2.5.2: Role of sea-level rise in Holocene coastal evolution
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Sea-level changes can affect the coastal zone very strongly. The effect of sea-level rise can be understood from the well known concept of Bruun (1954, 1962), which is briefly explained here and treated in more detail in Ch. 7. The Bruun rule assumes that the shoreface has a profile that is in equilibrium with the hydrodynamic forcing. Hence, it states that the shore profile is vertically invariant in space and time relative to mean sea level. Consequently, a sea-level rise results in a water depth that is too large to be in equilibrium with the forcing. In other words: extra space has become available for sediment accumulation, the so-called accommodation space. In the absence of sediment sources or sinks, equilibrium is again achieved by a landward and upward shift of the profile: the shoreline retreats and a new equilibrium profile forms at the new shoreline position by moving sediments to deeper water (Fig. 2.22).
The world’s coasts can be divided into two classes: transgressive coastal environments and regressive coastal environments. As the shoreline moves seaward or landward in response to sea-level changes, it either exposes or inundates coastal areas and, in so doing, causes the character of the coast to change. Additionally, the position of the shoreline influences coastal processes that shape the coastal environments. Inundation is also called transgression (or simply advance), whereas drying of the land is referred to as regression (or simply retreat). Transgressive coastal environments are characterised by lagoons and estuaries. We have seen before that estuaries are semi-enclosed coastal water bodies, which are on one side connected to the sea and, on the other side, have one or more rivers or streams flowing into it. Rias and fjords are estuaries formed through flooding of low-lying areas; rias are drowned river valleys (for instance Sydney harbour) and fjords are drowned valleys carved out by land ice (see Fig. 2.12). In regressive coastal environments the shoreline moves seaward and estuaries and lagoons that are present get filled in or abandoned.
The global distribution of transgressive and regressive systems is at first order determined by the (late) Holocene relative sea-level changes. Sea-level changes are relative
movements and thus vary from place to place as can be seen from for instance Fig. 2.18. Transgressive coastal environments are well developed in areas that have experienced isostatic subsidence, viz. mid to southern parts of North America, mid to southern parts of Europe and the Mediterranean region. In these regions relative sea level has risen at an average rate of 1 mm/yr over the last 7000 years. On the contrary, regressive coastal systems can be found (although not exclusively) in the regions that have been far from glaciers during the last ice age. This includes most of Asia, Oceania, cent- ral to southern parts of Africa and South America. In the beginning of the Holocene these areas have experienced a relative sea-level rise, but this had changed to relative sea-level fall further in the Holocene, resulting in a relative sea-level fall in the order of a few metres over the last 7000 years. Lagoons and estuaries that have formed in the beginning of the Holocene have been filled or abandoned later, so that now evidence of regression is found along those coasts.
Although the global distribution of transgressive and regressive coastal systems is at first order controlled by sea-level rise, the amount of sediment supply is very important as well in controlling transgression versus regression. Clearly, sediment supply may counteract the effect of shoreline retreat. For instance, if the rate of sediment sup- ply keeps up with the rate of sea-level rise, the accommodation space created by the sea-level rise is filled by incoming sediments and the profile moves upward only; the position of the shoreline remains unchanged. Sediment can be supplied to the coast by for instance rivers and erosion from coastal cliffs or headlands. Through alongshore and cross-shore processes, waves and tides will rework the sediment supplied to the coast, resulting at a certain location in either erosion or deposition of recycled (mainly riverine) sediment. At a very high level of aggregation these sources and losses can be summarised to be represented by the terminology ‘sediment availability’.
Figure 2.23 qualitatively summarises the effects of both relative sea-level change and sediment availability on the displacement of the coastline. This type of diagram is called Curray’s diagram after Curray (1964).
Curray’s diagram shows that in the absence of a net sediment source or sink regression occurs in the case of a falling sea level – called emergence in that case – and transgression occurs in the case of a rising sea level or submergence. With a net source of sediment the transition between regression and transgression shifts towards low levels of rising relative sea level, and alternatively with a net sink of sediment towards low levels of falling relative sea level. In other words, in areas with erosion the shoreline generally moves landward inundating the coast (transgression), in theory unless the relative sea-level fall is so large that the shoreline migrates seaward. Sediment supply (deposition) results in regressive shorelines moving seaward, except when the regression is counteracted by a large relative sea-level rise. This building out of the coast is also called progradation. The various names that are used to describe the coastal response are summarized in Intermezzo 2.1.
