Littoral Drift (2)

PARVATHY K G

ROLL NO 12MS12F

MARINE STRUCTURES

APPLIED MECHANICS AND HYDRAULICS DEPARTMENT

LITTORAL DRIFT ALONG DK COAST

LEARNING OBJECTIVES

• To understand the process of littoral drift

• To learn about the littoral drift along DK Coast

2

SOME BEACHES HAVE BEAUTIFUL GOLDEN SAND

WHEREAS OTHERS HAVE LESS ATTRACTIVE SHINGLES

BUT THERE IS A REASON WHY BEACHES LOOK SO AS THEY DO

LONG SHORE DRIFT

3

OVERVIEW

WHAT IS LSD?

HOW IT WORKS?

TYPES

FEATURES

DK COAST

4

WHAT IS LSD?

• LSD is a geological process by which sediments such as sand or other materials move along a beach shore

• Natural movement

5

HOW DOES LSD WORK?

• LSD works when a wave approaches the beach at a slight angle following the prevailing wind direction

• Material moved along the beach in a zigzag manner

• When waves approach the shoreline obliquely, refraction tends to turn the wave fronts so that they are almost parallel to the shoreline.

• At the same time, when approaching the breaker zone they undergo shoaling, which means that they become steeper and higher.

6

CONTINUED…

• Finally, the waves break.

• As the wave breaks the swash carries material up the beach at the same angle as the wave approached

• As the swash dies away any material carried by it falls back down the beach during backwash due to gravity

7

CONTINUED…

• During the breaking process, the associated turbulence causes some of the seabed sediments to be brought into suspension.

• These suspended sediments, plus some of the sediments on the seabed, are then carried along the shoreline by the longshore current, which has its maximum near the breaker line.

• The two transport modes are referred to as suspended transport and bed load, respectively. The sum of these is the littoral drift.

9

LONGSHORE DRIFTRIVER OF SAND

• Schematic map of longshore drift. Waves run up the beach at an angle but return to sea as a perpendicular backwash.

• Creates a longshore transport of water and sand along the coast. A river of sand.

LONGSHORE DRIFT FORMULAS

• Bijker formula (1967,1971)

• CERC Formula

• The Engelund and Hansen formula (1967)

• The Ackers and White formula (1973)

• The Bailard and Inman formula(1981)

• The Van Rijn formula (1984)

• The Watanabe formula (1992)[6]

12

http://en.wikipedia.org/wiki/Longshore_drift#cite_note-autogenerated1-5

CONTINUED…

• The most common factors taken into consideration in these formulas are:

1.Suspended and bed load transport

2.Waves e.g. breaking and non-breaking

3.The flow associated with waves.

13

THE CERC EQUATION

14

• The CERC equation is the most widely used approach to determine longshore transport sand volumes worldwide.

CONTINUED…

15

• k is a sediment transport coefficient

• H is breaking wave height,

• d is the depth of water where the waves break,

• p, is density of quartz sand,

• p is density of sea water, and

• is the angle of wave approach to the shoreline

MAGNITUDE OF THE LITTORAL TRANSPORT

• The magnitude of the littoral transport or drift, Q, depends on parameters, the most important of which are:

• Wave height. The littoral drift is proportional to the wave height to the power of approximately 3.

• Grain size. The littoral drift is inversely proportional to the grain size to the power of approximately 3.

• Wave incidence angle. The littoral drift is approximately proportional to sin2.5(2α), where α is the wave incidence angle.

16

CONTINUED…

• It can be seen that the littoral drift varies strongly with several parameters.

• It is therefore crucial to have exact data when making littoral drift calculations.

• It is an important point that the littoral drift over the coastal profile depends not only on the hydrodynamics but also very much on the variation of the sediment characteristics over the profile.

• Hence, the sediment distribution along the coastal profile should be taken into account whenever possible.

17

TYPES OF LSD?

