What is bruun effect?

Answer 1

The most widely cited method of quantifying the response of a shore to rising sea-levels is known as Bruun's rule. This was developed to describe the behaviour of sandy coasts with no cliff or shore platform. It assumes that the wave climate is steady and consequently the (average equilibrium) beach profile does not change, but does translate up with the sea-level. This rise in beach surface requires sand, which is assumed to be eroded from the upper beach and deposited on the lower beach. Thus as the profile rises with sea level it also translates landward, causing shoreline retreat. Note that despite the erosion of the upper beach no sand is actually lost; it simply translates a small distance down the profile. The Bruun rule has been the subject of some debate and criticism, but is still generally supported (e.g. Stive, 2004[1]) and a recent observational study by Zhang et al. (2004)[2] lends weight to it. They found that the Bruun rule modelled retreat of eastern USA shorelines well, although they recognised that it does not represent long-shore transport, and restricted their study to sites where this could be neglected.

Another constraint on the range of applicability of the Bruun rule results from its assumptions that the shore profile is entirely beach and loses no sediment. Along most Coastlines the beach is a surface deposit that can only be eroded by a limited amount before the land underlying it is exposed and attacked. Here the shore profile is composed of both beach and rock. The rock element of such composite shores complicates its behaviour because it can only erode (not accrete) and it is likely to contain material that is lost as fine sediment. In addition, being purely erosive and relatively hard, it will have a different equilibrium profile to that of the beach and will take longer to achieve it.

Modifications to the Bruun rule can be used to account for the loss of fine sediment (cfi Bray & Hooke 1997[3]) but not changes in profile form. Relatively little work has been done on the relationship between sea-level rise and the profiles of composite beach/rock shores. Recent results indicate that such profiles do change, becoming steeper as the rate of sea-level rise increases (Walkden & Hall, 2005[4]).

The Bruun rule predicts that rates of increase of sea-level rise and shoreline recession will be the same, i.e. R2/R1 = S2/S1 where R and S are the rates of equilibrium recession and sea-level rise respectively and 1 and 2 indicate historic and future conditions. Walkden & Dickson (2006)[5] predicted that low beach volume composite shores are rather less sensitive and that, for them, R2/R1 = sqrt(S2/S1), although, like the Bruun rule, this equation does not account for longshore interactions.

Dickson at al (2007) [6] modelled alongshore interactions along a 50 km stretch of composite beach/ rock coast under a range of sea-level rise scenarios. They demonstrated a marked increase in complexity of shore response to sea-level rise in areas where alongshore sediment transport was important, even observing some shoreline advance.

Shore wave heights are normally limited by water depth, so an increase in sea-level might be expected to increase waves at the shore. This appears to be true at composite beach/ rock shores, however it does not necessarily occur at beach shores. Bruun's model describes beach profiles remaining constant as they translate up and landward. This means that although the sea-level rises the water depth across the surf zone does not increase, and so larger waves can not be accommodated.