rip currents Archives

A safe harbour offers a place of refuge. Those in peril (or evading taxes), running before a storm, crossing a figurative bar to welcome respite. Non-figurative harbours, the coastal kind, have traditionally provided safe haven for mariners escaping inclement weather or foes.

Harbours fill and are emptied of seawater on the tide. Sea water that enters or exits is commonly focussed through narrow inlets. Here, powerful currents are generated that carry fish, sediment, flotsam, and unwary boats. Filling on an incoming tide is like a cleansing, a renewal; outgoing tides reveal channel arteries that keep alive the bars and broad flats of mud and sand, textured Kandinsky-like.

Northern New Zealand’s west coast has 6 harbours distributed along a 300 km stretch of coast. Each is protected by large sand barriers that have built over the last 2-3 million years with sand moved inshore by successive rises and falls of sea level.

Many of New Zealand’s harbours are drowned valleys, where rising sea level (following the last glaciation) has inundated dissected landscapes. Rising seas have crept up valleys, leaving the exposed high ground to front an intricately embayed coastline, islands, and estuaries that extend their marine fingers far inland.

New Zealand’s west coast is open to large swells, generated by westerly winds across a 2000 km expanse of Tasman Sea. Sea conditions along this coast are often rough. Access to the open sea via harbour inlets, requires sailors to ‘cross the bar’ – the zone of shallow, constantly moving sand. Strong tidal currents, particularly out-going tides can increase wave heights even further, as well as making wave conditions in general very choppy. The sea condition can change rapidly. Many a boat has come to grief across these west coast bars, a mix of bad luck and poor judgement (NIWA has real-time images of current bar conditions at several locations).

The oceanographic and geological term for sand bars at the entrance to harbours and lagoons is tidal delta. Tidal deltas can form on the seaward margin, in which case they are called ebb tidal deltas (because they are downstream of the outgoing tide). Those that form inside harbours and lagoons are flood tidal deltas where sand is deposited by incoming tides.

Raglan Harbour is small but it sports a very nice example of a symmetrical ebb tidal delta. The delta extends 1.5 – 2 km from the harbour mouth. Darker hues (image below) that mark the main channel contrast nicely the shallower sand bars on either side over which waves tend to break. These marginal sand deposits are called swash bars.

Westerly swells approach the coast with relatively straight crests. As they pass over the shallow delta platform, they move at a slower speed because they interact (friction) with the sea floor. Some of the wave energy is transferred to the sea floor such that sediment is moved as ripples and dunes. Slowing waves also build in amplitude (height); this is the region where waves break. However, the same waves in the adjacent, deeper water are moving at a faster pace – trace the crests of each wave and you will see it ‘bending’ around the delta.

Most of the tidal delta remains submerged even at low tide. Parts of the swash bar that are exposed during low tide show evidence for sand movement, mostly as ripples, large and small. Sand is moved during flood and ebb tides. The shape of these sand bars changes from one tide to the next, demonstrating that this is a dynamic environment.

The Raglan tidal delta consists almost entirely of sand. In contrast, Raglan Harbour and its estuaries contain a high proportion of mud. So where does all that sand come from?

The tidal delta is part of a much larger system of sand transfer – supply and demand from the adjacent continental shelf to the adjacent beaches, shallow sand bars (commonly formed by rip currents) and sand dunes. Sand in the inshore region is also moved along the coast by long-shore currents and it is this sand that continually feeds the delta. The delta in turn, via its main channel, moves sand back onto the shelf, completing the cycle.

The beach south of the tidal delta continually changes its profile. At times the profile is an uninterrupted swath of black sand along most of its length (about 3 km). At other times a significant volume of sand has been removed exposing ancient boulder deposits from nearby Karioi volcano; sand removal frequently occurs during stormy weather. The sand dunes also participate in this budgeting exercise. Sand transfer from the beach (and dunes) is probably a combination of movement directly offshore by rip currents and wave undertow, and long-shore movement towards the delta. Sand replenishment and removal from the beach, and addition to the tidal delta, is part of a much larger system of sand supply and demand – nature’s sand budget.

Sand moved onto the swash bars helps to replace sand that is removed by the deep, fast-moving channel. Channel flows in narrow inlets like the one at Raglan are commonly 4-6 km/hr (1-2 m/second), which may not sound fast (try swimming against it) but is sufficient to move large volumes of sediment during each tidal cycle. There are some small sand bars in the harbour itself, but the channel is an effective flushing mechanism that prevents the estuaries and tidal flats from clogging up.

Changes in sea level have a profound impact on coastal sand systems. If sea level falls, the beach and dunes would follow the retreating shoreline, the harbour would eventually become the domain of non-tidal rivers and swamps, and the main channel would be free to meander over a broad expanse of exposed continental shelf. Tidal deltas might be more ephemeral structures, constantly on the move. This was probably the scenario during the last glaciation, when sea level was more than 100m below its present position.

