Coastal Sedimentation

COASTAL SEDIMENTATION

L. R. RANGANATH SENIOR RESEARCH OFFICER Central Water and Power Research Station, Pune

1 INTRODUCTION This lecture addresses sedimentation and erosion engineering problems in estuaries and coastal seas and practical solutions of these problems based on the results of field measurements, laboratory scale models and numerical models. Often, the sedimentation problem is a critical element in the economic feasibility of a project, particularly when each year relatively large quantities of sediment material have to be dredged and disposed at far-field locations. For studying the problems posed by the sediment transport in any engineering projects, it is necessary to have the thorough knowledge of the flow conditions and type of bed sediment at the project site. Several experiments and studies were conducted over decades to understand the course of sediment movement. In recent past with the advent of digital computers, numerical models have developed to describe the flow field and sediment movement. Since the sediment movement is governed by several factors of the site conditions, even today it is not fully understood and is still a research problem. Although engineering projects are aimed at solving problems, it has long been known that these projects can also contribute to creating problems at other nearby locations (side effects). Erosion often occurs in places where sediment cannot be supplied by nature in sufficient quantities because it is trapped in another part of the system. The trapping can be due to natural causes or due to man-made changes in the system. Dredging of Navigational channels, construction of jetties, groynes and seawalls always results in the redistribution of sand within the local system. Engineering works should be designed in such a way that side effects (sand trapping, sand starvation, downdrift erosion) are minimum. Dramatic examples of side effects can be visualized at Chennai due to the development of Port. Nourishment and bypassing of sand are often required to mitigate the unavoidable side-effects of engineering works. These measures can be observed at Vishakapattanam and Paradip Ports in India. It is important to emphasize that engineering works should be designed and constructed or built in harmony rather than in conflict with nature. This ‘building with nature’ approach requires a profound understanding of the sediment transport processes in morphological systems.

2 SEDIMENT TRANSPORT PROCESSES

The erosion, transport and deposition of sediments in coastal zone depend on the physical and chemical properties of the bed sediments, prevailing flow conditions due to tide, wave and wind conditions and topography of the area. It is necessary to understand the sources of sediment available before going into its transport and deposition processes. Training Course on Coastal Engineering & Coastal Zone Management

Sources of Sediment for an Estuary • sediment laden rivers and streams (sr) • littoral drift and bank erosion (slb) • wind erosion of coastal dunes (sw) • erosion of near-shore continental shelf (so) • return of dredged soil (sd) • effluent and solid waste disposal, etc. (sp)

Factors Governing Sediment Transport

Forcing factors: Tidal currents Wave induced radiation stresses Bed shear stresses

Sediment resisting factors: Particle size Specific gravity Settling velocity Flocculation

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Modes of Sediment Transport

Bed Load Transport: The bed load is the part of the total load that is more or less in contact with the bed during the transport. It primarily includes grains that roll, slide, or jump along the bed. Thus the bed load movement is governed by the shear velocity at the bed and effective bed resistance on the bed surface.

Suspended Load Transport: The suspended load is the part of the total load that is moving in suspension without continuous contact with the bed as a result of the agitation of fluid turbulence. Many estuary deposits contain large proportion of fine sediments which readily set in motion by tidal current. The primary transport mode of fine sediments is in fact as suspended load and such sediment may amount to 75-95% of the total load in estuaries.

Wash Load Transport: It is the fine sediment generally oscillating near the shore and considered as negligible.

Computation of Bed Load Transport

Methods available for bed load transport computations Engelund-Hansen, Einstein, Bijkers and Kabib The generally used formula for bed load computations is Engelund-Hansen which gives sediment transport rate of per unit width per unit time as:

3/2 d 50 τ q = 0.05 γ V 2 | | (1) b s γ (γ -γ ) g( s - 1) s d 50 γ where, τ is the shear stress due to current only

In order to arrive at proper lay out of port with the proper alignment of the breakwaters orientation and depth of channels it is necessary to study the hydrodynamics and sediment movement pattern in the vicinity of project site.

3 SEDIMENTATION PROBLEMS

Sediment transport in the coastal zone is a physical phenomenon which is highly unpredictable and is a challenge to the Coastal engineers. Human interference in hydraulic systems often is necessary to maintain and extend economic activities related to ports and associated navigation channels. In many situations engineering structures are required to stabilize the shoreline, shoals and inlets, to reduce sedimentation, to prevent or reduce erosion, or to increase the channel depth to allow larger vessels entering the harbour basin. Coastal protection against floods and navigability are the most basic problems in many estuaries in the world. Examples of Engineering Works in Estuaries and Coastal Systems are shown in Fig. 1.

