Tidal trapping

Tidal trapping when parcels of water are advected into lateral traps during one phase of the tidal cycle and then moved out in another results in an overall upstream transport. 

Roscoe, D.E., L. Widjeskog, and W. Stansley. 1989. Lead poisoning of northern pintail ducks feeding in a tidal meadow contaminated with shot from a trap and skeet range. Bull. Environ. Contam. Toxicol. 42 226-233. 

Tidal barriers utilise the rise and fall of the tide (the tidal range) to trap seawater at high tide in a reservoir behind a barrage. As the water leaves or enters the reservoir in a constrained duct, submerged hydroturbines generate electricity, as in conventional hydropower. There is only one significant tidal barrier power plant, which is at La Ranee, Brittany, in France. 

With regards to solid phase, most of the estuaries are characterized by a "turbidity maximum". The fresh water flow generates an entrainment from the bottom water, the sea water which is entrained from the lower layer by this flow being compensated by a residual landward flow along the bottom. The river-borne particles which settle in this bottom layer are taken up by the residual landward current and trapped in the middle part of the estuary. This process greatly enhances the residence time of particles, especially in tidal estuaries. 

Figure 6.7 Illustration of particle trapping as it relates to resuspension in the mud reach and the location of the salt wedge over a tidal cycle. (Modified from Geyer, 1993.)...

The differences between these reefs probably reflect differences in their structural framework and variations in wave characteristics and tidal range in the two environments. Anaerobic reactions in reef interstitial waters may not progress far if reef structures are open and well flushed. If, however, the systems are nearly closed, little fresh reactant will enter via seawater exchange and mass transfer will be limited by the reactants trapped in the reef interstitial waters. An important conclusion of Sansone s studies was that thermodynamic disequilibrium among dissolved species such as CH4 and S04 implies microzonation of chemical reactions. Microzonation resulting in slight differences in reef interstitial water compositions may account for the coexistence of different cement mineralogies in reef structures. 

Alginates are a family of polysaccharides mainly extracted by brown algae. Alginate hydrogels are able to store up to 100 times their weight of water, trapped in a network of hydrophilic fibrils. This property accounts for their natural occiurence as water reservoirs in plants of the tidal area [6]. 

The Lower Series was deposited on a flooded erosion surface over a Paleozoic substrate, a marine surface on which were developed vast lagoons, volcanic flows, tidal flats and barriers. Truely marine conditions prevailed only over the northern part of the region. The sandstones of the Lower Series are not as uniformly continuous as those of the Tj and T. They are generally lenticular or sometimes form wedges interrupted by the volcanic rocks. A great potential for the accumulation of hydrocarbons in lithostratigraphic traps may develop under these conditions like in the sandstones in the upper part of the Lower Series in TKT-i, BKZ-i and BKZ-2. 

J. N. Marino and A. J. Mehta, Sediment trapping at Florida s east coast inlets. Hydrodynamics and Sediment Dynamics of Tidal Inlets, eds. D. G. Aubrey and L. Weishar (Springer-Verlag, 1988), pp. 284-296. 

If SLR forces positive accommodation in flood-dominant basins, surf zone generated sediment transport will be diverted into the tidal basin by flood currents. These sediments will be trapped in the flood delta. Typical barrier length scales are O (10 km). Therefore, typical values for shoreline changes in the vicinity of backbarrier sinks can be estimated as shown in Table 37.4. 

Spurious Opening of Either of Two Tidal Lock Cates Involving a Trapped Vessel... 

Hot water cleanup. When milder mediods fail to give the desired degree of cleanup, hot water can mobilize some of the oil still trapped within the polluted sand. This method is used as a last resort because of the damage it does to inter-tidal ecosystems. 

As discussed, submerged breakwaters are used effectively to dissipate waves and thus protect beaches from erosion. Where submerged breakwaters are located close inshore to beaches and where there is little tidal range (< 1 m) they may also act as siUs to trap sand and limit its removal from the beach. Sills are low, submerged breakwaters used to create and maintain perched beaches, as shown in Fig. 20.35. The sUl structure provides toe support for the perched beach slope which would otherwise be unstable and erode. Perched beaches are constructed first by building the submerged breakwater/sill and then by nourishing the area between the sill and land with sand to create the perched beach. 

---