Water in Emergencies: Swiftwater and Floods

We've discussed how to sanitize water for consumption, but what do you know about the risks associated with flooding? Real talk: This is important.

December 30, 2019


That's a nice floodplain you've got there... it'd be a shame if someone... built in it.
Map compliments of ESRI/ArcGIS.

Floods are one of those disasters that gets the old 'ho hum' from most people until they happen. They're deceptively common and fade into quickly into obscurity for those not directly impacted.

We've mentioned that water hazards claim more lives than any other natural disaster. Where we discussed some of the hazards of drinking contaminated water there, we will discuss threats to your physical health and safety here.

They're two sides of the same coin, but the way we mitigate the risks is entirely different.

Floods: Overview

Floods are one of the most frequent, severe, and costly natural disasters on the planet, and you really should be thinking about them.

For one, they can happen anywhere. It doesn't matter if it's a desert flash flood or a coastal deluge, floods lack the regionality of other disasters like volcanoes, earthquakes, or hurricanes.

They also often occur as a secondary effect from other natural disasters, such as rapid snowmelt from volcanic activity or flooding associated with hurricanes.

Worse, the population expansion has driven developers to take up riskier developments in known flood plains, which was put on display in San Antonio in 2015 during the Wimberley floods.

It's ok, though. Local officials make the flood insurance more expensive to compensate for the increased risk of living there. Sarcasm off: you're on your own.

City planners aren't out there making sure you won't be swept away in a 100 year flood and that term means nothing. For this reason, we want to bring the topic up so you've got a place to start if you want to harden yourself against floods.

"100 year floods"

When people throw out terms like "500 year flood" or "100 year flood", ask yourself "how were they collecting flood data back then and who's job was that?"

We haven't been keeping good records for very long. We've had a decent system of recording some basic weather data for 120 years or so, but not much more.

The answer is, in most reliable cases, the USGS - which is an excellent organization with some extremely bright minds. Looking at their literature on the topic, the concept of a "100 year flood" is sorta crap and it doesn't mean that a flood of that magnitude will only happen once every 100 years.

For example, at the time of this writing in Texas hill country: the Llano River rose 10 feet in 24 hours, causing some massive flooding. That's not far from Wimberly/San Antonio, so we have 2 major floods in reasonably close proximity within 3 years.

The 100 year flood projection is based on a per year chance of water exceeding a known boundary of 1%. That means that from the data gathered, most years won't see a flood like that... But, as with all statistics, emptor caveat; percentages don't work like that. From the USGS website:

The 1-percent AEP flood has a 1-percent chance of occurring in any given year; however, during the span of a 30-year mortgage, a home in the 1-percent AEP (100-year) floodplain has a 26-percent chance of being flooded at least once during those 30 years! The value of 26-percent is based on probability theory that accounts for each of the 30 years having a 1-percent chance of flooding.

Since weather patterns tend to shift gradually, record floods can happen over short periods. Using our Wimberly example from 2015 and the current situation in Burnet and Travis County Texas, we have 2 major floods in reasonably close proximity within 3 years. While they didn't affect the same flood plain, there's a good chance those same areas will have similar floods before another hundred years pass.

So don't slip into comfortable complacency because, 'hey, it's a 100-year flood plain!' It's tricky phrasing that opens the door to dangerous development practices. ‍

A Word on Prediction

Floods - like most other weather phenomenon - aren't predicted accurately with much lead time.

A good forecaster with the right tools can generally see the conditions coming 3-5 days in advance, so like other weather emergencies (heavy snow, hurricanes, wildfires) you've got a Type II emergency, often with a few days of lead time. While that can be a great benefit, if you take some steps now it'll greatly boost your chances of a positive outcome.

This is the aftermath of Hurricane Harvey in North Central Texas - some 200 miles away. It pays to get out early.

