Estuary Types
The New Zealand Estuary Environment Classification (Hume et al. 2007; 2003) provides a controlling factors classification based on broad scale physical components of the landscape or ‘controlling factors’ such as climate, oceanic and riverine conditions, and catchment characteristics that cause or ‘control’ differences and similarities in the physical and biological characteristics of estuaries. Level 2 of the Estuary Environment Classification subdivides estuaries according to differences in estuary-scale hydrodynamic processes.
Hydrodynamic processes are forced by the interaction of tides and ocean swell with freshwater inflow within the estuary basin, and wind acting on the surface of the estuary basin. These processes are controlled by the ocean at the estuary mouth, freshwater inflows at the headwaters and the morphometry of the estuary basin (i.e., whether it is deep, shallow or largely intertidal, broad and open, long and narrow or a complex branching network of arms). Together these forcing mechanisms produce: mixing, circulation, stratification, sedimentation, and flushing at the scale of the whole estuary. This conceptual model assumes that the net effect of the three factors on hydrodynamic processes is independent of the size of each estuary. Thus, small estuaries have similar characteristics to larger-scaled versions.
The Estuary Environment Classification defines 8 categories or types (Type A, Type B, Type C, Type D, Type E, Type F, Type G, Type H) at Level 2 that are based on distinctive hydrodynamic processes. These categories are diagnosed by particular combinations of the three controlling factors (ocean forcing, river forcing and basin morphometry). The estuary-scale hydrodynamic processes and resultant physical character of estuaries in each type are described below.
Type A estuaries
Type A estuaries.
Type A estuaries are very shallow basins (several metres depth), often elongate in shape and orientated parallel to the open coast shore. For the majority of the time there is no ocean (i.e., tidal or swell wave) forcing because the entrance to sea is for most of the time barred-off. Thus, these estuaries have zero intertidal area and are poorly flushed. Episodic flood events can open an entrance for several days or weeks each year, permitting exchange with the ocean. Such entrances are generally narrow, and close when littoral drift overwhelms the ability of tides and river inputs to flush sediment from the entrance. River inputs are small and may be ephemeral. Wind generated two-dimensional circulation and mixing occurs. Because these estuaries are shallow, wave suspension of bottom sediments is an important driver of whole-estuary sedimentation processes. These estuaries are characterized by muddy substrate. No ocean swell enters the system because the entrance is closed for most of the time and, when open, the narrow and shallow entrance filters out wave energy. Type A estuaries are representative of features commonly termed coastal lakes.
Type B estuaries
Type B estuaries.
Type B estuaries are elongate basins of simple shape and several to ten metres depth. The majority of the estuarine area is subtidal. The volume of river flow delivered during a tidal cycle is a significant proportion of the volume of the basin, and is greater than the tidal volume entering the basin. Thus, the estuary-scale hydrodynamic processes are dominated by river flows and these estuaries are well flushed. On shorelines with littoral drift, these estuaries form small sand bodies (bars) on the ocean side of the entrance. In deeper systems a circulation pattern (estuarine) can be set up where out flowing freshwater is balanced by the inflow of seawater entrained beneath freshwater and a salt wedge develops. Seawater intrudes a considerable distance up estuary on low gradient coastal plains. Large floods can expel much of the ocean water from the estuary. Wind generated two-dimensional mixing and wave driven resuspension are minor as wind fetch and waves are small and depths are largely too great for significant bed stress to be produced. Thus sediments tend to be muddy except in areas of high tidal flows. Type B estuaries are representative of features termed tidal river mouths.
Type C estuaries
Type C estuaries.
Type C estuaries occur where the mouth of a main river channel connects to shallow lagoons. While the main river channel is mostly subtidal, the lagoons can have significant intertidal area. The volume of river flow delivered during a tidal cycle is a significant proportion of the volume of the total basin and is greater than the tidal volume entering the basin. Thus, the estuary-scale hydrodynamic processes are dominated by river flows. However, river flows tend to bypass the lagoons. Thus in the deeper main arm a circulation pattern (estuarine) can be set up where out flowing freshwater is balanced by the inflow of seawater entrained beneath freshwater and a salt wedge develops. The main river channel is, therefore, well flushed, but seawater remains trapped in the lagoons where the flushing is comparatively poor. Wind generated two-dimensional mixing and wave resuspension of the substrate is minor in the main river channel, but greater in the lagoons because of the larger wind fetch and shallow depths. Wave resuspension produces coarser substrates in the lagoon. On shorelines with littoral drift, these estuaries have small sand bodies (bars) on the ocean side of the entrance. Type C estuaries are representative of features termed tidal river mouths.
Type D estuaries
Type D estuaries.
Type D estuaries are shallow, circular to slightly elongate basins with simple shorelines and wide entrances that are open to the ocean. They are mostly sub-tidal with small intertidal areas restricted to the headwaters (sheltered areas) of the more elongate types. There is little river influence and circulation is weak and ocean forced. The entrances are wide and open to the ocean, allowing swell to enter the bay and resuspend seabed sediments. Thus, the estuary-scale hydrodynamic processes are dominated by the ocean. There are no sand bodies (tidal deltas) on the ocean side of the entrance. Wind generated two-dimensional mixing and wave driven estuary-scale sedimentation occurs. As a result the substrate is sandy, except in areas where wave resuspension of the substrate is limited by depth. Type D estuaries are representative of features termed coastal embayments.
