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22 Factsheet 11: Wave set-up, run-up and overtopping

wave set-up

Photograph showing wave run-up
over the beach crest at East Clive
in Hawkes Bay.

Waves contribute to coastal inundation hazards by three consecutive processes:

  • wave set-up – after incoming waves break, the average level of the water inside the surf zone to the beach is set up higher than the sea level offshore from the breaker zone
  • wave run-up – the extra height that broken waves reach as they run up the beach and adjacent coastal barrier (natural or artificial), until the wave energy is finally expended by friction and gravity
  • overtopping – the spill-over of waves as they reach the crest of the coastal barrier or defence structure, resulting in flooding of the land and properties behind the barrier.  Depending on the overtopping flow and character of the barrier, the barrier may breach, increasing the potential for further inundation.  Wave spray or splash over a coastal defence structure can be hazardous for transport networks, but inundation volumes are relatively small.

Wave set-up
Text description of figure: Summary of water level variations as waves approach a beach. A slight lowering of the still water level, called wave setdown, occurs just seaward of where waves break. Landward of where waves break an increase in the still water level occurs, wave set-up. As waves dissipate energy on the beach, the wave run up the beach reaches to a certain level, which is called the wave uprush or run-up limit. 

Wave set-up is influenced by the offshore wave height and wave period, together with the nearshore seabed slope.  These factors may be similar over large stretches of coast in the district, which is why wave set-up is sometimes included in the storm-tide level.

Wave run-up and overtopping at any coastal locality is usually quite site-specific, depending on factors such as beach slope, roughness of the beach (sand, gravel or large rocks), wave height, exposure to ocean swell, how close inshore waves can penetrate before breaking, and the characteristics of the land above the beach (eg, dunes, seawall, low cliffs).

Waves also play a major role in causing coastal erosion, by:

  • the run-up of high-energy storm waves resulting in erosion of the dune or cliff toe
  • large quantities of sediment being de-stabilised and moved back and forth between the beach and nearshore bars.  Gentle swell and more quiescent waves following a storm usually assist in ‘re-stocking’ a beach by slowly combing sediment back onto the beach, helping the beach to recover.  Sequencing of moderate to severe storms that generate high wave activity is also an important factor in the susceptibility of a beach or cliff to severe coastal erosion
  • variations in the rate of longshore movement of sediment (the movement is due to waves approaching the coast at an angle to the shoreline).  Erosion can occur in this situation, especially if the drift is predominantly in one direction when any structure or natural feature traps sediment behind it, ‘starving’ the down-drift coast.

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