Science and technology
working with nature- civil and hydraulic engineering to aspects of real world problems in water and at the waterfront - within coastal environments
A harbor is a water basin of tranquil or tolerable wave and current climate, and of sufficient water depth in which a maritime or inland vessel (let us use this general term, but when Dead Weight Tonnage or DWT ≥ 500, a vessel is known as a ship) can operate safely. Maritime harbors are selected from the deep shoreline areas sheltered naturally, or are created artificially (see Flood Barrier Systems). The artificial harbors are configured and engineered within an ambient water body at the shoreline by dredging and installing suitable structures (see Breakwater). The purpose in each case is to locate a maritime port or marina within it (see Ship Motion and Mooring Restraints; and Propwash). For the convenience of design and operation, a harbor is classified and distinguished as deep-draft (water depth > 15 ft or 4.6 m), and shallow-draft or small-craft (water depth < 15 ft or 4.6 m). Many artificial harbors have one inlet to allow influx and efflux of water and sediment into the basin (a semi-enclosed basin that allows restricted/controlled entry and exit of matter and energy, see Upslope Events and Downslope Processes); and entry and exit of vessels. The layout of the structure – and the location, width and depth of the approach channel as well as of the harbor itself are designed by addressing such constraints as – ambient wave, current and sediment climates, and the largest allowable vessel designed to call at the port.
In this piece, let us attempt to discuss and understand the sedimentation rates of harbors in simple terms. Sediment transport dynamics and sedimentation pose a complicated problem. But ballpark estimates and numbers are always handy and useful to conceive and study the feasibility of a project. To that end, some methods and pieces of data are selected and blended in this piece. The purpose is to demonstrate the usefulness of some simple analytical models that can be used as a handy tool to picture a high-level impression of possible harbor sedimentation. The magnitude of sedimentation problem can be appreciated if one considers worldwide dredging operations. Maintaining enough water depth within the harbor and keeping the approach channels navigable – are some of the requirements that let flourishing of huge dredging industries. These two major demands, together with the erosion prevention and value-adding beach nourishment works, and others – have yielded the global dredging industry to an annual turnover of some $5.6 billion. I will try to come back to discussing different interesting aspects of dredging at a later time.
Among others, this piece is primarily based on: RB Krone 1962; Delft Hydraulics publications (E Allersma 1982; WD Eysink and H Vermass 1983 and WD Eysink 1989); R Soulsby 1997; USACE 2002 EM 1110-2-1100 (Part III) and 2006 EM 1110-2-1110 (Part II); I Smith 2006; and my own works on fine sediments and sedimentation (Fluid Mud 2008; and Settling Velocity of Natural Sediments 2004) published in the Journal of Hydraulic Engineering, and Journal of Waterway, Port, Coastal and Ocean Engineering, respectively; Seabed Roughness; and two papers presented at the International Symposiums on Coastal Ocean Space Utilization: COSU 1995 and COSU 1993, and my paper at the 24th International Conference on Coastal Engineering, Kobe, Japan, ICCE 1994. The Hydraulics of Sediment Transport and Resistance to Flow posted earlier laid out some fundamentals of sediment behavior and transport.
Configuring the layout of a harbor entrance needs careful optimization exercises and analyses – on the one hand, it has to provide effective diffractive energy dissipation of incoming waves – on the other, it has to minimize the formation and strength of current eddies at the entrance, and sedimentation inside the basin. Filling and emptying tidal currents at a harbor entrance are usually an order of magnitude less than the ambient tidal current. Their magnitudes depend on the size of the basin and entrance. Eddies – more vigorous during changing current directions – are undesirable for at least two primary reasons. The first is to minimize navigation hazards – to vessels entering and leaving the port. The second is to minimize scour and formation of sandy bars. Exercises to engineer a detailed and optimal layout include physical scale modeling and/or numerical modeling. Such exercises, especially the efforts of numerical modeling (see Water Modeling) are becoming increasingly common not only for optimizing harbor entrance layout, but also for visualizing the sediment morphodynamics, sedimentation and other aspects of harbor hydraulics (e.g. Ports 2013 paper).
Before moving on, let us have some words on tidal action. It is assumed that actions attributed to short-waves (see Ocean Waves and Linear Waves) and vessel generated wake-waves are minimal – a valid assumption for all harbors. The main concern of harbor sedimentation processes is the behavior of Suspended Sediment Concentration (SSC) that has a positive gradient from low at top to high at bottom of the water column. As flood and ebb currents reach threshold for erosion and resuspension during a tidal period – sediments are picked up from the seabed and are transported (coarser fraction close to the bed; fines up in the water column) back and forth by the current. Similar but opposite episodes happen, as flood and ebb currents slow down to reach deposition threshold. Suspended sediments have the opportunity to settle down during such slack water periods (SWP) – with more chances for sediments close to the bed than those up in the water column. However, there is often a settlement lag or incoherence between the slack water and actual deposition (see more in my 1990 Elsevier paper).
