# Characteristics of sedimentary shores

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## Introduction

This page gives an overview of morphological characteristics of different types of sedimentary shores. Rocky shores are excluded (see the corresponding article) as well as coasts strongly modified by human interventions. Coastal morphological characteristics are related to geological setting, sediment type, sediment supply, wave climate and tidal regime. Wave climate and tidal regime bear some relationship with the size of the adjacent sea. For each shore a short description is given below and some links to more detailed articles.

Several parts this article are drawn from Mangor et al. 2017 [1].

## Characteristic parameters

A useful empirical parameter for characterizing shore morphology is the so-called 'Dean-parameter', denoted $\Omega$. The Dean-parameter, introduced by Gourlay (1968) [2] and later exploited by Dean (1973) [3] and by Wright and Short (1984)[4], is the ratio of the significant wave height at the breaker line, $H_s$ and the downward settling distance of beach sediment during the peak wave period $T_p$,

$\Omega = H_s / (T_p w_s)$ ,

where $w_s$ is the sediment fall velocity.

It appears that the average cross-shore slope $\beta$ of the active coastal zone is negatively correlated with $\Omega$. Coarse sedimentary coasts subjected to low-energy swell waves ($\Omega$ of order 1 or less), are steep and partially reflect incident waves. Fine sedimentary coasts which are frequently subjected to high-energy sea waves ($\Omega$ of order 5 or more), are gently sloping and dissipate almost all the energy of incident waves. Intermediate between reflective and dissipative coasts are so-called intermediate coasts. Coasts subjected to swell waves are found on the open ocean at latitudes below 400 [5]. High-latitude coasts are frequently subjected to high-energy storm waves, see Fig.1.

Another characteristic parameter is the ratio $RTR$ of spring tidal amplitude, $H_{spring tide}$ and the significant wave height $H_s$,

$RTR= H_{spring tide} / H_s$.

This parameter reflects the influence of tides versus the influence of waves on shore morphodynamics. Wave action washes away fine sediments and tends to create a steep shore profile, whereas tidal action favours deposition of fine sediments and the development of a flat shore profile [6][7]. For small values of $RTR$, typically smaller than 3, the shore profile is wave-dominated. For small to medium values of $RTR$, typically between 3 and 10, the shore profile is tide-modified but wave-dominated. For large values of $RTR$, typically (much) larger than 20, the shore is tide-dominated and consists of tidal flats. Such large values of $RTR$ occur almost exclusively in estuaries and tidal lagoons.

The position of the surf zone is influenced by the tidal range. At low tide the surf zone is situated more seaward and at high tide more landward. The active coastal zone is thus larger for coasts with a large tidal range than for coasts with a small tidal range. A wide active coastal zone can accommodate more breaker bars than a narrow active coastal zone. Field data of a large number of beaches revealed that the number of breaker bars can be empirically related to the parameter $B_*$, representing a non-dimensional width defined as

$B_* = X_s / (gT^2 \tan \beta)$,

where $X_s$ is the width of the active coastal zone, $T$ the wave period, $g$ the gravitational acceleration and $\beta$ the average slope of the active coastal zone. For $B_*\lt 20$ no bar was present in the active coastal zone; for larger values of $B_*$ an increasing number of bars was found [8].

Fig.1. Global distribution of storm and wave climate. Blue: Coasts seasonally subjected to storm waves. Red: Coasts yearly subjected to tropical storms. Green: Coasts frequently subjected to ocean swell waves. Adapted from Davies (1973) [5]

## Sediment supply

Shore morphology also depends on sediment supply. Rivers are major sediment suppliers. Coasts situated in the vicinity of large river deltas are often directly supplied with fine fluvial sediments; this is the case of most mud coasts (see also Dynamics of mud transport). Sediment of fluvial origin is in general not supplied directly to the beach but derived from ancient fluvial deposits from which the present seabed is built. This is the case of most sandy coasts; they are sandy because wave action has washed away the fine sediment fraction. In the absence of rivers, sediment may also be delivered by cliff erosion or by erosion of offshore reefs. This is often the case of gravel beaches, which consist of coarse sand, gravel and/or pebbles. Gravel beaches are often found on mountain coasts.

## Exposed sandy shore

An exposed sandy shore ($\Omega$ of order 5 or more) is characterised by a wide sandy beach and a wide shoreface with several bars (generally 1-3); the beach is backed by dunes. Remnants of former geomorphologic environments, like glacial till, boulders or sandstone, may be exposed to some extent in the coastal profile. There are, for instance, many shorelines where a costal section is eroding and shapes a cliff. In such cases, the shore can be characterised as an exposed sandy cliff shore, as opposed to an exposed sandy dune shore. There can be a gradual transition between these two types of shores.

