**1. Introduction**

The Intergovernmental Panel on Climate Change forecasts rates of sea-level rise (SLR) between 8 and 16 mm/yr by the end of the century, resulting in a global sea-level rise between 0.52 to 0.98 m for a high greenhouse gas emission scenario [1]. Adding to the predicted increases of global mean sea level (GMSL), the upward or downward movement of the land surface—also known as vertical land motion (VLM)—may exacerbate relative sea-level (RSL) changes at local scales. Changes in RSL result from the combined effect of the mean sea-level height (i.e., GMSL) and VLM [2], and it is measured relative to a local tide gauge benchmark [3]. The most common triggers of VLM are glacial isostatic adjustments (GIA), earthquakes and tectonics, aquifer compaction and sediment consolidation [3,4]. Where VLM takes place near the coast, it may modify the coastline by the emergence or submergence of the terrain [2], resulting in, for example, shoreline progradation or retreat, respectively. Within the various causes of VLM, this work focuses on subsidence, defined as the downward movement of the land surface with respect to a datum or point of reference [5], and how it relates to coastal morphodynamics. Subsidence is driven by factors such as natural sediment compaction [6], fault displacements [5], or human actions (e.g., groundwater extraction [7], withdrawal of petroleum and natural gas [8], soil desiccation [9]).

**Citation:** Gómez, J.F.; Kwoll, E.; Walker, I.J.; Shirzaei, M. Vertical Land Motion as a Driver of Coastline Changes on a Deltaic System in the Colombian Caribbean. *Geosciences* **2021**, *11*, 300. https://doi.org/ 10.3390/geosciences11070300

Academic Editors: Germán Rodríguez, Carmen M. Rosskopf and Jesus Martinez-Frias

Received: 24 May 2021 Accepted: 15 July 2021 Published: 20 July 2021

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Deltas are common coastal landforms formed by the deposition and reworking of sediments from river systems where they enter larger bodies of water, such as oceans or lakes. Worldwide, most large deltas are subsiding at rates faster than the present global sea-level rise [10]. In the case that abundant sediment supply exists, a trade-off can be established between sediment delivery and subsidence in deltaic areas: the sediment deposition that triggers subsidence also helps compensate for or exceeds the subsidence. For instance, in some locations of the Mekong Delta in Vietnam, sediment accretion exceeds compaction rates, resulting in a net elevation gain of the delta surface [6]. However, when the delivery of fluvial sediment supply decreases, sediment compaction is no longer balanced by sediment deposition [6]. In this case, as the sediment matrix compresses and dehydrates, fluid pressure decreases and organic carbon-rich materials, if present, oxidize to carbon dioxide, resulting in a total mass and volume loss within the soil column [9] (Figure 1). Törnqvist et al. [11] noted that, adding to the sizeable natural component of subsidence due to isostatic flexure and the compaction of young sediments, subsidence caused by artificial drainage of wetlands and groundwater withdrawal can be up to an order of magnitude faster. For instance, using interferometric synthetic aperture radar (InSAR), VLM of up to −8 mm/yr have been measured in the coastal plain of the Selle River mouth (southern Italy) [12] and of up to −5 mm/yr in the Tiber Delta in Rome [13]. Additionally, Zhang et al. [14] reported average VLM rates of −5.1 mm/yr in the modern Yellow River Delta from 1992 to 2000.

**Figure 1.** Properties of soil structure before (left) and after (right) compaction takes place. Emission of CO2 occurs when dehydrated organic-rich soils are exposed to atmospheric oxygen (modified from Yuill et al. [8]).

Whilst the role of subsidence as a natural driver of delta growth and abandonment has been recognized and coined as the cyclic evolution of deltas [15], its influence in the evolution of coastal landscapes and the seaward progradation (i.e., accretion) or landward retreat (i.e., erosion) of coastlines has not been fully addressed. Despite subsidence being recognized as one of the key controls on coastline changes [13,16,17], the majority of research has focused on the added impact of current subsidence rates and rising sea levels for future scenarios of inundation and erosion (e.g., [12,18]). However, local-scale studies can help to better understand how VLM (subsidence or emergence) translates to coastline changes and related geomorphic responses. Accordingly, this study aims to support evidence-based policymaking by (i) testing the application of InSAR interferometry to estimate the rates and spatial distribution of VLM in a coastal setting, and (ii) linking VLM to recent coastline evolution derived from satellite imagery along a deltaic barrier.
