These maps use the “4.9 ft (2100 Intermediate-High) SLR Scenario (BCDC 2024)” layer from Todd Hallenbeck’s publicly available Flood Hazards dataset on the BCDC website. I chose this scenario because it sits between the lowest and highest projections in the California Sea Level Rise Guidance, and planning toward the upper end of the range gives jurisdictions the best chance of preparing for realistic future conditions. The Regional Shoreline Adaptation Plan (RSAP) documentation has been especially helpful for understanding the modeling behind these projections.
The hazard categories in this layer come from combining two data sources:
• ART Bay Shoreline Flood Explorer (tidal inundation and storm surge/extreme tide flooding)
• USGS CoSMoS shallow groundwater rise maps (shallow groundwater, very shallow groundwater, and emergent groundwater)
Todd and Katie’s GitHub page lays out the combined hazard logic clearly, and I learned a lot from reviewing it.
Below is a plain-language description of the eleven hazard categories included in the 4.9 ft (2100 Intermediate-High)layer:
- Emergent Groundwater
Sea level rise causes the groundwater table to rise to the surface, producing flooding “from below,” even far from the shoreline. - Moderately Shallow Groundwater (6–9 ft)
Groundwater rises to within 6–9 ft of the ground surface, affecting basements, buried utilities, pipes, and contaminated soils. - Shallow Groundwater (3–6 ft)
Groundwater rises to within 3–6 ft of the surface, increasing flooding vulnerability, reducing soil drainage, and potentially mobilizing pollutants. - Tidal Inundation + Emergent Groundwater
Both chronic tidal flooding and groundwater emerging at the surface occur in the same location. This is a compounded hazard from above and below. - Tidal Inundation with Storm Surge
Flooding from sea level rise combined with a 100-year storm surge event (approximately 3.5 ft of added water). - Tidal Inundation with Storm Surge + Emergent Groundwater
A severe combined hazard where both storm-driven flooding and emergent groundwater occur. - Tidal Inundation with Storm Surge + Moderately Shallow Groundwater (6–9 ft)
Storm surge + sea level rise flooding, with groundwater rising to within 6–9 ft of the surface. - Tidal Inundation with Storm Surge + Shallow Groundwater (3–6 ft)
Storm surge + sea level rise flooding, with groundwater within 3–6 ft of the surface. - Tidal Inundation with Storm Surge + Very Shallow Groundwater (0–3 ft)
Storm surge + sea level rise flooding, with groundwater within 0–3 ft of the surface — one of the highest-risk combined hazards short of full emergence. - Very Shallow Groundwater (0–3 ft)
The groundwater table rises to within 0–3 ft of the surface, saturating soils and increasing contamination and infrastructure vulnerability. - Existing Bay
Current permanent open-water areas of San Francisco Bay, included for reference.
For reference, here is a portion of the RSAP Coastal Flood Hazards atlas that is easiest to explore interactively:
Coastal Flood Hazards – RSAP Atlas