COASTAL PROCESSESold.cwc.gov.in/CPDAC-Website/Training/6_Coastall_Processes.pdf · illustrates the...

COASTAL PROCESSES

Dr. J. D. AGRAWAL, Senior Research Officer

Central Water & Power Research Station, Pune

1. INTRODUCTION

Coastal Processes is a collective term covering the action of natural forces on the shoreline and near shore seabed. Coastal engineers are associated with the task of construction of breakwaters, jetties, harbours and other structures on the coast and at the same time they have to ensure that the adjacent coast and beaches are not affected. In general these tasks are not compatible to each other. Commonly, construction of such structures has adverse effect on the adjacent beaches and may even affect beaches some distance away. The coastal engineer has to foresee this and try to prevent such damage by construction of shore protection structures such as seawalls, groins etc. For this purpose, an understanding of the processes involved in the near-shore region is required. This interest has led to important contributions to the understanding of the coastal processes.

2. TERMINOLOGY While describing the coastal processes, various terms are commonly used. Fig. 1 illustrates the typical beach profile,

Fig 1 (a) Typical Beach Profile

The definition of some of the commonly referred terms is given below – Alongshore: Parallel to and near the shoreline.

Training Course on Coastal Engineering & Coastal Zone Management

“Coastal Processes” by Dr. J. D. Agrawal, SRO , CWPRS 2

Armour layer: Protective layer on a breakwater or seawall composed of armour units / stones. Artificial Nourishment: The process of replenishing a beach with material 9usually sand) obtained from another location. Attenuation: A lessening of the wave height with distance from origin. Backshore: The zone of the beach profile extending landward from the sloping foreshore to the point of development of vegetation or change in physiography such as sea cliff, dune, etc. Barrier Beach: A bar essentially parallel to the shore, the crest of which is above normal high water level. Bathymetry: The measurement of water depths in oceans, seas and lakes. Beach: The zone of unconsolidated material that extends landward from the low waterline to the place where there is marked change in material or to the line of permanent vegetation. Breakwater: A man made structure protecting a shore area, harbour, anchorage or basin from waves. Beach Face: The sloping section of the beach profile below the berm, which is normally exposed to the action of the wave swash. Berm: A nearly horizontal portion of the beach or backshore formed by the deposition of sediment by the receding waves. Breaker Zone: The portion of the nearshore region in which the waves arriving from offshore reach instability and break. Chart Datum: A level to which soundings or tide heights are referred. Diffraction: The phenomenon by which energy is transmitted laterally along wave crest. Erosion: The wearing of land by the action of natural forces i.e. wave action, tidal currents, littoral currents etc. Groyne: Narrow, roughly shore-normal structure built to reduce longshore currents and / or to trap and retain littoral material. Foreshore: The sloping portion of the beach profile lying between a berm crest and the low-water mark of the backrush of the wave swash at low tide.



Inshore: The zone of the beach profile extending seaward from the foreshore to just beyond the breakwater zone. Irregular Waves: Waves with random wave period, which are typical for natural wind induced waves. Littoral Zone: Zone extending seaward from the shoreline to just beyond breaker zone. Longshore Bar: A ridge of sand running roughly parallel to the shoreline. Longshore Trough: An elongated depression extending parallel to the shoreline and any longshore bars that is present. There may be a series at different water depths. Offshore: The comparatively flat portion of the beach profile extending seaward from beyond the breaker zone to the edge of the continental shelf. Refraction: The process by which the direction of a wave moving in shallow water at an angle to the contour is changed. Rip Current: A strong surface current flowing seaward from the shore. Shoaling: Due to decrease in water, the transformation of wave profile as they propagate inshore. Shore: The strip of ground bordering the body of water, whether the ground is rock or loose sediment. Shoreline: The line of demarcation between water and the exposed beach. Significant Wave Height: The average height of the one third highest waves of a given wave group. Surf Zone: The portion of the nearshore region in which wave breaking takes place. This portion extends from the inner breakers shoreward to the swash zone. Tsunami: A long period water wave caused by an underwater disturbance such as volcanic eruption or earthquake. 3. REVIEW OF ENVIRONMENTAL PARAMETERS A wide range of environmental forces act on the beaches and coasts, which give rise to a variety of coastal processes in the near-shore region. Major environmental forces, which are important to a coastal engineer, are – (i) Waves (ii) Tides



(iii) Currents (iv) Wind and storm surge The above environmental parameters are described below. 3.1 Waves Waves are generated in the offshore region by the interfacial shear exerted by the wind blowing over the sea surface. The parameters, which govern the wave generation process, are: Fetch of water surface, which is subject to wind (F), Wind Speed (U), Duration of wind (t) The wind energy in the offshore region is transported to the coast in the form of waves. Fig. 2, shows a simple wave form with various characteristics such as wave height (H), wave period (T), wave length (L), wave amplitude (a) etc.

