Do you want to breach an embankment? Synthesis of the literature and practical considerations for breaching of tidally influenced causeways and dikes

Abstract Breaching of coastal causeways and dikes (embankments) is a common method used to restore estuaries and marshes to a more natural state. Here, we reviewed literature pertaining to embankment breaches to assess the effectiveness of this restoration method and to provide practical advice for future breaches. While all methods of embankment breaching can be successful, active measures appear to be more successful than allowing breaches to occur naturally. Restoring tidal flow sooner, and to a greater degree, will speed recovery of impacted systems. Overall, recovery is often quick, with hydrology changing immediately. Sedimentation and erosion patterns also change soon after breaching, but it can take up to 15 years for sediment elevation to stabilize, and 60–100 years for equilibrium conditions to develop. Marsh plant communities recover rapidly, with most systems almost fully recovering within 5–20 years. Fish, bird, and invertebrate communities also mostly recover within five years. Unfortunately, recovery is often not complete. Even with large investments of time and money, systems can still exhibit variation from reference habitats in their biotic and abiotic conditions up to 130 years post breach. In some cases, this may be due to embankment remnants still impacting the system. Conversely, the systems may have changed too much, and/or global climate change may have altered successional pathways to such a degree that recovery to pre-disturbance conditions is unlikely. Regardless, breaching of coastal embankments often restores conditions that are more natural and increases estuary similarity to reference systems. As such, breaching is a useful prescription for coastal restoration, one that can deliver a considerable return on investment.