Engineering Design Considerations for Sea Level Rise
Engineering Design Considerations for Sea Level Rise
By Azure Dee Sleicher, PE, Vice President Coastal Engineering
Sea level rise is occurring based on tide level recording stations around the United States. National Oceanic and Atmospheric Administration (NOAA) data from Station 8461490 in New London, CT shows the relative mean sea level in Long Island Sound to have increased approximately 2.6 mm/year based on data from 1938 to 2017. While this data can tell us what the historic trends have been, it is more difficult to forecast into the future to determine exactly how much sea level will change, by when, and what the consequences will be.
Many people ask themselves what are the impacts of sea level rise, whether they are required to or should include sea level rise (SLR) into the design of their project, and if so, how much? RACE takes a pragmatic approach to review and incorporate recommendations to account for SLR on a case by case basis depending on the nature of the project, type and function of the structure and overall risk tolerance of the owner.
SLR will exacerbate the risks already associated with living in coastal regions including increases to coastal flooding and erosion. Increased risk for land subsidence and salt-water intrusion into groundwater aquifer systems are also threats. As water levels rise, beach areas that are currently high and dry or perhaps only wet during high tides or storms will be submerged more frequently or at some point, entirely, which not only impacts recreational beach goers, but also critical habitat and the other functions of the coastal zone. Waterfront structures such as seawalls, piers or even non-elevated buildings will become closer to the water or eventually submerged.
The impacts of SLR include more than just higher water levels. As sea levels rise, nearshore areas will become deeper which allows larger wave heights to develop and break closer to shore, impacting structures with larger, more destructive hydrodynamic forces and causing increased erosion.
While many federal, state and local agencies support and advocate to anticipate, plan for, and adapt to risks of sea level rise, there are few mandated requirements to do so. In the case of federal requirements, Executive Order (EO) 13690 issued by President Obama in 2015 established more stringent flood risk management standards for federal projects but was later revoked in 2017 by President Trump under EO 13807.
The State of Connecticut passed Public Act No. 18-82 in May of 2018 that requires the state to consider sea level rise in municipal evacuation plans and state funded and initiated projects, but does not affect local zoning and private development.
The State of New York issued the Community Risk and Resiliency Act (CRRA) in 2014 which incorporated a new section of the Environmental Conservation Law which requires applicants for permits or funding in a number of specified state programs to consider and incorporate SLR. To date, we have seen this be applied to state-funded projects but the CRRA also requires DOS to develop model local laws that include provisions for SLR which will eventually trickle down to the municipalities.
Most building codes do not have a requirement to include projections for SLR into the design. Future SLR is not considered by FEMA in the determination of flood zones and base flood elevations although newer codes such as the 2018 version of Connecticut State Building Code, through reference to ASCE 24-14, have increased the elevation requirements above the Base Flood Elevation (BFE) (although not necessarily related to SLR).
Certainly, the design of critical infrastructure such as levees, bridges, energy and transportation projects should consider the impacts of SLR. These projects are costly, typically have a long design life consistent with the projections for SLR, contribute to the overall sustainability of the community and have life-threatening implications if they were to fail.
While not required in most cases, incorporating the risks of future sea level rise is a good idea for some projects such as shoreline protection (e.g., seawalls, bulkheads, and revetments). These structures typically are intended to have a long design life and with the primary function to protect the upland and other structures from the impacts of flood and erosion, design accommodations for SLR are prudent.
On the contrary, it may not be practical to design structures such as boat ramps, marinas and fishing piers for future SLR. The function of these structures is highly dependent on proximity and access to the water. For instance, incorporating additional freeboard on a fishing pier to accommodate several feet of sea level rise 50 years from now may render the structure impractical for fishing today. However, as mentioned above, the impacts of SLR include more than just increased water elevations. It is possible to design such structures to withstand the additional hydrodynamic forces associated with SLR today and include into the design considerations that will allow for future adaptations to account for increased freeboard required for SLR tomorrow.
There are many resources for sea level change predictions including The Nature Conservancy Coastal Resilience Mapping Portal, USACE, NOAA, and the CT Coastal Hazards Mapping Tool. Each of these sources typically report a range of sea level rise values for the same planning horizon (e.g., CT DEEP provides multiple sea level rise predictions for 2100; from 2.0 ft to 6.6 ft). This range in sea level change predictions is due to uncertainties in how the future may unfold. Generally, the low-end predictions assume a decrease in future emissions of greenhouse gasses and aerosols while the higher end predictions assume these emissions remain the same or increase.
USACE publication ER 1100-2-8162 (December 31, 2013) suggests the use of three scenarios, at a minimum, to estimate future sea levels. The “USACE low” curve is based on the extrapolation of the historical tide gauge rate of sea level change. Intergovernmental Panel on Climate Change (IPCC) projections for acceleration of global mean sea level and modified NRC projections with the local rate of vertical land movement added. The rate for the "USACE High Curve" is computed from the modified NRC Curve III also considering IPCC acceleration projections with the local rate of vertical land movement added.
The USACE curves are based on the nearest NOAA tide gauge. However, the USACE predictions deviate from the NOAA predictions beyond the “low” case. NOAA predictions are higher than the USACE due to assumptions regarding ocean warming and ice sheet loss.
The Connecticut Institute for Resiliency and Climate Adaption (CIRCA) developed recommendations for sea level rise in coastal Connecticut in the Sea Level Rise in Connecticut report prepared by James O’Donnell. Similar to NOAA and USACE, O’Donnell presents several sea level rise scenarios including a low, intermediate low, intermediate high and a high projection for sea level rise. The low projection is based off of tide gage observations, the intermediate low is based off of the IPCC RCP 4.5 model simulations near Long Island Sound, the intermediate high projection is based off of a semi-empirical model prediction, and the high projections are based off of ice mass balance projections. The report recommends consideration of 20 inches of SLR by 2050 in CT.
Concerned about the impacts of SLR and what you can do to protect your assets? RACE coastal engineers have a solid understanding of the ramifications of SLR and can provide viable solutions to reduce risks. Call us for a consultation and assessment today.