This type of shore typically occurs in relation to shorelines bordering medium large to large water bodies with dimensions from 100 km and more, storm-dominated wave climates (3m significant wave height exceeded at least 12 hours a year), micro to moderate tidal regimes, and moderate to macro storm surge regimes. There will often be a positive correlation between onshore wind speeds/wave heights and storm surge. The annual gross littoral transport is generally large, from 50,000 m3 up to more than 1 million m3 per year. The active coastal zone is wide, typically from 300 m and up to more than 1km.

Exposed sandy shores are characterised by great mobility, which often causes serious management problems. Management of such shorelines requires great care (see Human causes of coastal erosion and Natural causes of coastal erosion).

Typical examples of exposed sandy shores are found along the European coasts bordering the North Sea, the North Atlantic and to some extent the Mediterranean and the North- and the South American coasts bordering the Atlantic and the Pacific Oceans as well as the coasts of Africa and Asia bordering the Indian Ocean, the Persian (Arabian) Gulf, the Arabian Sea, the Bay of Bengal and the South China Sea, etc. A typical exposed shore and the associated exceedance curve for waves is shown in Fig.2.

 Fig.2a. Typical exposed littoral dune shore, the Danish North Sea Coast Fig.2b. Corresponding wave height distribution.

## Moderately exposed sandy shore

A moderately exposed shore ($\Omega$ of order 3 or less) is characterised by a narrow beach and active coastal zone, normally with no bar or a single bar. The shore is backed by cliffs or low dunes.

This type of shore occurs when coasts are bordering relatively small bodies of water with typical dimensions less than 100 km, such as shallow inland seas and bays. They also occur on larger water bodies, such as the Gulf of Mexico or the Mediterranean Sea, where wave heights do not exceed 3m, where tides are small and where storm surges correlate positively with onshore winds/waves. A typical example is shown in Fig.3. The gross littoral transport is relatively small in both situations, in the order of 10,000 – 50,000 m3 per year, and the shoreface zone is relatively narrow, typically 50 to 300 m.

 Fig.3a. Typical moderately exposed littoral cliff shore, the Danish coast at the Sound. Fig.3b. Corresponding wave height exceedence distribution.

## Sheltered or marshy shore

Fig.4. Typical sheltered shore, Rødby Lagoon, Denmark.

The sheltered shore is characterised by a narrow beach or the complete lack of a sandy beach, see illustration Fig.4. The shoreface is narrow and without any bars; the coast is often covered with vegetation right out to the beach and sometimes it is marshy with only very low cliff-like scars in the coastal formations. This type of coast occurs under arctic, temperate and subtropical conditions. The littoral transport is small (typically less than 5,000 m3 per year) and there will be hardly any shoreface zone.

This type of coast usually occurs in relation to shorelines out to small bodies of water, for example fjords, estuaries and lagoons with typical dimensions of less than 10 km and micro to moderate tidal regimes.

However, such shores can also occur out to larger bodies of water if one or more of the following conditions are fulfilled:

• The wind and wave climate is mild
• The geology of the area has provided a very shallow nearshore zone, which protects the shore against severe wave action
• Strong onshore winds are correlated with low water (negative surge)

The sheltered shore will normally not erode, but it can be exposed to flooding. Sheltered shores will normally be of poor recreational value, but are often of great environmental value.

## Tidal flat shore

Fig.5. Tidal flat shore, the Danish Wadden Sea.

Tidal flat shores are characterised by a very wide and mildly sloping foreshore, the so-called tidal flats, see illustration Fig.5. This type of coastal profile develops when tidal processes dominate over wave processes. As a rule of thumb tidal flats will develop when the parameter $RTR$ is large (20 or more). Tidal flats often occur for combined macro-tidal and macro-surge conditions. Negative correlation between storm surge and wave conditions may lead to the formation of a very flat shoreface similar to a tidal flat. The width and character of the backshore of a tidal flat mainly depends on storm surge conditions and the general geology and morphology of the area. If the coast is low, it will often be exposed to flooding during combined high tide and storm surge. In this case the hinterland is usually protected by sea defences, such as dikes. If the coast is high, often a sandy backshore is present and the shore is backed by cliffs.

Tidal flat shores are most frequently found in relation to moderately exposed to sheltered conditions combined with non-tropical climates. Under tropical conditions, the tidal flat will often be vegetated by mangroves, which changes the character of the tidal flat (see #Muddy mangrove shore).

Under sheltered conditions, such as in estuaries, tidal flats will often consist of fine sand and mud, whereas under more exposed conditions, the tidal flat mainly consists of sand. However, many variations of tidal flat shores exist; the specific characteristics strongly depend on the type and amount of sediments supplied from nearby rivers.

