Submitted by Suanne Z. Thamm
Reporter – News Analyst
September 16, 2016 2:11 p.m.
The City of Fernandina Beach has been fortunate to be able to tap into a vast reservoir of expertise among local residents in drafting ordinances that affect all aspects of life in the city. The most recent example is the painstaking work done by island resident Robert Prager to help the city and its citizens understand the complex world of civil engineering and the regulatory framework imposed by all levels of government to insure the health and safety of citizens while not imposing unreasonable burdens on industry.
As a co-founder of an engineering firm specializing in the restoration of streams and rivers in flood-prone urban areas, Robert developed his skills in reconciling the built environment with sustainable natural resources and local cultural values. Robert Prager’s effectiveness as a technical advisor is based, in part, on his ability to appreciate the perspective of project owners, designers, residents and other vital stakeholders. Robert brings over 40 years of experience as a designer of large civil and environmental projects. His technical specialty is designing sustainable, constructible, cost-effective water resource projects. Robert’s experience also includes project management, scheduling, cost control and relationship management of stakeholders, regulatory agencies, contractors and clients. Robert is the reviewer for a textbook published by the ASCE Press and has authored over two dozen technical articles in the civil and environmental restoration fields. He has designed hundreds of kilometers of successful river and stream stabilization projects and over 20 major reservoirs and dams, several near 30 meters high. His career began with the forensic investigation of a major hydroelectric dam failure and continued with design of large water resource projects including responsible engineer positions on four hydro-electric and two coal-fired power plants. He was a response engineer for a nuclear power station.
Robert is also an experienced technical reviewer of a broad array of projects including environmental restoration, water and wastewater, flood reduction and river management, pump stations, pipelines, tunnels, dam safety and rehabilitation, conservation site design, buildings, processes, and security. In transportation, his projects include highways, tollways, airports, waterways, locks, railroads and railheads.
Robert is a volunteer mentor with Engineers Without Borders. In this capacity he guides young engineers as they improve the health, safety and well-being of people throughout the developing world.
He has provided the information below in an attempt to help commissioners and the public better understand engineering requirements as discussed in the recent PAB and FBCC deliberations and decisions regarding proposed ordinances on storage of hazardous materials in the 100-year floodplain.
We thank Robert for his participation in this sometimes contentious process to craft new city code. His report appears below:
Purpose of this document: Provide background data to assist in making an informed decision.
Risk: Risk is comprised of two parts, the likelihood of occurrence and the consequences of occurrence. Each or both of these can be mitigated to reduce risk. A common example to illustrate the two parts is Russian Roulette. If one is foolish enough to play Russian Roulette, there is a 1 in 6 chance of shooting oneself and the consequences are dire. If the revolver is replaced with a toy gun that shoot sponge-like bullets there is still a 1 in 6 chance of shooting oneself but the consequences are greatly reduced.
The probability of flooding for any span of years
“In the 1960’s, the United States government decided to use the 1-percent annual exceedance probability (AEP) flood as the basis for the National Flood Insurance Program. The 1-percent AEP flood was thought to be a fair balance between protecting the public and overly stringent regulation. Because the 1-percent AEP flood has a 1 in 100 chance of being equaled or exceeded in any 1 year, and it has an average recurrence interval of 100 years, it often is referred to as the “100-year flood”.” (USGS, Robert R Holmes, Jr.)
“The “100-year flood” is an estimate of the long-term average recurrence interval, which does not mean that we really have 100 years between each flood of greater or equal magnitude. Floods happen irregularly. Consider the following: if we had 1,000 years of streamflow data, we would expect to see about 10 floods of equal or greater magnitude than the “100-year flood.” These floods would not occur at 100 year intervals. In one part of the 1,000-year record it could be 15 or fewer years between “100-year floods,” whereas in other parts, it could be 150 or more years between “100-year floods.”
“Admittedly, use of such terms as the “100-year flood” can confuse or unintentionally mislead those unfamiliar with flood science. Because of the potential confusion, the U.S. Geological Survey, along with other agencies, is encouraging the use of the annual exceedance probability (AEP) terminology instead of the recurrence interval terminology. For example, one would discuss the “1-percent AEP flood” as opposed to the “100-year flood.”” (USGS, Robert R Holmes, Jr.)
