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| 05/11/2009 - Updated 07:36 PM ET |
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Flood safety and awareness
Tuesday, March 21, 2:00 PM ET
Flooding causes more deaths and property damage in the USA than any other severe weather related event. Join us for a chat with Tom Graziano, Ph.D., the acting chief of the National Weather Service's Hydrologic Services Division and an expert on flood safety and awareness. March 20-24 is Flood Safety Awareness Week.
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| Comment from Tom Graziano: Flood Safety Awareness Week serves to heighten the understanding of how dangerous floods are and what can be done to protect life and property. Flooding can occur in all 50 states and is a potential threat in all months of the year. Moreover, flooding is responsible for more fatalities than any other severe weather related phenomena. The average annual deaths attributable to flooding exceed those from lightning or tornadoes. For the 20 year period 1984-2004, floods on average are responsible for $4.6 billion in damage and more than 100 deaths. Interestingly, more than half of all flood-related deaths result from vehicles, including pickup trucks and SUVs being swept away by water covering roads. |
| Silver Spring, MD: What is the difference between river floods and flash floods? |
| Tom Graziano: We define a flash flood as a rapid and extreme flow of high water into a normally dry area, or a rapid water level rise in a stream or creek above a predetermined flood level, beginning within six hours of the causative event (e.g., intense rainfall, dam failure, ice jam). However, the actual time threshold may vary in different parts of the country. Ongoing flooding can intensify to flash flooding in cases where intense rainfall results in a rapid surge of rising flood waters. The term flash is used to signify a sense of urgency and indicates that immediate action should be taken to mitigate the impacts of the flood. In contrast, a flood or river flood is any high flow, overflow, or inundation by water which causes or threatens damage (typically beginning more than six hours after the causative event). The impacts of both flash and river floods can be devastating. |
| York Springs, PA: I have seen your "turn-around-don't-drown" web page. What is with that pink sign? |
| Tom Graziano: I’m glad you’ve noticed. The National Weather Service, in partnership with the National Safety Council, worked with the Department of Transportation’s Federal Highway Administration to come up with that sign. The Federal Highway Administration has official standards for all highway signs. The Turn Around Don’t Drown sign is considered an incident sign. All incident signs are required to be fluorescent pink with black lettering. You can find our more about the Turn Around Don’t Drown sign and our public awareness campaign by going to http://tadd.weather.gov. We are in the process of getting the “word” out to emergency managers all across the nation about this officially sanctioned Department of Transportation road sign so that more signs will be deployed on an event driven basis when it floods. On average, over the past few decades, the majority of the over 100 flood-related deaths in United States each year are the result of motorists attempting to drive through water covered roadways. It's very difficult for motorists to assess the depth of the water and whether the road bed beneath the flood waters has been washed away. Vehicles float because of buoyancy (Archimedes' Principle). In fact, most vehicles are swept away in as little as 18-24 inches of water. Consequently, our advice is simple, make the smart choice, heed the pink incident sign and "Turn Around, Don't Drown." |
| Charlotte, NC: I'm a teacher and am always looking for weather-related educational material for my classes. Do you know where I can find it? |
| Tom Graziano: Sure. The first place I would look is at www.weather.gov. In the left column, look under both Weather Safety and Education/Outreach. I hope those help. |
| Grand Forks, ND: What is a "snowmelt flood" and where can find information about snow cover? |
| Tom Graziano: When melting snow is a major source of the water involved in a flood, it's considered a snowmelt flood. Snowpacks store water. Unlike rainfall, which reaches the soil almost immediately, snow stores the water for some time until it melts, delaying the arrival of water at the soil for days, weeks, or even months. Once it does reach the soil, water from snowmelt behaves much as it would if it had come from rain instead of snow -- the water either infiltrates into the soil or it runs off (or both). Flooding can occur whenever the rate of water input exceeds the ability of the soil to absorb it or when the amount of water exceeds natural storage capacities in soil, rivers, lakes and reservoirs.
The National Weather Service provides extensive information about snowpack conditions across the country. Local weather forecast offices routinely provide flood forecasts, whether or not they're related to snowmelt. A wide array of ground, airborne and satellite observations are used to monitor snow conditions. These data are collected and provided by our National Operational Hydrologic Remote Sensing Center and are available at the following Web site: www.nohrsc.noaa.gov/
Also, each Spring, NOAA's National Weather Service provides a National Hydrologic Assessment to communicate the risk of spring flooding across the USA. Go to the following web site for more information: www.nws.noaa.gov/oh/hic/nho/index.shtml
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| Chicago, IL: What can I do to avoid being caught in a flood? |
| Tom Graziano: There are many things you can do. We recommend you follow the safety rules below:
- Monitor the NOAA Weather Radio All Hazards, National Weather Service forecasts on the internet at www.weather.gov or your favorite news source (radio, television) for vital weather-related information.
