Beach
Content
Beach is defined as a narrow body of wave worked sediments elongated parallel to the shoreline. Mostly sediments are wave reworked in the beach with wave action. Generally Beach is long, narrow, accumulation of sand aligned parallel to the shoreline and attached to the land. Beach (as defined by Langwall & flint) is “the wave worked sediments along the shoreline extending throughout the surf zone”. Beach is the most unlikely of the bed form to be found facing the open sea. They are the morphological imprints between the highest & lowest level of spring tide. They are mainly pile of loose sand, & yet they manage to remain intact on coastline where the waves can reduce concrete sea walls to rubble in very short time. The geomorphic success lies in the very fact that they are only loose sand. Thus beach can adapt their shape very quickly to change in wave energy & also despite this energy in minor adjustment of the position of each sand grains. The beach therefore becomes able to maintain a dynamic equilibrium with the environment due to inherent mobility of its sediments. Land word limit of the beach, the upper limit of the wave action & its sea word limit is the zone of shoaling transformation Beach is the most dynamic of all depositional environments. Wave energy is the major factor controlling the development and changes in the beach. For a given grain size a beach slope will increase with decreasing wave action. Alternatively, under similar wave conditions coarser sand will always form a steeper slope than finer sand. Beaches are best developed on wave-dominated coasts by wave-dominated processes, which include wave swash, storm waves and near shore currents such as the long shore current and the rip current, where tidal ranges are small. Morphology of beach is controlled by long shore current and rip currents that develop within the Surf zone (zone where the waves break). If waves reach the beach at rather low angles, stronger long shore currents are produced. This results in an active sediment transport parallel to the coast. In Chandipur, the coast near the confluence of the river Buribalam with the Bay of Bengal is characterized by a narrow beach. The seaward side of the beach is bounded by an extensive tidal flat. During high tide levels, waves are generated by the shear action of wind over the water, resulting in the formation of this narrow beach. The beach sediments are mostly those eroded from the Aeolian dune fields. The general trend of beach varies between 50˚-55°. The average width of the beach is 45-55m.The beach facies divided in two sub-facies – Foreshore and Backshore, having their own characteristics. The contact between beach & tidal flat is sharp in Chandipur. On the other hand contact between the beach & aeolian dune field is defined by an erosional berm. However the contact between backshore & foreshore is not well defined in the absence of runnel (a depressed area). Runnel is formed by wave action during storm. A shallow alongshore trough is present. This may be produced due to erosion by long shore current.
The beach can be divided into 2 distinct zones-
FORESHORE
The foreshore of high energy and of low energy beaches is more or less flat, without any morphological features, corresponding to the zone of wave swash. The foreshore region of Chandipur extends from the shoreline up to the landward limit of wave propagation that is zone of wave swash. This part belongs to intertidal zone and here sediments are mainly reworked by wave. It has a sharp contact with the tidal flat. Its average slope is about 65o.
LITHOLOGY:
Sediments of foreshore consist of fine to medium grained. The grain size is of fine silty sand to medium grained sand and increases landward and towards the bar. Heavy mineral concentration is low and the sediments are wet and some-what cohesive.
SEDIMENTARY STRUCTURES:
Foreshore region is constantly under the wave action thus a number of wave included sedimentary structures are well preserved here. These are -
CURRENT CRESCENT:
These are horse-shoe shaped troughs around the obstacles scattered on the foreshore region was a very common feature on the beach surface. These obstacles tend to accumulate sand on their down-current side in the shape of a small ridge which tapers down current. A semicircular depression also results on the up current side of the obstacle. These features are known as current-crescents. The opening direction of the scour is slope controlled and hence is towards the sea. The measurements of the opening direction of current crescent are expressed by the rose diagrams. The size of the current crescent is controlled by the size of the obstacles, and the geometry of the marks is determined by secondary flow patterns induced within the main flow by the obstacles themselves.
Current crescent
SWASH RIPPLES:
The front of the dying waves during backwash leave behind a pattern of tiny imbricating sand ridges with their convexity landward that marks the line of further encroachment of the waves. They were composed of very fine sand. Usually coarser but hydrodynamically lighter materials like broken shells, wooden fragments accumulated along their curved ridges. Swash marks are excellent indicators of the shoreline of a sea or lake.