• CONSTRUCTIVE

• DESTRUCTIVE

18

CONSTRUCTIVE

• Process which leads to the formation of spits bars tombolos

• LSD generally considered to be a constructive process

• Continuing process that nourish the beach carry sand along the shore and deposit at the end of a spit so that it grows in length and size

19

DESTRUCTIVE

• In a storm condition the process can cause significant erosion to beaches

• Threat to Coastal communities tat rely on their beaches(tourism)

20

FEATURES OF SHORELINE CHANGE

• Longshore drift plays a large role in the evolution of a shoreline

• As if there is a slight change of sediment supply, wind direction longshore drift can change dramatically, impacting on the formation and evolution of a beach system or profile.

• Any interurruption to the littoral drift either through manmade barriers like bw entrance channels of harbours;natural barriers like river outlets :deposition on the updrift side & erosion on the downdrift side

21

CONTINUED….

• BEACH

• SPITS

• BARRIERS

• TOMBOLOS

22

HUMAN INFLUENCES-GROYNES

• Groynes are shore protection structures, placed at equal intervals along the coastline in order to stop coastal erosion

• Three most common groyne designs consisting of:

• zig-zag groynes, which dissipate the destructive flows that form in wave induced currents or in breaking waves.

• T-head groynes, which reduce wave height through wave diffraction.

• ‘Y’ head, a fish tail groyne system

25

SINGLE LONG GROIN

• When the long shore sediment transport threatens to cause a problem such as siltation of harbour entrance etc., a long groin can be constructed just slightly up drift from the harbour entrance or river mouth. Though, it prevents sediment movement, it can cause erosion on the other side

26

SERIES OF GROINS

• Another way of using groins is to build a series of small groins (groin field) at shorter intervals along the affected coast.

• This will tend to stabilize the entire coast by keeping the sand trapped between them.

27

CONTINUED….

30

• Mappleton located to the NE of England Holderness coast suffer from a large amount of coastal erosion due to LSD

• LSD takes away the sand that protects the cliff face

• Stone groynes built..By this means cliff wall is protected

TOM ABBOTT, BIDDULPH HIGH SCHOOL AND MADE AVAILABLE THROUGH WWW.SLN.ORG.UK/GEOGRAPHY AND ONLY FOR NON COMMERCIAL USE IN SCHOOLS


Direction of swash


Direction of swash

Bac

kwas

h


Direction of swash

Bac

kwas

h

Longshore Drift


Direction of swash

Longshore Drift


Direction of swash

Longshore Drift

Groyne traps sediment

Build up of sedimentagainst the groyne

BREAKWATER

• Built parallel to coast to provide protection

• Stops waves from hitting the beach. Sand is deposited behind and on the upside of the breakwater.

• Needs permanent dredging to keep the harbor or sheltered area open.

PORTS AND HARBOURS

• The creation of ports and harbours throughout the world can seriously impact on the natural course of longshore drift]

• The major influence the creation of a port or harbour can have on longshore drift is the alteration of sedimentation patterns, which in turn may lead to accretion and/or erosion of a beach or coastal system.

88

http://en.wikipedia.org/wiki/Longshore_drift#cite_note-35

CONTINUED…..

• The creation of a port in Timaru, New Zealand in the late 1800s led to a significant change in the longshore drift along the South Canterbury coastline. Instead of longshore drift transporting sediment north up the coast towards the Waimataitai lagoon, the creation of the port blocked the drift of these (coarse) sediments and instead caused them to accrete to the south of the port at South beach in Timaru The accretion of this sediment to the south, therefore meant a lack of sediment being deposited on the coast near the Waimataitai lagoon (to the north of the port), which led to the loss of the barrier enclosing the lagoon in the 1930s and then shortly after, the loss of the lagoon itself. As with the Waimataitai lagoon the Washdyke Lagoon, which currently lies to the north of the Timaru port is undergoing erosion and may eventually breach causing loss of another lagoon environment.

89

EROSION AND ACCRETION NEAR JETTIES•

Where there is net littoral drift, jetties can block the flow of sand, causing erosion and accretion.

• Jetties - Are used to protect inlets. Jetties keep the river or inlet openings from migrating.

•Sand builds up on upstream jetty, and severe erosion takes place on the down-current side of the opposing jetty.