We can streamline certain aspects of preparedness by:

  • Having equipment staged in a common area.‍
  • Having a solid load plan for the equipment that fits our vehicle.‍
  • Creating a plan and checklist for who's responsible for loading what. A clipboard near your supplies works well.‍
  • Discussing a plan ahead of time that includes a PACE (primary, alternative, contingency, emergency) location your family can move to if displaced by a disaster. Think about low roads with flooding!‍
  • Paying close attention to the areas affected to adjust your route, ensuring you're not caught up in a mass exodus or the path of destruction.‍
  • Getting out ahead of the crowd.

Often as not, simply being ready to do these things is enough. Likewise, I know that people have low confidence in meteorologists.

Try and keep in mind that those forecasters are putting almost unimaginable amounts of data together to provide you a free, cautionary service. Most truly have your best interest in mind, though allow for the fact that meteorology has become a reality TV show on most stations. For them, it's better to forecast terrible conditions and get 'bad' than it is to forecast bad and get terrible.

If you want the weather without the fluff, go to the National Weather Service, and the Storm Prediction Center.

Swift Water (and some Rescue)

Photo Credit Ohio DNR

One of the first things they teach you during swift water rescue is the force water is capable of generating. If we like one thing, it's nerding out on our readers with facts, and then making them approachable.

Water weighs 8 pounds per gallon, and 62 pounds per cubic foot. This water exerts constant force, which means fatigue quickly sets in. Along with cold and debris, we want you to take away a key point:

The more time passes, the lower your chances of a successful recovery.

We also like honesty: you can't learn this from a blog or article.

That said, we can help you make good decisions around swift water so that you don't need to be rescued.

Water creates a dramatic amount of force, but what we want to focus on is the Bathymetry, which is underwater topography, of a flooding channel, and how we can read the water surface to detect threats.

Reading the Water - Flow
A simple example of flow - a flood stage river will be more complex. ISG 2018

When we discuss the way water flows, to determine left and right banks, we always look downstream. The banks themselves define the waterway's "control", or the mechanism keeping it bound. Controls aren't important for causal interest, but once the waterway exceeds the natural controls, it enters flood stage. Here are some of the features we see, and what they mean:

Hydraulics occur behind underwater barriers and along gradients where water drops sharply.

Hydraulic: A hydraulic is a condition created by water circulating downstream from an obstruction. There are many types of hydraulics, but for this article, just keep in mind they're dangerous and you want to avoid them. Weak hydraulics are referred to as holes.

Laminar Flow (A): Marked by flat, consistent velocities free from eddies and turbulence, laminar flow is typically a sign of "no trouble". Even if you're in swift water, laminar flow can be used to swim hard to shore. When it happens in man-made structures, like waterways where escape is difficult, it can still be very dangerous and difficult to escape.

Riffle (B): Riffles are a indicator of shallower, wider areas of flow. They're often found immediately upstream from a hydraulic, below which escape can be difficult (or impossible). Riffles generally occur in very shallow water, and if you see them during flood conditions, they should be a big red flag that you need to get to shore, or prepare for a drop. These drops on roadways often include strainers - so best bet: get out as fast as you can.

Frown (C): The frown (as viewed looking downstream) indicates swift water dropping sharply, which can push you under and keep you there. Frowns can capture a victim and trap them in the hydraulic. This is more of a concern the deeper the drop and the strongest reversal of the current occurs on the sides, where you can easily be pushed towards the faster flow in the center. A smile, conversely has faster flow on the corners, which can provide a path around obstacles or hydraulics.

Turbulence (D): Turbulence is created when as the gradient increases and laminar flow is disturbed. It doesn't always indicate trouble, but it does mean there will be irregular currents.

Eddy/Eddy Line (E): The eddy line is the boundary where the flow reverses and velocities drop off sharply. The eddy typically can be used to get out of the water, rest, or grab ahold of some terra firma. Short of being tossed a rescue device, the eddy offers one of your best chances for self-rescue. If you can, swim hard to get to the eddy line. Even if you can't find a place to pull yourself out, you'll likely be out of the path of debris and more dangerous water.

