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The USGS Coastal Storm Modeling System (CoSMoS) team has extensively studied overland flooding and coastal change due to rising seas and storms. Interactions with coastal stakeholders have elucidated another important question; will rising seas also intrude into coastal aquifers and raise groundwater tables? The CoSMoS-Groundwater (CoSMoS-GW) modeling effort seeks to provide initial insight into this question for the entire California coastline, as well as San Francisco Bay.

Why CoSMoS-GW

As sea levels rise, the shallow groundwater table in coastal communities will also rise (Befus et al. 2020). This slow but chronic threat can flood communities from below, damaging buried infrastructure, flooding below grade structures, reducing storm sewer capacity, liberating pollutants, compromising foundations, and emerging above ground as an urban flood hazard that can amplify overland storm flooding. As communities develop climate adaptation plans to address sea level rise and extreme storm events, it is important to consider this additional hazard.

An initial study by Hoover et al. 2017 found that groundwater shoaling (or the rising of groundwater toward the ground surface) due to sea-level rise could result in significant hazards in three coastal California sites with shallow well data, but risk assessment for other areas was limited by a lack of data.

Through continued discussions with coastal California stakeholders, it became clear that more detailed information and modeling for this hazard was imminently needed. In response, the USGS Coastal Climate Impacts project team partnered with Dr. Kevin Befus (U. Arkansas) to model the impacts of rising seas on shallow coastal groundwater. Building on the initial work of the Coastal Storm Modeling System (CoSMoS), the team developed the model-driven, observation-validated CoSMoS-Groundwater (CoSMoS-GW) tool to predict representative coastal groundwater conditions today and into the future.

Modeling Details

CoSMoS-GW mapping is based on steady-state groundwater flow modeled in three dimensions using the USGS MODFLOW program. Water tables for present-day to +5 meters are simulated separately over 12 sea level rise increments, allowing water tables to equilibrate to each sea-level scenario based on two tidal datums, local mean sea level (LMSL) and mean higher high waters (MHHW). These datums bracket the range of likely elevations where groundwater would discharge at the coast. Interannual, seasonal, and daily fluctuations are not considered, nor are any human activities (e.g., pumping, drains, augmentation).

A series of overlapping groundwater flow models, ten within the Bay Area alone, provide high-resolution (10 m by 10 m) predictions that are merged for continuous groundwater results. Model inputs include very high resolution (2m by 2m) digital land surface models (DEMs) derived primarily from LiDAR topography-bathymetry data, gridded effective average groundwater recharge rates from 2000-2013, ocean salinity, and tidal datum elevations for setting present-day and higher sea levels. Unknown 3D hydrogeology is represented in the models by varying the hydraulic conductivity three orders of magnitude (0.1-10 m/day) while setting the lower impermeable boundary as a flat layer at -50 m NAVD88.

Two maps illustrate two states of computer modeling scenarios around a large bay and coastal area.

Examples of CoSMoS-GW mapping products that show (left) modeled current groundwater tables with red depicting sites of emergent groundwater and (right) groundwater tables with 6 feet of sea-level rise. Blue indicates coastal-driven flooding and inundation. Both figures are for a horizontal hydraulic conductivity of 1.0 m/d and LMSL tidal datum.

Modeled present-day water table elevations (i.e., hydraulic heads) are validated against State Water Resources Control Board GeoTracker, USGS, and California Department of Water Resources well observations, totaling approximately 3000 locations.

The modeled steady state (i.e., equilibrium) groundwater surface represents the long-term average elevation of the groundwater table that would occur for groundwater discharging along the coast at the tidal datum used (LMSL or MHHW).  The resulting water table can be viewed as a baseline that then would be overprinted by seasonal, tidal, and other transient signals such as storms. Areas of emergent groundwater (when the groundwater table rises to or above the surface of the ground and create surface flooding) are likely to experience chronic ‘sunny day’ surface flooding (i.e., surface flooding in the absence of heavy precipitation) and compound flooding from surface runoff during storms.

A group of people sit together around a table with laptops and notepads.

Workshop attendees discuss the emerging phenomenon of a rising groundwater table due to sea-level rise. They addressed the challenges currently faced, and anticipated future challenges with rising water tables.

Photo credit: USGS

Stakeholder Engagement

The development of this model was informed by a workshop with a broad group of technical experts who have studied this hazard in other coastal settings (see technical advisory team members below). Initial findings from the research were discussed with key coastal and San Francisco Bay partners during a workshop in November 2019. We continue to work closely with partners across California to share this science and make it usable and accessible to coastal managers for their coastal planning needs. 

The results from CoSMoS-GW are publicly available on the USGS ScienceBase data catalog and on HydroShare.  The results will also be presented on the Our Coast Our Future web tool, managed by Point Blue Conservation Science, and associated socioeconomic analysis will be presented on the USGS Hazards Exposure Reporting and Analytics web tool.

CoSMoS-GW Technical Advisory Experts

  • Tiffany Anderson, University of Hawaii, Manoa
  • Chip Fletcher, University of Hawaii, Manoa
  • Shellie Habel, University of Hawaii, Manoa
  • Andy Fisher, University of California, Santa Cruz
  • Graham Fogg, University of California, Davis
  • Kristina Hill, University of California, Berkeley
  • Michelle Hummel, University of Texas at Arlington
  • John Masterson, USGS
  • Ferdinand Oberle, USGS
  • Chris Smith, USGS
  • Kris May, Slivestrum
  • Peter Swarzenski, International Atomic Energy Agency