Science Serving Maryland's Coasts

R/HAZ-1a

Long-term Impacts of Different Techniques for Shoreline Stabilization in the Maryland Chesapeake Bay

Principal Investigator: 

Lawrence Sanford

Start/End Year: 

2012 to 2014

Institution: 

Horn Point Laboratory, University of Maryland Center for Environmental Science

Co-Principal investigator: 

Evamaria W. Koch, Cindy M. Palinkas, Court Stevenson, Horn Point Laboratory, University of Maryland Center for Environmental Science; Jeffrey Halka, Maryland Geological Survey

Topic(s): 

Strategic focus area: 

Viable coastal communities and economies

Description: 

OBJECTIVES: The primary purpose of the research proposed here is a comparative synthesis of the physical and ecological effects of different shoreline stabilization techniques in the Maryland Chesapeake Bay. By comparing similar metrics, both physical and ecological, across a wide variety of shoreline stabilization sites, we will identify differences or similarities that have not been possible to discern previously. We will also investigate differences in physical forcing that might explain variability within and between different stabilization techniques. We will include unprotected eroding shorelines as controls, representing the effects of no action. We will work with Maryland state planners and regulators to make sure that our results are useful for them, and will make them available to the interested general public as well. This research directly addresses the Sea Grant RFP focus area "Develop an understanding of immediate and long-term effects of climate change and its associated hazards to coastal communities."

METHODOLOGY: We will select 17 study sites in consultation with coastal managers at Maryland DNR and MDE, and building on our previous or ongoing studies. These will encompass a variety of different shoreline protection measures, structural as well as "living," and include baseline sites with no stabilization. We will fill in data gaps with selected field work and re-analysis of previously collected data. Methods will include shoreline and bathymetric surveys for comparison to pre-installation information, comparison of pre- and post-construction SAV coverage, surveys of SAV and marshes adjacent to stabilized and natural shorelines, and collection of cores to determine changes in sediment texture, organic content and sedimentation rate. We will also assemble available estimates of wave and tides near each site to construct wave-sea level climatologies and will explore their use in a semi-empirical model of erosion potential. Finally, we will construct an integrative index of shoreline protection for management use.

RATIONALE: Currently 69% of Maryland's shoreline is eroding and 12% is hardened with increasing rates of hardening occurring as development progresses. Shoreline erosion rates are likely to increase and community needs for shoreline protection to become more important as rates of sea level rise increase with climate change, constituting a serious coastal hazard. However, the effects of different shoreline stabilization structures on erosion and nearshore water quality and habitat are complex, making appropriate management decisions difficult. A variety of stabilization techniques are used in the Maryland Chesapeake Bay, and while the qualitative effects of the different techniques are generally known, there is little quantitative, long-term information available. This study will develop a comprehensive data set comparing long-term impacts of different shoreline stabilization techniques on both the physical environment and habitat, and will construct indices of erosion potential and shoreline protection that will be provided directly to Maryland state coastal managers.

Impact/Outcome: 

This section describes how this project has advanced scientific knowledge and/or made a difference for coastal residents, communities, and environments. Maryland Sea Grant has reported these details to the National Oceanic and Atmospheric Administration (NOAA), one of our funding sponsors.

Summary: A study found that different types of shoreline stabilization techniques in Chesapeake Bay, such as hardened and natural shorelines, are associated with different patterns of local sedimentation. This project contributed evidence and understanding of an aspect of ecosystem dynamics that is highly complex and variable. Improved information may eventually help planners minimize harmful effects of human-driven shoreline modifications on water quality and aquatic vegetation.

Relevance: Sixty-nine percent of Maryland’s shorelines are eroding. Erosion rates and coastal flooding are likely to increase due to sea level rise caused by climate change. A variety of shoreline stabilization techniques are used in Maryland’s portion of Chesapeake Bay. But there are few quantitative, long-term data available about how well these techniques control erosion and promote near-shore water quality and abundance of submerged aquatic vegetation (SAV) — information that is useful to resource managers and land use planners. 

Response: Building on previous studies, researchers from the University of Maryland Center for Environmental Science collected data from 17 sites in Maryland representing a variety of shoreline stabilization mechanisms. These included riprap, landscaped vegetated shorelines (a.k.a. “living shorelines”), and offshore breakwaters as well as baseline sites with no stabilization. At each site, the researchers measured sedimentation rates and sediment types before and after stabilization techniques were applied and characterized whether the changes observed were likely to increase or diminish SAV growth. In addition, a graduate fellow supported by Maryland Sea Grant, Jia Gao, conducted related research to create an updated wind climate model for the Maryland portion of the Chesapeake Bay, which the research team used to study stabilized shoreline sites.

Results: The researchers made a novel set of findings about associations between different kinds of shoreline hardening and sedimentation patterns. Riprapped and living shorelines appeared to decrease the transport of fine sediments (clays, silts) and organic material from land into the adjacent nearshore waters. Riprap projects also may increase the supply of coarser (sand) sediments to the nearshore when sand is used to fill in gaps in these built barriers. These conditions are beneficial for SAV growth because SAV prefers coarser (sand) sediments to finer (mud) ones as a growth substrate. The study also found that offshore breakwaters promoted accumulation of finer sediments on their landward side, which could prove detrimental to SAV. The researchers documented a range of sedimentation responses associated with variations in local conditions. By identifying a range of variability and uncertainty in these responses, this project has identified important questions for further study. The scientists presented these findings at national scientific meetings and a regional summit on living shorelines and to the Chesapeake Bay Program Office (CBPO) Modeling Workgroup. In addition, Cindy Palinkas, one of the co-principal investigators, is participating in a CBPO project to produce a revised technical synthesis report about SAV restoration in Chesapeake Bay. This project’s findings will inform a section of the report about effects of land use, including shoreline hardening, on SAV. Jia Gao’s modeling work is relevant to refining existing models to predict the effects of sea level rise and wind conditions on erosion rates along the estuary’s shorelines.