“Coastal Sedimentation” by L. R. Ranganath , SRO, CWPRS 4

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Fig. 1 Examples of Man-Made Structures in River, Estuary and Coastal Systems

Sedimentation problems generally occur at locations where the sediment transporting capacity of the hydraulic system is reduced due to the decrease of the steady (currents) and oscillatory (waves) flow velocities and related turbulent motions. Examples are: the expansion of the flow depth and width due to natural variations or artificial measures (dredging), the presence of vortex or eddy zones, flow separation zones, dead water zones and lee zones of structures. Expansions of the navigational depth will reduce velocities inducing shoaling. Similarly, the expansion of the width of turning and mooring basins inside a harbour area will reduce velocities stimulating shoaling conditions. Piers or piling structures create eddies resulting in increased shoaling. Sedimentation problems are most often associated with human interference in the physical system such as the construction of artificial structures or the dredging of sediment from the bed to increase the flow depth or width. However, sedimentation (as well as erosion) also is a basic phenomenon of nature dealing with loose sediments within the transporting cycle from source to sink locations. Natural sedimentation areas are known as shoals, flats, banks, sheets, bars, etc. Human interference in these natural sedimentation areas will always lead to relatively large maintenance cost and should therefore be avoided as much as possible. Sedimentation problems are herein classified, as follows:

Channel Basin sedimentation Shoreline sedimentation sedimentation

1. Navigation channels. 1.Harbour and port basins, 1. Updrift area of 2. Inlet channels. Docks. groynes and 3. Entrance channels of 2.Open settling basins, breakwaters harbours, docks and turning and mooring normal to shore. water intakes. basins, mining pits. 2. lee-side area of 4. Trenches for tunnels, 3.Water intake basins. offshore pipelines and cables. 4.Flood plains and reservoirs. breakwaters parallel to shore.

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3.1 Navigation Channels

Ports are of vital importance for the economy of coastal countries. The increasing draft of vessels requires the dredging of deep-draft channels connecting the port to deep water. Generally, these channels suffer from sedimentation requiring maintenance dredging to ensure safe passage of the ships under most conditions. The costs related to capital and maintenance dredging often is critical in the economic functioning of ports. Therefore, the channel should be designed in such a way that sedimentation is minimum. When the flow passes a channel, the velocities decrease due to the increase of the water depths in the channel and hence the transport capacity of bed load and suspended load decreases. As a result the bed-

Fig.2 Channel sedimentation load particles and a certain amount of the suspended sediment particles will be deposited in the channel (Fig. 2).

When waves are present, this process is considerably enhanced due to the sediment stirring action of the wave motions in the near-bed region resulting in larger sediment concentrations, which are subsequently transported by the flow.

Factors enhancing sedimentation are:

• Deep and wide channel;

• Orientation almost normal to the flow;

• Strong flows and large waves passing the channel;

“Coastal Sedimentation” by L. R. Ranganath , SRO, CWPRS 6

Training Course on Coastal Engineering & Coastal Zone Management

• Fine sediment (fine sand and mud);

• Alignment through shoaling areas.

Natural navigation channels in estuaries often suffer from the generation of bars/shoals at the transition of flood-dominated and ebb-dominated channels.

Inlet Channels

Natural tidal inlet channels generally suffer from heavy sedimentation due to wave- induced longshore input of sediments thereby reducing the navigability of the channel. Jetties (long dam-like structures) are commonly built to eliminate the input of sediment by the longshore current, creating an inlet channel (Fig. 3). The jetties should be so long that the sedimentation at the entrance of the channel due to longshore bypassing of sediments is minimum. Sediment accumulation will generally take place on the updrift side of the jetties and erosion on the downdrift side. Mechanical bypassing of sediment may be required to reduce downdrift erosion. Jetty design requires the following considerations to reduce sedimentation:

• The length of the jetties should extend beyond the littoral transport zone;

• The jetty spacing should be narrow, but not leading to excessively large channel velocities undermining the jetties (wide jetties may lead to shoaling and meandering of the channel);

• The jetties should be impermeable to prevent lateral passage of water and littoral sediments;

Fig.3 Sedimentation of inlets, entrances and intakes

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• The jetties should be parallel, rather than curved or tapered (narrow entrance, channel widening); parallel jetties tend to confine the ebb flow, raising ebb velocities and thereby Flushing sediment out of the channel into the sea;

• Three jetties creating two parallel channels, may be build to separate the inlet channel from the river outflow;

• A settling basin/trap may be dredged at the entrance of the channel as a buffer for sedimentation to ensure navigability of the channel.

