DSD-INT 2025 Quantifying Flood Mitigation Strategies Under Sea Level Rise - Hard, Soft, and Hybrid Approaches for San Francisco Bay - Nederhoff

DSD-INT 2025 Quantifying Flood Mitigation Strategies Under Sea Level Rise - Hard, Soft, and Hybrid Approaches for San Francisco Bay - Nederhoff

• 1.
  Kees Nederhoff1, RohinSaleh², Patrick Barnard³, Mark Stacey⁴, Stendert Laan1 ¹Deltares USA, ²Aquaflows, ³ University of California, Santa Cruz, ⁴University of California, Berkeley November 12, 2025 Quantifying Flood Mitigation Strategies Under Sea Level Rise: Hard, Soft, and Hybrid Approaches for San Francisco Bay
• 2.
  Rising Risk: TheVulnerable Estuary Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 2 San Francisco Bay Under Pressure: the Bay Area faces a perfect storm • Sea level projected to rise 0.5-2 meter by 2100 (CA SLR Guidance 2024) • Intensifying atmospheric river events (Wang et al., 2023) • $150 billion in property at risk by 2100 (Barnard et al., 2019) • Critical infrastructure vulnerability (airports, highways) • Environmental justice concerns in low-lying communities
• 3.
  Research Question Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 3 How doshoreline hardening, wetland restoration, and floodgates affect flood hazards under sea level rise?? Shoreline Hardening Traditional "gray" infrastructure protecting urban edges with levees, seawalls, and riprap Wetland Restoration Nature-based solutions restoring tidal marshes to absorb wave energy and store water Strategic Floodgates Engineered barriers that close during extreme events to prevent inland flooding
• 4.
  Modeling Framework: Delft3DBay-Delta Community Model Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 4 • Use the open-source & open-access Delft3D San Francisco Bay Community Model platform but add a model focused on extreme water levels and SLR • Delft3D modelling of San Francisco Bay-Delta goes back to 2006 as part of USGS science collaboration that goes back to ~2000. • Delft3D is open-source and trusted by 38,000+ members worldwide from academia to consultancy • CASCaDE: Computational Assessments of Scenarios of Change for the Delta Ecosystem − Impact of climate change scenarios on San Francisco Bay-Delta eco-system (water quality, eco-systems, etc) − Funding: 8 MUSD funded by CalFed and USGS (2011- 2016)
• 5.
  Community modeling website:2014-onwards Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 5 • Community model: http://www.d3d-baydelta.org/ • The community model concept creates a standard model setup for everyone in which models and software are available free of charge. Research Consultancy Networking
• 6.
  Next generation ofthe Community Model: SFBD-SWL Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 6 • Open-source Delft3D and open-access model setup. Available via www.d3d- baydelta.org • Include relevant engineering-scale features included and scientifically peer-reviewed paper in Nederhoff et al. (2021). Used operationally for forecasting of high-water levels by NOAA and USGS as part of the AQPI project • Model domain covers SF Bay and Delta and can thus be used for regional studies • 1D-2D hydrodynamic model with subgrid features for efficient and accurate model simulations (RMSE < 10 cm) • Used for reanalysis by Deltares USA and Alameda FCD to perform 70-years of continuous simulation for a robust estimate of extreme values and return periods statistics
• 7.
  What is availableon the Community Website? www.d3d-baydelta.org Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 7 1. Download the Model: grid, boundary conditions, and wind forcing for WY1941-2024. 2. Download processed model results: processed model results for 1244 points across the SF Bay for WY1941 - WY2024 for those wishing to access the data without running the SWL Model (for using data without running model) 3. Download extreme, still, and total water level data: includes shapefiles and analysis results for extreme, still, and total water level across the SF Bay. Third parties can run, amend, copy, and distribute this SFBD-SWL and other SFBD Community Models under a Creative Commons Attribution-Share Alike 4.0 International License
• 8.
  Tidal Dynamics Todayand Sea Level Rise Effects Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 8 • M2 tide grows deeper inside estuary → this tidal amplification drives extremes • With SLR → tidal amplification decreases → slower-than-expected rise in extremes (damping)
• 9.