- If flooding occurs, get to higher ground. Get out of areas subject to flooding. This includes dips, low spots, canyons, washes, flood plain areas, etc.
- Avoid areas already flooded, especially if the water is flowing fast. Do not attempt to cross flowing streams and NEVER attempt to drive through water-covered roadways. Road beds may be washed out under flood waters. Always make the smart choice and "Turn Around Don't Drown"
- Do not camp or park your vehicle along streams and washes, particularly during threatening conditions.
- Be especially cautious at night when it is harder to recognize flood dangers.
For a detailed explanation of what to do before, during and after a flood, please see a safety brochure entitled "Floods the Awesome Power" which we at NOAA's NWS developed in coordination with the National Safety Council, the American Red Cross, and FEMA. You can find this safety brochure at:
www.weather.gov/floodsafety/resources/FloodsTheAwesomePower_NSC.pdf |
| Clinton, MD: I hear all of these terms: flood advisories, flood watch, flood warning. What do they all mean? |
| Tom Graziano: Here's some commonly used terminology/products and their meaning:
--Urban and Small Stream Advisory means that the flooding of small streams, streets and low-lying areas, such as railroad underpasses and urban storm drains, is occurring or is imminent . Advisories are issued when such events warrant notification of the public in a product less urgent than a warning.
--A Flood Watch is issued when flooding is possible – typically within a 6-48 hour time frame before the event.
--A Flood Warning is issued when flooding conditions are actually occurring or are imminent in the warning area.
--A Flash Flood watch means flash flooding is possible in the watch area. Flash Flood Watches are generally issued for flooding that is expected to occur within 6 hours of the causative event, which could be heavy rainfall or a dam or levee failure.
--A Flash Flood Warning is issued when flash flooding is actually occurring or imminent in the warning area. Flash flood warnings tend to be fairly localized areas such as a county or small group of counties, and the specific locations threatened within those areas are often highlighted. Flash Flood Warnings are issued for short-term events which require immediate action to protect lives and property, such as dangerous small stream flooding or urban flooding and dam or levee failures. |
| Washington, D. C.: I'm going to retire in Phoenix. Does it ever flood there? |
| Tom Graziano: Congratulations on your retirement. Yes, it does flood in Phoenix, typically in the summer months when the monsoon season kicks in. During monsoon season, it’s not unusual for locations in the Desert Southwest to receive thunderstorm-generated downpours during the afternoon, which can result in flash floods. Because the downpours tend to be fairly intense, much of the rain runs off quickly, often into arroyos and low water crossings, potentially placing motorists and others at risk. |
| Uniondale, NY: How much rain is too much? |
| Tom Graziano: All life needs water. The atmosphere distributes water from oceans and lakes to land areas where we live. Some areas, like deserts, are naturally dry, while others, like tropical rain forests, are wet. "Too much" water depends on where you are. When there is more water (whether from rain or snow melt) than the ground can handle, there is too much water and floods occur. The amount of water the ground can absorb depends on the type of surface (from concrete to sand), the type of vegetation (from bare ground to wheat fields to forests), and the amount of water already in the soil. When rain falls faster or snow melts quicker than the ground can absorb it, water pools in low spots. Typically, excess water on the surface flows into rivers and streams which are some of the lowest areas. A small amount of excess water can be nothing more than an annoyance. Much too much water causes flooding that produces significant damage. On average, for the past 20 years ending in 2004, flooding has caused $4.6 billion in losses in the USA. |
| Austin, Texas: How fast can water move? How heavy is it? |
| Tom Graziano: For reference, one gallon of water weighs 8.35 pounds and a cubic foot of water weighs 62.43 pounds. Water, like all other things on Earth, is affected by gravity. Given this, the speed at which water moves is primarily a function of the slope of the terrain over which it is flowing. So, in general, the greater the slope, the greater the speed. It should be noted, however, that I'm not accounting for surface/channel roughness. In areas where the channel roughness is high, and the flow of water is impeded somewhat by rocks, boulders, etc, the speed of the water will be reduced. |
| Cary, NC: I've always been very interested in the weather and follow your NWS forecasts closely. How can I become one of your volunteer weather observers who records rainfall in their own backyard? |
| Tom Graziano: Glad you asked. You can find out all you need to know about becoming a weather and/or river cooperative observer at the following web site: www.weather.gov/om/coop/index.htm |
| Pflugerville, TX: Is it true that flooding kills more people each year than tornadoes? |
| Tom Graziano: Generally, each year, more deaths occur due to flooding than from any other severe weather related hazard (see fatality statistics at http://www.nws.noaa.gov/om/hazstats.shtml).