Perhaps of more immediate concern is a rise in sea level (the present situation) which would erode older foreshore beach and dune deposits, and destabilise some cliff areas south of the Harbour. The Surf Club at the south end of Ngarunui Beach would need to move – yet again. The Harbour area flooded at high tide would increase, resulting in a greater volume of seawater entering and exiting the narrow inlet. To accommodate this, the inlet would need to expand, or the speed of current flow would need to increase. Changes such as these would have an immediate effect on the size and shape of the tidal delta.

A rocky mound, 1500m above, and 119.44 km straight-line distance from the sea, is about as far as one can get in New Zealand from either the Tasman and Pacific coasts. The location is in central Otago, southern New Zealand, a region better known for its Pinot Noir. No one lives at this farthest point from the beach. In fact, most New Zealand folk live only a few kilometres from the sea. Hence the summer exodus. Any nostalgia for those snow-bound, icicled, northern hemisphere winter solstice festivities is short-lived, banished by squealing kids and crying gulls.

I don’t like putting a damper on this general sense of merriment but, despite all the signs that warn, all the cautions and reminders of potential dangers, people drown. New Zealand has one of the worst records for preventable drownings of any OECD country (113 in 2015). Most drownings occur at seaside beaches where rip currents are the leading cause of strife for swimmers – there were over 1300 rescues in 2015 (NZ), and most of those were plucked from rip currents. These statistics are repeated the world over.

As waves approach the shore they begin to interact with the sea floor, growing in amplitude (wave height) until they break. Waves in the surf zone move the water mass onto the beach. Gravity requires that all this water then moves down the beach slope, back to the surf zone. On all beaches, the return flow, or backwash, produces an undertow that flows beneath the incoming waves. Undertow occurs everywhere along a beach. Its influence is generally confined to the surf zone, and for the most part is not dangerous (although it can be quite a strong flow). Undertow IS NOT the same process as a rip current. Rip currents are not that same as tidal currents.

As water moves back into the surf zone, it commonly shifts sideways across the beach, a process referred to as along-shore drift. Most swimmers will have experienced this ‘drift’ when they find themselves farther along the beach from where they started (this is usually where the local surf lifeguard starts waving at you to return to the flagged swimming area). Rip currents form when the returning seawater is diverted and focused by shallow holes and sand bars on the sea floor. If the sand bars extend onto the beach, then the waterline will have a kind of point-like shape along the beach (as shown in the images here). These channel-like currents frequently extend beyond the surf zone. Rip currents are narrow flows (a few 10s of metres wide) that move rapidly offshore; current speeds of 4m/second have been recorded, speeds that are well beyond the ability of even the strongest swimmer. The currents are powerful because so much water is being focused through a relatively narrow gap. Rips can appear suddenly on any beach where there is appreciable wave activity. They can also form adjacent to rocky promontories.

Rip currents are best viewed from an elevated vantage; they are not easily seen from the water’s edge. Useful identifying features include:

The surface waters of a rip tend to be relatively calm or rippled,
The current cuts through the surf and is usually clear of large breaking waves,
Currents commonly carry flotsam (including swimmers) or sediment offshore, and hence may appear cloudy, and
A point-like, or cuspate shape to the waterline along the beach may indicated submerged sand bars – such features increase the likelihood that rip currents will form.

While swimmers and life guards tend to view rip currents with (respectful) dislike, people who study coastal processes see them as one process among many, that shape coastlines. On sandy coasts, sediment is constantly being transferred among the deeper offshore regions of the sea bed, the shore and beach, and sand dunes. Sometimes the beach or dunes are in sand deficit, and at other times in surplus. Breaking waves tend to move sand onshore, whereas undertow and rip currents tend to move sand seaward. There is generally a balance between the onshore and offshore transfer of sand, but this can be disrupted by seasonal changes in tidal currents and storm tracks, by decadal cycles in the movement of ocean water masses, and by longer-term rises or falls in sea-level. Rip currents, despite the risks they pose, are an important part of coastal sand budgeting and transfer . They are a geological phenomenon.

The advice normally given to anyone caught in a rip is DON’T PANIC (perhaps more easily said than done), and don’t try to swim against the current – you will not win that contest. Swim parallel to the beach and you will eventually exit the current. Surf life-saving folk put up flags where they assess the safest stretch of beach to be. It is common for these flags to be moved up or down the beach, as rips come and go. But these common-sense warnings are ignored by too many – if they’re lucky they won’t enter the books as another statistic.

Surf Life Saving Australia has produced this useful video

Geological Digressions

Earth science resources, images, glossaries, and SCICOMM for students and non-specialists

Tag Archives: rip currents

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