“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.” (USGS, Robert R Holmes, Jr.)
The graph below can be used to relate the probability of flooding for any given period of years for the 100-year and the 500-year flood levels. As stated above the 100-year recurrence interval flood has a 26 percent chance of occurring at least once within a 30-year period while the 500-year flood level only has a 5.8 percent chance of occurring at least once within a 30-year period.
The graph below can be used to relate the probability of flooding for any given period of years for the 100-year through the 500-year flood levels.
An analogy that might be useful to understand this is:
If there is a bowl with 99 white balls and 1 red ball, a blindfolded individual has a 1 in 100 chance of selecting the red ball each time they reach into the bowl. However, each additional time they reach into the bowl the probability of selecting the red ball increases. Like the flood example, on the 30th attempt to grab the red ball they have a 26 percent chance of success. Looking at the first graph and using the 100-year line, they have a 39.5 percent chance of selecting the red ball at 50 attempts and at 100 attempts they have a 63.4 percent chance.
If 400 more white balls are added there is a total of 500 balls. Using the 500-year line, at 30 attempts they have a 5.8 percent chance, at 50 attempts a 9.5 percent chance and at 100 attempts they have an 18.1 percent chance.
How much protection
How much more protection that is afforded for each one-foot increase in elevation is a complicated question. The recent FEMA flood model only calculated the 100-year flood and the 500-year flood elevations. The 500-year flood is approximately 3 feet higher than the 100-year flood elevation or about the 100-year plus 3 feet.
The 100-year plus 3 feet is about 5 times less likely to occur than the 100-year flood in any year; the 100-year plus 3 feet is about 4 times less likely to occur than the 100-year flood in a 50-year interval; and the 100-year plus 3 feet is about 3.5 times less likely to occur than the 100-year flood in a hundred-year interval.
The recent Flood Insurance Study dated 01/15/2016 calculated stillwater elevations for a range of flood events including the 10, 25, 50, 100, 500-year annual chance. The results are presented in Table 17. For the west side of the City of Fernandina Beach the differences for the 100-year and 500-year flood elevations is 2.5 to 2.6 feet for the stillwater elevations. For mapping purposes the elevations are rounded up. The difference would be 3 feet.
Sea Level Rise
The US Army Corps of Engineers developed a Sea Level Change Curve Calculator (2015.46) that calculates sea level rise at NOAA Gauges. The results relative to the NAVD88 datum for the Fernandina Beach Gauge is presented below.
The intermediate value for sea level rise in about 50 years is 0.43 feet.
Other considerations
The City’s Floodplain Ordinance requires that facilities shall be designed and constructed in accordance with the flood load and flood resistant construction requirements of ASCE 24. ASCE 24-14 Flood Resistant Design and Constructions sets minimum elevations for structures by Flood Design Class and Flood Zone. Highlights of ASCE 24-14 is attached. From Table 1-1 on page 5, a tank with hazardous materials would be Flood Design Class 3. From the Table on page 4, for Zone A not identified as Coastal A Zone the minimum elevation for equipment would be the Base Flood Elevation plus 1 foot (BFE+1) The 100-year flood elevation is the BFE.
The above discussion addresses the effects of flood water level but doesn’t address other hazards. As the tank is elevated, there will be more wind loading and susceptibility to damage by flying debris. As the base of the tank is raised it is more susceptible to damage by wave driven floating debris. Damage to the base of the tank will spill more than damage higher. Wind loading could be from microbursts or tornadoes not associated with high flood waters.
There are probably also operational risks. Hazardous materials will be added to the tank a lot more frequently than major flooding occurs.
Editor’s Note: Suanne Z. Thamm is a native of Chautauqua County, NY, who moved to Fernandina Beach from Alexandria,VA, in 1994. As a long time city resident and city watcher, she provides interesting insight into the many issues that impact our city. We are grateful for Suanne’s many contributions to the Fernandina Observer.
We are fortunate to have experienced and highly qualified folks that have chosen to live here – like Robert Prager – who share their knowledge and make hard decisions possible. Also a well deserved thanks to Suanne Thamm who repeatedly has made difficult subjects easier to understand.