For the 30-year period 1975-2004, on average, more than 100 people died each year due to floods. During the same period, on average, tornadoes killed 65 people annually. In a very general sense, the explanation for this disparity is two-fold: 1) tornado warnings have become steadily better through the years, which save lives, and 2) while flash flood and flood warnings have also improved, people often underestimate the destructive force and power of water. Interestingly, more than half of all flood-related deaths result from vehicles, including pickup trucks and SUVs, being swept downstream.
Contrary to popular belief, many people don’t realize that two feet of water on a bridge or highway can float most vehicles, including SUVs. If the water is moving rapidly, the car, truck or SUV can be swept downstream with disastrous results. Think about it. Why does a 97,000-ton aircraft carrier float? The answer is buoyancy. Flowing water can even wash out the road bed. The best advice when approaching a water covered-road is “Turn Around Don’t Drown.” |
| Tucson, AZ: What is the Advanced Hydrologic Prediction Service, and who does it benefit? |
| Tom Graziano: AHPS is an ongoing effort by NOAA's NWS to modernize our hydrologic services. AHPS includes state-of-the-art forecasting tools that cover the whole spectrum, from flash floods on small streams to floods in larger rivers to droughts. AHPS extends the range and quantifies the certainty of NWS hydrologic forecasts, thereby enabling users to make more informed, risk-based decisions to mitigate the impacts of floods and droughts and optimize the management of water resources.
Everyone who makes decisions based on water, including farmers, emergency manages, municipal water supply officials, river boat pilots and dam operators benefits from AHPS. An independent assessment by the National Hydrologic Warning Council (http://nhwc.udfcd.org/) indicates that the federal government’s one-time investment of $60 million in AHPS will return an annual recurring benefit to America of more than three quarters of a billion dollars. AHPS information is accessible in easy to use graphical formats at http://www.weather.gov/ahps/ . |
| Fargo, ND: We typically get a lot of snow here in the winter. What causes snow melt floods? |
| Tom Graziano: Snowmelt floods occur in your area as well as other areas in the northern USA. Most events are relatively minor and affect localized areas, but not all. Eight of the most significant floods of the 20th century (in terms of area affected, property damage and deaths) were related to snowmelt. The Northeast and North Central USA, and some areas of the West are particularly susceptible to snowmelt flooding.
Common causes include:
-- High soil moisture conditions prior to snowmelt: Rainfall during the late fall is particularly important because there is less evapotranspiration and less time for the soil to drain and dry before it freezes.
-- Ground frost or frozen soil: Deep, hard ground frost prevents snowmelt from infiltrating into the soil. Cold temperatures prior to heavy snowfall and normal or above normal soil moisture contribute to this.
-- Heavy winter snow cover: Unseasonably heavy snow cover means there is more water stored and available for snowmelt. Also, when heavy snowcover is widespread, it usually keeps air temperatures cooler and delays spring warming, which increases the potential for more rapid snowmelt and for spring rains occurring with snowmelt. In some parts of the country, the heaviest snowfalls usually occur in late February or March.
-- Widespread heavy rains during the melt period: Rain at this time contributes more water for flooding. Also, heavy rain can warm up cold snowpacks, causing them to begin melting earlier than they would otherwise. “Rain-on-snow” events are watched carefully for this reason.
-- Rapid snowmelt: Most often, snowmelt is a relatively slow phenomenon. Snowmelt rates are usually comparable to light-moderate rainfall. Important exceptions to this can occur, especially during unusually warm periods and when nighttime temperatures remain above freezing. Snowmelt rates can be much higher than normal under these conditions. |
| Park Ridge, Illinois: How many inches of rain in the Chicago area would it take to initiate a flood warning from the National Weather Service?