3.2 Entrance Channels of Harbours, Docks and Water Intakes

The entrance to a harbour basin, dock or water intake basin generally suffers from sedimentation due to the reduction of flow velocities and wave activity (Fig. 3). Often, a circulation cell is generated in the entrance of the basin due to the geometry involved, which attracts sediments by exchange processes with the main flow system outside the entrance. The breakwaters should be streamlined or training walls may be built to reduce eddies and dead-water areas. Harbour entrances should never be built on the inside of bends, where natural shoaling processes (point bars) generally occurs.

Fig. 4 Sediment deposition pattern

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Entrance design requires the following considerations to reduce sedimentation:

• The entrance should not be located in shallow depth on the inside of a bend or close to other natural shoaling areas (Fig. 4); • The entrance should be narrow and streamlined to reduce the generation of eddies, and circulation zones; • The breakwaters should be impermeable to prevent lateral passage of water and sediments; • A settling basin/trap may be dredged at the entrance of the channel as a buffer for sedimentation.

3.3 Harbours and Ports

Sedimentation in harbours and ports is a problem that exists as long as harbours and ports exist and is related to their basic function, providing shelter by creating quiescent conditions. In general, a harbour is a place that provides shelter and mooring for ships, whereas a port is a place where cargoes are loaded and unloaded from ships. Both types of places require quiescent conditions protected from wave penetration and strong currents.

The design and construction of harbour basins is one of the oldest branches of engineering. One of the oldest known artificial harbours was PHAROS, located on the open coast of Egypt at about 2000 B.C. It had two parallel breakwaters, each about 2.5 km long which consisted of rubble-mound structures of very large blocks of rock and this harbour undoubtedly suffered from sedimentation.

There are various types of natural harbours:

• Well protected bays; • Lee areas of islands or headlands;

• Lee areas of reefs; • Lee areas in river mouths. Artificial harbours can, in principle, be made at any place along the water front, but should preferably be located in areas where the sedimentation is observed or estimated to be minimum.

Artificial harbour basins can be classified into: • open basins; basins or berthing places which are open to flow and/ or waves; these basins can be situated in nearshore coastal areas and along river banks, see Fig. 5; the basin generally is substantially deeper than the surrounding area so that an acces channel is required for vessels to enter the basin; mooring is only possible in conditions with weak cross-currents and mild wave motions;

• sheltered basins (enclosed); these types of enclosed basins are typified by a single entrance to reduce the flow and wave motion as much as possible; three subclasses can be identified: “Coastal Sedimentation” by L. R. Ranganath , SRO, CWPRS 9

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- coastal basin with a controlled entrance, generally consisting of twin-armed breakwaters or jetties, the breakwaters may block the longshore sediment transport requiring sediment bypassing methods; eddies and turbulence may be produced in the entrance, a semi-enclosed basin is obtained when only one breakwater or jetty is present on the updrift side.

- coastal dredged lagoon with an uncontrolled entrance (Fig. 5), which may migrate along the shore or may suffer from severe sedimentation due to wave-induced processes or may even be closed by wave- induced sedimentation during storms; the entrance may suffer from migrating channels and bars; relatively strong currents may be present due to tidal filling of the basin; entrance may also be controlled by jetties.

- interior basin (docks) along esturial or river channels (Fig. 5) and/or lakes; eddies are generally present in the entrance area, where sedimentation of silty and muddy materials usually is maximum.

Fig.5 Types of Harbour basins.

The most important and difficult part of a sheltered harbour basin is the entrance. Many harbour entrances have been found to be difficult for ships to navigate owing to currents, waves and morphology (sedimentation).

From observed sedimentation rates it can be concluded that harbour sedimentation in fresh water conditions is much less than in salt and brackish water conditions. The generation of stratified flow with a clear salt water wedge is a well known phenomenon in the tidal zone of major rivers (tidal volume about equal to fresh water volume over tidal period). The maximum silt and mud concentrations are generally found in the area where the wedge of the salt water front is moving up and down the river channel. This zone where soft fluid mud layers are formed due to deposition at slack tides (especially neap tides) is known as the turbidity maximum. Harbour basins should preferably be situated outside this zone to avoid that the deposited fluid mud layers penetrate into nearby harbour basins.