Swash ripples
RILL MARKS:
Rill marks are bifurcating dendritic erosional sculptures made by flow of a thin layer of water on the sediment surface during the process of sinking of ground water level. They have very low preservation potential and are generally not found in the rock record. Rill
marks are controlled by the local morphology, slope of the sediment surface, and grain size. These acquire diverse forms such as, branching rill marks, meandering rill marks, tooth shaped rill marks, comb shaped rill marks, bifurcating rill marks etc. These are small dendritic channels or grooves that form on the beaches by the discharge of pore waters at low tide or by small streams debouching onto sand. The whole foreshore was marked by profuse development of rill marks. It resembles a river network exhibiting a micro-delta at the terminal part. This is mainly found in the junction of tidal flat and beach.
rill marks, in the fore shore
ANTI DUNES
These are ripple marks on the fore-shore formed due to rapid flow of water over the sediment surface. Erosion occurs on their gentler stoss or down-current sides and deposition occurs in their steeper lee or up current sides. Hence they migrate in upstream direction. They are bed forms in-phase with the flow.
Anti dune formation
Parting lineation
PARTING LINEATIONS
Parting lineations are characterized by the ridges alternating with grooves, forming two sets of lineations. It is the product of bidirectional nature of wave both swash & backwash flow. These are few millimeters in width and many centimeters in length. There is an acute angle between the two sets of parting lineations. The acute angle may vary from low to moderate. Because these two sets are formed by swash and back swash. Swash and back swash are not parallel to each other. It is a very good indicator of the palaeo-current direction because of its parallel alignment to the current direction. The opening directions of the parting lineations are very much helpful in this regard. But it cannot indicate which is upcurrent or down current direction. Parting lineations are found in the plane beds indicating the upper flow regime.
RHOMBIC
RIPPLES
Rhombic ripples are one of the most common structures that found on the beach formed by swash and the backwash currents. During the swash a straight crested ripple is formed and during backwash another similar ripple is formed at an angle to the previous ripple and forms a rhomboid structure. These ripples develop under a very thin layer of water, usually on the seaward slopes of the beaches. The trends of the ripples give us the direction of the swash ad backwash on the beach. This is formed due to the angle between swash and back swash current.
Rhombic ripple
Movement of waves from two opposite directions produces this ripple pattern, so they are characteristics of wave pattern.
BIOGENIC STRUCTURES
BORROWS AND BIOTURBATION STRUDTURES
In the foreshore region there are large numbers of burrows, made by mostly red crabs, but the diameter is small at about 1cm and they are mostly sub vertical, having depth less than 5.5 cm. Very few of them show larger diameters. The concentration of burrow is very high and the area is highly bioturbated with pseudo fecal pellets. Crabs take food from sediments and throw them on the surface. This sediment looks like pellet.
Fore shore burrows associated with the pseudofecal pellets
SUBSURFACE FEATURES OF FORE-SHORE
Trench sections in the fore-shore region, about 20-25 cm deep, reveal alternating laminations of sand and heavy mineral rich layers. The sand was much darker, brownish yellow in colour, and wet and slightly cohesive as compared to the back-shore sands. In some fore shore trenches, those alternating laminations were interrupted by vertical and inclined burrows of different sizes, though the fore shore burrows towards the Buribalam estuary show less burrowing evidences. In the trench in the fore-shore region of the crescent shaped bar, different type of shell fragments concentrate to form a totally shell hashed layer. They are known as coquina layers. The layer was only a few cm thick. Shore perpendicular section in the same trench showed sea-ward dipping.
Fore shore trench section showing seaward dipping low angle cross strata
Fore shore trench section showing Shell hashed layer
A fore shore trench trench
Litho-log of fore shore
BACKSHORE
The backshore represents the upper part of a beach which remains normally dry, except under unusually high water conditions, when it can be flooded and acted upon by waves and weak currents. However, most of the times these parts remain exposed to wind activities. It is bounded on the landward side by the vegetation line & dune & on the seaward side high tide mark. It is a supra-tidal area. Its width varies from a few to 30 m and has an average inclination of 6-7o.