Boil: A boil is a spot of diffluence in upwelling that pushes against the current and downstream, creating a "roll". Often times, these indicate loose or shifting bed, but boils can occur downstream of hydraulics and low head dams, which can trap a victim underwater. If you see a boil, try and avoid it as it signals rapidly sinking and rising water.

A strainer and trapped raft; if rescue trained personnel were not present, this could have severe consequences. Photo Credit.

Strainer: A strainer is a feature in the water that has the potential to 'catch' a victim, pinning them between the force of the water and the obstacle. Common strainers are guardrails along the sides of roads, fences, and downed trees. It's typically advisable to go "feet first" if you find yourself in swift water, but if you see a strainer, swimming hard and deliberately towards it with the objective of going over the top is the best chance of survival. Strainers are one of the most threatening features of the swift water environment, and should be taken very seriously.

Other Hazards

There's a lot to consider with flow and the hazards swift water creates, but it doesn't end there. Some of the more obvious hazards to consider are debris and thermal injury, but it's important to know that hydraulics can exist behind low head dams and drainage culverts. If you find yourself in one, chances of survival are slim, so as difficult as it is, you've got to be aware of what's coming if you're trapped in flood waters.

Low water crossings are likewise a major concern; every time there's a flood some dunce tries to cross in a Prius thinking 'it's probably not as bad as it looks'. It's likely worse than it looks and *any* water crossing you undertake should be done with as much information as possible.

If you do have to ford a water crossing, knowing the depth, width, and having a general idea of how swift the water is will go a long way in determining how much danger you're putting yourself in. The adage "turn around, don't drown" isn't a joke, so if you're unfamiliar with water crossings and you didn't plan ahead by having an alternative route or destination, don't make your situation worse by sacrificing your car, equipment, or life.

One of my old sites: this guy got caught in a low water crossing that started taking his car off the road. He managed to shimmy to the roof, where he was pinned for what people tell me was a half hour. The rescue team that pulled him out said that no sooner than they tossed him a rope, the car dislodged and floated down stream.


Most of the equipment used for swift water rescue is esoteric; that is, if you're not part of a rescue team, it'll do you exactly no good. One exception is a throw bag.

With a little training, these can make a big difference to someone trapped in swift water. You still need to be trained in it's use and be able to instruct the victim so that they can work with you for extraction, but we're mainly trying to stay out of trouble.

Secondary Concerns and Conclusion

In addition to the hazards we've already identified, we need to keep in mind that there are also mudslides, sinkholes, and contaminated standing water to consider when we look at water hazards. Don't forget the impact that the Type II emergency has on availability of resources. During Hurricane Harvey, gas was sold out from Dallas to Waco.

Topographic maps are easy to buy these days and establishing a firm understanding of your local area's likely water traps is a wise preventative measure. Keep in mind that floods can happen anywhere, whether the product of volcanic snow melt or torrential rains.

The purpose of this article is to illustrate that of all the emergencies people ignore, floods are probably the most complicated, common, costly, and significant. No one wants to talk about them because they aren't exciting (if you're not dealing with them) and we get bored with things that aren't outlandish. Don't get complacent. Localized flooding is a classic Type II Emergency, with embedded Type I's, so give it some thought. The preparations we make to dodge severe flooding are common with the other Type II's, so apart from familiarizing yourself with the specific hazards and getting some topographic maps, having an understanding of this material is easy lifting.

We hope it's been helpful. 



Author's Note:

This is not to be construed as, or used in place of, swift water rescue training. It's largely made of notes from Swiftwater Rescue training attended by the author and experiences in and around flooding. As such, most of this information is to be used to help identify and mitigate risks *not* actually participate in a rescue. Seek professional training if you intend to get involved with swift water rescue.


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