Type E estuaries
Type E estuaries.
Type E estuaries are shallow, circular to slightly elongate basins with simple shorelines and extensive intertidal area. They generally have a narrow entrance to the sea that is usually constricted by a spit or sand barrier. Sand bodies occur as ebb and flood tidal deltas at the mouth on littoral drift shores. On zero drift shores funnel shaped entrances with no sand bodies occur. The tidal prism is a large proportion of the estuary basin volume. The volume of river flow delivered during a tidal cycle is very small compared to the total volume of the estuary. Thus, estuary-scale hydrodynamic processes are dominated by ocean forcing. Wind generated two-dimensional circulation, mixing and resuspension occur at high tide. Type E estuaries have good flushing because much of the water leaves the estuary on the outgoing tide. The combination of wave resuspension of the substrate and flushing result in Type E estuaries having generally homogeneous and sandy substrates. These estuaries are also well mixed because strong flushing, wind mixing and the shallow depths prohibit density stratification. Salinity is close to that of the sea. Ocean swell can resuspend sediment in the entrance of estuaries with wider mouths at high tide when screening from the ebb tidal delta is minimised. Type E estuaries are representative of features termed tidal lagoons or barrier enclosed lagoons.
Type F estuaries
Type F estuaries.
Type F estuaries share similarities with Type E estuaries having shallow basins and narrow mouths, usually formed by a spit of sand barrier. However Type F estuaries have complex shorelines and numerous arms leading off a main basin. As a consequence, the extensive intertidal area of Type F estuaries tends to be cut by deep channels caused by drainage from the arms. Sand bodies in the form of ebb and flood tidal deltas occur at the mouth on littoral drift shores. Funnel shaped entrances occur on low littoral drift shores. The tidal prism makes up a large proportion of the tidal volume. River inputs over the tidal cycle are very small compared to the total volume of the estuary. Thus, estuary-scale hydrodynamic processes are dominated by the tides. Wind generated two-dimensional circulation, mixing and wave resuspension of the substrate is less pronounced than for Type E estuaries because the narrow arms means that fetch is small. As a result, the main body of these estuaries is characterised by sandy substrate, with a transition to muddy substrate in the upper portion of the arms. The planform complexity means that Type F estuaries are not as well flushed as Type E estuaries because water is trapped in the arms. Although the main body of these estuaries are reasonably well mixed, the upper reaches of narrow arms are characterised by weak stratification and salt wedges. Salinity is close to that of the sea in the main body of the estuary with a transition lower salinity in the arms. Ocean swell can resuspend sediment in the entrance of estuaries with wider mouths at high tide when screening from the ebb tidal delta is minimised. Type F estuaries are representative of features termed barrier enclosed lagoons or drowned valleys.
Type G estuaries
Type G estuaries.
Type G estuaries are very deep (up to 100’s of metres), narrow, elongate and largely subtidal basins. They are characterised by sills at the mouth and along the length of the estuary that were formed as terminal moraines of glaciers. Both river and tidal inputs over the tidal cycle are very small proportions of the total volume of the basin. Water movement is controlled primarily by thermohaline forcing, that is the circulation is maintained by the large density differences produced by the salinity contrast between freshwater and oceanic water. The resulting circulation pattern is characterised by out-flowing freshwater, which is balanced by the inflow of seawater entrained beneath freshwater. Wind may modify this circulation and two-dimensional and three–dimensional wind driven circulation may become a dominant force on occasions, but it is not responsible for the mean circulation over extended periods of time. Consequently, these estuaries are characterized by poor flushing, particularly in more complex shaped (multiple arm) systems. The very deep basin and partitioning by sills means that flushing takes place in a relatively thin layer of fresh water, which moves over the top of a ‘dead zone’ of saline water. Substrate resuspension by ocean swell or wind waves is not an important estuary-scale hydrodynamic process because of the large depth of the basin. As a consequence, the substrate is generally fine sand or mud. Type G estuaries are representative of features termed fjords or sounds.
Type H estuaries
Type H estuaries.
Type H estuaries are deep (10’s of metres), narrow, elongate basins and largely subtidal. Both river and tidal inputs over the tidal cycle are very small proportions of the total volume of the basin. Water movement is controlled primarily by thermohaline forcing; that is the circulation is maintained by the large density differences produced by the salinity contrast between freshwater and oceanic water. The wind may modify the circulation and become a dominant force on occasions, but it is not responsible for the mean circulation over extended periods of time. A circulation pattern (estuarine) is set up where out flowing freshwater is balanced by the inflow of seawater entrained beneath freshwater. There is a strong longitudinal gradient (head to mouth) in hydrodynamic processes with riverine forcing and stratification dominating in the headwaters and ocean forcing and vertical mixing to depth near the entrance. The systems are characterized by poor flushing. The flushing is poorer in the more complex shaped (multiple arm systems) systems. Ocean swell and wind waves are unimportant in substrate resuspension processes because of the large depth of the basin. The substrate is generally fine sand or mud. Type H estuaries are representative of features termed sounds, drowned valleys, rias or fjords.