Many people have heard about the 100-year-floodplain or the 500-year-floodplain. These terms can be very confusing, and do not accurately reflect the risk of a property that is near to a river or stream.
The 100-year and 500-year floodplain are terms used in the National Flood Insurance Program, and do not relate to the random nature of actual floods. The massive devastation in the Delaware River Basin in 2004, 2005 and 2006 were in many cases only 35, 45 or 55-year floods according to the regulatory floodplain demarcation, but they were still dramatic, dangerous and damaging events. Damage from these sorts of storms is very common: approximately 30 percent of all flood claims come from outside of the 100-year floodplain.
We use the term "flood zone" to indicate an area that is either at high, medium or low, but still possible, risk of flooding. It is important to determine whether you are in a flood zone. The Federal Emergency Management Agency has mapped the 100 and 500-year floodplains in most areas of the country, and these "flood insurance rate maps" or "flood maps" are available here. They can be a good starting point, but are not all that you need to consider. Property owners should investigate their flood risk by researching the history of their property, through conversations with neighbors, and by investigating the geography and terrain around the property. It is also important to know whether access roads to the property flood or if they flood sooner than the property does, as this will affect evacuation plans. Even if an individual property doesn't flood, if many homes and access roads in your neighborhood do flood, it might be wise to evacuate at the same time, to avoid being isolated in a potentially dangerous storm.
Other distinctions to make
Properties can be at higher or lower risk within a FEMA floodplain area, depending where they are located. For example, one house in the "50-year floodplain" may have their property flood 2 feet deep during a storm, but their neighbor deeper in the floodplain may flood 6 feet deep.
Another very important consideration is the nature of the terrain that gathers rainfall upstream of your property. If that area has steep hillsides and little vegetation or thin rocky soils that don’t absorb water easily, you could be subject to flash floods that can be much more powerful and dangerous than slow rising water.
If you have questions about your flood risk, contact someone from your city or county who can help you understand the potential hazards you may face. Many communities have certified floodplain managers who are trained to help you understand these issues. Do you live, work or drive in a flood zone?
The National Weather Service has a suite of flood forecast and warning tools that can provide you with critical advance notice of floods. This video will provide you an overview of how to sign up for "flood alerts" using an RSS feed.
These tools can share with you predictions of flood levels (i.e., how high the river will get) as well as flood timing (i.e., how high it will be at a certain time of day).
To sign up for alerts, you will first need to identify the stream gage nearest you. You can do that here. Once you zoom in and find the gage nearest you, click on the button for that gage, and you will see a "hydrograph," or a chart that shows you current river levels. If a flood is predicted in your area, the hydrograph will also show you predicted flood levels and timing. (In some locations, the hydrograph will estimate future river levels regardless of whether a flood is predicted). It is a good idea to bookmark this page and check it regularly during times when the river level is rising or expected to flood. Additionally, if you sign up for the alerts, you will receive notice to your Internet browser, RSS reader, email, and if you have one, smart phone email, when a flood watch or warning is issued. National Weather Service employees themselves say not to wait until a formal "Flood Warning" is issued to start getting prepared: that may be too late. Instead, keep abreast of river levels, so you have plenty of time to prepare your go-kit and evacuate if needed. Sign up for flood alerts!
What's In a Flood Plan?
A flood plan is simply the advance methods your family, business or community have put together to help you respond quickly in the event of a flood near your property. Planning in advance can afford you extra critical time when a flood is coming, and can help you increase the odds of protecting your valuable documents, your real estate, and your personal property - including cherished belongings. A "rapid-response" plan can be as simple as a one-page plan that answers the following questions:
1. How will we find out about a coming flood?
The first part of a Flood Plan is putting yourself in a position to get some advance warning of an unfolding situation. Large scale flooding on the main stem of a river may occur over many hours or several days, but flash floods can strike in minutes.
Signing up for flood alerts and monitoring weather patterns and local conditions are important steps. People living along the Eastern seaboard need to be mindful of the flood potential from tropical moisture that can move up the coast and deposit huge amounts of rain (or snow that melts quickly) inland. Summertime thunderstorms are often the cause of flash floods, but such events can happen any time of year.
Sign up for National Weather Service Flood Alerts NOAA Watch
You can also monitor river levels via the USGS gage here: NOAA Watch
FEMA has general flood hazard maps online here, which serve as a starting point for determining your property's relationship to nearby waterways.
2. At what river level does our property begin to flood?
First, determine "What's Your Number?" This means visiting the National Weather Service's Advanced Hydrologic Prediction Services website to learn the flood stage at the stream gage nearest you. Click here to learn more. Then, determine the level at which flood waters begin to affect your property. This step may take research or personal experience to determine, but it is worth talking to neighbors to find out how high the river was during recent floods, and at what point flooding began in your neighborhood. Each neighborhood and each property has its own unique terrain and placement to consider when determining this factor, and it is safest to err on the side of caution. Give yourself plenty of time to evacuate.
