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 “Coastal Sedimentation” by L. R. Ranganath , SRO, CWPRS 3 Training Course on Coastal Engineering & Coastal Zone Management 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 Training Course on Coastal Engineering & Coastal Zone Management 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. “Coastal Sedimentation” by L. R. Ranganath , SRO, CWPRS 5 Training Course on Coastal Engineering & Coastal Zone Management 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