A Methodology for Assessing the Impact of Sea Level Rise on Representative Military Installations in the Southwestern United States

Dr. Bart Chadwick | SPAWAR Systems Center

RC-1703

Objective

The objective of this project was to develop an analysis framework and methodologies for evaluation of coastal military installation vulnerabilities and test them under prescribed scenarios of increased local mean sea level (0.5 m, 1.0 m, 1.5 m, and 2.0 m) over the next century. Methodologies were developed to assess the potential scope and magnitude of impacts from physical effects of flooding (wetting that occurs infrequently), inundation (wetting that occurs regularly), erosion, seawater intrusion, and alteration of tidal flows. Assessment methodologies targeted potential vulnerabilities of buildings, civil infrastructure, training areas, and waterfront and coastal structures. The project focused on conditions in the southwestern United States and utilized the key coastal military installations at Naval Base Coronado (NBC) and Marine Corps Base Camp Pendleton (MCBCP) to test the approach.

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Technical Approach

The technical approach was organized around five tasks. The first task focused on developing a generalized sea level rise (SLR) vulnerability assessment framework for application to coastal military installations. The second task encompassed developing methods to project future trends in sea level and sea level variability, and then combining these underlying sea level characteristics into realistic assessment scenarios for a range of regional sea level conditions. In the third task, methods were developed to compile, analyze, and integrate critical biogeophysical and infrastructure data for each installation within a three‐dimensional Geographic Information System (GIS) modeling environment. Using the range of scenarios developed under task two as test cases, task four focused on developing methods to characterize the expected physical effects of SLR within the Southwest region. These results were then incorporated into the GIS modeling framework. Finally, the framework and tools were then used in task five to explore the application of these methods to assess SLR vulnerability at the two installations.

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Results

The assessment framework adopted a source‐pathway‐receptor conceptual model in which a source is a sea level related hazard, a pathway is the process that links a sea level related hazard and a military installation element that is subject to harm from that hazard, and a receptor is a military installation element or class of elements that is subject to harm from a sea level related hazard. The framework reflects the evolution of the field from strategies to support broad‐scale, qualitative screening assessments, toward application at regional and local scales. This enables more quantitative assessment of specific vulnerability questions at Department of Defense installations, evaluation of a range of plausible future scenarios, and identification of potential responses at the source, pathway, and receptor level.

Sea level rise projection methods were successfully developed based on a superposition of mean sea level rise (MSLR) scenarios with increases of 0.5, 1.0, 1.5, and 2.0 m by 2100 relative to 2000, astronomical tide heights, non‐tide residual (NTR) water level variability from general circulation models (GCMs), and wave‐driven runup on beaches. Using these time series, robust regional scenarios were developed for water level extremes at MCBCP and NBC using extreme value methods. Geospatial basemodels of the terrestrial and marine topography were constructed for both of the installations. This included the development of methods to accommodate future conditions by superimposing revised beach or beach/cliff elevation sub‐models into the changed domain of the basemodel using the results of the physical response models. An infrastructure model defining six key receptor categories of training areas, buildings, civil infrastructure, waterfront structures, and coastal structures was integrated with the terrain model such that accurate locations and elevations for the infrastructure could be extracted to evaluate interactions with erosion, inundation, and flooding.

Physical response models were developed to describe exposure pathways including inundation, flooding, erosion, and seawater intrusion. Primary pathways for this study were classified by exposure under categories for exposed shorelines, protected shorelines, and groundwater. New modeling systems were developed that enabled the long‐term topographic response of these beach and cliff/beach systems to SLR to be integrated with short‐term storm wave response changes. Evaluation of inundation and flooding along exposed shorelines incorporated changes to the underlying elevation model due to erosion, spatially varying total water level exposures, and requirements for complete hydraulic connectivity. A density‐dependent groundwater‐flow and solute‐transport model was used to explore the influence of seawater intrusion in the Santa Margarita River Basin at MCBCP and the resulting potential impacts to water quality and future extraction capacity.

Sea level rise vulnerability at NBC and MCBCP was assessed through application of these methodologies using two levels of analysis: receptor‐level and component‐level. The receptor‐level methodology encompassed the breadth of the data compilation, modeling, and analysis methods and included installation‐ and exposure‐specific SLR source scenarios, pathway‐specific physical response of the coastal system, and characteristic sensitivities and operational thresholds for the installation receptors. The analysis illustrated the ability of these methods to resolve the increasing level of vulnerability of the installation to erosion as a function of increasing sea level, as well as the sensitivity of some receptors to short-term wave driven erosion events. At NBC training areas, this translated into frequent (weekly return period) conditions with remaining available area reduced to about 53% of baseline for 1.0 m SLR, and further reductions to a remaining area of about 23% of baseline for 2.0 m SLR. Training areas at MCBCP are generally backed by erodible cliffs, and the landward boundary of the beach training area was allowed to retreat inland (autonomous adjustment) at the rate of retreat of the cliff base. MCBCP also had a higher underlying sand imbalance, and together these factors resulted in frequent (weekly return period) conditions with remaining area reduced to about 41% of baseline for 1.0 m SLR and further reductions to a remaining area of 27% of baseline for 2.0 m SLR. Component‐level assessment examples also were illustrated for NBC training areas, buildings, waterfront structures, coastal structures, and civil infrastructure receptor classes.

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Benefits

Based on the objective to develop a robust analysis methodology that provides a reliable means to identify and plan for vulnerabilities under both currently projected sea level scenarios, and scenarios that may be considered in the future, this project achieved a number of key accomplishments. New methodologies for the development of Southwest United States‐relevant SLR scenarios and cyclical events were successfully developed, as well as a capability to project these at 100 m increments along the shoreline. These scenarios were successfully applied to the development and application of a range of “beyond the bathtub” pathway response models that link these sea level scenarios to potential vulnerabilities to coastal military installations. Based on the projection of these physical responses, this project was able to illustrate their application to an assessment of the responses of two key military installations, with an emphasis on military-specific receptors including beach training areas and waterfront infrastructure, and to contrast the results. As part of this research and development effort, a number of products were developed that served to advance the research and provide a testing ground for the methodologies. In addition, these products may serve future uses, particularly for the installations where the analysis was conducted, but also potentially as models for application to other areas with similar requirements and conditions.

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Points of Contact

Principal Investigator

Dr. Bart Chadwick

SPAWAR Systems Center

Phone: 619-553-5333

Fax: 619-553-3097

Program Manager

Resource Conservation and Resiliency

SERDP and ESTCP

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