Technical Bulletins on Climate Adaptation in Civil and Environmental Engineering
ASCE Committee on Adaptation to a Changing Climate
Climate Adaptation Engineering Bulletin No. 5
Rising Groundwater Levels due to Sea Level Rise and Its Effects
Sea level rise (SLR) will have a significant impact on coastal groundwater levels. Civil and environmental engineers will need to consider rising groundwater levels in a variety of different design applications.
Climate Adaptation Engineering Bulletin No.1 (Using Federal Sea Level Rise Projections in Engineering Practice) addresses projected increases in mean sea level through the end of this century. Robust technical resources developed by the U.S. Interagency Task Force for Sea Level Rise are available to practicing engineers providing ranges of SLR projections in the vicinity of each local data point (Sea Level Rise Viewer). The historic and projected SLR levels include allowances both for rising sea levels and observed land subsidence, both of which vary by coastal region. Net sea level rise projections to the end of the century vary from 2 feet to more than 6 feet depending upon the specific location and climate scenario used to inform the projection.
Near-Coast Groundwater Levels Will Rise with SLR – How Much Will Vary
Changes in groundwater levels along the coast reflect a balance between recharge (the flux of surface water to the water table reflecting evaporation, transpiration, and infiltration), withdrawals for human use and sea level rise. In coastal settings where seawater is in hydraulic communication with the aquifer (i.e., typically in the unconfined surficial aquifer), the difference in density between fresh groundwater and seawater results in the development of a saltwater wedge beneath the discharging groundwater. In this instance, groundwater surface elevations will rise with SLR, though that rise may be non-linear due to damping through surface water discharge. Groundwater regions and factors affecting groundwater levels are described in detail in the USGS publication Ground-water Regions of the United States.
The magnitude of the groundwater rise and the extent to which increases extend inland depend on highly localized, variable conditions (Coastal Effects: Chapter 9, The 5th National Climate Assessment). Where there is substantial topographic relief, the groundwater rise may not significantly affect infrastructure design. However, in relatively flat coastal plains, the groundwater rise can have a significant effect. A modeling effort of a New England coastal drainage basin (Modeling Groundwater Rise Caused by Sea-Level Rise in Coastal New Hampshire | University of New Hampshire) based on a USGS assessment (Assessment of Ground-Water Resources in the Seacoast Region of New Hampshire ) concluded that “in much of the areas within 1.25 miles of the coast, SLR-induced groundwater rise will be at least half of the SLR”. Other literature sources have indicated an inland range of 1-10 miles of affected groundwater levels. Projections of coastal groundwater hazards for California are presented on the Our Coast Our Future web viewer (www.ourcoastourfuture.org), and for the Pacific Northwest and Southeast U.S. coasts on the USGS Hazards Exposure Reporting and Analytics web tool.
In Some Coastal Areas, Groundwater is below Mean Sea Level
Saltwater intrusion due to over-pumping of groundwater is a significant problem in many areas of the US resulting in groundwater levels that are below mean sea level in about 15 percent of the U.S. coastline,. Saltwater intrusion will be exacerbated by sea level rise, Saltwater intrusion can also occur due to an advancing tongue of saltwater beneath outward flowing fresh water, far upstream in some rivers. High sodium and/or chloride concentrations can render water supplies unusable both for agricultural and potable uses. USGS provides guidance on saltwater intrusion in coastal areas (Saltwater Intrusion | U.S. Geological Survey).
Projecting the inland extent of groundwater rise due to SLR is complicated by many factors related to geology, topography and development. Definitive projections will often not be simple due to the complex hydrogeology of many coastal settings and the lack of comprehensive geophysical data and well logs to fully understand these systems. Still, rising groundwater from SLR needs to be considered by engineers in design.
Effects of Groundwater Rise due to SLR on Civil Engineering Practice
How might SLR-induced groundwater rise affect engineering practice? There are many ways, affecting many different landforms and uses including urban and rural, peninsulas and islands, and inland saltwater waterways.
Roadway Bases and Subbases – The presence of groundwater in roadway subbase materials has a deleterious effect on roadway pavement performance (Geotechnical Aspects of Pavements). Groundwater in proximity to the pavement surface can result in reduced strength in the base and subbase soils, leading to rutting, cracking and pumping, reducing pavement performance and shortening service life. In cold climates, freeze/thaw effects on pavement increase with higher groundwater. Groundwater rise needs to be considered in coastal areas when designing roadway improvements and maintenance and repair requirements need to be monitored to inform when improvements may be advisable.
Contaminated Sites – Many landfills and industrial and commercial waste sites, including Superfund and State-regulated sites, have been remediated by installing impermeable capping materials, leaving waste materials and contaminated soils in vadose zones (unsaturated soil above the water table) which may not have been subject to leaching in decades. Rising groundwater levels associated with SLR may create the potential for release of contaminants (Rising Coastal Groundwater as a Result of Sea‐Level Rise Will Influence Contaminated Coastal Sites and Underground Infrastructure). Whether such increases are significant with respect to offsite contamination and the impact on sensitive receptors is highly site specific, but water quality monitoring and site investigations may be warranted in areas where the water table has risen or is projected to rise significantly into contaminated soils or waste materials.
Water Supplies – Public and private water supplies in near coastal areas may need to monitor chloride concentrations to detect saltwater intrusion.
Utilities – The groundwater in very close proximity to the coast will be salty. This can have a negative effect on pipe materials and components of buried utilities that are susceptible to corrosion. Sewer infiltration rates will increase in most systems.
Building Foundations – Buildings in very close proximity to the tidewater interface may be impacted by saltwater in rising groundwater, affecting the corrosion resistance of foundation materials. Rising groundwater levels may affect structural loads, risk of liquefaction, and, in cold climates, freeze-thaw stresses.
Stormwater management – In coastal areas, rising water tables associated with SLR can exacerbate stormwater flooding, especially during high tide. Changes in groundwater table elevations associated with future SLR in the coming decades and their effects on storage should be considered in stormwater design.
Sump Pump Flow and Frequency – In near coastal areas, groundwater rise may result in the frequency of sump pump operation to increase significantly.
Septic Systems – The functionality of wastewater leach fields is highly dependent upon lower groundwater elevations. Rising groundwater due to SLR may result in system failure depending upon topography and depth to groundwater in the future.
Site Development Engineering – Groundwater rise in close proximity to tidewater can affect roadway design, the functioning of on-site wastewater disposal systems, and land use potential in areas that were historically dry land but may transition to freshwater wetlands in the future. Such considerations may not be reflected in local or State regulations.
Engineers engaged in the design of projects within several miles of coastal areas need to be cognizant of the long-term effects of rising groundwater in conjunction with overland flooding.
Engineering Bulletins on Climate Change in Civil and Environmental Engineering are prepared by the American Society of Civil Engineers Committee on Adaptation to Climate Change (CACC), Dan Walker, Ph.D., and Craig Musselman, P.E., editors who are responsible for its content. Technical input was provided by William Kelly, Ph.D., Patrick Barnard, Ph.D., Jayne Knott, Ph.D. and Daniel Urish, Ph.D. Manuscripts are reviewed by assigned members of the CACC Committee on Climate Intelligence in Standards and Practice. NOTE: These Engineering Bulletins are intended to alert the reader to emerging topics related to climate change and civil engineering practice and are NOT intended to act as a substitute for any sources cited herein.