3. Where is our family go-kit located? What does it contain?
Preparing your household for a flood involves steps that will help you be prepared for many different types of disasters. An emergency kit, or Family Go-kit, might include first aid supplies, a 3 day supply of non-perishable food, bottled water, a battery-powered radio, extra batteries and flashlights. Also, personal items like rubber boots and a rain jacket, warm clothes plus hygiene and sanitation products can be very helpful. An example of contents for a "Family Go-Kit" can be found here.
4. How will we learn about evacuation orders?
Find out how your community notifies residents of floods and how it makes evacuation orders. Make a commitment to follow evacuation orders the first time, to prevent emergency personnel from having to return for a rescue when travel is no longer safe.
5. What access roads can we use to evacuate in the case of rising waters?
Research indicates that the majority of flood-related fatalities occur as cars become trapped on roads that are known to flood. Talk to neighbors, emergency personnel and others to determine which access roads to your home flood, and when. Know what roads you regularly travel and whether or not they will flood, and plan alternate routes when needed.
6. What steps should we take to prepare our property?
Research the flood-proofing options available to you. Can you install a quick-disconnect furnace, or elevate electrical and mechanical equipment? Are there steps you can take to alleviate pressure on your structure if flooding does occur, to prevent extensive damage to doors and windows?
7. Where should our family meet if we are separated during a flood event?
Is our child's school or spouse's workplace in a flood zone, or are access roads subject to flooding? If so, what provisions should we make for their safety?
A written plan is essential for helping individuals and households to think through important issues in advance. You should also investigate whether there are similar plans in place for work, daycare and school and see how they work with your plan.
Expect roadways to be blocked. Using a cell phone (sparingly, so as not to clog the network), you can determine whether other friends and family are safely sheltered where they are, and whether it is safer to wait out the storm than to risk dangerous travel across town through flood-prone roads.
Remember, the most common things people regret planning to save are: pets, photographs, and computers. Can you pack all these in a vehicle and drive to high ground in time?
Helpful Flood Terms
Flood Watch: Flooding is possible. Tune in to NOAA Weather Radio, commercial radio, or television for information and check the flood alert sites on the Internet.
Flash Flood Watch: Flash flooding is possible. Be prepared to move to higher ground; listen to NOAA Weather Radio, commercial radio, or television for information.
Flood Warning: Flooding is occurring or will occur soon; if advised to evacuate, do so immediately.
Flash Flood Warning: A flash flood is occurring; seek higher ground immediately.
Find additional information on how to plan and prepare for floods, what to do during and after a flood, and learn about available resources by visiting the following:
Federal Emergency Management Agency
Floods happen. Lessen the loss. (A project of Nurture Nature Foundation)
Understanding your flood risk means more than just knowing whether your property is in a FEMA-designated flood zone. It also means knowing the natural factors that affect flooding, and the technology available to help preserve properties and protect lives.
"Understanding Flood Risk in the Delaware River Basin" features a series of six interviews with flood authorities and experts who discuss the factors that affect flood risk, and the decision-making processes and new technology that can help reduce losses. Hear from authorities on flooding, discussing flood frequency and the misleading phrase "100-year-storm," climate and flooding, flood forecast and warning, community planning responses to flooding, and more. Companion text, graphics, linked resources and discussion questions make this a great teaching tool or basic introductory guide to the issues that surround flood risk in the Delaware River Basin and beyond.
Introduction to Understanding Flood Risk
A brief introduction to the six chapter series, Understanding Flood Risk.
Understanding Flood Risk: Chapter 1.
An overview of some of the weather and terrain factors that increase flood risk in the Delaware River Basin. This segment is chapter 1 of 6 in the Understanding Flood Risk series.
Understanding Flood Risk: Chapter 2.
An interview with Pennsylvania State Climatologist Dr. Paul Knight about weather and climate impacts on flood risk. This segment in chapter 2 of 6 in the Understanding Flood Risk series.
Understanding Flood Risk: Chapter 3.
Nurture Nature Foundation Director of Environmental Outreach Rachel Hogan Carr interviews Dr. Dru Germanoski, geology professor at Lafayette College, about regional flood frequency, and the challenges of the "100-year-flood" concept. This segment is chapter 3 of 6 in the Understanding Flood Risk series.
Understanding Flood Risk: Chapter 4.
Hydrologist Peter Ahnert, of the National Weather Service's Middle Atlantic River Forecast Center, discusses the use of new "flood alert" technology that, in the case of an impending flood event, sends flood watch and warning messages to users who have signed up. This segment is chapter 4 of 6 in the Understanding Flood Risk series.
Understanding Flood Risk: Chapter 5.
Becky Bradley, Director of Planning and Codes for the City of Easton, Pennsylvania, discusses how the City changed its land use codes and began to regulate the 500-year-floodplain after repeated flooding - and how it has benefited the city's safety and economic development. This segment is chapter 5 of 6 in the Understanding Flood Risk Series.
Understanding Flood Risk: Chapter 6.
Floodplain resident John Mauser discusses the steps he takes to keep his property and family safe during recurrent flood events, including monitoring river levels, and having detailed evacuation and readiness plans. This segment is chapter 6 of 6 in the Understanding Flood Risk series.
Understand Your Flood Risk