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Aquatic Barrier Removal at the Head-of-Tide

Michael Chelminski
Principal
Stantec Consulting Services Inc.
Topsham, ME

Tidally influenced waterways are diverse and important habitats at the interface of the marine and riverine environments. These waterways are often influenced by the presence of head-of-tide barriers, such as culverts or dams, that are located within the freshwater/saltwater interface. Due to their location, these head-of-tide barriers can have a significant influence on seaward (downstream) ecohydraulic characteristics. Removal of head-of-tide barriers can be effective at providing a more natural hydraulic regime and restoring intertidal ecosystems landward (upstream) of the former barrier.

However, these projects present unique ecosystem restoration opportunities and challenges. Specific opportunities include restoration of connectivity for biotic (e.g., diadromous fish) and abiotic processes (e.g., sediment transport), and improved resiliency of tidal habitats with sea level rise by accommodating landward migration of fauna, flora and associated habitats. Specific challenges include alteration of existing habitats and impacts to existing habitats and fauna as well as social perceptions. This presentation provides general guidance to address associated opportunities and challenges.

Primary considerations associated with head-of-tide barrier removal projects are 1) addressing the potential for landward propagation of tidal influences and 2) identification of beneficial and adverse impacts that can result from a proposed project. Approaches for evaluating the potential for tidal exchange landward of the barrier are discussed as part of the presentation. Analysis of tidal water surface elevation data examined within the context of potential adverse impacts that can result from restoration of saline tidal exchange landward, including potential impacts to existing vegetation and alteration of habitat, is also presented. Finally, improved resiliency of infrastructure and natural resources are primary opportunities associated with head-of-tide barrier removal projects, which are examined through examples of small dam removals and replacement of undersized culvert projects. A related consideration with these projects is the potential for increased tidal flooding landward, which is discussed within a general context with emphasis on changes to existing flood risks.