Sediment transport processes on tidal flats are complicated as they are influenced both by tidal currents and by waves. This means that there are considerable longshore as well as cross-shore transport processes. Furthermore, tidal flats are cut by tidal channels and creeks of different dimensions. Both non-cohesive as well as cohesive sediments are often involved in the transport processes, as well as numerous biotic processes, which further complicates the dynamics.

These types of shores are seldom used for traditional beach recreation, as they provide no attractive beaches; however, the tidal flats often constitute important habitats. Coastal erosion is normally not a problem, whereas flooding is often a major problem.

## Monsoon shore or swell shore

Monsoon wave climates and swell wave climates are characterised by persistent wave exposure with relatively small waves, typically with average wave heights less than one meter, and extreme waves less than a few meters. This provides a persistent and uniform exposure at water depths up to 3–4 m, and hardly no exposure beyond that depth. See illustration Fig.6.

Regions with monsoon and swell wave climate are often situated in tropical zones with an abundant supply of sand and mud from tropical rivers. The combination of available mixed sediments and a fairly constant wave climate results in a very pronounced sorting of the sediments. This results in the formation of a narrow sandy beach and a sandy shoreface out to a water depth of 3–4 m. The width of the shoreface is often less than 200-300 m.

The beach and shoreface material often consists of well-sorted medium sand. Beyond this sandy shoreface, the bed material often shifts abruptly to silt or mud and the slope of the profile becomes considerably more gentle. Fine offshore bed material is only suspended and transported during the rougher part of the monsoon period. Under these conditions, there is considerable transport of fine material in addition to the littoral transport of sand.

The monsoon wave climate is seasonal, whereas this is seldom the case for swell climates. The monsoon shores are normally only used for recreation in the calm period as the waves are too rough in the monsoon period. Furthermore, the water is often turbid during much of the monsoon period.

Monsoon and swell shores are often exposed to erosion if there is no supply of sand to the coast. Other typical problems are sedimentation in ports and the closure of river mouths due to the seasonal pattern of precipitation and wave conditions.

 Fig.6a. Monsoon shoreline, SW coast of Sri Lanka Fig.6b. Corresponding wave height exceedance distribution.

## Muddy mangrove shore

Fig.7. Mangrove shore, Belize.

A muddy coast is characterised by a muddy shoreface, sometimes in the form of muddy tidal flats, and the lack of a sandy shore, see illustration Fig.7. The area exposed to the tidal variation, or part of it, is vegetated by mangroves.

This type of shoreline occurs in tropical climates where rivers supply abundant fine material to the coastal zone. Wave exposure is generally low to moderate; waves are damped by a bed layer of fluid mud [9]. The tidal regime can be any type.

The coast generally consists of low wetlands exposed to flooding. Dikes are often built in such areas.

Mangroves occupy an important part of the active coastal zone; they highly contribute to the stability of the coast and provide important protection against storm waves produced by tropical cyclones. Cutting of mangroves for the creation of shrimp ponds or salt pans or for other uses causes severe problems as it decreases bio-diversity and reduces resistance to erosion and flooding.

## Coral shore

Coral exist both in temperate and tropical waters, but shallow-water reefs only form in a zone extending from 300 N to 300 S of the equator. Tropical corals do not grow at depths of over 50 m. The optimum temperature for most coral reefs is 26-27 0C and few reefs exist in waters below 18 0C. Most shallow-water corals live within the boundary of the 20 0C isotherm.

Fig.8. Fringing reef along the Egyptian Red Sea or Gulf coasts.

There are three main types of coral reefs: fringing reefs, barrier reefs and atoll reefs. Fringing reefs grow directly from the shore, with typically a reef flat or a relatively shallow lagoon between the reef edge and the shore. Barrier reefs are located parallel to the shore and well separated from the shore by a lagoon. An atoll is a roughly circular oceanic reef surrounding a large deep central lagoon. Coral shorelines and carbonate beaches are mainly associated with fringing reefs.

The growth of warm-water corals not only requires high temperatures, but also sunlight, clear and clean shallow ocean water. These conditions are often associated with nutrient-poor water, which is especially found in arid climates. The requirement of clear water also implies that the water is without suspended material. Furthermore, the corals require a firm seabed, to which they can adhere. Corals are small animals, which produce a hard skeleton composed of carbonate. The permanent growth of these skeletons, on top of the old skeletons, produces a coral reef. Coral reefs and related features are very fragile, as they are composed of the skeletal remains of living coral animals, which means that their survival are dependent on both physical and biological conditions. The most common type of coral shore is the fringing coral reef type (Fig.8), the development of which is schematically illustrated in Fig.9.

Fig.9. Typical evolution stages of a fringing reef: 1) Early stage with little carbonate accumulation, 2) Stage with shallow reef flat, 3) Late stage with lagoon and back-reef developed. Heavy dark areas show good coral growth. Adapted from [10].

