Coastal Wetland Surface Elevation Post-Construction: Application of Observational Method to Calibrate Consolidation Models

The US Army Corps of Engineers seeks to beneficially use 70% of sediment dredged by 2030, which equates to approximately 30 million cubic yards annually. One option to achieve this is the beneficial use of dredged material (BUDM) to supplement marsh elevation, bolstering the long-term stability of the natural infrastructure. In the BUDM design process, target elevation throughout the design life must be determined, which requires a holistic understanding of external factors such as tidal ranges, local hydrology, subsidence, accretion, and projected sea-level rise rates. However, determining a target elevation is only half the problem as one must also determine the magnitude and rate of consolidation. Hydraulically dredged sediment is deposited in a fluid state and will undergo large deformations. Engineering uncertainty is highly prevalent in coastal projects due to the difficulty in quantifying the engineering properties of soft deltaic soils. In 2020, the USACE Philadelphia District (NAP) began a multi-phase BUDM project to use sediment from a waterway maintenance event to supplement a back-bay island. The observational method was utilized to calibrate consolidation models against data collected during the first two placements to better predict elevations following a future placement. The comparison against elevation data showed that base models underpredicted the magnitude of consolidation which would have compounded to a 17% difference between the base and calibrated models following the third placement.