Let us now dive down into the core issue of this piece. A harbor faces at least two types of major sedimentation problems. The first is the formation of localized shoals or sandbars at and around the entrance due to the scouring actions of eddies, and the sudden drop in flow velocities. These shoals mostly of sandy materials are often attached to the shoreline as a side bar or develop as middle bar(s). They mostly develop when the harbor entrance is located on littoral shores (see Managing Coastal Inlets) – and are usually termed as flood-tidal and ebb-tidal deltas (see Coastal River Delta and Managing Coastal Inlets). The dynamics of such sandy shoals, bars or deltas can best be discerned from the piece on The Hydraulics of Sediment Transport.
The focus of this piece is on the second type of sedimentation problem. It is the sedimentation of fine sediments within the harbor basin. This sedimentation (a phenomenon of suspended sediments having very low settling velocities) is somewhat uniform due to the relatively weak circulation within the harbor basin – but is often less in areas of relatively high currents than in remote areas of stagnant water. It is highly problematic when a harbor is located within Turbidity Maximum (TM) zone (1990 Elsevier paper). The presence of TM in the tide-dominated east shore channels and waterways of the Ganges-Brahmaputra-Meghna (GBM) River mouth has shown very high siltation rates of fine sediments (1997 Taylor & Francis paper). Observations at the mouth of Karnafuli River estuary showed a positive correlation between the Surface Suspended Sediment Concentration (SSSC) and tidal range (TR) – indicating that the resuspension actions of tidal currents are directly related to tidal range. This correlation ends up yielding an exponential relation between SSSC and TR (ICCE 1994). The fitted relation shows, for example, that at mean neap-tidal TR = 1.7 m, SSSC = 154 mg/L; and at mean spring tidal TR = 3.8 m, SSSC = 1912 mg/L.
The gradual but slow filling up of the basin is highly dependent on the concentration of sediment suspended (in textures of fine sand, silt and clay) of influx water. For the convenience of discussion, let us spilt the piece in two: (1) the first is on sedimentation of granular (silt-sized particles) materials; and (2) the second is on sedimentation of silty/clayey materials that are affected by aggregation and flocculation. The provided estimates represent only a high-level first-order magnitude – afforded by some approximations and assumptions. And to be simple yet realistic of a deep-draft harbor, let us use most of the same inlet/basin/tide parameters (inlet depth 15 m; harbor depth 10 m; semi-diurnal tidal period 12.42 hours; and tidal amplitude 1 m) as described in Managing Coastal Inlets – for a large harbor area of 1 million square meter; and an inlet length and width of 100 m and 300 m, respectively. A tide of this amplitude at 15 m water depth, causes a passing peak depth-averaged current of about 0.81 m/s in front of the harbor. A rough estimate shows that for a harbor of this size at this tidal condition – the turnover time (the time required to tidal flushing out 63% of the harbor water volume) is about 3.2 days.
Sedimentation of Suspended Particulates
This part of the piece is based mostly on my published papers: the 1994 ICCE 24th; and the COSU 1993 and 1995 papers. The published works are based on some site-specific information; and are therefore primarily applicable for situations and conditions in which they were derived. But the approach and methodology can be applied elsewhere with some assumptions for cases – of tide-dominated estuaries, bays and waterways dominated by fine seabed sediments. Let us attempt to see some applications of the gained experience in simple terms. They are supplemented by my discussions in 2004 and 2008 publications.
Before finishing I like to tell a story the Buddha (563 – 483 BCE) told to a congregation of monks and lay people. He did this in context of the 4th Buddhist precept of Right Speech (see Revisiting the Jataka Morals – 2). Once an angry and hateful person used very harsh and abusive words to the Buddha. Instead of getting provoked, the Buddha calmly listened and told the man to take back his abusive outburst. The man was dumbfounded hearing such an unusual reaction. The Buddha then said: Hold it there. If a person gives a gift to another, and if the second person refuses to accept the gift, to whom the gift belongs? The man replied: it belongs to the first person. The Buddha said: so, my friend you must take back the abusive language you have used, because it belongs to you and I refuse to accept it. Then the Buddha delivered some words of wisdom to the congregation: if someone spits against the sky, the spittle returns back to the spitter. So, be mindful. If you use an abusive or unwholesome speech, it gets back at you.
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- by Dr. Dilip K. Barua, 23 November 2020