Fig. 2 : Wave Characteristics

The waves that are at generating area are Sea waves and are under the influence of wind and are steep and short period. Once these waves leave the generating area they are free from influencing forces. They are referred as swells. As waves approach the coastline, they undergo transformation due to the processes of refraction, shoaling, diffraction, wave breaking etc. The wave height and angle of approach of waves at the coastline play an important role in governing the coastal processes. 3.2 Tides Tides are defined as the periodic rise and fall of water level in the ocean caused by the gravitational attraction between the celestial bodies. Though many celestial bodies influence the tides, the moon and the sun are the main contributors to the gravitational pull compared to others. Gravitational pull causes water mass in one part of the ocean to rise with corresponding decrease in water level on the other part of the ocean. This rise and fall of water level in ocean is a regular phenomena and occurs daily. The magnitude of the rise and fall of water level at a given location is linked with the relative positions of the moon, the earth and the sun and



is influenced further by the geography of the location on the earth. Fig.3 shows the types of tides.

Fig. 3: Types of Tides

The tides are classified as (i) diurnal (ii) semi-diurnal and (iii) mixed. The diurnal tides have one high water and one low water in each lunar day i.e. 24 hours and 50 minutes. The semi-diurnal tides occur twice a day with a tidal period of 12 hours and 25 minutes. The mixed tides are characterized by a combination of diurnal and semidiurnal tides. It may be noted that the tidal range is maximum on full moon and no moon days, which are called spring tides. During the first and last quarter of the moon, the tidal range is small, which are called neap tides. 3.3 Currents In the ocean, currents are generated due to various reasons. Some of these, which are of concern to coastal engineers, are as follows:

a) Ocean circulation b) Tidal currents c) Nearshore currents

1. Longshore currents 2. Rip currents 3. On shore – off shore currents



3.3.1 Ocean Circulation Exchange of energy between the atmosphere and the ocean surface is complex. The atmospheric factors which generate circulation in the ocean are, wind and heating of the ocean. Wind Driven Circulation: The atmosphere receives large solar radiation in the equatorial region compared to the polar region. This differential heating causes air circulation, which is influenced by a phenomenon known as, Coriolis force. Thermohaline Circulation: This is a process, which occurs in the deep waters and is caused by the variation in the density of the seawater. This circulation is mainly a convection process wherein the cold water at high latitudes sinks and flows towards the equator. In essence, the major current systems in the oceans are due to the combined effects of wind blowing on the ocean surface and the variation in the density of seawater. Ocean currents to some extent influence the nearshore currents. A typical pattern of ocean circulation is shown in Fig.4.

Fig. 4: Ocean Circulation 3.3.2 Tidal Currents These currents are generated due to rise and fall of water level caused by tides. The tidal currents play an important role in the coastal processes. In the estuaries and near the river mouths, change in direction of flow every six hours is experienced due to tides. The flow reversal is caused due to rise and fall of water level in sea and the currents generated in the process are known as tidal currents.



In addition, tidal currents are quite strong along the region whenever the coastline exhibits drastic change in geometric shape. A typical current pattern due to tides is shown in Fig.5. 3.3.3 Nearshore Currents

In the nearshore region meteorological factors such as sunshine, wind and precipitation have strong influence on the coastal waters compared to that of deep ocean. Seasonal variations in water temperature, salinity and silt charge are comparatively large in coastal waters. As the coastal waters are influences by fresh water supplied by rivers, the seasonal change in temperature, salinity and silt charge cause density difference and generate currents.

Fig. 5 : Current Pattern due to Tide

In addition, predominant currents in the coastal region are generated due to waves. Depending upon the direction of wave approach, longshore current is generated. The longshore current continues to occur for some distance along the coast and returns to the sea in the form of rip currents. The rip currents are predominantly perpendicular to the coast and are very strong. Fig.6 shows the longshore and rip currents.