3.4 Turning, Mooring and Settling Basins

Expansions of width and depth are often required outside and inside harbours or docks for navigational reasons. The flow in these basins is reduced considerably in proportion to the expansion dimensions resulting in rapid siltation of fine material. This principle can also be used for the design of a settling basin in areas of rapid shoaling to create a buffer (overdepth) for sediment from which dredging activities can be performed to remove the sediment from the system. “Coastal Sedimentation” by L. R. Ranganath , SRO, CWPRS 10

Training Course on Coastal Engineering & Coastal Zone Management

3.5 Shorelines

Shorelines may suffer from large-scale and small-scale sedimentation and erosion processes. Structures such as groynes or breakwaters perpendicular or oblique to the shoreline (Fig. 6) will lead to sedimentation on the updrift side of the structure due to the blocking of the sediment transport (current-induced and wave-induced transport of sediment). Similarly, sedimentation will occur in the lee of a shoreparallel breakwater (Fig. 6). Generally, erosion will take place on the downdrift side, because the sediment transport will gradually restore itself eroding sediment particles from the Fig.6 Shoreline accretion and erosion Near coastal structures. downdrift side. Mechanical bypassing of sediment may be necessary to deal with the downdrift erosion near structures.

Large scale shoreline sedimentation and erosion are generally caused by:

• Obstruction of the sediment transport processes due to the presence of natural barriers or artificial barriers. • Episodic flood or storm-induced processes. • Fluctuations in sediment supply and transport. • Onshore/offshore-directed sediment transport to or away from the shoreline. • Mining/dredging and dumping of sediment.

The available options to deal with typical erosion problems, are: (1) to accept retreat in areas where the shorelines/banks are wide and high; (2) to maintain the shoreline at a fixed position (by hard and/or soft structures or by dredging activities) and (3) to bring the shoreline forward by reclaiming land.

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4 CASE STUDIES (CWPRS,Pune)

4.1 Mathematical model studies for developments in Salaya harbour proposed by m/s Essar power Gujarat Limited at Salaya, Gujarat

The ESSAR group of Industries propose to develop 1200MW power project near Salaya, Jamnagar District, Gujarat with marine facilities like jetty for handling bulk cargo and containers, seawater intake and outfall systems. The Salaya creek is part of the Gulf of Kutch along the coastline of Gujarat. It has wide tidal flats on both sides and deep pockets along the creek. The proposed project envisages construction of about 900m long jetty with an approach channel and a turning circle to cater to the vessels up to 150,000 DWT for bulk cargo and container cargo with desired draft more than 17m. The proposal also includes construction of 10 m wide solid approach bund of about 7.2 km and an additional 1 km trestle up to the jetty. In this regard model studies were referred to CWPRS by WAPCOS to suggest the suitable layout of the channel and the turning circle for the proposed jetty. Accordingly following studies were carried out and single comprehensive technical report was submitted.

1. Mathematical model studies for tidal hydrodynamics 2. Mathematical model studies for wave tranquility 3. Mathematical model studies for moored ship behaviour at berth 4. Mathematical model studies for ship navigation for optimization of channel width 5. Mathematical model studies for Siltation in the approach channel and dredged material disposal studies.

These model studies were conducted to investigate the feasibility of the channel alignment evolved during the mutual discussions with ESSAR. Hydrodynamic studies were carried out for both existing and with proposed developments. From the tidal hydrodynamics studies it was found that there would be no significant change in the flow conditions after the proposed dredging of the channel except that the flow gets streamlined along the channel. Study shows that the orientation of the proposed jetty is in line with the flow direction and there would be no circulations near the jetty as well as in the turning circle. Wave data analysis shows that the predominant wave directions are from South, SSW, East (2250N to 2700N) and wave propagation studies indicate that there would be no significant wave disturbance at the jetty throughout the year.

From the studies on ship motions at berth for the determination of the mooring rope tensions and fender deflections of the 1,50,000 DWT vessel moored at the proposed NE – and SW - Berths at Salaya it is found that the ship motions at the berths are within the limits for safe working conditions as recommended by PIANC. Navigation studies indicated that under severe environmental conditions, considering a safety margin, uniform channel width of 260m is recommended for safe manoeuvering of the design ship. Sediment transport studies showed that the depth of deposition along the outer channel and at turning circle varies from 0.1 to 0.2m during monsoon months. Considering the depth of deposition in the different “Coastal Sedimentation” by L. R. Ranganath , SRO, CWPRS 12

Training Course on Coastal Engineering & Coastal Zone Management regions it is found that the sedimentation in the outer channel and turning circle would be 0.2 to 0.3 Mm3 per annum. Based on the sediment disposal studies it is suggested to dispose the dredge material beyond 20 m contour in the gulf of Kutch to avoid reentry of the sediments in to the channel or near jetty location. Further the sedimentation pattern in the region reveals that the depositions are mainly in the tidal flats adjacent to channel and marginal accretion along the outer creek banks. Thus from the studies carried out, it was inferred that there would be no significant changes in the existing flow conditions due to proposed developments at Salaya, as the orientation of the proposed jetty and the approach bund are in line with the flow direction and there would be no adverse morphological changes due to siltation and disposal of dredge material.

