Hydrodynamics

Useful links

References

Ref.1. U.S. Army Cops of Engineers. COASTAL ENGINEERING MANUAL – PART II (30 April 2002). Chapter 6: Hydrodynamics of Tidal Inlets.

Ref.2. Botes, W.A.M. & Le Roux, M. Milnerton Estuary: An assessment of the effects for reduced flows from the Potsdam WWTW. Reference: PW56/CMC/Milnerton_01. Submitted to: CMC/BVI. July 2004.

Main page	Definition of an estuary	Useful links
HYDRODYNAMICS

		A demonstration of estuary mouth dynamics, estuary dynamics and the concepts of numerical modelling
	Factors influencing estuarine circulation
	A hydrodynamic analysis of an estuarine system requires a good understanding of those factors influencing estuarine circulation. Circulation in estuaries can be complex and mainly dependent on: Magnitudes of tidal variations Freshwater inflow Gravitational forces caused by density differences. The Coriolis acceleration (to a lesser extent for smaller estuaries). Short term wind and wave conditions. The mixing characteristics of an estuary depend on the magnitude of the above.
	Impact of tides on an estuary
	The geometry of an estuary will influence the progressing of a tidal wave, for example a convergent estuary will tend to increase the tidal amplitude and boundary friction tends to reduce the amplitude. For a long shallow estuary the tidal amplitude may gradually diminish to zero. Tide terminology: High water is the water level at its highest extent during one cycle. Low water is the same for the lowest water level. A tidal current that is flowing landward is called a flood current, while one flowing seaward is called an ebb current. There are mainly three types of tides, which are a result of numerous tide-generating forces and location on the earth. Diurnal tide: one high and one low water level in a lunar day (24.84 hours) Semidiurnal tide: produce a tidal cycle (high and low water) in one-half the lunar day (12.42 hours) or two nearly equal tidal cycles in one lunar day Mixed tide: a combination of diurnal and semidiurnal characteristics Every 14.3 days (twice a lunar month) the earth, moon, and sun are aligned in phase. At this time the gravitational forces act together to form higher than average tides, that is "spring tides." Also twice a month the moon and sun are at right angles to the earth and the interaction of forces forms lower than normal tides called "neap tides."
	Hydrodynamics of the estuary (Ref.1. and Ref.2.)
	The hydrodynamics at the estuary inlet can vary from simple tidal inflow (flood) / outflow (ebb) conditions to a complex process which includes tidal variations, hydrological fluxes, wind wave processes, wind stresses and man made interferences such as effluent discharges and storm water flows. Entrance flows may be very complex due to the combined interaction of the tidal currents and currents due to breaking waves at the inlet, wind-stress currents, and variable longshore currents approaching the inlet due to wave action on adjacent beaches.
	Typically the time for a shallow water wave to propagate to the upstream end of the estuary is: T = L_E/(gD_av)^1/2 Where: L_E = length of the estuary (m), D_av = Average depth of the estuary (m) and g = 9.81 m/s²
	Although inlet configurations can be complex, a number of useful ‘tools’ for the analytical assessment of hydraulic behavior of the inlet can be applied to understand the hydrodymic behaviour of an estuary for example the "*Tidal Prism*"
	Tidal Prism
	The exchange of water in an estuary, generally refer to the term Tidal Prism with many definitions, such as:
	Volume of water that flows into a tidal channel and out again during a complete tide, excluding any upland discharges The volume of water present between mean low and mean high tide. A volume of water exchanged between an estuary or a lagoon and the open sea during one tidal period. The tidal prism is defined by USACE as the volume of water that enters through the inlet channel during flood flow and then exits during ebb flow.
	A simple approximation of the Tidal Prism is: P = 2 a_b A_b Where: a_b = average amplitude of the tide in the estuary (m) and A_b = Surface area of the estuary (m²)
	As long as the inlet area is large in proportion to the area of the estuary, currents will be small and tidal wave propagation will predominate. If the cross-sectional area of the inlet becomes small, tidal wave propagation will be negligible and the flow through the inlet will be hydraulic, which means that the water surface along the inlet channel will have a slope, resulting in a hydraulic flow. The well-known relationship between the minimum cross-sectional flow areas at an inlet and the tidal prism (P) , by O’Brien in 1931 for numerous estuaries provides a good indication of the stability of an estuary mouth as shown below.

	Estuary mouth functioning
	The estuary inlet is the significant element for the controlling of the hydrodynamic behaviour of an estuary. For clarity on terminology, let us consider the inlet as the entire sandy seabed between the banks at both ends of the shoreline boundary of the estuary, which connects the estuary with the open ocean and the ‘mouth’ (or ‘throat’) which is the section in the inlet with the smallest cross-sectional area and the highest velocities (during ebb and flood). The sill is the lowest level of the mouth. The mouth may consist of more than one meandering channel, varying during the tidal cycle. The flow (velocities) in the mouth is a function of the water levels in the estuary and in the sea with external inflows (river flows) superimposed on it. When the estuary mouth closes, the mouth disappears and a ‘bar’ is formed (lowest level of the sand bar is the sill). During inflows, sediment will be transported into the estuary and deposition will occur (flood ‘shoals’ in the estuary will be formed) when the flow diverges and decrease in the wider sections. During outflows (ebb conditions) sediment will be transported to the sea and deposited as ebb shoals. The shoreline from which the majority of wave energy arrives at the mouth will provide a sediment source, which will effect the estuary mouth (geometry) and the quantity of sediment transported into the estuary, depending on the wave and tidal conditions. Depending on the long shore sediment loads, wave and tidal conditions, encroachment upon the mouth may eventually affect the mouth hydraulics as such that it will start to ‘choke’ and eventually closes. Opening of the mouth will then only occur during breaching of the bar, when sufficient external inflows to the estuary (river flow) cause the water levels to rise above the sill level of the sand bar. Mechanical breaching can also be introduced – skimming the sill of the bar until the sill level is less then the water level in the estuary.