Fig 6: Currents Generated by Waves



Also on-shore offshore currents are generated across the beaches, which are caused by the waves breaking in the nearshore zone. The strength of these currents depends on the wave characteristics and the beach profile. 3.4 Winds and Storm Surge As seen earlier, the winds and storms generate waves in the sea, which travel to the coastline and influence the coastal processes. In addition, they cause rise in water level at the coast, which is known as storm surge. Due to the increase in water level at the coast due to the storm surge, higher waves can reach the coastline, which influence the coastal processes. 4. COASTAL SEDIMENTS The supply of sediments to the coastal zone is mostly through rivers and partially by disintegration of coastal rock outcrops due to continuous wave action. For making a sediment budget in coastal region, an attempt similar to the one described in Fig.7 could be considered. The sediment budget consists of littoral drift, sediment gain from river sources and cliff erosion, losses down a canyon and shoreward into sand dunes.

Fig. 7 : Sediment Budget In addition to the natural supply of sediments, the following human activities accelerate the sediment supply at a particular place.



a) Disposal of dredged material. b) Pumping of coastal sand for artificial nourishment of coastline. c) Dumping of inland-quarried sand for beach replenishment. 5. CHARACTERISTICS OF INDIAN COASTLINE At this stage, it is appropriate to look at the characteristics of the Indian coastline. The east coast of India is about 2650 km long whereas the west coast is 2900 km long. Important features of the Indian coastline are given below – West Coast

East Coast

• Flat sea bed slopes (1:100 to 1:500) • Wide continental shelf (about 250 km) • Tidal range : 1 to 6 m • Strong tidal currents (especially in the

Gulfs). • Wave climate less severe • About 2 storms per year • Southwest monsoon (May to Sept) • Littoral drift negligible • Only two major rivers meet the

Arabian Sea • Bed material : Clay, Silty-Clay

• Steep seabed slopes (1:30 to 1:100)

• Narrow continental shelf (about 20 km)

• Tidal range : 1 to 1.5 m • Peak tidal currents • Severe wave climate • Frequent storms (about 5 per

year) • Two monsoons : Southwest (May

to Sept) and Northwest (Oct to Jan)

• Large littoral drift : 0.5 million cum at Chennai to 1.5 million cum at Paradip

• Almost all rivers meet the Bay of

Bengal : High source of sediment

• Bed Material : fine sand

6. BEACH PROCESSES The morphological processess that the beaches undergo depends upon the interaction between external environmental forces and beach sediments. The beaches adapt their profiles, both transverse and longitudinal, according to the environment. Thus, beach maintains a dynamic equilibrium with the environmental conditions depending on the characteristics of the sediments on the beach.



6.1 Longshore Transport In the nearshore region, currents parallel to the shore are generated due to oblique incidence of waves, which are termed as longshore currents. Due to energy imparted by the breaking waves, littoral material is transported parallel to the shore, which is termed as littoral drift (Fig.8). At a particular beach site, transport may be to the right (looking sea ward) during part of the year and to the left during remainder of the year. If the left and right transports are denoted respectively Ql and Qr with Qr being assigned a positive quantity and Ql assigned a negative value., then net annual transport is defined as Qnet = Ql + Qr . The net longshore sediment transport rate is therefore directed right and positive if Qr > Ql

and to the left and negative if Qr <|Ql |. The gross annual transport is defined as Qgross = Qr +|Ql|, the sum of the temporal magnitudes of littoral transport irrespective of direction. These two contrasting assessment of longshore sediment movements have different engineering applications. For example, the gross longshore transport may be utilized in predicting shoaling rates in navigational channels and uncontrolled inlets, whereas the net longshore transport more often relates to the deposition versus erosion rates on opposite sides of jetties or breakwaters.

Fig. 8: Sediment Movement due to Oblique Attack of Waves

6.2 Quantitative Indicators of Longshore Transport Magnitude Analyses of the aerial photographs of the longshore growth of sand spits have been used to establish approximate rates of sediment transport. For reasonable estimates, a survey span of a decade or longer is necessary. The blockage of longshore sediment transport by jetties and breakwaters and the resulting growth and erosion patterns of the adjacent beaches yielded reasonable evaluation of the net transport rates at many coastal sites. Sand tracer has also been used to make short-term estimates of longshore sand transport.



