Tidal prism

{{Short description|Volume of water in an estuary or inlet between mean high tide and mean low tide}} A '''tidal prism''' is the volume of water in an [[estuary]] or [[inlet]] between mean [[high tide]] and mean [[low tide]],<ref name="Luketina, D 1998. pp. 77-84">Luketina, D. 1998. Simple tidal prism models revisited. [[Estuarine, Coastal and Shelf Science]]; Vol. 46. pp. 77–84.</ref> or the volume of water leaving an estuary at [[wikt:Ebb tide|ebb tide]].<ref>Davis, R., D.M. Fitzgerald. 2004. Beaches and Coasts. Blackwell Science Ltd. Malden, MA.</ref>

The inter-tidal prism volume can be expressed by the relationship: ''P=H A'', where ''H'' is the average [[tidal range]] and ''A'' is the average [[surface area]] of the basin.<ref name="Lakhan, V.C. 2003">Lakhan, V.C. (ed). 2003. Advances in Coastal Modelling. Amsterdam, the Netherlands; Elsevier B.V. pp.</ref> It can also be thought of as the volume of the incoming tide plus the river discharge.<ref name="Hume, T.M 2005. pp. 981">Hume, T.M. 2005. Tidal Prism. Encyclopedia of Coastal Science. Springer Netherlands. M.L. Schwartz, editor. pp. 981. Accessed via Springerlink database October 13, 2009.</ref> Simple tidal prism models stated the relationship of [[river]] discharge and inflowing [[ocean]] water as Prism=Volume of ocean water coming into an estuary on the flood tide + Volume of river discharge mixing with that ocean water; however, there is some [[controversy]] as to whether traditional prism models are accurate.<ref name="Luketina, D 1998. pp. 77-84"/> The size of an estuary's tidal prism is dependent on the basin of that estuary, the tidal range and other [[friction]]al forces.

==Applications of tidal prism== Calculations of tidal prism are useful in determining the [[residence time]] of water (and [[pollutant]]s) in an estuary. If it is known how much [[water]] is exported compared to how much of the estuarine water remains, it can be determined how long pollutants reside in that estuary. If the tidal prism forms a large proportion of the water in an estuary at high tide, then when the tide ebbs, it will take with it the majority of the water (this occurs in shallow estuaries) and any pollutants or [[sediment]]s suspended in that water. This means that the estuary has a good [[flushing time]], or that the residence time of water in that estuary is low.<ref name="Hume, T.M 2005. pp. 981"/> On the contrary, in deeper estuaries, the amount of water that is influenced by the tides forms a smaller proportion of the total water. The difference between high tide and low tide is not as great as in shallower estuaries creating a smaller tidal prism and a longer residence time.

The size of an inlet or estuary is determined, according to O’Brien<ref name="Brien, M.P 1931. pp. 738-739">O’Brien, M.P. 1931. Estuary tidal prisms related to entrance areas. Civil Engineer; Vol. 1. pp. 738–739.</ref> by tidal prism. Tidal prism [[Magnitude (mathematics)|magnitude]] can be calculated by multiplying the area of the estuary by the tidal range of that estuary.<ref name="Davis, R. 2004">Davis, R., D.M. Fitzgerald. 2004. Beaches and Coasts. Blackwell Science Ltd. Malden, MA</ref> During spring or fall tides, when sea level is relatively high and floods backbarrier areas that are normally above tidal inundation, the cross sectional area at the entrance of the estuary increases as tidal prism increases.<ref name="Brien, M.P 1931. pp. 738-739"/> Since tidal prism is largely a function of area of open water and tidal range, it can be changed by alterations of the basin area of estuaries and inlets as in dredging; however, if the estuary or inlet is dredged, or the size changed, the channel will fill in with sediment until it has returned to tidal prism [[:wikt:equilibrium|equilibrium]].<ref name="Davis, R. 2004"/>

==Sand transport== Additionally, there are correlations between tidal prism and amount of sediment deposited and exported in an estuary or inlet. The Walton and Adams<ref name="Walton, T.L. 1976">Walton, T.L., W.D. Adams. 1976. Capacity of inlet outer bars to store sand. In Proceedings of the 15th Coastal Engineering Conference, Honolulu, HI: ASCE, pp. 1919–37.</ref> relationship shows a strong relationship between the magnitude of the tidal prism and the volume of [[sand]] in ebb dominated [[River delta|deltas]]. The larger the tidal prism, the larger the amount of sand that is deposited in deltas in ebb-dominated estuaries.<ref name="Walton, T.L. 1976"/> Inlets with small tidal prisms have too little power to remove sand deposited from adjacent shores. Inlets with large tidal prisms can erode sand and deposit it in ebb-tidal deltas in deeper waters (National Research Council). The size of ebb tidal deltas is proportional to tidal prism.<ref name="Walton, T.L. 1976"/> If tidal prism increases, there is an increase in deltas and [[shoal]]s formed by sand transport during ebb tide.<ref>National Research Council (U.S.). Committee on Engineerings Implications of Changes in Relative Mean Sea Level. 1987. Responding to changes in sea level. Washington DC, United States. National Academy Press.</ref>

==Tidal prism models and assumptions== There are assumptions that go along with tidal prism models. The first is that they are applied to smaller estuaries (less than a few kilometers wide) and secondly, that the estuaries are internally well mixed.<ref name="Lakhan, V.C. 2003"/> Additionally, it is assumed that the water entering the estuary is of oceanic [[salinity]] mixing with the fresh river discharge, and that the mixed water will be exported on the ebb tide. Officer provides a model for simple tidal prism theory where the estuary is represented by a box with the [[inflow (hydrology)|inflow]] as the volume of river discharge at a salinity of 0, within the estuary, the river discharge mixes with the volume of the tide flooding in (Vp) from the ocean at oceanic salinity (So) and the mixed VR + VP) water flows out at ebb tide.<ref>Officer, C. B. 1976. Physical Oceanography of Estuaries (and Associated Coastal Waters). Wiley, London.</ref>

==References==  {{Reflist}}

{{DEFAULTSORT:Tidal Prism}} [[Category:Tides]] [[Category:Estuaries]]