California Climate Action Team Sea-Level Rise Guidance Document – for use by Caltrans and other State Agencies

The State of California Sea-Level Rise Guidance Document was developed by the Coastal and Ocean Working Group of the California Climate Action Team (CO-CAT) to provide guidance for incorporating sea-level rise (SLR) projections into planning and decision-making in California. The guidance provides ranges of SLR for all California agencies, including the Department of Transportation (CalTrans), to be used for risk-based scenario planning and assessments, such as long term transportation planning and vulnerability assessments.

California Executive Order S-13-08 required all state agencies planning construction projects in areas vulnerable to future SLR to "consider a range of sea level rise scenarios for the years 2050 and 2100 in order to assess project vulnerability and, to the extent feasible, reduce expected risks and increase resiliency to sea level rise." The State of California Sea-Level Rise Guidance for project planning and decision making was developed in 2010 to support this directive. It was updated in 2013 to incorporate the findings of the National Academy of Sciences (NAS) report on Sea-Level Rise for the Coasts of California, Oregon, and Washington, released in June 2012. CalTrans developed its own Guidance on Incorporating Sea Level Rise in 2011, based on the 2010 State of California Interim Guidance Document. 

The updated guidance is based around eight key recommendations: First, the guidance recommends using the SLR projections presented in the June 2012 NAS report as a starting place to select SLR values, incorporating specific considerations of risk tolerance and adaptive capacity based on the agency and context. The guidance offers separate projections for two regions of California - north and south of Cape Mendocino. The regional differences are due to geologic forces that can cause sudden vertical land movements in the northern part of the state. Using 2000 as a baseline, SLR ranges are projected over three time periods:

2000-2030:

  • North of Cape Mendocino:  -4 to 23 cm (-0.13 to 0.75 ft)
  • South of Cape Mendocino: 4 to 30 cm (0.13 to 0.98 ft)

2000-2050:

  • North of Cape Mendocino:  -3 to 48 cm (-0.1 to 1.57 ft)
  • South of Cape Mendocino: 12 to 61 cm (0.39 to 2.0 ft) 

2000-2100:

  • North of Cape Mendocino:  10 to 143 cm (0.3 to 4.69 ft) 
  • South of Cape Mendocino: 42 to 167 cm (1.38 to 5.48 ft)

The 2030 and 2050 projections are similar to but have a wider range than those presented in the 2010 Interim Guidance Document, while the 2100 projections here are lower than those in the Interim Guidance Document because of different modeling approaches and regional considerations.

Second, the guidance recommends that agencies consider project timeframes, risk tolerance, and adaptive capacity in selecting SLR estimates. Uncertainty regarding SLR estimates rises considerably after 2050, making these considerations even more important for selecting the appropriate SLR estimate for projects with long timeframes.

Third, the guidance suggests that state agencies consider storms and extreme weather events in their analyses. Tides, storm surges, and El Niño‐Southern Oscillation are all relevant considerations for infrastructure planning decisions. It recommends that the agencies use future sea level as a starting point, but also combine those projections with extreme oceanographic conditions to design projects that can withstand the impacts from extreme events.

Fourth, the guidance recommends that state agencies coordinate with each other in selecting SLR estimates, seeking consistency and efficiency whenever feasible and appropriate. 

Fifth, the guidance warns that SLR projections should not be based on linear extrapolation of past sea level patterns. Because of non‐linear increases in global temperature and the unpredictability of a complex natural system, linear extrapolation is inadequate and likely to underestimate SLR for projections beyond one or two decades. Instead, agencies should use the NAS projections in their planning.

Sixth, it recommends that agencies consider changing shorelines, as California’s coastline is dynamic and will evolve due to SLR and erosion. Areas of high sediment supply, for instance, could undergo less coastal change due to SLR, whereas the coast may recede more rapidly in low-sediment areas.  For reference, it points to a U.S. Geological Survey report on shoreline changes for California’s beach habitat and report on shoreline changes for California’s bluff habitat

Seventh, the guidance recommends that agencies also consider tectonic activity predictions. As highlighted in the 2012 NAS report, the region north of Cape Mendocino is at significant risk of a large earthquake, which could cause significant land subsidence and instantaneous relative SLR. More specific recommendations concerning changes in SLR due to tectonic activity are beyond the scope of this guidance.

Finally, the guidance recommends that agencies consider trends in relative local mean sea level in making predictions of future sea levels at specific locations. As coastal land elevation in California changes due to tectonic activity and subsidence, sea level relative to the elevation of the land will also change, affecting projected SLR impacts. The guidance suggests that agencies use relative sea level trend data from the National Oceanic and Atmospheric Administration (NOAA) to incorporate relative sea level into their analyses.   

 

This Adaptation Clearinghouse entry was prepared with support  from the Federal Highway Administration. This entry was last updated on March 25, 2014.

 

Publication Date: March 2013

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