The main conclusion from Curray’s diagram is:
Whether a shoreline migrates seaward or landward is determined by the combination of relative sea-level changes and amount of sediment supply or loss. Seaward migrating coasts are also referred to as regressive coastlines; landward migrating coastlines are termed transgressive coastlines.
On the basis of coastal response to substantial sea-level changes various classifications have been proposed in literature. They are invariably based on the balance between sea-level change and sediment supply. Some to the terminology used to describe the coastal response is given here (Fig. 2.24):
|Regresssion||seaward shift of the shoreline \(\to\) former sea bottom exposed|
|Transgression||landward shift of shoreline \(\to\) inundation|
|Progradation||sediment is deposited such that shoreline moves seaward|
|Retrogradation||sediment is deposited but shoreline moves landward|
|Emergence||land emerges out of the water due to relative sea-level fall (e.g. due to tectonic uplift: Chile)|
|Submergence||inland regions are flooded due to relative sea-level rise|
The classification of Valentin (1952) is a clear example. He simply divides the world’s coasts into coasts that have advanced (due to emergence and/or deposition) or coasts that have retreated (due to submergence and/or erosion). The various definitions are also indicated in Curray’s diagram.
If the amount of erosion due to sea-level rise is small compared to the sediment deposition, the shoreline may migrate seaward even under conditions of relative sea-level rise. The Mississippi delta (see also Intermezzo 2.2) and the central Holland coast are good examples of regressive systems that have developed despite their location in regions with Holocene relative sea-level rise. In regressive coastal environments typically deltas and extensive strand plains (sandy beach and shoreface systems) and chenier plains (muddy tidal flats) are found.
Most of the present active deltas are geologically very young features; some are only a few hundred years old. Because a delta develops at the coast, its existence is, in part, controlled by the sea level. It therefore was and still is, vulnerable to sea-level rise, too. During the periods of extensive glaciation, sea levels were much lower and rivers traversed the present continental shelves, dumping their sediment loads at or near the outer shelf edges. This suspended sediment cascaded down the continental slopes in turbulent, high-density flows called turbidity currents. New deltas did not form during this period, and deltas that had previously existed near the positions of present-day coasts were abandoned and entrenched by rivers as they flowed across the continental shelves. Melting glaciers brought a rapid rise of sea level, and river mouths retreated so rapidly that deltas could not develop. Finally, about 7000 years ago, the Holocene sea-level rise slowed, and in some parts of the world it stabilised at approximately its present position. Where conditions were appropriate, deltas began to develop as large quantities of river sediment accumulated.
Not all present-day deltas are only up to a few thousand years old. Many of them have formed on ancestral deltas built up during previous interglacial periods. A few, such as the Mississippi and Niger Deltas, are underlain by ancestral deltas that formed tens of millions of years ago. The upper regions of these mature deltas are also ancient, but their active delta lobes are only between 3000 and 6000 years old. The lower Mississippi Delta includes 16 detectable lobes. A new lobe forms whenever the location of the river mouth changes. The channels of abandoned lobes fill up with sediment, contributed both by the river, by the waves and by the tides of the coast. The present delta lobe of the Mississippi dates back only 600 years; its most active portion has developed since New Orleans was founded in 1717.
It is important to realise that the characteristics of a coast show imprints of different episodes in the history of its development. Australia for instance, has a large number of estuaries and lagoons although the relative sea level fell during the late Holocene. This is related to the dry climate and therefore very low sediment supply. The estuaries that have formed during the rising sea levels of the early Holocene can still be found at present, since not enough sediments were supplied to fill them in during late Holocene sea-level fall. Therefore the present-day coastline still shows the signs of early Holocene transgression and does not bear the typical characteristics of regression. Classification apparently implies an integration of the developments over a certain time-period. At different moments in time a certain coast might occupy a different position in the quadrant of Fig. 2.23. The determined classification class is therefore dependent on the integration period. An example of a situation where progradation and transgression have alternated over the geological history is the central Holland coast.