A Forensic Analysis of the Performance of Ecologically-Enhanced Shorelines along the Hudson River and the Bronx, New York

As part of The Hudson River Sustainable Shorelines Project, a multi-year effort lead by the New York State Department of Environmental Conservation Hudson River National Estuarine Research Reserve (http:\\www.hrnerr.org), a forensic analysis was performed to investigate the site conditions of six alternative shoreline stabilization treatments, that were impacted by Tropical Storms Irene and Lee in 2011 and Post-Tropical Storm Sandy in 2012. Project objectives included: determining patterns among those structures that survived and those that did not; determining which aspects of structural maintenance lead to the failure/survival; determining the potential impacts from extreme waves, water levels, and increased currents; and determining the impact of vegetation on structure failure/survival. The following tasks were completed as part of the forensic analysis: a review of previous shoreline stabilization attempts, a characterization of the conditions at each site, the collection of all available engineering/design data, post-storm field data collection, and hind-casting of the storm conditions. Factors that were found to lead to the failure of structures included the use of undersized or improperly graded stone, the implementation of inappropriate slopes, and immature vegetation. Severe erosion and scour were caused by large floating debris and were determined to be the primary cause of damage at several of the sites. Impacts were more severe along shorelines with steep slopes, with large waves and heavy debris scouring away soil and vegetation. Common factors amongst those structures that survived the storms with minimal damage included sloping terraces and adequately sized stones, as well as the presence of engineered structural elements for armoring. Mature vegetation with established root systems was determined to be a critical factor for the survival of the plantings. In addition, project maintenance and where necessary, adaptive management was also identified as a key factor in determining performance.