easy and in many cases, we do not have the needed data (for instance soil seepage rates).
Q: How did or how do flood waters travel down the river system, and how did the waters rise so quickly in neighborhoods close to these rivers for example?
Carol Wicks, department chair and professor in the Department of Geology & Geophysics: The Amite River and its tributaries including the Comite River and Bayou Manchac flow into Lake Maurepas and eventually into the Gulf of Mexico. We have good maps of the courses of the streams and rivers. During normal rainfall events, we can "watch" the flood pulse move downstream. The water levels in the Comite rise and fall and then as that water flows downstream and into the Amite River, the water levels in the Amite rise and then fall. At any location along a river, the river receives water from all the tributaries that feed into it upstream of that location and from along all of the land that drains into it in between tributaries.
As the water levels in the Comite rose, the Comite overflowed its banks. All that water (the water in the river and the water that overflowed) then had to flow downstream into the Amite, which had been simultaneously receiving 20 inches of rain. Yet it had to accommodate the flood waters from the Comite. There was not enough "room" in the river channel, so the Amite flooded. The same thing happened for all of the tributaries. You can imagine how dynamic these river systems are - one tributary floods, that flood water has to be carried downstream, and then water that floods the land has to flow back into the river and downstream. This repeats and repeats and repeats.
Q: In the aftermath of the flooding, do Louisiana residents need to be concerned with mosquitoes or other disease-carrying insects in or around their homes?
Rebecca Christofferson, assistant professor, LSU SVM Department of Pathobiological Sciences: Yes. Flood waters bring more mosquitoes. Maybe not in the immediate days after, but certainly a week or so following. Culex mosquitoes carry West Nile virus and Eastern equine encephalitis virus and bite during the day as well as night. Aedes albopictus or Asian Tiger mosquitoes can also carry West Nile virus, and bite during the day.
Rebecca Christofferson’s expertise is in infectious disease transmission, quantitative methodologies for surveillance and transmission modeling, and data communication.
Q: Does the flooding affect mosquito populations?
Rebecca Christofferson, assistant professor, LSU SVM Department of Pathobiological Sciences: Flood waters flush out mosquito larva that have already been there, but leave behind lots of standing water that is prime real estate for mosquito breeding. For Culex species, they love flood waters, ponds, etc. Aedes mosquitoes love containers. They will breed in anything, including bottle caps that hold water. As mentioned above, the mosquito population will experience a short lull because of mortality, but in about a week, I expect there will be an explosion of the populations. Couple this with the potential for interruption of normal mosquito abatement activities in some areas and the population numbers might get relatively high. People cleaning up can help mitigate this by getting rid of any sources of standing water they see in or around their homes.
Q: What should residents be aware of in terms of protecting themselves from mosquitoes in flood-affected areas, and are mosquito-borne illnesses more of a concern in the aftermath of the storm?
Rebecca Christofferson, assistant professor, LSU SVM Department of Pathobiological Sciences: Some people in Louisiana (myself included at times) have this attitude of “it’s just mosquitoes” because we’ve grown up being bitten. Some may not think they’re even being bitten, because they do not itch. However, this is not necessarily true. So to everyone: wear a mosquito repellent with DEET. The EPA has a website that will match you with the appropriate repellent. Re-apply as you sweat and dump out standing water around your homes.
With more mosquitoes, obviously there is a higher likelihood of being bitten. Also, people will be spending more time outdoors, windows and doors will be open and mosquitoes will be inside, so there will be an increase in contact between people and mosquitoes relative to a “normal” day where ACs are running and mosquitoes are less likely to come into your home. To add insult to injury, this is usually the time of year when we see an increase in the number of human West Nile cases because the bird-biting mosquitoes start to seek out other blood sources as nestlings leave the nests. This also has the effect of increasing the contact rate between those particular mosquitoes (Culex) and the human populations. However, if precautions are taken, there should be no increase in risk to individuals.
While there has been no local transmission of Zika virus, these precautions will protect you against the (very) remote possibility of introduction of that virus to these areas. Louisiana has some of the most sophisticated mosquito abatement programs, staffed by very smart and insightful individuals. They are aware of the effects of flood water and will be responding to the best of their abilities.
Q: Should people be concerned about alligators or snakes in returning to areas that were flooded?
Christopher C. Austin, professor in the Department of Biological Sciences and head curator of amphibians and reptiles in the Museum of Natural Science: Alligators are dangerous animals and should not be approached or apprehended. Even small alligators can cause serious harm. What you see on TV and YouTube is not a recipe for how to handle wildlife. Call your local animal control office to have a professional remove any wayward alligators. If the roaming alligator is near a bayou, pond or lake, they will likely naturally retreat to their original habitat when the water recedes.
Q: How does the flooding affect the distribution of other wildlife, and what can people expect over the next few days / weeks as far as spotting wildlife in areas around their homes?
Christopher C. Austin, professor in the Department of Biological Sciences and head curator of amphibians and reptiles in the Museum of Natural Science: People should be aware that the recent flooding may have displaced all sorts of wildlife. Larger wildlife like alligators and deer should be left alone and will likely move back to where they came from relatively quickly. Smaller reptiles and amphibians also get displaced by rising waters but many, including many snakes, are quite good at climbing trees to avoid flood waters. Smaller animals also have the potential to get inside flooded houses, garages and sheds. When moving flood or home debris from inside and around your house, be careful of snakes and other wildlife that may be taking refuge underneath.
Unless you are absolutely positive that a snake is not venomous, do not pick it up. Killing of snakes should be avoided as this is harmful to our natural wildlife and importantly it increases the probability of getting bitten. People often get bitten by venomous snakes when they are trying to kill them or when they are removing the body (yes a dead venomous snake can still inflict a medically serious bite). In addition, snakes eat pest rodents so are good to have around.
Frogs, turtles and lizards can simply be placed outside in a shaded moist area to allow them to find a good place to call home like a nearby wooded area. Remember frogs and lizards eat many pest insects and are great to have around the yard and neighborhood.
Q: Are there things we can do or that researchers can do to also help wildlife in the flooded areas "get back to normal," and to prevent any human-animal conflict or harmful interactions over the next few weeks?
Christopher C. Austin, professor in the Department of Biological Sciences and head curator of amphibians and reptiles in the Museum of Natural Science: The single best thing to do is let the animals choose which habitats they want to retreat to as refuge. They don't want to be in your living room any more than you want them there. If they can be safely moved outside they will retreat to nearby wooded areas.
For more information about the 2016 Louisiana Flood and post-flooding issues, see the following links:
- August 2016 Flood Relief: http://www.lsu.edu/floodrelief/
- Clean and sanitize flooded kitchens [LSUAgcenter: link]
- Steps for mold removal and prevention after a flood [LSUAgcenter: link]
- Prevent further damage to your flooded home and your health [LSUAgcenter: link]
- LSU Experts Available to Speak on Historic Louisiana Flooding [link]