Ready for a Century of Sea Level Rise

Climate Change
Written By Ted Caplow

by Ted Caplow, PhD

Before explaining how Caplow Manzano designs houses for the future of Miami, it is helpful to define a few basic terms:

Sea Level - we use this term to refer to what is called the mean highest high water. That is the highest tide that is normally experienced in a year.

Base Flood Elevation - we use this term to refer to the federally designated Base Flood Elevation, ¹ which is measured above a reference point that is close to sea level. These elevations differ slightly at each property. For many areas that are currently inland or located on high ground, the flood elevation is undefined, but over time, the Federal Emergency Management Agency (FEMA) revises their maps, and in general, the areas that are officially prone to flooding move upward and inward. These maps frequently become the basis for local zoning ordinances and flood Insurance programs, as is the case in Miami.

The federal Base Flood Elevation represents the elevation of a flood that is estimated to have a 1% chance of happening in any given year. ² This flood may represent the combined effect of storm surge, rain, and waves, depending on the location. Mathematically, there is about a 63% chance of the Base Flood Elevation being exceeded by flood waters at least once over a 100 year period. ³

In coastal areas, many newer homes in Miami are at least somewhat elevated against storm surge.  In fact, the zoning code requires that houses be built above the Base Flood Elevation (where one has been defined), and in Miami the Base Flood Elevation is typically from 8 to 10 feet above sea level. ⁴

At Caplow Manzano, we pursue sustainable design that minimizes the consumption of non-renewable natural resources. We also pursue resilient design that maximizes the ability of our built environment to withstand the impacts of climate change and the tests of time. To address both goals, we seek a long design life for our buildings. In fact, we design homes to last at least 100 years. Throughout the world, we find buildings of this age or older support strong communities, enduring cultures, and the accumulation of value. Buildings with shorter design lives are associated with instability, lack of investment, and the waste of materials.

With a changing climate, we are observing rising seas, and scientists expect this trend to accelerate over the next 100 years. When we design a house to be ready for a century of rising seas, the question we face is, how high is high enough?

The four counties of southeast Florida (Broward, Miami-Dade, Monroe, and Palm Beach) work together to interpret the best and latest scientific evidence available, and use it to formulate a set of guidelines for infrastructure development in this region (first published in 2011 and updated in 2015 and 2019). ⁵ As the chart below shows, this work produced a range of estimates for sea level rise in 2120. For reasons that are explained in the report, the “NOAA Intermediate High” prediction model is recommended for use in the design of long term structures that cannot adapt later to changing sea levels. Our homes fall into this category. The model predicts just under eight feet of sea level rise (“92 inches”) by 2120:

These projections start from zero in year 2000 and are referenced to mean sea level at the Key West tide gauge. Based on the 5-year average of mean sea level, approximately 3.9 inches of sea level rise has occurred from 2000 to 2017 (see historic sea level section of guidance document). The projection includes global curves adapted for regional application: the median of the IPCC AR5 RCP 8.5 scenario (Growing Emissions Scenario) as the lowest boundary (solid thin curve), the NOAA Intermediate High curve as the upper boundary for short-term use until 2070 (solid thick line), the NOAA High curve as the upper boundary for medium and long-term use (dash dot curve). The shaded zone between the IPCC AR5 RCP 8.5 median curve and the NOAA Intermediate High is recommended to be generally applied to most projects within a short-term planning horizon. Beyond 2070, the adaptability, interdependencies, and costs of the infrastructure should be weighed to select a projection value between the IPCC Median and the NOAA High curves. The NOAA Extreme curve (dash curve) brackets the published upper range of possible sea level rise under an accelerated ice melt scenario. Emissions reductions could reduce the rate of sea level rise significantly. Chart from Southeast Florida Regional Climate Change Compant, “Unified Sea Level Rise Projection Southeast Florida: 2019 update”, 2020, p. 10.

Eight feet of sea level rise, if that prediction proves accurate, would put large areas of today's Miami underwater in 2120; even greater areas will be right around sea level, and additional areas will be just a foot or two above sea level. Of course, streets will likely be filled in and elevated before that happens, but houses cannot rise so easily. We must plan ahead.

Below is an example of planning ahead from Caplow Manzano. Our home in the Silver Bluff neighborhood sits on land that is about 7 feet above sea level today. The house is called CM1. In 2120, the sea will be above the current backyard at CM1. Perhaps the ground will have been raised with fill, and the water won't be seen, but in the porous limestone bedrock of south Florida, the water will still be there.

If the sea level itself were the only concern, we would only need to build a few feet off the ground, as many of our neighbors do. However, we must not forget about flooding! Storm surge and rain are expected to be just as significant in 2120 as they are today (if not more so), it is logical to assume that the Base Flood Elevation of 2120 will be about the same 8 to 10 feet above sea level that we see today. Indeed, as new FEMA maps are adopted by our area in 2024, many inland areas are slated to receive their first Base Flood Elevations. A typical example is found at another property that Caplow Manzano plans to develop near South Miami. There, at 5332 Sunset Drive, there is no flood elevation today. But later this year, due to changes in the FEMA maps, the Base Flood Elevation will become 8 feet…above sea level.

At CM1, we expect the Base Flood Elevation in 100 years to sit 16 to 18 feet above today's sea level, as seen on the sketch. The living room of this house sits only about 3 feet higher. We think it's just enough. The house might last even longer than 100 years (we hope so), or storms might intensify, or sea levels might rise a little faster than anticipated.

What about coastal areas? Recently revised zoning code in the City of Miami permits houses be built with their first floor up to 9 feet above the Base Flood Elevation (provided the Base Flood Elevation is already defined). In coastal areas (and future coastal areas!) we believe in using as much of that 9 feet as possible. After all, in 100 years, that safety margin may have been swallowed by the sea.

Notes: NAVD88 is the current datum used for elevations in Miami. In 2020, MHHW (the highest normal tidal excursion of the sea, what we call Sea Level above) was 0’4” NAVD88. ⁶ Coincidentally, sea level rise since 2000 in our area is also about four inches. The Unified Sea Level Rise ⁷ projection (2019 edition) indicates a rise of 92 inches from 2000 to 2120, corresponding to a 2120 MHHW prediction of 7’10” NAVD88, which we have rounded to eight feet of sea level rise over the next 100 years for the sake of simplicity.

Ted Caplow
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