How is this base flood determined? It\u2019s a matter of looking at the past to forecast the future.<\/p>\n\n\n\n
FINDING THE FLOODPLAIN<\/h2>\n\n\n\n![\"Walnut](\"https:\/\/ncseagrant.ncsu.edu\/coastwatch\/wp-content\/uploads\/sites\/13\/2017\/03\/AERIAL2016.4131_web.jpg\")
Floodplain is a general term for a normally dry area subject to flooding from natural waterbodies, including rivers, streams, lakes and the ocean, as a result of storms and sea-level rise. Walnut Creek \u2014 and its floodplain \u2014 cut through NC State University’s Centennial Campus. Photo by Roger Winstead\/NC State.<\/figcaption><\/figure><\/div>\n\n\nRisk distributions rely on historical recordkeeping, so the longer the records the better the predictions. The longest-running tide gauge in North Carolina has been active for 81 years, but only a few gauges have continuously collected data \u2014 the newest gauge for only 13 years. Most river gauges have been around a little longer than those on the coast.<\/p>\n\n\n\n
Scientists apply models to the short-term existing data to approximate the history we do not have. Then, they test the model outputs against real-life events to verify their accuracy.<\/p>\n\n\n\n
In New Hanover County, for example, the storm-surge model closely predicted Hurricane Fran in 1996, roughly a 100-year flood. Hurricane Hazel closely matched the same water levels in 1954.<\/p>\n\n\n\n
Now observed through 2017, just two 100-year-equivalent events in New Hanover County have happened within a span of more than 63 years \u2014 indicating the modeling predictions are in the right ballpark.<\/p>\n\n\n\n
However, when the press reports a river reaching a 1,000-year flood, or when a property owner in Avon observes his house flooded by the \u201cthird annual Storm of the Century,\u201d something is amiss. Either those locations were very unlucky, or more likely, our modeling was inaccurate.<\/p>\n\n\n\n
Eventually, the new flooding data will enter the historical record and improve the accuracy of the models. This work is done by the North Carolina Floodplain Mapping Program<\/a>, a formal partnership between the state and the Federal Emergency Management Agency<\/a>, created after Hurricane Floyd in 1999 to modernize the state\u2019s floodplain maps.<\/p>\n\n\n\nIn addition to producing updated, digital flood-hazard data, the program is tasked with maintaining and updating the floodplain maps. As more data are collected, or floodplains change due to nature or development, they can re-evaluate flood frequency. You can view current maps and learn more about the program at ncfloodmaps.com<\/a>.<\/p>\n\n\n\nACCUMULATING ODDS<\/h2>\n\n\n\n
Whether your home or business lies in or out of the 100-year floodplain, it\u2019s important to understand your risk. With enough rain or a big storm surge, almost any location can flood. And the chances of flooding increase the closer you get to the water source, as well as the longer the period of time you consider.<\/p>\n\n\n\n
Let\u2019s reconfigure that original wager: What if you got 30 tries to pull out a specific numbered tile, with the only stipulation being that after each turn the chosen tile is returned to the bag?<\/p>\n\n\n\n
The odds may not be in your favor but they are much better: You now have about a one-in-four chance of hitting the mark.<\/p>\n\n\n\n
The decisions we make in designing buildings last for the lifetime of the house \u2014 an estimated 70 years for wood-framed buildings in the Southeastern United States. Most of us don\u2019t build or buy a house expecting to occupy it for just one year, and many of us choose to finance our home-sweet-home with a conventional 30-year mortgage.<\/p>\n\n\n\n
If you have a 1 percent chance of a flood occurring each year, the resulting probability of experiencing the 100-year flood during a 30-year mortgage compounds to 26 percent. Over the 70-year average life of the house, the likelihood of a 100-year flood increases to 51 percent \u2014 the flip of a coin. Does it get wet? Heads or tails!<\/p>\n\n\n\n
Floodplain managers and insurance agents use the 100-year floodplain to determine construction standards and flood insurance premiums. Flood safety is one of several hazards addressed in building codes. Compared to standards for wind, fire and earthquakes, the building requirements for floods are actually the least protective in statistical terms.<\/p>\n\n\n\n
