Tidal Restrictions

This dataset depicts tidal restrictions and their potential impact on upstream salt marshes along the North Atlantic coast of the United States (Maine to Virginia). Tidal restrictions include undersized culverts and bridges, tide gates, dikes, and other structures that interfere with normal tidal flushing in estuarine systems. Each potential tidal restriction has been assigned a value ranging from 0 (no effect from the restriction) to 1 (severe effect). The value represents an estimate of the proportion of salt marsh lost (converted to a different ecosystem type) due to the tidal restriction. The dataset is intended to be used in conjunction with the dataset “Tidal Restrictions Metric, Version 3.0, North Atlantic U.S. Coast,” which depicts the tidal areas upstream of the restrictions. This dataset is one of a larger set of results developed by the Designing Sustainable Landscapes project [https://scholarworks.umass.edu/designing_sustainable_landscapes/ ]led by Professor Kevin McGarigal of UMass Amherst. Funding for this project was provided by the North Atlantic Landscape Conservation Cooperative and Department of the Interior Project #24, Decision Support for Hurricane Sandy Restoration and Future Conservation to Increase Resiliency of Tidal Wetland Habitats and Species in the Face of Storms and Sea Level Rise. Intended Uses This dataset, and the associated Tidal Restrictions Metrics dataset, are intended to identify tidal restrictions that may be adversely impacting tidal wetlands and to provide information about the potential magnitude of the impacts. Because the datasets cover a large geographic area while using a relatively simple analysis, they may be best used as a screening tool to identify restrictions that merit more in-depth, field-based investigation. Used in this way, the datasets may aid in identifying candidate restrictions that would benefit from restoration or replacement to restore more natural tidal regimes. Description and Derivation The derivation of the Tidal Restrictions Metric and Tidal Restrictions (Points) datasets are described in detailed technical documentation [https://scholarworks.umass.edu/cgi/viewcontent.cgi?filename=0&article=1028&context=data&type=additional]. Briefly, the analysis encompassed the coastal zone where elevation was less than or equal to 5 m in elevation, as defined by a Digital Elevation Model derived from the NOAA Sea Level Rise Viewer. The tide range was modeled using NOAA’s VDatum. Tidal restrictions were identified as locations where roads or railroads (based on Open Street Maps) crossed streams (based on a high resolution version of the National Hydrography Dataset) in the coastal zone. Tidal areas were predicted based on elevation and tide range, using a logistic regression. At each tidal restriction point, a salt marsh loss ratio was calculated as 1 - (mapped salt marsh area / potential salt marsh area), where the potential salt marsh area included all areas predicted to be tidal, excluding open water. If there was only one tidal restriction downstream of a coastal area, then the salt marsh loss ratio for that restriction was assigned to the entire upstream coastal area for the Tidal Restrictions Metric dataset. (Specifically, values were assigned to locations mapped as estuarine wetland, freshwater wetland, nonforested uplands, and non-freshwater rivers.) If more than one restriction was situated downstream of a coastal area, then the most severe (highest-value) tidal restriction was assigned to the area. Known Issues and Uncertainties As with any project carried out across such a large area, the tidal restrictions datasets are subject to limitations. The results by themselves are not a prescription for on-the-ground action; users are encouraged to verify, with field visits and site-specific knowledge, the importance and impacts for any tidal restrictions and coastal areas identified in the project. Known issues and uncertainties include the following (see the technical documentation [https://scholarworks.umass.edu/cgi/viewcontent.cgi?filename=0&article=1028&context=data&type=additional ]for a more complete discussion): * The analysis does not take into account all of the effects of tidal restrictions on ecosystems; it focuses specifically on the potential loss of salt marsh habitat. The effects of tidal restrictions on freshwater tidal systems are ignored. * In situations where there is a chain of multiple road-stream crossings in a basin, the model has only limited ability to assign responsibility to particular potential restrictions. Therefore, the values assigned to individual restrictions should be regarded cautiously and it may be preferable to consider the restrictions collectively. * The tidal restrictions metric relies on a number of assumptions, including that salt marshes are adequately mapped; tides data are accurate; tidal restrictions are the only cause of saltmarsh conversion to freshwater wetland or upland; dams are not present; and crossings adequately represent tidal restrictions. * Salt marsh loss ratios are poorly estimated in extremely small basins. * In numerous areas, parts of basins were incorrectly omitted from the model due to known errors in the flow grid derived from the Digital Elevation Model (DEM) or due to gaps in coverage in the DEM. * The quality of the model compared to field conditions was not assessed. * Tide gates not associated with roads are excluded as potential tidal restrictions, as they are not comprehensively mapped throughout the region. * Bridges with a watershed of greater than 1,350 km2 were assumed not to act as tidal restrictions.