90

91

• Need to be built out just the right length to create a large enough tidal prism (in-and-out volume) of seawater to keep them clear.

• Extend well beyond the breaker zone and interfere with longshore drift.

EAST COAST VS WEST COAST

• The wave induced sediment transport, 'littoral drift' takes place along the coast as well as normal to the shore, and however, the former mode of transport is predominant along the east coast of India in general and along the Tamilnadu coast in particular.

• The approximate rate of net littoral drift is 1.2 M m^3/ year along the Tamilnadu coast which is directed towards North.

93

CONTINUED…

• As we proceed northwards of Tamilnadu coast, the net drift reduces due to the interception of its movement by the breakwaters of Visakhapatnam and Paradeep ports. The said quantity is probably one of world's highest rates

• In Madras, depositions on the southern has caused the wide Marina beach ;erosion on the northern side has caused severe retreat of the shoreline in Royapuram area

94

CONTINUED…

• One eg, the inlets near Madras ie Adyar river inlet ,Ennore river inlet are constantly closed

• Costly dredging to keep it open

• Stability of inlet in the east coast is controlled by heavy littoral drift

95

CONTINUED….

• The calculated quantum of gross littoral drift in the westcoast of India about 1.28 million cubic metres/year, (Mani, 1972) with an annual discharge of sediments to the sea along Indian coast of the order of 1.2 x 10^12 kg (Chandramohan et al)

• Littoral drift is less along west coast comparitively

96

LITTORAL DRIFT ALONG DAKSHINA KANNADA COAST

97

GENERAL FEATURES :DK COAST

• Coastal area extends from Mangalore in the south to Udupi in the north

• Arabian sea on the west and western ghats on the east

• Under the influence of southwest monsoon

98

CONTINUED…

• Straight coast mostly sandy beaches except at one or two points rocky outcrops are seen

• LD Data for the DK Coast is very scanty

• Reported datas does not appear to be reliable

• Littoral drift on the DK coast is negligbly small.No significant influence on the coastal erosion phenomenon

100

CONTINUED…

• Longshore current principally responsible for longshore sediment transport

• Velocities of these current have mean values of 0.3 m/s or less during nonmonsoon

• Monsoon season 0.45 m/s directed north

101

Littoral Drift Rate Along Dk Coast

• Manohar(1958) gives an estimate of 200,000 tonnes of sediment movement per yr as LD past Mangalore

• No mention how the magnitude is obtained

• Nayak & Chandramohan (1992) estimates a net southernly drift of 0.707 million m^3yr

• (1.069 Mm^3 to the south & 0.362Mm^3 to the north

• Best way to estimate the magnitude and direction of the littoral drift through the measurements of sediments accreted at any littoral barrier

102

• Development of ports and harbours can cause drastic changes in the coastal morphology

• The changes can be adverse where littoral drift is as heavy as on the east coast of India

• Fortunately on Karnataka coast as the littoral drift is very small the adverse effects of manmade structures has not been serious

103

104

• Breakwaters are stone or concrete structures built from the shore extending into the sea to prevent a beach from washing away.

• The breakwaters at Old Mangalore Port have also been responsible for the present severe erosion problem in Kotepura as they block the littoral drift.

• The study also revealed the erosion in Kotepura is accentuated by building sea walls, breakwaters, small dams across the neighbouring river and due to sand mining on riverbeds and beaches.

• In DK district the bw of NMPT extends beyond the littoral zone

• Northern side of port in accretion and southern side of port in erosion .Littoral drift direction from north to south

105

106

•Studies have also shown that breakwaters—a popular measure along the Karnataka coast—may protect the immediate beach concerned, but would cause severe damage to adjacent areas.

•A T-groyne or sea wall would protect Ullal but obstruct the littoral drift southwards and erode the adjacent beach

107

• Submerged breakwaters is a good option with comparatively lower side effects on neighbouring beaches, as it does not obstruct the littoral drift

• Submerged breakwaters are structures built in the shallow waters parallel to the shore that break waves and reduce their impact on the shore.

• It has been implemented in US and Canadian beaches, and scientists say it is the most natural way to ease erosion.