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| Tom Graziano: Good question. The amount of rain required is a function of a number of important factors. First, the existing conditions and what type of land surface that exists in your area are important considerations. The amount of water the ground can absorb depends on the type of surface (from concrete to sand), the type of vegetation (from bare ground to wheat fields to forests), and the amount of water already in the soil. In areas where the soil is wet and vegetation is lacking, a higher percentage of the rain that falls will run off. In urban areas like Chicago, where a high percentage of the surface is paved or concrete, more of the rain that falls will run off. When rain falls faster than the ground can absorb it, water pools in low spots. Hence, the rainfall rate is another important consideration. That is to say, the more rain that falls in an area during a given period of time, the more likely flooding is to occur. For the case of current conditions in the Chicago area, approximately 2 to 2.5 inches of rain in a 3-hour period will likely result in flooding and cause a flash flood warning to be issued. NWS forecasters monitor and forecast the threat of floods and flash floods 24 hours a day, 365 days of the year, and when necessary, issue warnings to protect life and property. For the latest weather related warnings issued by NOAA's NWS, visit weather.gov. |
| Columbia, Maryland: Can you tell me the top 5 things I should know or need to do with my family in case we find ourselves in a flood situation? |
| Tom Graziano: Below I've listed a few recommended actions to take should you find yourself in a flooding situation.
-- If advised to evacuate, do so immediately! Families should use only one vehicle to avoid getting separated and reduce traffic jams. Move to a safe area before access is cut off by flood water. Continue listening to NOAA Weather Radio, radio, or television for information concerning the flooding. Don't drive if you don't have to.
-- Get out of areas subject to flooding. This includes dips, low spots, canyons, washes etc. Do not attempt to cross flowing streams.
-- Never try to walk, swim, drive or play in flood water. You may not be able to see how fast the flood water is moving or see holes or submerged debris.
-- Do not attempt to drive through a flooded road. The depth of water is not always obvious. The road bed may be washed out under the water, and you could be stranded or trapped. Our advice is “Turn Around, Don’t Drown.” Rapidly rising water may engulf the vehicle and its occupants, sweeping them away. Vehicles can be swept away by as little as two feet of water.
-- Children should NEVER play around high water, storm drains, viaducts or arroyos. It is very easy to be swept away by fast-moving water.
For a detailed list of actions, please see a safety brochure entitled "Floods the Awesome Power" which we in NOAA's NWS developed in coordination with the National Safety Council, the American Red Cross, and FEMA. You can find this safety brochure at:
www.weather.gov/floodsafety/resources/FloodsTheAwesomePower_NSC.pdf |
| Ellicott City, Maryland: Dr. Graziano,
I was a kid in 1972 living in Silver Spring, Md., when Hurricane Agnes hit our area. I remember our street and everyone's basement in our neighborhood flooding. The rain was unbelievable. What is the probability that we will see a hurricane like that come through the Chesapeake Bay area and cause that type of flooding in the Ellicott City/Baltimore area? Thanks for your reply and keep up the good work. |
| Tom Graziano: Good question. This is close to home for me as I live in nearby Elkridge, Md. The possibility exists that another Agnes-like storm will impact the mid-Atlantic states in the future. We are currently in a period of heightened hurricane activity, and as such, the risk that a tropical storm/hurricane will impact the mid-Atlantic is increased. This past hurricane season was particularly active. In fact, during the 2005 Atlantic hurricane season, we had 27 named storms and 3 tropical depressions (unnamed).