  With SLR, 100-yearextremes in the Bay increases less than proportional relative to SLR (1/3) Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 9 • Extreme vary in SF Bay and are typically higher in South and North Bay than near SF
• 10.
  With SLR, 100-yearextremes in the Bay increases less than proportional relative to SLR (2/3) Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 10 • Extreme vary in SF Bay and are typically higher in South and North Bay than near SF • With SLR, daily, annual and 100-year water levels are increasing.
• 11.
  With SLR, 100-yearextremes in the Bay increases less than proportional relative to SLR (3/3) Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 11 • Extreme vary in SF Bay and are typically higher in South and North Bay than near SF • With SLR, daily, annual and 100-year water levels are increasing. • However, in contrast to other work, increase is less than SLR due to reduction tidal amplification
• 12.
  Extreme Water Levels changesin 10-yr water levels as function of SLR Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 12 - Absolute water levels rise with SLR, but relative increases are damped due to tidal dynamics - Regional variability: South Bay & Suisun Bay most affected (relative reduction >1 ft)
• 13.
  Adaptation Scenarios Tested Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 13 1.Wetland Restoration • 740 potential restoration sites identified • Based on Baylands Ecosystem Habitat Goals 3. Hybrid Approach • Combination of strategic hardening with restoration 2. Shoreline Hardening • Levees positioned at the urban edge • Represents traditional engineering approach • Modeled as non-erodible barriers 4. Strategic Floodgates • Four locations tested: Mill Valley, Dumbarton, Napa, San Leandro Bay • Modeled as closable barriers during extreme events • Various operation scenarios for closures
• 14.
  How do theextremes change? Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 14 Hummel and Stacy (2021) found negative effects of ~ 20inch (50 cm) => this work (Nederhoff et al., 2025) suggests <20 cm (or <8 inches) 4.9 ft of SLR wl [ft NAVD88]
• 15.
  Extreme water levelsduring this event reach ~9.5 ft+NAVD88. Subregional barrier can reduce water levels when, for example, closing at 8 ft + NAVD88 Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 15
• 16.
  The reduction ofthe extreme water levels depend on when the subregional barrier is closed. The lower the threshold the lower the extreme water level Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 16
• 17.
  Subregional barrier workswell for 3 ft SLR (1m) & reduces extremes in San Leandro Bay from ~12.5 to ~9.5 ft when closing at 9 ft with limited negative effects Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 17
• 18.
  Combination of resultsfor closure at 8.5ft Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 18 SLR 0.0 ft 0 cm 0.5 ft 15 cm 1.0 ft 30 cm 1.5 ft 45 cm 2.0 ft 60 cm 2.5 ft 75 cm 3.0 ft 90 cm How often [#] 1.17 6.61 31.16 85.94 175.69 268.05 319.56 How long per year [hr] 1.7 9.3 49.3 163.1 410.7 827.3 1406.0 Average closure [hr] 1.43 1.41 1.58 1.90 2.34 3.09 4.40
• 19.
  Key Takeaways Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 19 1. Floodextremes rise slower than sea level • Due to natural tidal damping, we have slightly more time to adapt than previously thought. However, this doesn't eliminate the urgent need for action. 2. Wetland restoration offers strongest benefits • Nature-based solutions reduce flood levels while providing ecological, carbon, and recreational benefits. Priority should be given to South Bay and Suisun Bay restoration. 3. Avoid shoreline hardening in isolation • Traditional levees and seawalls exacerbate regional flooding and should only be implemented in conjunction with wetland buffers whenever possible. 4. No single strategy is sufficient • Beyond 1m or 3ft SLR, local measures become inadequate. Regional strategies and managed retreat from highest-risk areas will be necessary components of long-term adaptation.
• 20.
  Thank You &Stay Connected Quantifying Flood Mitigation Strategies for SLR in San Francisco Bay 20 Kees Nederhoff, PhD Coastal Scientist, Deltares USA Email: Kees.Nederhoff@deltares-usa.us Website: www.d3d-baydelta.org • Nederhoff et al. (2025). Mitigating Flood Risks in Urban Estuaries: Tidal Dynamics, Shoreline Hardening, Nature-Based Solutions, and Floodgates in San Francisco Bay • Journal of Waterway, Port, Coastal, and Ocean