GULF OF KUTCH

DHANI BET

KARUMBHAR REEF

JETTY LOCATION

SALAYA

3-D VIEW OF COMPUTATIONAL MODEL FLOW FIELD DURING PEAK EBB

Fig.7 Some of the salient features of the studies.

4.2 Mathematical model studies for prediction of siltation near outfall and inner canal of samudra khal at Junput, Purba Medinipur, West Bengal

Junput is situated in the Purba Medinipur district of coastal West Bengal and is noted for its high aquatic salinity due to its proximity to the Bay of Bengal. The confluence of Samudra Khal is located at Junput in the district of Purba Medinipur along the coastline of West Bengal which is gradually silting up and reducing navigability in all tidal conditions. This tidal inlet is known for traditional fishing activity with good number of fishing vessels operating in the area. However, these vessels are experiencing difficulties in their movement due to inadequate draft even during ebb tide conditions. To facilitate continuous movement of fishing vessels all through the tidal phase, West Bengal Fisheries Corporation (WBFC) Ltd, proposes dredging in the channel and at the mouth to improve the navigational conditions. WBFC proposes to have two lane movement of fishing vessels as per I.S.Code No. IS 4651(Part-V) 1980 having a length of 13 M, width of 2.6 M and a draft of 1.5 M. Presently the entrance is nearly 600m wide and has shallow depths. The beaches on either side are quite flat. The inshore-offshore movement of sediments due to tidal currents seems to be the main cause for siltation at the entrance. To over come the above situation and finding out a proper way to keep the Samudra Khal navigable at all tidal conditions WBFC approached CWPRS to

“Coastal Sedimentation” by L. R. Ranganath , SRO, CWPRS 13

Training Course on Coastal Engineering & Coastal Zone Management study hydrodynamics and sedimentation aspects at the entrance and inside the Samudra Khal. Accordingly Mathematical model studies were conducted for simulating tidal hydrodynamics and sedimentation at the entrance and inside the Samudra Khal.

The studies revealed that by dredging and resizing the channel to a depth of - 1.1m RL with a total channel depth of 2.1m at low water (+1.00 HRD) and a bottom width of about 30m would provide continuous flow in the channel all through the tidal cycle. The dredging of the channel on the upstream side and on the offshore side may be carried out as per the recommendations in IS code. Sedimentation study indicated that the sediment deposition is mainly observed at the mouth/entrance of the channel, which is required to be dredged every alternate year. The loss in depth in the channel at the entrance would be of the order of 0.10 to 0.30m after the monsoon. The quantum of maintenance dredging works out to be 15,000 cum to 20,000 cum annually but this has to be monitored for couple of years after implementation of the project. The maintenance dredging is required to be carried out every alternate year so that the mouth remains open for navigation. The Capital dredging for the deepening and resizing of the channel works out to be of the order of 0.5 Mcum. The dredged material can also be utilized by reclaiming suitable area near Ganga mandir to utilize it in future for storage of fish and fish drying activity.

Fig.8 2-D View of the computational model with dredged channel.

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Fig.9 Capital Dredging Zone.

REFERENCES Coastal Engineering Manual (CEM), 2003,CHL, US Waterways Experiment Station – Vicksburg, Mississippi “Coastal Sediment Processes” Part III, 2003

Komar P D and Moore J R (Ed. 1983), CRC handbook of Coastal Processes and erosion, CRC Press Inc.

Ranganath L R, Krishna B and Kanetkar C. N, (2010) “ A numerical model study for development of a harbour layout in a creek with wide tidal flat ” International Conference on Hydro-Science and Engineering (ICHE 2010),IIT Madras, Chennai.

Ranganath L R, Krishna B and Kanetkar C. N, (2009) “Numerical model studies for prediction of siltation near outfall and inner canal of a fishing harbour: A case study” Speciality conference on River Hydraulics,Maharishi Markandeshwar University, Maulana.

Van Rijn, L.C., 2004, Principles of Sedimentation and Erosion Engineering in Rivers, Estuaries and Coastal Seas.

Van Rijn, L.C., 2004, ESTUARINE AND COASTAL SEDIMENTATION PROBLEMS, Proceedings of the Ninth International Symposium on River Sedimentation October 18 – 21, 2004, YICHANG, CHINA

Van Rijn, L.C., 1986, ‘Sedimentation of dredged channels by current and waves, Journal of waterway. Port, coastal and ocean engineering, Vol. 112, No.5, ASCE.

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