Coastal Sediment Transport PDF

Coastal sediment transport  Modification of the coast takes place through: Erosion Transport Deposition of material that is either eroded by waves and currents or brought to the coast, e.g. by rivers. Transport mechanism Fine sediments (silt and clay) do not occur in appreciable...

Coastal sediment transport  Modification of the coast takes place through: Erosion Transport Deposition of material that is either eroded by waves and currents or brought to the coast, e.g. by rivers. Transport mechanism Fine sediments (silt and clay) do not occur in appreciable amounts in the inner nearshore and surf zones on energetic coasts. They tend to be placed in suspension and diffuse uniformly through the water column, maintained by turbulence. They are then removed offshore or alongshore where they settle out of suspension in deep water - in the outer shoreface or further offshore. Fine sediments may also be brought into estuaries, bays and lagoons where they are deposited in quiet water, often promoted by the presence of vegetation such as seagrasses, salt marsh plants and mangroves.  4 types of transport mechanisms: i. Solution ii. Suspension iii. Saltation iv. Traction/Rolling  Coarser particles of sand and gravel are exchanged readily between the inner nearshore and surf zones and the beach, and may be transported alongshore in appreciable quantities. It is usual to distinguish between processes that lead to the net transport of sediment onshore or offshore (shore normal transport) and those tending to move sediment alongshore (longshore sediment transport), though both processes occur simultaneously. On-offshore sand transport  On many beaches, onshore and offshore transport tends towards an equilibrium or balance over a period of months or years, with offshore movement dominating during periods of intense storms, and onshore movement during non-storm conditions. The net transport at any elevation can be obtained from integrating the co-spectrum of velocity and sediment concentration which thus permits inclusion of all forms of water motion from the oscillatory component of waves, to long waves, undertow and rip currents. Such an approach assumes that sediment moves at the same speed as the water. Onshore and offshore transport Sediment transport modelling Summary of numerical modelling procedure Sediment transport modelling Sediment transport modelling Sediment transport modelling Net Shore Drift (NSD) direction from Teluk Chempedak to Kuala Pahang  (a) (b) Hjulstorm Diagram: a) Teluk Chempedak b) Sepat c) Kuala Pahang (c)  Sediment transport – low wave condition Under low waves, little sediment will reach high enough to be moved in the offshore direction and offshore flow speeds are low. Net transport is likely to be onshore because of the asymmetry or skewness in the strength of the oscillatory motion of shoaling waves and the small net drift velocity onshore. Under low waves, sand tends to be moved onshore.  Sediment transport – high wave condition As the wave action increase, sediment will reach higher into the water column and at the same time the strength of the undertow will increase. Initially only the total transport may increase, but eventually the higher concentration within the area influenced by the undertow will result in a change in the direction of net transport from onshore to offshore. This may be enhanced by the effect of vortex generation by plunging breakers that will dramatically increase suspended sediment concentrations in the middle of the water column (Voulgaris and Collins, 2000). Under high waves and onshore winds, net sand transport is offshore. Measurement of sediment transport Water filtration method - TSS Streamer trap The fine mesh bag is attached to a rectangular frame which keeps it open. Several of these are fixed to a vertical post for deployment in the water Measurement of sediment transport OBS – Orbital Backscatter Laser diffraction instruments are commercially available to measure the particle size and concentration of suspended sediments -measuring turbidity & suspended solids LISST – Laser In-Situ Scattering and Transmissometery Longshore sand transport Longshore sand transport Longshore sediment transport in the beach and nearshore zone results from the operation of three sets of processes: a) Beach drifting – on the swash slope which is driven primarily by oblique wave action b) Surf zone transport – transport by wave-generated longshore currents c) Transport seaward – of the breaker zone by residual tidal currents and wind-driven currents Beach drift Beach drifting results from wave run-up (swash) on the beach foreshore, where waves approach at an oblique angle to the local beach orientation. The swash occurs at the angle of wave approach, but the return flow in the backwash is influenced primarily by gravity, and thus tends to return straight down the slope. Sediment entrained by waves is transported in the direction of wave advance in a series of saw tooth motions. Beach drift Where the swash slope is steep and a substantial fraction of the swash does not infiltrate into the bed, some of the alongshore momentum in the wave uprush may carry over to the backwash so that some alongshore movement occurs during this phase as well, with the result that the transport rate is enhanced. The process is easily observed on sandy beaches with a steep foreshore slope under moderate wave conditions. Importance of longshore transport Longshore sediment transport is extremely important on most coastlines. Locally, it is important in the removal of sediment from the base of bluffs and cliffs, thus promoting further erosion and cliff retreat, and from river mouths and deltas. The sand and gravel component of this sediment is then transported alongshore to create beach and dune systems and large depositional features such as spits, baymouth barriers and barrier islands.  The direction of net longshore sediment transport may be manifested from natural features and by the accumulation of sediment behind obstacles such as breakwalls and groynes. Interference with the natural longshore sediment transport is the cause of many human induced problems involving locally enhanced erosion and sedimentation and is critical to the evaluation of many coastal engineering works. Longshore sediment transport is also a key factor in determining local beach sediment budgets and in the definition of littoral cells. Littoral sediment budget and littoral drift cells Littoral sediment budget The littoral sediment budget is an accounting technique in which the volume (or mass) of all sediment inputs (sources) and outputs (sinks) to and from the beach and nearshore zone are assessed for a section or reach of the shoreline. Schematic of the common natural sources and sinks for a littoral sediment budget Littoral sediment budget A source can be defined either as a point source (e.g. river mouth) where the input is confined to a local area, or a line source (such as cliff erosion) where the input takes place over a long stretch of the shoreline. On a world-wide basis, inputs from rivers are probably the most important source, followed by cliff erosion, and in the tropics biogenic inputs from coral reefs may dominate.  Littoral cells An ideal littoral cell is a section of coast that contains a source, or sources of sediment, a well-defined and continuous zone of alongshore sediment transport and a downdrift sink or zone where sediment is either deposited or is lost offshore.