LITHOLOGY
The grain size of the sand ranges from medium to coarse grain towards land, which are well sorted. Concentration of heavy mineral is high, derived during storms as suspended load. Hence they settle rapidly to generate alternate layer of sand and thin lamina of dark coloured heavy minerals. These layers contain minerals like tourmaline and magnetite. These sands are dry and fluffy. As we move eastward from PWD bungalow towards bar the colour of the sand becomes paler & the adjacent to the bar the colour was almost whitish.
SEDIMENTARY STRUCTURES
Here also we got sand and heavy mineral laminas forming alternating layers. Sedimentary structures at the backshore region were not well preserved due to local disturbances by human activity causing destruction of the imprints of wind action, though we encountered some large vertical and inclined burrows, interrupting those laminations.
BIOGENIC STRUCTURES
The domain also includes sparsely distributed disarticulated or dead molluscan shells. Hence, there are high concentrations of crab made burrows with higher diameter up to 45cm as compared to the foreshore burrows. The Backshore burrows are also much deeper, about 25-30 cm deep.
Back shore burrow
SUB-SURFACE FEATURES OF BACK-SHORE
The subsurface features were studied on the basis of trenches. Within the well preserved ripple-zone of backshore we dug up two sections to construct two planes perpendicular to each other i.e. one was shore-parallel and another is perpendicular to it. The trenches were nearly 55-60cm. deep. We have encountered different interesting features in the back shore trenches. In the upper parts of the back shore trenches deformational features (most probably anthropogenic). In some cases, deformation results in the penicontemporaneous folding of the layers. The pressure during deformation caused the high density minerals to settle in the bottom part.
Different deformational features in the upper parts of the trenches
Now we are going to discuss about the different types of burrows- identified in the trench sections. Large burrows (with respect to the fore shore burrows) - both vertical and inclined (landward) were found, interrupting the „black and white‟ sequence of the trench. Those burrows were different in shapes and sizes. Some burrows were filled up by totally sand, some by totally heavy minerals and few others were found to be filled up by non-laminated mixture of sand and heavy minerals. These are passively filled burrows. But some burrows are filled by laminated sediments. These are actively filled burrows. Some burrows were walled off by heavy minerals.
Different types of burrows in the back shore trenches
As heavy mineral content is relatively high away from the river estuary, hence most of the burrows of this area were found either completely filled up by heavy mineral or partitioned by the heavy minerals.
However, from multiple trench sections at the back shores we may come to a conclusion and it reveals that the average heavy mineral content in the beach reduces laterally from PWD bungalow towards the river estuary. Trench sections towards the river estuary show lesser number of heavy mineral lamina. In our second last trench- dug up at the bar flank, we encountered heavy mineral layers, restricted only in the upper parts and those layers have less thickness with respect to the early back shore trenches, away from the estuary. Introduction of mud with alternating sandy layers was first observed at approximately 200 meter riverward from the place of beach festival. Increase in mud content was observed towards the Buribalam river mouth.
Back shore trench away from river mouth mouth
Back shore trench nearer to river
Fig-1
Fig-2
Here are two litho logs of the back shore trenches. The first one is of the trench near PWD Bungalow and the second one is of the trench, dug up at the bar trench. So the main two changes in the back shore lithology are- decrease in heavy mineral content and increase in mud content towards the Buribalam river estuary.
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The beach can be divided into 2 distinct zones-
FORESHORE
The foreshore of high energy and of low energy beaches is more or less flat, without any morphological features, corresponding to the zone of wave swash. The foreshore region of Chandipur extends from the shoreline up to the landward limit of wave propagation that is zone of wave swash. This part belongs to intertidal zone and here sediments are mainly reworked by wave. It has a sharp contact with the tidal flat. Its average slope is about 65o.
LITHOLOGY:
Sediments of foreshore consist of fine to medium grained. The grain size is of fine silty sand to medium grained sand and increases landward and towards the bar. Heavy mineral concentration is low and the sediments are wet and some-what cohesive.
