IMPACT OF A SMALL ANTHROPOGENIC ALTERATION ON A HIGHLY ENERGETIC ESTUARY WITH MULTIPLE TIDAL INLETS

A relatively minor anthropogenic alteration has affected the development of an estuary by preventing the main tidal channel from migrating landward thus altering sediment transport pathways, multiple tidal inlets and estuarine flushing. In January 1987 a tidal inlet formed through a barrier spit during an extratropical storm. As a result a series of small revetments were built as an exception to a stringent ‘no-hard-structures’ state law and local bylaw. These smaller revetments were later consolidated and today have become a single 750 m revetment. This structure represents 0.7% of the 115 km estuarine shoreline and has changed the way the 1987 inlet has evolved, altering the hydrodynamics of an estuary with a semi-diurnal tidal prism of 58,800,000 m³.

Another inlet formed 4 km updrift of the 1987 inlet in April 2007. This was seen as part of a documented 140-year cycle of spit elongation, barrier thinning and new spit formation. Observations over the last decade have documented inlet evolution and estuarine development. Sediment transport pathways were documented via past field studies including direct measurement of tidal current velocities and intertidal bedforms, sidescan sonar surveys and USACE bathymetric lidar. These surveys, coupled with analysis of aerial photos, helped document recent morphodynamic and hydrodynamic conditions. A phase-measuring bathymetric sidescan sonar was used to map the tidal channel and revetment in July 2014, which was then compared to the 2010 bathymetric lidar to help elucidate the short-term evolution of the channel and structure.

This relatively small, anthropogenic alteration in a large, highly energetic system has had a dramatic impact. The main tidal channel has migrated directly against the revetment and deepened from 6-7 m pre-1978 inlet to a current depth of >20 m. We suggest that the deepening of the channel has slowed, and likely prevented to date, the closing of 1987 inlet. The failure of the 2007 inlet to become the primary inlet, as seen in previous cycles, has altered flushing rates and tides higher up in the estuary. In addition, an outcome that becomes increasingly likely going forward is a catastrophic failure of the revetment, which could result in a loss of property and endanger lives as many houses are immediately landward of the revetment.