Beach

Summary

A beach is a landform alongside a body of water which consists of loose particles. The particles composing a beach are typically made from rock, such as sand, gravel, shingle, pebbles, etc., or biological sources, such as mollusc shells or coralline algae. Sediments settle in different densities and structures, depending on the local wave action and weather, creating different textures, colors and gradients or layers of material. Though some beaches form on inland freshwater locations such as lakes and rivers, most beaches are in coastal areas where wave or current action deposits and reworks sediments. Erosion and changing of beach geologies happens through natural processes, like wave action and extreme weather events. Where wind conditions are correct, beaches can be backed by coastal dunes which offer protection and regeneration for the beach. However, these natural forces have become more extreme due to climate change, permanently altering beaches at very rapid rates. Some

Official source

https://en.wikipedia.org/wiki/Beach

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Even if the total length of shorelines in low-wave environments like bays, sounds, lagoons and estuaries greatly exceed that of open-ocean coasts, the morphodynamics of such systems are poorly understood. Only few studies developed models to predict changes in these environments and these systems are often managed with the same policies as open ocean beaches. The Pittwater estuary is located in the north of Sydney, it is a drowned valley estuary with a well developed ood-tidal delta. The present study investigates the morphodynamics of four beaches in the northern part of this estuary, at two dierent timescales. The long term stability of these beaches have been examined using historical aerial photographs that allowto determine the position of the shoreline for the last decades. At the monthly to seasonal timescale, the morphological changes have been studied from beach surveys that are perform every one or two months since May 2016. Given that the survey campaign began just before a major storm that occurred in June 2016, this data set allow to study the post-storm recovery of these sandy estuarine beaches. The results of the present study showthat the beaches within the Pittwater estuary are evolving dierently according to parameters like distance from the inlet, exposure to ocean generated waves or beach shape. Examination of the aerial photographs suggest that two out of the four studied beaches are experiencing changes visible over several decades. Concerning the monthly to seasonal changes, detailed analysis of beach surveys indicated that sandy estuarine beaches can be strongly eroded during a storm event and that post-storm recovery can take several years up to decades. The comparison with two beaches located along the open coast in the Sydney area conclude that estuarine beaches have a much slower recovery rate.

2018

Decadal shoreline erosion and recovery of beaches in modified and natural estuaries

Vincent Henri De Staercke

Sandy beaches in estuaries and bays (BEBs) are common landforms on the coasts of many major cities. They exist under a wide range of settings and their morphology is controlled by their distance from the estuary/bay entrance, exposure to different types of waves (e.g., ocean swells vs locally generated wind waves), proximity to flood-tide delta/shoals, and anthropogenic interventions (e.g., dredging, groynes). Both swell waves propagating into estuaries/bays and locally generated wind waves can erode BEBs. However, more understanding of BEB storm erosion and recovery over decadal timescales is needed, as they typically respond slower than open coast beaches. Here we present decadal shoreline behaviours of nine BEBs from two estuarine systems in SE Australia are presented in this study, using 76 years of aerial imagery (1941-2017). We quantify and compare decadal behaviour between beaches, developing a new typology of BEBs based on shoreline evolution. We identify four decadal behaviours: prograding, quasi-stable, retreating and storm relict - and assess the influence of flood-tide deltas, river mouths, distance from the ocean entrance, and anthropogenic interventions. Swell-exposed BEBs near the entrance are quasi stable and recover after storms at rates comparable with open coast beaches (

ELSEVIER2021

To understand how organisms adapt to novel habitats, which involves both demographic and selective events, we require knowledge of the evolutionary history of populations and also selected alleles. There are still few cases in which the precise mutations (and hence, defined alleles) that contribute to adaptive change have been identified in nature; one exception is the genetic basis of camouflaging pigmentation of oldfield mice (Peromyscus polionotus) that have colonized the sandy dunes of Florida's Gulf Coast. To quantify the genomic impact of colonization as well as the signature of selection, we resequenced 5000 1.5-kb noncoding loci as well as a 160-kb genomic region surrounding the melanocortin-1 receptor (Mc1r), a gene that contributes to pigmentation differences, in beach and mainland populations. Using a genome-wide phylogenetic approach, we recovered a single monophyletic group comprised of beach mice, consistent with a single colonization event of the Gulf Coast. We also found evidence of a severe founder event, estimated to have occurred less than 3000 years ago. In this demographic context, we show that all beach subspecies share a single derived light Mc1r allele, which was likely selected from standing genetic variation that originated in the mainland. Surprisingly, we were unable to identify a clear signature of selection in the Mc1r region, despite independent evidence that this locus contributes to adaptive coloration. Nonetheless, these data allow us to reconstruct and compare the evolutionary history of populations and alleles to better understand how adaptive evolution, following the colonization of a novel habitat, proceeds in nature.

Wiley-Blackwell2012