• But this too will make waves converge on neighbouring beaches, thereby gradually eroding them further

MAGNITUDE OF MAINTANENCE DREDGING IN NMPT

• Entrance channels of harbours generally cross the littoral zone

• Annual maintanence dredging in the entrance channel indication of the littoral drift in the area

• As per data (1982-1987) avg volume of dredging the entrance channel is 1.57 million cubic m.

108

EVIDENCES AT THE RIVER MOUTH

• Inlets where river joins the sea migrates in the direction of the LD

• Large no of inlets ;but no consistent trend of their migration

• Cyclic tendency with migration to the north for some years & to the south for a few years(Netravathi Gurupur inlet at mangalore)

• Bengre sand spit at Mangalore oriented to the south

• Mulky Pavanje sandspit to the north

109

CONTINUED…

• Geomorphological formations at the river mouths are not the result of littoral drift but interplay of river discharges both water and sediment and wave action.

• Sediment size analysis done by KREC team confirms this observation

• Monsoon discharges in the West coast rivers are so high

110

CONTINUED…

• Changes in rivermouth like migration of sandspit very high in this season

• Large part of sediments by river & remaining by littoral drift

• Once the Monsoon is over disharges in the river of DK district drastically reduces

• Inlets are seen opened throughout the year ;Indicative that there is no significant LD in DK district

111

NETRAVATHI SPIT COMPLEX

112

• Part of Karnataka coast and associated with Netravathi Gurupur river system

• Spit grows in the direction of net shore drift irrespective of seasonal drift

• Northward growth of ullal spits indicates that effective shore drift is towards north(1910-1993)

• Supply of sediments for these spit is from longshore drift and river discharge

CONTINUED…

113

• In Ullal spit coarse to medium sand whereas in mangalore spit it is moderately sorted

• From nature of sorting its understood that a major amt of sediment contributed by Netravathi Gurupur river system for the growth of spit

• In 1967 -1993 Ullal spit drastically eroded mainly due to construction a shore normal break water on Mangalore spit

• This construction which extends beyond the nearshore environment impedes the normal longshore drift & causes deposition at updrift side which might have caused the widening of Mangalore spit

CONTINUED…

114

• Other area where interruption of littoral drift is occuring beaches adjacent to Udaiyavara river mouth where for the Malpe fisheries harbours , two bw has been constructed

• These breakwaters for guiding the flood waters of river in a confined way

• This would create sufficient velocity in monsoon months to flush the river channel from the sediments brought by the river

• This is necessary to maintain the required depth for the entry of fishing craft

CONCLUSION

• During monsoon seasons(June to September);wind direction normal to the coast:waves follow wind direction

• Monsoon waves approaching with their crest parallel to the shore ;negligible alongshore component of wave energy flux; corresponding LD small

• Due to littoral drift there is no major problems of coastal sustainability

115

SUMMARY

• Littoral drift phenomenon can be described as river of sand

• Littoral drift process can be constructive as well as destructive

• In east coast littoral drift rate is very high compared to west coast

• In DK coast effect of littoral drift is negligible.

• Its only a minor reason for the coastal erosion in Southern Karnataka

116

REFERENCES

• Danida-Dee-Mangalore Study On Coastal Erosion (Dakshina Kannada District):KREC:1994

• Wikipedia free encyclopedia http://en.wikipedia.org/wiki/Longshore_drift

• Prof. V. Sundar & Prof. R. Sundaravadivelu:Protection measures for Tamilnadu coast-

• J.A.G. Cooper1 & O.H. Pllkey; Longshore drift: trapped in an expected universe school of Environmental Studies, University of Ulster, Coastal Studies Research Group, Coleraine, BT52 ISA, Northern Ireland, U.K.

• B R Raghavan, B T Vinod K A Dimple T R Sreedhara Murthy: Netravathi spit complex,west coast of India:Dept of Marine Geology ,Mangalore 2001

• Tom Abbott, Biddulph High School and made available through www.sln.org.uk/geography

117

118

THANK YOU

Littoral Drift (2)

Documents

Transcript of Littoral Drift (2)