The formation and closure of the Big Breach of Sacalin spit associated with extreme shoreline retreat and shoreface erosion

Abstract

We document a case of exceptionally large natural breaching of a sandy spit (Sacalin spit, Danube Delta) using multiannual to seasonal surveys of topography and bathymetry on successive cross-barrier and shoreface profiles, LiDAR data, satellite imagery, and wind and wave data.The large breach, which quickly reached 3.4 km in May 2014, is attributed to morphological preconditioning of the narrow (50-150 m) barrier, which was susceptible to breaching even during moderate storm conditions. The event switched the barrier's decadal evolution from low cross-shore transport to high cross-shore transport over the barrier, which is an order of magnitude larger than during the non-breach period. Upper shoreface erosion, as indicated by the extensive erosion down to À4 m, indicates that this zone is a significant source for the rebuilding of the barrier. Barrier recovery and widening trigger a negative feedback which limits the back-barrier sediment transfer. As a result, back-barrier deposition decreases whilst the barrier aggradation through overwash becomes more frequent.The Big Breach (TBB) closed naturally in three years. The very high deposition rate of sediment in the breach is a testimony of the high sediment volumes supplied by the longshore transport and the high sediment released through shoreface retreat, and resulted in widening the barrier to a maximum of 1 km. Since the newly-formed barrier shoreline retreated 500 m, this reveals that barrier breaching is an important mechanism which significantly accelerates the landward migration of the barrier system and is a proof of the highly non-linear morphodynamics involved in the barrier island translation.

Sand spit dynamics in a large tidal‐range environment: Insight from multiple LiDAR, UAV and hydrodynamic measurements on multiple spit hook development, breaching, reconstruction, and shoreline changes

“…(iv) Finally, breaching and destruction of the extremities of hooks are regularly observed in the evolution of spits with or without hooks (e.g. Sanchez‐Arcilla and Jimenez, 1994; Chaumillon et al ., 2014; Williams et al ., 2015; Safak et al ., 2016; Sadio et al ., 2017; Zainescu et al ., 2019; Fruergaard et al ., 2020). In addition to these points, the three stages of decadal evolution of the Agon spit, involving hook destruction followed by construction, also show that spit hooks can evince geomorphic resilience in the sense of Kombiadou et al .…”

“…Waves lead to destruction of the spit and its platform during storms, as observed in this study between January and March 2016, and as reported in the literature (e.g. Sanchez‐Arcilla and Jimenez, 1994; Cloutier and Héquette, 1998; Williams et al ., 2015; Safak et al ., 2016; Zainescu et al ., 2019). Waves are therefore the main agent driving topographic changes in the spit and its delta. Finally, waves produce longshore transport that nourishes the shore‐parallel oriented bar and its evolution into a new hook (hook #9) with its platform.…”

“…Morphological differences are the result of numerous controlling factors that can interact and influence spit development processes at various timescales. Among these factors are hydrodynamic context and conditions such as waves (Allard et al ., 2008; Ashton et al ., 2016), tidal range and prism (Powell et al ., 2006; Robin et al ., 2007), inlet discharge and migration (Chaumillon et al ., 2014; Adams et al ., 2015), and storm‐induced breaching (Sanchez‐Arcilla and Jimenez, 1994; Zainescu et al ., 2019), as well as sediment availability (Héquette and Ruz, 1991; Firth et al ., 1995), sea‐level rise (van de Plassche and van Heteren, 1997; Billy et al ., 2018a), and geological processes and framework (Riggs et al ., 1995; Billy et al ., 2018b; Cooper et al ., 2018). In the last few decades, anthropogenic impacts have assumed increasing importance (Garel et al ., 2014; Miselis and Lorenzo‐Trueba, 2017; Sadio et al ., 2017; Kombiadou et al ., 2019a), especially in terms of sediment budget applications and engineering interventions (Psuty et al ., 2014; Stéphan et al ., 2018).…”

The formation and morphodynamics of complex multi‐hooked spits and the contribution of swash bars

“…Recurved or hooked spits can be observed in a wide variety of environments ranging from micro-mesotidal coasts (e.g. Hine, 1979;Nahon et al, 2019;Z ainescu et al, 2019b;Zenkovich, 1967) to macro-megatidal coasts (Bateman et al, 2020;Fruergaard et al, 2020;Poirier et al, 2017;Robin et al, 2009bRobin et al, , 2020Thomas et al, 2014).…”

Coastal morphodynamics: nearshore, beach and dunes

“…Local bedrock topography and sediment supply are therefore deemed the dominant controls on variability in barrier structure. Zăinescu et al (2019) showed that barrier evolution can be highly nonlinear, with barrier breaching playing an important role on the landward migration of barrier systems. They investigated a large natural barrier breaching event at the Danube Delta, where a narrow section of the barrier breached and reached 3.4 km under relatively moderate storm conditions, hence highlighting the significance of pre-breaching barrier morphology.…”