SEDIMENTARY STRUCTURES:
Foreshore region is constantly under the wave action thus a number of wave included sedimentary structures are well preserved here. These are -
CURRENT CRESCENT:
These are horse-shoe shaped troughs around the obstacles scattered on the foreshore region was a very common feature on the beach surface. These obstacles tend to accumulate sand on their down-current side in the shape of a small ridge which tapers down current. A semicircular depression also results on the up current side of the obstacle. These features are known as current-crescents. The opening direction of the scour is slope controlled and hence is towards the sea. The measurements of the opening direction of current crescent are expressed by the rose diagrams. The size of the current crescent is controlled by the size of the obstacles, and the geometry of the marks is determined by secondary flow patterns induced within the main flow by the obstacles themselves.
Current crescent
SWASH RIPPLES:
The front of the dying waves during backwash leave behind a pattern of tiny imbricating sand ridges with their convexity landward that marks the line of further encroachment of the waves. They were composed of very fine sand. Usually coarser but hydrodynamically lighter materials like broken shells, wooden fragments accumulated along their curved ridges. Swash marks are excellent indicators of the shoreline of a sea or lake.
Swash ripples
RILL MARKS:
Rill marks are bifurcating dendritic erosional sculptures made by flow of a thin layer of water on the sediment surface during the process of sinking of ground water level. They have very low preservation potential and are generally not found in the rock record. Rill
marks are controlled by the local morphology, slope of the sediment surface, and grain size. These acquire diverse forms such as, branching rill marks, meandering rill marks, tooth shaped rill marks, comb shaped rill marks, bifurcating rill marks etc. These are small dendritic channels or grooves that form on the beaches by the discharge of pore waters at low tide or by small streams debouching onto sand. The whole foreshore was marked by profuse development of rill marks. It resembles a river network exhibiting a micro-delta at the terminal part. This is mainly found in the junction of tidal flat and beach.
rill marks, in the fore shore
ANTI DUNES
These are ripple marks on the fore-shore formed due to rapid flow of water over the sediment surface. Erosion occurs on their gentler stoss or down-current sides and deposition occurs in their steeper lee or up current sides. Hence they migrate in upstream direction. They are bed forms in-phase with the flow.
Anti dune formation
Parting lineation
PARTING LINEATIONS
Parting lineations are characterized by the ridges alternating with grooves, forming two sets of lineations. It is the product of bidirectional nature of wave both swash & backwash flow. These are few millimeters in width and many centimeters in length. There is an acute angle between the two sets of parting lineations. The acute angle may vary from low to moderate. Because these two sets are formed by swash and back swash. Swash and back swash are not parallel to each other. It is a very good indicator of the palaeo-current direction because of its parallel alignment to the current direction. The opening directions of the parting lineations are very much helpful in this regard. But it cannot indicate which is upcurrent or down current direction. Parting lineations are found in the plane beds indicating the upper flow regime.
RHOMBIC
RIPPLES
Rhombic ripples are one of the most common structures that found on the beach formed by swash and the backwash currents. During the swash a straight crested ripple is formed and during backwash another similar ripple is formed at an angle to the previous ripple and forms a rhomboid structure. These ripples develop under a very thin layer of water, usually on the seaward slopes of the beaches. The trends of the ripples give us the direction of the swash ad backwash on the beach. This is formed due to the angle between swash and back swash current.
Rhombic ripple
Movement of waves from two opposite directions produces this ripple pattern, so they are characteristics of wave pattern.
BIOGENIC STRUCTURES
BORROWS AND BIOTURBATION STRUDTURES
In the foreshore region there are large numbers of burrows, made by mostly red crabs, but the diameter is small at about 1cm and they are mostly sub vertical, having depth less than 5.5 cm. Very few of them show larger diameters. The concentration of burrow is very high and the area is highly bioturbated with pseudo fecal pellets. Crabs take food from sediments and throw them on the surface. This sediment looks like pellet.
Fore shore burrows associated with the pseudofecal pellets
SUBSURFACE FEATURES OF FORE-SHORE
Trench sections in the fore-shore region, about 20-25 cm deep, reveal alternating laminations of sand and heavy mineral rich layers. The sand was much darker, brownish yellow in colour, and wet and slightly cohesive as compared to the back-shore sands. In some fore shore trenches, those alternating laminations were interrupted by vertical and inclined burrows of different sizes, though the fore shore burrows towards the Buribalam estuary show less burrowing evidences. In the trench in the fore-shore region of the crescent shaped bar, different type of shell fragments concentrate to form a totally shell hashed layer. They are known as coquina layers. The layer was only a few cm thick. Shore perpendicular section in the same trench showed sea-ward dipping.