6.3 Longshore Sediment Transport Predicting Methods In engineering applications, the longshore sediment transport rate is expressed as the volume rate having units cubic metre per day or per year. Various methods, which are used in the long shore transport determination, are as follows: 1. Energy Flux method. 2. Longshore current method. 3. Using hindcast wave data. 4. Littoral drift Roses. 5. Cross-shore distribution of longshore sediment transport. 6. Empirical shoreline models. 7. Analytical longshore sand transport shoreline change models. One of the empirical methods is as follows The rate of littoral drift (Q) can be computed as: Q (m3/yr) = 1290 Ρ Ρ = ρg H2 Cb Sin 2 θb / 16 Where ρ = mass density of water G = acceleration due to gravity H = wave height Cb = celerity of breaking wave θb = breaking wave angle. The littoral drift is thus a function of wave climate, sediment characteristics, beach profile and configuration of the shoreline. The littoral drift along the coast of India is shown in Fig. 9. It is seen that on the east coast of India, the general direction of littoral drift is from south to north, which is practically nil at Tuticorin and increases gradually to 1 to 1.5 million cum per year at Paradip. The littoral drift is less on the west coast having magnitude of 0.1 to 0.2 million cum per year and the direction varies depending on the local configuration of the coastline.



Fig. 9: Littoral Drift Along the Coast of India

The effect on the shoreline due to construction of structures in the coastal area prone to littoral drift is shown in Fig.10.

Fig. 10 Effect on Shoreline due to Littoral Drift

7. ONSHORE - OFFSHORE TRANSPORT The sediment transport in transverse direction to the coastline due to wave action is termed as onshore-offshore transport. This is especially predominant during storms, when high waves are encountered.



Onshore-offshore transport of sand associated with profile changes from storm to swell conditions (Fig.11) is correlated with the wave steepness H / L, the ratio of the wave height (H) to the deep-water wave length (L). Storm waves have high steepness values, due to both their greater heights and shorter periods, while long swell waves have low steepness values. In general for wave steepness of less than 0.025 (as in fair weather conditions) onshore movement of sand takes place, which results in building of beach whereas when wave steepness is greater than 0.025 (as in storm conditions), offshore movement of sand taken place resulting in erosion of the shoreline.

Fig. 11: Profile Change due to Onshore-Offshore Transport 8. TIDAL INLETS

A tidal inlet is a waterway connecting the sea and a bay, a lagoon or a river entrance through which tidal and other currents flow. The flow of currents into and out of a bay through an inlet provides natural flushing to maintain good water quality and reasonable salinity levels. Tidal inlet is a classical example, which demonstrates various coastal processes due to the combined effect of waves, tides, currents and wind. The typical features of a tidal inlet are shown in Fig.12. Tidal inlet technology includes a number of special topics, including tidal hydraulics, tidal inlet stability problems, and density & pollution problems. Tidal hydraulics, tidal inlet stability problems have been discussed here.

Fig. 12 : Typical Features of Tidal Inlet



8.1 Tidal Inlet Hydraulics Keulegan (1967) made an attempt to find out the inlet hydraulics by introducing a parameter ‘ K ‘ by relating it to maximum inlet current, the tide range of the bay and the phase lag of the bay tide relative to the tide in the ocean in terms of parameters which can be easily measured or determined, including inlet cross-sectional area, bay surface area, ocean tide amplitude and period, length of channel, and head loss coefficients. After applying few assumptions Keulegan developed a solution for velocity and resulting bay tide, which contained a parameter ‘K’ known as coefficient of filling, which is defined below:

++

=

RfLkka

gA

TA

exenb

avg

4

22

K0

π

Where Aavg, = average area over the channel length. Ab = surface area of bay g = acceleration due to gravity R = inlet hydraulic radius T = tidal period a0 = ocean tide amplitude ken = entrance energy loss coefficient kex = exit energy loss coefficient L = inlet length Keulegan K values can be used to understand effect of gross geometric variations on inlet hydraulics. Fig 13 shows the ocean and bay tide curves and velocities in the channel during tidal cycle phasing and bay tide amplitude for various Keulegan K values. Three K values are represented, lower the K, the lower the tide range in the basin and the greater the phase lag in the ocean and high water in the bay. As the K increases, and bay fills more completely, peak ebb and flood flows tend to occur near the same tide level. These variations have implications with regard to sediment transport. Mota Oliveira (1970) found that, for 0.6<K<0.8, the inlet had maximum sediment flushing ability. When k>0.8, there was flood dominance of sediment transport capacity, meaning net bay ward transport and when K<0.6, there was ebb dominance of sediment transport capability.