The original national standard for wind was based on a local 50-year wind speed with additional \u201csafety factors\u201d added to better ensure damage would not occur within those 50 years.<\/p>\n\n\n\n
Recent changes to the standards moved some of the safety factors into the wind-speed maps. Most new buildings now are designed for a minimum 700-year wind speed \u2014 hospitals for at least a 1,400-year wind speed.<\/p>\n\n\n\n
For life-safety purposes, the building standards require a more stringent 2,475-year recurrence period for earthquakes, and a roughly 3,000-year rate for fire safety. These projections generally extend well beyond the expected lifetime of a building, but were judged to be necessary for human safety should the threat occur.<\/p>\n\n\n\n
BETTING THE HOUSE<\/h2>\n\n\n\n
In many cases, flood damage also can occur with water levels below the 100-year prediction because the building codes usually address floodwater reaching the lowest floor elevation of a house. Everything below the floor \u2014 insulation, ductwork and wiring \u2014 is susceptible to flooding below the standard.<\/p>\n\n\n\n
Building above the 100-year flood standard is the best way to improve your odds of staying dry now, and in the future. Using freeboard \u2014 additional height above the base flood elevation \u2014 is a common option for new and existing homes to achieve a greater margin of safety and reduction in insurance premiums.<\/p>\n\n\n\n
Adding up to 4 feet of freeboard can reduce National Flood Insurance Program premiums up to 70 percent in some areas. Many coastal communities in the state already require the use of freeboard as a locally implemented safety factor.<\/p>\n\n\n\n
Despite our efforts to plan and prepare for floods, the waters will surely rise again. You can bet on this.<\/p>\n\n\n\n
North Carolina Sea Grant works closely with coastal communities to effectively prepare for and improve resilience to natural hazards like floods. In Hyde County, for example, Sea Grant partnered with the county, Georgia Sea Grant, the University of Georgia\u2019s Carl Vinson Institute of Government and UGA Marine Extension Service to study community resilience and adaptation options for flooding. The final residents\u2019 flood-preparedness guide, as well as a video created for Hyde County about flood planning, can be viewed at ncseagrant.ncsu.edu\/floodguide<\/a>.<\/p>\n\n\n\nThis article was published in the Spring 2017<\/a> issue of <\/em>Coastwatch.<\/em><\/p>\n\n\n\n
Risk distributions rely on historical recordkeeping, so the longer the records the better the predictions. The longest-running tide gauge in North Carolina has been active for 81 years, but only a few gauges have continuously collected data \u2014 the newest gauge for only 13 years. Most river gauges have been around a little longer than those on the coast.<\/p>\n\n\n\n
Scientists apply models to the short-term existing data to approximate the history we do not have. Then, they test the model outputs against real-life events to verify their accuracy.<\/p>\n\n\n\n
In New Hanover County, for example, the storm-surge model closely predicted Hurricane Fran in 1996, roughly a 100-year flood. Hurricane Hazel closely matched the same water levels in 1954.<\/p>\n\n\n\n
Now observed through 2017, just two 100-year-equivalent events in New Hanover County have happened within a span of more than 63 years \u2014 indicating the modeling predictions are in the right ballpark.<\/p>\n\n\n\n
However, when the press reports a river reaching a 1,000-year flood, or when a property owner in Avon observes his house flooded by the \u201cthird annual Storm of the Century,\u201d something is amiss. Either those locations were very unlucky, or more likely, our modeling was inaccurate.<\/p>\n\n\n\n
