Abstract
The 350-km long coast of Suriname, South America, is part of a unique system in the world characterized by large-scale mud supply from the Amazon and exposed to Atlantic waves. Large banks migrate alongshore from the Amazon to the Orinoco delta, separated by 'inter-bank' zones. Banks dissipate waves, partially weld onshore, and are colonized by mangroves, whereas waves in inter-bank areas cause shoreline erosion, mitigated where rare cheniers develop. Cheniers assure coastal protection and recreational and ecosystem services, notably providing nesting sites for marine turtles. Cheniers are also under pressure from sand mining. In order to gain a better understanding of how these cheniers form and evolve, a study was conducted on Braamspunt beach, a major turtle-nesting chenier in Suriname constructed from sand supplied by the Maroni River. Satellite images between 1987 and 2018 show a reduction of the alongshore extent of the chenier, following sand supply cut-off from the Maroni by a mud bank migrating westward towards the mouth of the Suriname River, exacerbated by mud-blanketing of sand. Commercial sand mining has further affected the beach, but mined volumes are not known. Field surveys (high-resolution topography, drone photogrammetry, wave measurements) conducted in February, 2016 highlight two chenier types depending on sand availability, transport and wave reworking: a high-tide reflective/low-tide dissipative and longshore transport-dominated type 1 exhibiting berm scarping, and a low, overwash-dominated type 2. As the mud-bank's leading edge impinges on the inter-bank trailing edge, sand sequestering by mud and storage in type 2 washovers entail less available sand downdrift. Type 1 lost over 4200 m(3) of sand in just 3-4 days. An increasingly deficient budget induces progressive change to type 2, morphodynamically better adapted to diminution of available sand, and epitomizing chenier fragmentation and isolation within the muddy Guianas coastal plain. By affecting the morphology and sand budget of Braamspunt, these changes lower opportunities for turtle-nesting. Further reduction of the length of exposed chenier is apparently halted near the Suriname River mouth where mud is liquefied by the outflowing river and tidal jet, but the continued existence of this preserved beach could be endangered by sand mining.
Highlights
The term “chenier” designates a body of wave-reworked coarsegrained sediment resting stratigraphically on a muddy substrate (Otvos and Price, 1979; Otvos, 2018)
Unlike beach ridges (Scheffers et al, 2012; Tamura, 2012), cheniers are not common because their genesis depends on a specific balance between sediment availability and wave action, and, they will only develop where cohesive sediments are available in large volumes with enough sand, shelly deposits, or gravel that can be reworked by waves (Nardin and Fagherazzi, 2018)
Cheniers are similar to any other wave-formed beach, except that their dynamics and preservation in coastal plains are conditioned by the supply of bedload-sized sediment in a context of ambient mud abundance, such as in river deltas (e.g., Saito et al, 2000; Hori et al, 2001; van Maren, 2005; McBride et al, 2007), and adjacent to estuaries (e.g., Anthony, 1989; Hein et al, 2016); but they develop where sand, or gravel and shells, are reworked and segregated from mud (e.g., Woodroffe and Grime, 1999) which serves as a substrate over which the chenier develops
Summary
The term “chenier” designates a body of wave-reworked coarsegrained sediment resting stratigraphically on a muddy substrate (Otvos and Price, 1979; Otvos, 2018). Limitations in the supply of appropriate sediment result in cheniers generally undergoing more or less active landward migration over their mud substrate This occurs by overwash as waves top the lowelevation beach and transfer sediment from the active beach face to the back-beach. As waves rich in temporarily suspended sediments overwash the beach, water rapidly infiltrates into the beach, depositing lobes of sand or shells that form washovers Since such sand remains segregated throughout from the ambient mud (i.e., mud on the foreshore, mud underlying the beach sand, and mud on the backshore in such situations of limited sand supply) as a result of wave action, the landward-migrating chenier tends to maintain its integrity and shape across-shore and alongshore. Cheniers serve as settlement areas, while abandoned inland cheniers are commonly sites for coast-parallel routes, and provide aggregate for building materials in situations where this is not pernicious to their role in coastal protection
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