Tidal inlet seafloor changes induced by recently built hard structures

Abstract

Tidal inlets are extremely dynamic environments that are often strongly modified by anthropogenic intervention. In this study, we describe the rapid evolution of a highly human-impacted tidal inlet, studied through repeated high-resolution multibeam surveys and geomorphometric analysis. We document the rapid change induced by new hard coastal structures built to protect the historical city of Venice (Italy). A new breakwater erected between 2011 and 2013 induced the formation of large scour holes with the consequent erosion of about 170 · 103 ± 15.6% m3 of sediment until 2016. The construction of a new island in the middle of the inlet and the restriction of the inlet channel caused a general change of the inlet sedimentary regime from depositional to erosive with a net sediment loss of about 612 · 103 ± 42.7% m3, a reduction of the dune field area by more than 50% in about five years, and a coarsening in the sediment distribution. Our results give new insight on the tidal inlet resilience to changes, distinguishing two different phases in its recent evolution: (i) a very rapid response (from 2011 to 2013) of the seafloor morphology with scour-hole erosion at the new breakwater tips at a rate of about 45⋅103 m3/year and the disappearing of dune fields at a rate of 104⋅103 m2/year; and (ii) a general slowdown of the erosive processes from 2013 to 2016. Nevertheless, the erosion continues at the breakwater, though at a reduced rate, possibly representing a threat to the hard structure. In view of global mean sea level rise and consequent proliferation of hard structures along the coast all over the world, the combined use of very high resolution multibeam surveys and repeatable geomorphometric analysis proposed in this study will be crucial for the monitoring and future management of coastal environments.

Spatial and Temporal Changes of Tidal Inlet Using Object-Based Image Analysis of Multibeam Echosounder Measurements: A Case from the Lagoon of Venice, Italy

“…Change detection statistics presented in Table 5 suggests a very high dynamic of sediment substrates located in the Chioggia inlet, which is in line with other results found in the literature concerning bathymetry changes in 2002-2013 [37], and in 2004-2014 in the neighboring Lido inlet, located in the north of the Venice Lagoon [15].…”

“…Such hard anthropogenic structures located in the inlets could substantially affect the hydrodynamics, sediment transport, and the benthic habitat distribution of the lagoon [36]. In particular, it was shown that the recent construction of the hard structures had a direct impact on the seafloor [15,37]. Hence, there is a pressing need to understand the spatial and temporal response of the seafloor.…”

“…On the landward side of the inlet, the presence of larger structures like flood-and ebb-tide deltas was documented [38,39]. The Chioggia area of interest (AOI) was greatly modified by human activity over the centuries, though it underwent the most significant changes due to the development of the MoSE project, similar to the Lido inlet [15]. The southernmost inlet of the lagoon had an average depth of around 8 to 12 m in artificially maintained navigation channel areas [36].…”

“…Despite the recent rapid development of hydroacoustic devices and tools for geomorphometric analysis, detection of spatial and temporal changes of benthic habitats over time was only recently investigated [10][11][12][13][14][15]. The main measuring device used for high-resolution spatial mapping of the seafloor is the multibeam echosounder (MBES) [16].…”

Nearshore Sandbar Classification of Sabaudia (Italy) with LiDAR Data: The FHyL Approach

“…The bathymetric position index (BPI), representing the marine version of the Topographic Position Index (TPI) introduced by [58], has been applied to several sedimentary [59], geomorphological [60] and seafloor habitat mapping [61] studies. The BPI has been recently used for benthic habitat mapping studies [62][63][64][65][66].…”