Influence of hydrodynamic and sedimentary processes on tidal wetland landscape evolution for Making Room for Wetlands in the Bay of Fundy

Managed dyke realignment is increasing being implemented in the Upper Bay of Fundy Canada in response to increased vulnerability of dyke infrastructure to erosion and overtopping.  The Making Room for Wetlands project was initiated in 2017 to develop an evidence-based framework for implementing managed realignment (MR) and tidal wetland restoration in this hypertidal estuary.  MR was conducted at two sites: Converse (near mouth of Missiguash River) and Belcher St. (near head of Cornwallis River) and included both pre and post restoration monitoring of hydrology, soils & sediments, vegetation and morphology.  Earthworks included removal of an aboiteau structure, channel excavation, inner dyke construction and levelling of old dyke infrastructure.  Detailed hydrodynamics, sediment transport and deposition data were collected seasonally at the Converse site since first tidal waters were introduced in Dec 2018.  Ecomorphodynamic changes were quantified using repeat high resolution RPAS surveys, field measurements and RSET stations at both sites.  The rate of evolution of tidal wetland landscape differed between the two sites.  High sedimentation rates (~10 cm/yr) associated with the turbidity maximum at Belcher provided a disturbance surface rapidly colonized by annual halophytes and was fully vegetated by Year 3 with a mix of tidal brackish species (equivalent to reference site).   Sedimentation also played a role at Converse, filling in the borrow pit used to construct the inner dyke within the first two years, facilitating the development of a shallow tidal creek network.  Additional sedimentation over the former agricultural surface aided in slower development of a hybrid creek network incorporating the relict ditches.  Large spring tides play an important role in sediment supply to the marsh platform.  Positive sediment flux values into the site were recorded in association with erosion of the inlet channel which typically occurred during higher spring tide events.  A preliminary model linking sediment flux at the inlet with deposition on the marsh surface was developed.  While some halophytic vegetation was established within the first two years at Converse, vegetation established increased markedly in Year 4.   Erosion of the tidal inlet during high spring tides provided important subsidies of sediment in addition to baseline concentrations within tidal waters to the marsh platform that facilitated the evolution of the tidal wetland landscape.  While the establishment of tidal marsh vegetation was slower at Converse than Belcher both are providing ecosystem services.  Implications for modelling the trajectory of tidal wetlands after managed realignment including MR design, timing of earthworks and position within the estuary are discussed.