To quote the NWS Director, Brigadier General David L. Johnson, from written testimony before the Committee on Science, U.S. House of Representatives in October 2005, “We believe this heightened period of hurricane activity will continue due to multi-decadal variance, as tropical cyclone activity in the Atlantic is cyclical and tied to fluctuations in sea-surface temperatures and other characteristics of the coupled ocean-atmosphere system. The 1940’s through the 1960’s experienced an above-average number of major hurricanes, while the 1970’s into the mid-1990’s averaged fewer hurricanes. The current period of heightened activity could last another 10-20 years." |
| Sacramento, CA: What is a stream gage and how are they used for river forecasting? |
| Tom Graziano: Well, a stream gage is a little complicated to define because there are quite a few types. The U.S. Geological Survey operates over 7,000 stream gages nationwide. The data provided by these stream gages are critical to the river forecast and warning mission of NOAA’s NWS. Some stream gages continuously measure river level and river flow (that is cubic feet per second, gallons per minute, etc.). This type of stream gage is used by the National Weather Service forecast models to forecast the height the river might rise after a given rainfall in that river's basin upstream of the stream gage. NWS flood warnings are used by millions of people in the USA to get people and property out of harm's way. Many U.S.G.S. stream gages also measure the quality of water in the nation's rivers. (For example, how salty, how dirty - that is, how much sediment is in the river, how much oxygen is available for fish and other species to breath, and many other water quality parameters). |
| Tucson, AZ: I'm a high school senior and I'm interested in becoming a hydrologist. Do you know of a web site that lists universities that offer degrees in hydrology? |
| Tom Graziano: It’s great that you want to become a hydrologist. The American Society for Civil Engineers lists dozens and dozens of colleges that offer degrees in civil engineering (http://www.asce.org/community/educational/instlist.cfm ) |
| East Grand Rapids, MI: How do I know how severe a flood will be? |
| Tom Graziano: The impacts of floods vary locally. For each National Weather Service river forecast location, flood stage and the stage associated with each of the NWS flood-severity categories are established in cooperation with local public officials. Increasing river levels above flood stage constitute minor, moderate, and major flooding. Impacts vary from one river location to another because a certain river stage (height) in one location may have an entirely different impact than the same level above flood stage at another location.
Within flood warning products, the National Weather Service conveys the magnitude of observed or forecast flooding using flood-severity categories. These categories include minor flooding, moderate flooding, and major flooding. Each category has a definition based on property damage and public threat.
--Minor Flooding: Minimal or no property damage, but possibly some public threat or inconvenience
--Moderate Flooding: Some inundation of structures and roads near streams. Some evacuations of people and/or transfer of property to higher elevations are necessary.
--Major Flooding: Extensive inundation of structures and roads. Significant evacuations of people and/or transfer of property to higher elevations. |
| Dallas, TX: why would people drive over a flooded road? I wouldn't even consider it. Can't the water damage your car? |
| Tom Graziano: I'd be more concerned about the water damaging you and any passengers you may have with you. As I stated earlier, the majority of flood deaths result from people attempting to cross water-covered roadways. To protect both you and your car, "Turn Around, Don't Drown." |
| Salt Lake City, UT: I noticed on your flood safety web page a section about ice jams. How serious are they and where do they occur? |
| Tom Graziano: According to the U.S. Army Corps of Engineers: In many northern regions, ice covers the rivers and lakes annually. The annual freeze up and breakup commonly occur without major flooding. However, some communities face serious ice jam threats every year, while others experience ice-jam-induced flooding at random intervals. The former often have developed emergency plans to deal with ice jam problems, but the latter are often ill-prepared to cope with a jam event when it occurs.
The Cold Regions Research and Engineering Laboratory Ice Jam Database (IJDB) contain data for ice events in 43 states. Ice jams have been reported most frequently in Montana and New York, each with more than 1,400 ice events. Two additional states have reported more than 1,000 ice events (Pennsylvania and Minnesota), and 24 states have reported more than 100 ice events. Even mountainous regions as far south as New Mexico and Arizona experience river ice. Ice jams affect the major navigable inland waterways of the United States, including the Great Lakes. A study conducted in Maine, New Hampshire and Vermont identified over 200 small towns and cities that reported ice jam flooding over a 10-year period (USACE 1980). In March 1992 alone, 62 towns in New Hampshire and Vermont reported ice jam flooding problems after two rainfall events. |
| Harpers Ferry, WV: Flooding is a big problem where I live. What is the National Weather Service doing to make better river forecasts? |
| Tom Graziano: A lot. The National Weather Service is modernizing its hydrologic services. Over the past decade or so, the National Weather Service underwent a modernization by restructuring its field offices, implementing new Doppler radars, deploying new satellites and implementing other technologies. The hydrologic services program is leveraging those investments to enhance its forecast and warning services nationwide through a new program called the Advanced Hydrologic Prediction Service (AHPS).
AHPS is the National Weather Service’s frontline solution to provide improved river and flood forecasting and water information across the USA. AHPS improves river and flood forecasts and water information across the country to protect life and property and ensure the nation’s economic well-being. AHPS graphical products are available online at: http://www.nws.noaa.gov/rivers_tab.php |
| Comment from Tom Graziano: Thanks for having me on today. Remember that floods are the number one severe weather related hazard in the U.S. To access information on current information on all weather related hazards including floods, visit www.weather.gov. Always remember when you encounter a water covered roadway, take our advice and "Turn Around, Don't Drown."
- Tom |
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