Fig 13 : Hydraulic Response Of Inlet And Bay Tide Phasing And Bay Tide Amplitude For Various Keulegan ‘K’ Values.

8.2 Hydrodynamics and Sediment Interaction at Tidal Inlets Waves, current and sediment interact at tidal inlet over a large physical area, with varying effects over a tidal cycle. At the inlet, tidal currents are predominant and forcing agent interacting with sediments. Wave energy on the edges of the inlet channel can contribute sediment from adjoining beaches, with flood tidal current tending to move bayward or the ebb current jetting sediment sea ward to settle on the ebb shoal. Sediments may again be moved to adjoining beaches by the combination of waves and currents, thus ‘bypassing’ the inlet.



8.3 Tidal Prism Channel Area Relationship O’Brien (1931,1969) originally determined a relationship between minimum throat cross-sectional area of an inlet below mean tide level and the tidal prism (i.e. the volume of water entering or exiting the inlet on ebb and flood tide) at spring tide. For dual jettied inlet (O‘Brien) Ac = 7.489 * 10–4 P 0.86 Dean (1971) found that velocity equal to 1m/sec might be interpreted as a level of velocity necessary to maintain an equilibrium flow area. Escoffier (1977) proposed a diagram (Fig.14) for inlet stability analysis in which two curves are initially plotted. The first is the velocity versus the inlets cross sectional flow area Ac . As the area approaches zero velocity also approaches zero due to increase in frictional forces. As the channel area increases, frictional forces are reduced but, on the far right side of the curve, velocity decreases as the tidal prism has reached a maximum. And any area increases just decreases velocity. If channel area increases (move right on curve from point b) velocity will fall and more sediment can fill in the channel to bring it back to equilibrium. If area decreases, velocity will increase scouring back to the equilibrium point

Fig.14 : Maximum Velocity And Equilibrium Velocity Versus Inlet Cross-Sectional Area

Inlet stability ratings can also be judged based on the relationship of a ratio P/M, where P is the tidal prism and M is the total annual littoral drift.



150≥MP

Conditions are relatively good, little bar and good flushing.

150100 ≤≤MP

Conditions become less satisfactorily, and offshore bar formation becomes more pronounced

10050 ≤≤MP

Entrance bar may be rather large, but there is usually a channel through the bar

5020 ≤≤MP

All inlets typical bar-bypassers

20≤MP

Descriptive of cases where the entrance becomes unstable over flow channels rather than permanent inlet.

9. CONCLUDING REMARKS It is seen from the foregoing that the beach is under a state of dynamic equilibrium with the environmental parameters of waves, tides, currents, and winds etc, which are prevalent in the coastal area. The response of the beach is a function of shoreline configuration, beach profile and sediment characteristics. Also, the coastal processes are influenced by sources / sinks of sediment materials. Thus, the morphological processes in coastal areas are quite complex and need comprehensive understanding of the various coastal processes involved. The coastal engineer has to plan development activities duly considering the coastal processes in that region. 10 References: 1. Coastal Engineering Manual, U S Army Corps of Engineers, 2003 2. Dean R G 1971, ’Hydraulics of Inlets’ COEL/UFL-71/019, Univ. of Florida,

Gainsville. 3. Escoffier F F 1977,’ Hydraulic and stability of tidal inlets’ GITI Report 13, US

Army Engineer Waterways Exp. St. Vicksburg, MS. 4. Keulegan G H 1967,’ Tidal flow in entrance water level fluctuations of basins

in communications with seas’. Technical Bulletin No 14, US Army Engineer Waterways Exp. St. Vicksburg, MS.

5. Mota Oliveira I B 1970 ‘Natural flushing ability in tidal inlets’, Procee. of the Twelfth Coastal Engg. Conf. ASCE pp 1827-1845

6. O’Brien M P 1969 ‘Equilibrium flow areas of inlets on sandy coasts’ Journal of the Waterways Harbors division, ASCE No. WWI pp 43-52.

7. Water Wave Mechanics for Engineers and Scientist, Robert G Dean and Robert G Dalrymple, World Scientific Publishing Company, 2004.

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