Eventually, the new flooding data will enter the historical record and improve the accuracy of the models. This work is done by the North Carolina Floodplain Mapping Program<\/a>, a formal partnership between the state and the Federal Emergency Management Agency<\/a>, created after Hurricane Floyd in 1999 to modernize the state\u2019s floodplain maps.<\/p>\n\n\n\n In addition to producing updated, digital flood-hazard data, the program is tasked with maintaining and updating the floodplain maps. As more data are collected, or floodplains change due to nature or development, they can re-evaluate flood frequency. You can view current maps and learn more about the program at ncfloodmaps.com<\/a>.<\/p>\n\n\n\n Whether your home or business lies in or out of the 100-year floodplain, it\u2019s important to understand your risk. With enough rain or a big storm surge, almost any location can flood. And the chances of flooding increase the closer you get to the water source, as well as the longer the period of time you consider.<\/p>\n\n\n\n Let\u2019s reconfigure that original wager: What if you got 30 tries to pull out a specific numbered tile, with the only stipulation being that after each turn the chosen tile is returned to the bag?<\/p>\n\n\n\n The odds may not be in your favor but they are much better: You now have about a one-in-four chance of hitting the mark.<\/p>\n\n\n\n The decisions we make in designing buildings last for the lifetime of the house \u2014 an estimated 70 years for wood-framed buildings in the Southeastern United States. Most of us don\u2019t build or buy a house expecting to occupy it for just one year, and many of us choose to finance our home-sweet-home with a conventional 30-year mortgage.<\/p>\n\n\n\n If you have a 1 percent chance of a flood occurring each year, the resulting probability of experiencing the 100-year flood during a 30-year mortgage compounds to 26 percent. Over the 70-year average life of the house, the likelihood of a 100-year flood increases to 51 percent \u2014 the flip of a coin. Does it get wet? Heads or tails!<\/p>\n\n\n\n Floodplain managers and insurance agents use the 100-year floodplain to determine construction standards and flood insurance premiums. Flood safety is one of several hazards addressed in building codes. Compared to standards for wind, fire and earthquakes, the building requirements for floods are actually the least protective in statistical terms.<\/p>\n\n\n\n The original national standard for wind was based on a local 50-year wind speed with additional \u201csafety factors\u201d added to better ensure damage would not occur within those 50 years.<\/p>\n\n\n\n Recent changes to the standards moved some of the safety factors into the wind-speed maps. Most new buildings now are designed for a minimum 700-year wind speed \u2014 hospitals for at least a 1,400-year wind speed.<\/p>\n\n\n\n For life-safety purposes, the building standards require a more stringent 2,475-year recurrence period for earthquakes, and a roughly 3,000-year rate for fire safety. These projections generally extend well beyond the expected lifetime of a building, but were judged to be necessary for human safety should the threat occur.<\/p>\n\n\n\n In many cases, flood damage also can occur with water levels below the 100-year prediction because the building codes usually address floodwater reaching the lowest floor elevation of a house. Everything below the floor \u2014 insulation, ductwork and wiring \u2014 is susceptible to flooding below the standard.<\/p>\n\n\n\n Building above the 100-year flood standard is the best way to improve your odds of staying dry now, and in the future. Using freeboard \u2014 additional height above the base flood elevation \u2014 is a common option for new and existing homes to achieve a greater margin of safety and reduction in insurance premiums.<\/p>\n\n\n\n Adding up to 4 feet of freeboard can reduce National Flood Insurance Program premiums up to 70 percent in some areas. Many coastal communities in the state already require the use of freeboard as a locally implemented safety factor.<\/p>\n\n\n\n Despite our efforts to plan and prepare for floods, the waters will surely rise again. You can bet on this.<\/p>\n\n\n\n North Carolina Sea Grant works closely with coastal communities to effectively prepare for and improve resilience to natural hazards like floods. In Hyde County, for example, Sea Grant partnered with the county, Georgia Sea Grant, the University of Georgia\u2019s Carl Vinson Institute of Government and UGA Marine Extension Service to study community resilience and adaptation options for flooding. The final residents\u2019 flood-preparedness guide, as well as a video created for Hyde County about flood planning, can be viewed at ncseagrant.ncsu.edu\/floodguide<\/a>.<\/p>\n\n\n\n This article was published in the Spring 2017<\/a> issue of <\/em>Coastwatch.<\/em><\/p>\n\n\n\nACCUMULATING ODDS<\/h2>\n\n\n\n
BETTING THE HOUSE<\/h2>\n\n\n\n