Wave, Mixed Energy, and Tide Dominated Hydrographic Regimes

Wave, Mixed Energy, and Tide Dominated Hydrographic Regimes

Our understanding of waves and tides now sets the stage for a more thorough understanding of how they interact to shape a coastal zone. In the context of the hydrographic regime, there are two fundamental things that must be considered: the wave and tidal energy of a coast.

1. Wave energy (E) is a function of wave height (H) with the energy (E) of a wave approximately equal to the square of the wave height (H2).
2. Tidal energy can be equated with tidal range because it is the rise and fall of the tides that produce tidal currents. Tidal range (TR) is the vertical difference between high and low tides, which are commonly categorized into three ranges:

Microtidal: TR < 2.0 m

Mesotidal: 2.0 m < TR < 4.0 m

Macrotidal: TR > 4.0 m

The relative dominance of wave versus tidal energy dictates coastal morphology, and three general regimes exist along depositional coasts:

1. Wave Dominated Coasts
2. Mixed Energy Coasts
3. Tide-dominated Coasts

Wave Dominated Coasts

Along wave-dominated coasts, the role of the waves and, thus, longshore transport processes is the dominant factor controlling the coastal morphology. Because longshore transport carries sediment along the shore, these types of coasts tend to have long linear barrier islands and beaches with relatively few inlets or passes between the open ocean and the more interior bays and waterways. The reason for this is that the longshore transported sediment tends to fill in these inlets and passes. Since the tidal currents are relatively weak, there is no energy available to keep the inlets open as a connection between the open ocean and interior waterways.

Satellite image of the Nile delta of Egypt. The wave energy that dominates this coastal environment smoothed the shoreline where the Nile River enters into the Mediterranean Sea. Longshore transport by waves reworks sediment along the coast.

Credit: NASA: EO Images

Mixed Energy Coasts

Mixed energy coasts are a product of both wave and tidal processes and neither process exerts an excessive influence on the coastal morphology, although some mixed energy coasts show tidal or wave dominance. Rarely in nature are processes in equilibrium. These types of coasts typically contain short stubby barrier islands, have numerous tidal inlets and passes, and are separated from a mainland by extensive marshes and an array of tidal creeks.

Satellite image of barrier islands on the northeast coast of Brazil. Note the short stubby appearance of the sandy shorelines, the expansive wetland environments on the landward side of the sandy shorelines, and the many tidal channels that are present on the landward or backbarrier side. This coastal morphology suggests a mixed energy hydrodynamic regime.

Credit: NASA: EO Images: Brazil

Satellite image of the Copper River delta in Alaska, U.S.A. showing a morphology that suggests a mixed energy environment because of the many inlets and passes between the relatively short barrier islands. Although this is a mixed energy environment, the tides have slightly more influence in this location, and the region is considered a tide-dominated, mixed energy environment.

Credit: NASA: Copper River Delta

Tide Dominated Coasts

Tide dominated coasts are those where strong tidal currents act to redistribute sediment along the vector of falling and rising tidal currents, which are orientated perpendicular to the trend of the coastline. Quite often, the highest energy tidal currents develop in locations where large tidal ranges are present in conjunction with a shoreline morphology that accentuates tidal flow by constricting tidal currents into funnel-shaped embayments. Sediment is reworked by the resulting tidal currents to create subaerial and subaqueous tidal shoals and islands that are aligned parallel to the tidal flow direction, perpendicular to the coastline. Moreover, in areas with very large tidal ranges, extensive tidal flats, and marsh or mangrove systems form along the borders of the embayment and can cover large areas.

1:250,000 scale map of the Fly River delta in Papua New Guinea. The islands that are aligned perpendicular to the coastline are the result of tidal currents reworking sediment delivered to the area by the Fly River. The spring tidal range at the mouth of the Fly River is as much as 5 m.

Credit: Fly River Delta-txu-oclc-6552576-sc54-4 by Army Map Service (LU), Corps of Engineers, U.S. Army, Public Domain, via Wikimedia Commons

Muddy tidal flat in Schleswig-Holstein Wadden Sea National Park of northern Germany. This area represents some of the most continuous tidal flats on Earth and is part of a UNESCO World Heritage Site. Although the tidal range here is only approximately 2 m, the low gradient of the coastal zone means that large areas can be exposed during falling or ebbing tide and covered by water during rising or flooding tides.

Credit: Wikipedia: Tidal Flats (Public domain)