Fore shore trench section showing seaward dipping low angle cross strata
Fore shore trench section showing Shell hashed layer
A fore shore trench trench
Litho-log of fore shore
BACKSHORE
The backshore represents the upper part of a beach which remains normally dry, except under unusually high water conditions, when it can be flooded and acted upon by waves and weak currents. However, most of the times these parts remain exposed to wind activities. It is bounded on the landward side by the vegetation line & dune & on the seaward side high tide mark. It is a supra-tidal area. Its width varies from a few to 30 m and has an average inclination of 6-7o.
LITHOLOGY
The grain size of the sand ranges from medium to coarse grain towards land, which are well sorted. Concentration of heavy mineral is high, derived during storms as suspended load. Hence they settle rapidly to generate alternate layer of sand and thin lamina of dark coloured heavy minerals. These layers contain minerals like tourmaline and magnetite. These sands are dry and fluffy. As we move eastward from PWD bungalow towards bar the colour of the sand becomes paler & the adjacent to the bar the colour was almost whitish.
SEDIMENTARY STRUCTURES
Here also we got sand and heavy mineral laminas forming alternating layers. Sedimentary structures at the backshore region were not well preserved due to local disturbances by human activity causing destruction of the imprints of wind action, though we encountered some large vertical and inclined burrows, interrupting those laminations.
BIOGENIC STRUCTURES
The domain also includes sparsely distributed disarticulated or dead molluscan shells. Hence, there are high concentrations of crab made burrows with higher diameter up to 45cm as compared to the foreshore burrows. The Backshore burrows are also much deeper, about 25-30 cm deep.
Back shore burrow
SUB-SURFACE FEATURES OF BACK-SHORE
The subsurface features were studied on the basis of trenches. Within the well preserved ripple-zone of backshore we dug up two sections to construct two planes perpendicular to each other i.e. one was shore-parallel and another is perpendicular to it. The trenches were nearly 55-60cm. deep. We have encountered different interesting features in the back shore trenches. In the upper parts of the back shore trenches deformational features (most probably anthropogenic). In some cases, deformation results in the penicontemporaneous folding of the layers. The pressure during deformation caused the high density minerals to settle in the bottom part.
Different deformational features in the upper parts of the trenches
Now we are going to discuss about the different types of burrows- identified in the trench sections. Large burrows (with respect to the fore shore burrows) - both vertical and inclined (landward) were found, interrupting the „black and white‟ sequence of the trench. Those burrows were different in shapes and sizes. Some burrows were filled up by totally sand, some by totally heavy minerals and few others were found to be filled up by non-laminated mixture of sand and heavy minerals. These are passively filled burrows. But some burrows are filled by laminated sediments. These are actively filled burrows. Some burrows were walled off by heavy minerals.
Different types of burrows in the back shore trenches
As heavy mineral content is relatively high away from the river estuary, hence most of the burrows of this area were found either completely filled up by heavy mineral or partitioned by the heavy minerals.
However, from multiple trench sections at the back shores we may come to a conclusion and it reveals that the average heavy mineral content in the beach reduces laterally from PWD bungalow towards the river estuary. Trench sections towards the river estuary show lesser number of heavy mineral lamina. In our second last trench- dug up at the bar flank, we encountered heavy mineral layers, restricted only in the upper parts and those layers have less thickness with respect to the early back shore trenches, away from the estuary. Introduction of mud with alternating sandy layers was first observed at approximately 200 meter riverward from the place of beach festival. Increase in mud content was observed towards the Buribalam river mouth.
Back shore trench away from river mouth mouth
Back shore trench nearer to river
Fig-1
Fig-2
Here are two litho logs of the back shore trenches. The first one is of the trench near PWD Bungalow and the second one is of the trench, dug up at the bar trench. So the main two changes in the back shore lithology are- decrease in heavy mineral content and increase in mud content towards the Buribalam river estuary.
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