The coral shores will themselves produce coral debris and coral sand, whereby originally hard shorelines over time are converted into sandy beaches fronted by coral reefs. In this way they enter into a category of sandy shores, the so-called carbonate beaches. A typical carbonate beach is shown in Fig.10.

Fig.10. Coral shore with carbonate beach, Mexico.
Fig.11. Diagram showing a fringing reef along the Red Sea Coast . A dry river, a Wadi, and a local bay, a sharm, is also shown.

The coral shore normally occurs in originally rocky areas in the tropics, either in rainforest climates, but away from sediment-carrying rivers, or in arid climates (desert climate such as the Red Sea and the Arabian Gulf). This means that coral shores in a rainforest climate are most often seen on small islands, where there is no supply of fine sediments to the nearshore zone. The typical situation for a fringing coral reef shoreline along the Red Sea coast is presented in Fig.11.[11].

Due to the presence of the reef, the shoreline will hardly be exposed to waves, as they break on the reef edge. This results in a narrow and not very attractive beach, which is less suitable for bathing, as it is fronted by the shallow reef flat, as illustrated by the Red Sea coast in Fig.12.

Fig.12. Breaking waves at reef edge south of Safaga (Red Sea). The site is exposed but the beach is protected by the shallow reef resulting in a narrow beach fronted by a shallow shoreface.
Fig.13. Attractive natural Red Sea beach at sharm location near Utobia Beach Resort in Bir Assal Center area.

In the hot arid climate rivers are normally dry, a so-called wadi. However, during the rare but violent thunder showers in the mountains, the wadies are transformed to violent rivers carrying lots of sediments. The sediment damages the reef at the outlet of the wadi whereby a small semi-sheltered bay may be formed, a so-called sharm or marsa.

Such locations are attractive locations for local fishermen, as they provide partly sheltered natural harbours, where boats can be moored/landed safely. Furthermore, these locations are attractive from a recreational point of view, as they provide semi-sheltered beaches and sufficient water for swimming. In such environments the beaches often consist of sand carried to the coast by the wadies, which means that the beach does not consist of carbonate sand, as illustrated in Fig.13. In coastal profiles along the Red Sea sediments therefore consist of both carbonate sand from the fringing reefs and silicate sand supplied by the wadies.

## Arctic shores

Fig.12. Arctic shore, Photograph by Agata Weydmann

Shores above the North Polar Circle, which are exposed annually for more than six months to freezing, are considered Arctic coast (Fig.12). Their characteristic feature is the importance of ice forms (glacial ice – growlers, sea ice – ice pack and winter local ice – ice foot) for their ecology and evolution. Ice act as a limiting factor for the occurrence of infaunal and epifaunal organisms (ice scouring, ice melt and freezing are all stressful processes for coastal macrofauna). Even in the high Arctic, macrofauna and macroalgae can survive winter ice in rock crevices. In places with very cold water, where permafrost surfaces at the intertidal zone, the specific type of Arctic shore appears. This so-called cryolittoral shore has a glaciated seabed, often covered with stones and algal debris (northernmost parts of the Siberian coast and its islands). Soft sediment Arctic shores are eroding very fast due to the combined effect of ice melt, ice scouring and waves action.

## Related articles

Shoreface profile
Coastal and marine sediments
OzCoasts website

Komar, P. D. 1998. Beach processes and Sedimentation. Prentice-Hall, NJ, 2nd edn., 544 pp.
Woodroffe, C. D. 2002. Coasts: Form, Process and Evolution.
Cambridge University Press, 623 pp.
Bird, E.C.F. 2008. Coastal Geomorphology. John Wiley & Sons, 410 pp.
Davidson-Arnott, R. 2010. An Introduction to Coastal Processes and Geomorphology. Cambridge University Press, 442 pp.
Dronkers, J. 2016. Dynamics of Coastal Systems. World Scientific Publ.Co., 735 pp.

## References

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 The main author of this article is Mangor, KarstenPlease note that others may also have edited the contents of this article. Citation: Mangor, Karsten (2020): Characteristics of sedimentary shores. Available from http://www.coastalwiki.org/wiki/Characteristics_of_sedimentary_shores [accessed on 28-11-2020] For other articles by this author see Category:Articles by Mangor, Karsten For an overview of contributions by this author see Special:Contributions/Karsten

 The main author of this article is Ulrik LumborgPlease note that others may also have edited the contents of this article. Citation: Ulrik Lumborg (2020): Characteristics of sedimentary shores. Available from http://www.coastalwiki.org/wiki/Characteristics_of_sedimentary_shores [accessed on 28-11-2020] For other articles by this author see Category:Articles by Ulrik Lumborg For an overview of contributions by this author see Special:Contributions/UlrikLumborg